feat: NCS-21 Write Scripts to automate certain tasks (#15)

Co-authored-by: shahdlala66 <shahd.lala66@gmail.com>
Reviewed-on: #15
Co-authored-by: Janis <janis.e.20@gmx.de>
Co-committed-by: Janis <janis.e.20@gmx.de>

.claude/CLAUDE.MD

@@ -1,120 +0,0 @@
-# Claude Code – Working Agreement
-
-## Workflow: Plan → Write Tests → Implement → Verify
-
-### 1. Plan First
-Before writing any code, produce an explicit plan:
-- Restate the requirement in your own words to confirm understanding.
-- List every file you intend to create or modify.
-- Identify risks or unknowns upfront.
-- Wait for confirmation **only** when the plan reveals an ambiguity that cannot be resolved from context. Otherwise proceed immediately.
-
-### 2. Write Tests
-Before implementing, write tests that should cover the new behaviour.
-Only write tests for the new behaviour.
-
-### 3. Implement
-Follow the plan. Do not add scope beyond what was agreed.
-
-### 4. Verify Every Requirement
-After implementation, go through each requirement one-by-one and confirm it is satisfied:
-- Run the relevant tests (unit, integration, or build check) for every changed module.
-- If a requirement **cannot** be fulfilled, do **not** silently skip it — document it immediately (see *Unresolved Requirements* below).
-
----
-
-## No Code Without Verification (Testing)
-
-- Every new behaviour must be covered by at least one automated test before the task is considered done.
-- Every bug fix must be accompanied by a regression test that fails before the fix and passes after.
-- Run `./gradlew :modules:<module>:test` (or the appropriate Gradle task) and confirm a green build before marking work complete.
-- If a test cannot be written for a legitimate reason, document it in `docs/unresolved.md` with an explanation.
-
----
-
-## Automatic Bug Fixing
-
-- When a test or build step fails, attempt to fix the root cause immediately — do **not** ask for permission.
-- Apply the fix, re-run the verification, and continue until green.
-- If the same failure persists after **three** fix attempts, stop, log the issue in `docs/unresolved.md`, and surface a concise summary.
-
----
-
-## Unresolved Requirements → `docs/unresolved.md`
-
-When a requirement or bug cannot be resolved, append an entry to `docs/unresolved.md`:
-
-```markdown
-## [YYYY-MM-DD] <Short title>
-
-**Requirement / Bug:**
-<What was requested or what failed>
-
-**Root Cause (if known):**
-<Why it cannot be resolved right now>
-
-**Attempted Fixes:**
-1. <What was tried>
-2. …
-
-**Suggested Next Step:**
-<What a human engineer should investigate>
-```
-
-Create the file if it does not exist. Never delete existing entries.
-
----
-
-## Project Structure
-
-```
-.                        ← Repository root (multi-project Gradle setup)
-├── build.gradle.kts     ← Root build file (shared plugins, dependency versions)
-├── settings.gradle.kts  ← Gradle settings (declares all subprojects)
-├── modules/             ← One subdirectory per microservice
-│   └── <service>/
-│       ├── build.gradle.kts
-│       └── src/
-└── docs/                ← Architecture Decision Records, API docs, unresolved issues
-    └── unresolved.md
-```
-
-### Conventions
-- All microservices live under `modules/{service-name}`. Never place service code in the root.
-- Shared configuration (dependency versions, plugin setup) belongs in the **root** `build.gradle.kts` or in `buildSrc` / a version catalog.
-- `settings.gradle.kts` must include every module via `include(":modules:<service>")`.
-- Architecture decisions go in `docs/adr/` as numbered Markdown files (`ADR-001-<title>.md`).
-- API contracts live in `/docs/api/`.
-- Unit tests extend `AnyFunSuite with Matchers` — no `@Test` annotations, no `: Unit` requirement
-- Integration tests use `@QuarkusTest` with JUnit 5 — `@Test` methods must be explicitly typed `: Unit`
-- Always exclude scala-library from Quarkus deps to avoid Scala 2 conflicts
-
-## Coverage Conventions
-- Branch coverage must be at least 90% - unless there is a good reason not to.
-- Line coverage must be at least 95% - unless there is a good reason not to.
-- Method coverage must be at least 90% - unless there is a good reason not to.
-- To check coverage use jacoco-reporter/scoverage_coverage_gaps.py modules/{service}/build/reports/scoverageTest/scoverage.xml
-- IMPORTANT: modules/{service}/build/reports/scoverage/scoverage.xml is not used for coverage TEST calculation. Do not use it.
-
-## Agent Routing Rules
-
-### Use agents in PARALLEL when:
-- Tasks touch different, independent microservices
-- No shared files or state between tasks
-- Example: "implement service-user AND service-orders simultaneously"
-
-### Use agents SEQUENTIALLY when:
-- Tasks have dependencies (architect → implementer → test-writer)
-- Shared API contracts are involved
-- Example: design API first, then implement, then test
-
-## Quick-Reference Checklist
-
-Before considering any task done, confirm:
-
-- [ ] Plan was written and requirements restated
-- [ ] All planned files were created / modified
-- [ ] Automated tests cover the new behaviour
-- [ ] `./gradlew build` (or scoped task) is green
-- [ ] Each requirement has been explicitly verified
-- [ ] Any unresolved items are logged in `docs/unresolved.md`

.claude/agents/scala-implementer.md

@@ -2,9 +2,10 @@
 name: scala-implementer
 description: "Implements Scala 3 + Quarkus REST services, domain logic, and persistence"
 tools: Read, Write, Edit, Bash, Glob
-model: sonnet
+model: inherit
 color: pink
 ---
+
 You do not have permissions to write tests, just source code.
 You are a Scala 3 expert specialising in Quarkus microservices.
 Always read the relevant /docs/api/ file before implementing.

.claude/agents/test-writer.md

@@ -2,9 +2,10 @@
 name: test-writer
 description: "Writes QuarkusTest unit and integration tests for a service. Invoke after scala-implementer has finished."
 tools: Read, Write, Edit, Bash, Glob, Grep, WebFetch, WebSearch, NotebookEdit
-model: sonnet
+model: haiku
 color: purple
 ---
+
 You do not have permissions to modify the source code, just write tests.
 You write tests for Scala 3 + Quarkus services.
 
@@ -12,12 +13,11 @@ You write tests for Scala 3 + Quarkus services.
 - Unit tests: `extends AnyFunSuite with Matchers` — use `test("description") { ... }` DSL, no `@Test` annotation, no `: Unit` return type needed.
 - Integration tests: `@QuarkusTest` with JUnit 5 — `@Test` methods MUST be explicitly typed `: Unit`.
 
-Target 95%+ conditional coverage.
+Target 100% conditional coverage if possible.
 
 When invoked BEFORE scala-implementer (no implementation exists yet):
   Use the contract-first-test-writing skill — write failing tests from docs/api/{service}.yaml.
 
 When invoked AFTER scala-implementer (implementation exists):
   Run python3 jacoco-reporter/jacoco_coverage_gaps.py modules/{service-name}/build/reports/jacoco/test/jacocoTestReport.xml --output agent
-  Use the jacoco-coverage-gaps skill — close coverage gaps revealed by the report.
   To regenerate the report run the tests first.

.claude/memory/MEMORY.md

@@ -0,0 +1,9 @@
+# Memory Index
+
+## Feedback
+- [feedback_keep_structure_updated.md](feedback_keep_structure_updated.md) — Update structure memory files whenever source files are added, removed, or changed
+
+## Project Structure
+- [project_structure_root.md](project_structure_root.md) — Top-level layout, modules list, VERSIONS map, navigation rules (skip `build/`, `.gradle/`, `.idea/`)
+- [project_structure_api.md](project_structure_api.md) — `modules/api`: all files and types (Board, Piece, Square, GameState, Move, ApiResponse, PlayerInfo)
+- [project_structure_core.md](project_structure_core.md) — `modules/core`: all files and types (GameContext, GameRules, MoveValidator, GameController, Parser, Renderer)

.claude/memory/feedback_keep_structure_updated.md

@@ -0,0 +1,16 @@
+---
+name: keep-structure-memory-updated
+description: Always update the project structure memory files when adding, removing, or changing source files
+type: feedback
+---
+
+After any change that adds, removes, renames, or significantly alters a source file, update the relevant structure memory file:
+
+- New/renamed/deleted file in `modules/api` → update `project_structure_api.md`
+- New/renamed/deleted file in `modules/core` → update `project_structure_core.md`
+- New module, dependency version change, or new top-level directory → update `project_structure_root.md`
+- New module added → create a new `project_structure_<module>.md` and add it to `MEMORY.md`
+
+**Why:** Structure memories are the primary navigation aid. Stale entries cause wasted exploration.
+
+**How to apply:** Treat the structure memory update as part of completing any implementation task — do it in the same session, not as a follow-up.

.claude/memory/project_structure_api.md

@@ -0,0 +1,51 @@
+---
+name: module-api-structure
+description: File and type overview for the modules/api module (shared domain types)
+type: project
+---
+
+# Module: `modules/api`
+
+**Purpose:** Shared domain model — pure data types with no game logic. Depended on by `modules/core`.
+
+**Gradle:** `id("scala")`, no `application` plugin. No Quarkus. Uses scoverage plugin.
+
+**Package root:** `de.nowchess.api`
+
+## Source files (`src/main/scala/de/nowchess/api/`)
+
+### `board/`
+| File | Contents |
+|------|----------|
+| `Board.scala` | `opaque type Board = Map[Square, Piece]` — extensions: `pieceAt`, `withMove`, `pieces`; `Board.initial` sets up start position |
+| `Color.scala` | `enum Color { White, Black }` — `.opposite`, `.label` |
+| `Piece.scala` | `case class Piece(color, pieceType)` — convenience vals `WhitePawn`…`BlackKing` |
+| `PieceType.scala` | `enum PieceType { Pawn, Knight, Bishop, Rook, Queen, King }` — `.label` |
+| `Square.scala` | `enum File { A–H }`, `enum Rank { R1–R8 }`, `case class Square(file, rank)` — `.toString` algebraic, `Square.fromAlgebraic(s)` |
+
+### `game/`
+| File | Contents |
+|------|----------|
+| `GameState.scala` | `case class CastlingRights(kingSide, queenSide)` + `.None`/`.Both`; `enum GameResult { WhiteWins, BlackWins, Draw }`; `enum GameStatus { NotStarted, InProgress, Finished(result) }`; `case class GameState(piecePlacement, activeColor, castlingWhite, castlingBlack, enPassantTarget, halfMoveClock, fullMoveNumber, status)` — FEN-compatible snapshot |
+
+### `move/`
+| File | Contents |
+|------|----------|
+| `Move.scala` | `enum PromotionPiece { Knight, Bishop, Rook, Queen }`; `enum MoveType { Normal, CastleKingside, CastleQueenside, EnPassant, Promotion(piece) }`; `case class Move(from, to, moveType = Normal)` |
+
+### `player/`
+| File | Contents |
+|------|----------|
+| `PlayerInfo.scala` | `opaque type PlayerId = String`; `case class PlayerInfo(id: PlayerId, displayName: String)` |
+
+### `response/`
+| File | Contents |
+|------|----------|
+| `ApiResponse.scala` | `sealed trait ApiResponse[+A]` → `Success[A](data)` / `Failure(errors)`; `case class ApiError(code, message, field?)`; `case class Pagination(page, pageSize, totalItems)` + `.totalPages`; `case class PagedResponse[A](items, pagination)` |
+
+## Test files (`src/test/scala/de/nowchess/api/`)
+Mirror of main structure — one `*Test.scala` per source file using `AnyFunSuite with Matchers`.
+
+## Notes
+- `GameState` is FEN-style but `Board` (in `core`) is a `Map[Square,Piece]` — the two are separate representations
+- `CastlingRights` is defined here in `api`; the castling logic lives in `core`

.claude/memory/project_structure_core.md

@@ -0,0 +1,48 @@
+---
+name: module-core-structure
+description: File and type overview for the modules/core module (TUI chess engine)
+type: project
+---
+
+# Module: `modules/core`
+
+**Purpose:** Standalone TUI chess application. All game logic, move validation, rendering. Depends on `modules/api`.
+
+**Gradle:** `id("scala")` + `application` plugin. Main class: `de.nowchess.chess.Main`. Uses scoverage plugin.
+
+**Package root:** `de.nowchess.chess`
+
+## Source files (`src/main/scala/de/nowchess/chess/`)
+
+### Root
+| File | Contents |
+|------|----------|
+| `Main.scala` | Entry point — prints welcome, starts `GameController.gameLoop(GameContext.initial, Color.White)` |
+
+### `controller/`
+| File | Contents |
+|------|----------|
+| `GameController.scala` | `sealed trait MoveResult` ADT: `Quit`, `InvalidFormat`, `NoPiece`, `WrongColor`, `IllegalMove`, `Moved`, `MovedInCheck`, `Checkmate`, `Stalemate`; `object GameController` — `processMove(ctx, turn, raw): MoveResult` (pure), `gameLoop(ctx, turn)` (I/O loop), `applyRightsRevocation(...)` (castling rights bookkeeping) |
+| `Parser.scala` | `object Parser` — `parseMove(input): Option[(Square, Square)]` parses coordinate notation e.g. `"e2e4"` |
+
+### `logic/`
+| File | Contents |
+|------|----------|
+| `GameContext.scala` | `enum CastleSide { Kingside, Queenside }`; `case class GameContext(board, whiteCastling, blackCastling)` — `.castlingFor(color)`, `.withUpdatedRights(color, rights)`; `GameContext.initial`; `extension (Board).withCastle(color, side)` moves king+rook atomically |
+| `GameRules.scala` | `enum PositionStatus { Normal, InCheck, Mated, Drawn }`; `object GameRules` — `isInCheck(board, color)`, `legalMoves(ctx, color): Set[(Square,Square)]`, `gameStatus(ctx, color): PositionStatus` |
+| `MoveValidator.scala` | `object MoveValidator` — `isLegal(board, from, to)`, `legalTargets(board, from): Set[Square]` (board-only, no castling), `legalTargets(ctx, from)` (context-aware, includes castling), `isCastle`, `castleSide`, `castlingTargets(ctx, color)` — full castling legality (empty squares, no check through transit) |
+
+### `view/`
+| File | Contents |
+|------|----------|
+| `Renderer.scala` | `object Renderer` — `render(board): String` outputs ANSI-colored board with file/rank labels |
+| `PieceUnicode.scala` | `extension (Piece).unicode: String` maps each piece to its Unicode chess symbol |
+
+## Test files (`src/test/scala/de/nowchess/chess/`)
+Mirror of main structure — one `*Test.scala` per source file using `AnyFunSuite with Matchers`.
+
+## Key design notes
+- `MoveValidator` has two overloaded `legalTargets`: one takes `Board` (geometry only), one takes `GameContext` (adds castling)
+- `GameRules.legalMoves` filters by check — it calls `MoveValidator.legalTargets(ctx, from)` then simulates each move
+- Castling rights revocation is in `GameController.applyRightsRevocation`, triggered after every move
+- No `@QuarkusTest` — this module is a plain Scala application, not a Quarkus service

.claude/memory/project_structure_root.md

@@ -0,0 +1,55 @@
+---
+name: project-root-structure
+description: Top-level project structure, modules list, and navigation notes for NowChessSystems
+type: project
+---
+
+# NowChessSystems — Root Structure
+
+## Directory layout (skip `build/`, `.gradle/`, `.idea/`)
+
+```
+NowChessSystems/
+├── build.gradle.kts          # Root: sonarqube plugin, VERSIONS map
+├── settings.gradle.kts       # include(":modules:core", ":modules:api")
+├── gradlew / gradlew.bat
+├── CLAUDE.md                 # Project instructions for Claude Code
+├── .claude/
+│   ├── CLAUDE.MD             # Working agreement (plan/verify/unresolved)
+│   ├── settings.json
+│   └── agents/               # architect, code-reviewer, gradle-builder, scala-implementer, test-writer
+├── docs/
+│   ├── Claude-Skills.md
+│   ├── Security.md
+│   └── unresolved.md
+├── jacoco-reporter/          # Python scripts for coverage gap reporting
+└── modules/
+    ├── api/                  # Shared domain types (no logic)
+    └── core/                 # TUI chess engine + game logic
+```
+
+## Modules
+
+| Module | Gradle path | Purpose |
+|--------|-------------|---------|
+| `api` | `:modules:api` | Shared domain model: Board, Piece, Move, GameState, ApiResponse |
+| `core` | `:modules:core` | TUI chess app: game logic, move validation, rendering |
+
+`core` depends on `api` via `implementation(project(":modules:api"))`.
+
+## VERSIONS (root `build.gradle.kts`)
+
+| Key | Value |
+|-----|-------|
+| `QUARKUS_SCALA3` | 1.0.0 |
+| `SCALA3` | 3.5.1 |
+| `SCALA_LIBRARY` | 2.13.18 |
+| `SCALATEST` | 3.2.19 |
+| `SCALATEST_JUNIT` | 0.1.11 |
+| `SCOVERAGE` | 2.1.1 |
+
+## Navigation rules
+- **Always skip** `build/`, `.gradle/`, `.idea/` when exploring — they are generated artifacts
+- Tests use `AnyFunSuite with Matchers` (ScalaTest), not JUnit `@Test`
+- No Quarkus in current modules — Quarkus is planned for future services
+- Agent workflow: architect → scala-implementer → test-writer → gradle-builder → code-reviewer

.idea/AndroidProjectSystem.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="AndroidProjectSystem">
+    <option name="providerId" value="com.android.tools.idea.GradleProjectSystem" />
+  </component>
+</project>

.idea/gradle.xml

@@ -12,6 +12,7 @@
             <option value="$PROJECT_DIR$/modules" />
             <option value="$PROJECT_DIR$/modules/api" />
             <option value="$PROJECT_DIR$/modules/core" />
+            <option value="$PROJECT_DIR$/modules/ui" />
           </set>
         </option>
       </GradleProjectSettings>

.idea/misc.xml

@@ -1,4 +1,3 @@
-<?xml version="1.0" encoding="UTF-8"?>
 <project version="4">
   <component name="ExternalStorageConfigurationManager" enabled="true" />
   <component name="FrameworkDetectionExcludesConfiguration">

.idea/scala_compiler.xml

@@ -5,7 +5,7 @@
       <option name="deprecationWarnings" value="true" />
       <option name="uncheckedWarnings" value="true" />
     </profile>
-    <profile name="Gradle 2" modules="NowChessSystems.modules.core.main,NowChessSystems.modules.core.scoverage,NowChessSystems.modules.core.test">
+    <profile name="Gradle 2" modules="NowChessSystems.modules.core.main,NowChessSystems.modules.core.scoverage,NowChessSystems.modules.core.test,NowChessSystems.modules.ui.main,NowChessSystems.modules.ui.scoverage,NowChessSystems.modules.ui.test">
       <option name="deprecationWarnings" value="true" />
       <option name="uncheckedWarnings" value="true" />
       <parameters>

.idea/vcs.xml

@@ -6,6 +6,16 @@
       <inspection_tool class="CommitNamingConvention" enabled="true" level="WARNING" enabled_by_default="true" />
     </profile>
   </component>
+  <component name="IssueNavigationConfiguration">
+    <option name="links">
+      <list>
+        <IssueNavigationLink>
+          <option name="issueRegexp" value="(?x)\b(CORE|NCWF|BAC|FRO|K8S|ORG|NCI|NCS)-\d+\b#YouTrack" />
+          <option name="linkRegexp" value="https://knockoutwhist.youtrack.cloud/issue/$0" />
+        </IssueNavigationLink>
+      </list>
+    </option>
+  </component>
   <component name="VcsDirectoryMappings">
     <mapping directory="" vcs="Git" />
   </component>

ARABIAN CHESS/license.txt

@@ -0,0 +1,7 @@
+YOU CAN:
+- Edit and use the asset in any commercial  or non commercial project 
+- Use the asset in any commercial  or non commercial project
+
+YOU CAN'T:
+- Resell or distribute the asset to others
+- Edit and resell the asset to others - - Credits required using This link: https://fatman200.itch.io/

CLAUDE.md

@@ -1,58 +1,58 @@
-# CLAUDE.md
+# CLAUDE.md — NowChessSystems
 
-This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+## Stack
+Scala 3.5.x · Quarkus + quarkus-scala3 · Hibernate/Jakarta · Lanterna TUI · K8s + ArgoCD + Kargo · Frontend TBD (Vite/React/Angular/Vue)
 
-## Build & Test Commands
+### Memory
 
+Your memory is saved under .claude/memory/MEMORY.md.
+
+## Structure
+```
+build.gradle.kts / settings.gradle.kts   # root; include(":modules:<svc>") per service
+modules/<svc>/build.gradle.kts + src/
+docs/adr/  docs/api/  docs/unresolved.md
+```
+Versions in root `extra["VERSIONS"]`; modules read via `rootProject.extra["VERSIONS"] as Map<String,String>`.
+
+## Commands
 ```bash
-# Build everything
 ./gradlew build
-
-# Build a single module
-./gradlew :modules:<service>:build
-
-# Run tests for a single module
-./gradlew :modules:<service>:test
-
-# Run a specific test class
-./gradlew :modules:<service>:test --tests "de.nowchess.<service>.<ClassName>"
+./gradlew :modules:<svc>:build|test
+./gradlew :modules:<svc>:test --tests "de.nowchess.<svc>.<Class>"
 ```
 
-The only current module is `core` (`modules/core`).
+## Workflow
+1. **Plan** — restate requirement, list files, flag risks. Proceed unless genuine ambiguity.
+2. **Tests first** — cover only new behaviour.
+3. **Implement** — no scope creep.
+4. **Verify** — check each requirement; confirm green build.
 
-## Architecture
+## Scala/Quarkus Rules
+- `given`/`using` only (no `implicit`); `Option`/`Either`/`Try` (no `null`/`.get`)
+- `jakarta.*` only; reactive I/O (`Uni`/`Multi`), no blocking on event loop
+- Always exclude `org.scala-lang:scala-library` from Quarkus BOM
+- Unit tests: `extends AnyFunSuite with Matchers` — no `@Test`, no `: Unit`
+- Integration tests: `@QuarkusTest` + JUnit 5 — `@Test` methods need explicit `: Unit`
 
-**NowChessSystems** is a chess platform built as a Scala 3 + Quarkus microservice system.
+## Coverage
+Line = 100% · Branch = 100% · Method = 100% · Regression tests · document exceptions
+Check: `jacoco-reporter/scoverage_coverage_gaps.py modules/{svc}/build/reports/scoverageTest/scoverage.xml`
+⚠️ Use `scoverageTest/`, NOT `scoverage/`.
 
-- Multi-module Gradle project; every service lives under `modules/{service-name}`.
-- Shared dependency versions live in the root `build.gradle.kts` under `extra["VERSIONS"]`.
-- Each module reads versions via `rootProject.extra["VERSIONS"] as Map<String, String>`.
-- `settings.gradle.kts` must `include(":modules:<service>")` for every module.
+## Bug Fixing
+Fix failures immediately without asking. After 3 failed attempts → log in `docs/unresolved.md` + surface summary.
 
-### Stack (ADR-001)
-| Layer | Technology |
-|---|---|
-| Language | Scala 3.5.x |
-| Backend framework | Quarkus + `quarkus-scala3` extension |
-| Persistence | Hibernate / Jakarta Persistence |
-| Frontend (TBD) | Vite; React/Angular/Vue under evaluation |
-| TUI | Lanterna |
-| Container orchestration | Kubernetes + ArgoCD + Kargo |
+## Agents (new service)
+Sequential: architect → scala-implementer → test-writer → gradle-builder → code-reviewer (review only, no self-fix)
+Parallel: only when services are fully independent (no shared contracts/state).
 
-### Key Scala 3 / Quarkus Rules
-- Use `given`/`using`, not `implicit` (no Scala 2 idioms).
-- Use `Option`/`Either`/`Try`, never `null` or `.get`.
-- Jakarta annotations only (`jakarta.*`), never `javax.*`.
-- Use reactive types (`Uni`, `Multi`) for I/O; no blocking calls on the event loop.
-- **Always exclude `org.scala-lang:scala-library` from Quarkus BOM** to avoid Scala 2 conflicts.
-- **Unit tests use `extends AnyFunSuite with Matchers`** — ScalaTest DSL, no `@Test` annotations needed.
-- **Integration tests use `@QuarkusTest` with JUnit 5** — explicit `: Unit` return type still required on `@Test` methods.
+## Unresolved (`docs/unresolved.md`)
+Append only, never delete:
+```
+## [YYYY-MM-DD] <title>
+**Requirement/Bug:** **Root Cause:** **Attempted Fixes:** **Next Step:**
+```
 
-### Agent Workflow (for new services)
-1. **architect** → writes OpenAPI contract to `docs/api/{service}.yaml` and ADR to `docs/adr/`.
-2. **scala-implementer** → reads contract, implements service under `modules/{service}/`.
-3. **test-writer** → writes `@QuarkusTest` integration tests and `AnyFunSuite with Matchers` unit tests.
-4. **gradle-builder** → resolves any build/dependency issues.
-5. **code-reviewer** → reviews; reports findings back without self-fixing.
-
-Detailed working agreement (plan/verify/unresolved workflow) is in `.claude/CLAUDE.MD`.
+## Done Checklist
+- [ ] Plan written · files created/modified · tests green · requirements verified · unresolved logged

build.gradle.kts

@@ -1,5 +1,6 @@
 plugins {
     id("org.sonarqube") version "7.2.3.7755"
+    id("org.scoverage") version "8.1" apply false
 }
 
 group = "de.nowchess"
@@ -28,7 +29,10 @@ val versions = mapOf(
     "SCALA_LIBRARY"         to "2.13.18",
     "SCALATEST"             to "3.2.19",
     "SCALATEST_JUNIT"       to "0.1.11",
-    "SCOVERAGE"             to "2.1.1"
+    "SCOVERAGE"             to "2.1.1",
+    "SCALAFX"               to "21.0.0-R32",
+    "JAVAFX"                to "21.0.1",
+    "JUNIT_BOM"             to "5.13.4"
 )
 extra["VERSIONS"] = versions
 

clean

@@ -0,0 +1,4 @@
+#! /usr/bin/env bash
+set -euo pipefail
+
+./gradlew clean

compile

@@ -0,0 +1,4 @@
+#! /usr/bin/env bash
+set -euo pipefail
+
+./gradlew classes

coverage

@@ -0,0 +1,10 @@
+#! /usr/bin/env bash
+set -euo pipefail
+
+./gradlew test
+
+if [ "$#" -eq 0 ]; then
+  python3 jacoco-reporter/scoverage_coverage_gaps.py
+else
+  python3 jacoco-reporter/scoverage_coverage_gaps.py "modules/$1/build/reports/scoverageTest/scoverage.xml"
+fi

docs/unresolved.md

@@ -0,0 +1,20 @@
+## [2026-03-31] Unreachable code blocking 100% statement coverage
+
+**Requirement/Bug:** Reach 100% statement coverage in core module.
+
+**Root Cause:** 4 remaining uncovered statements (99.6% coverage) are unreachable code:
+1. **PgnParser.scala:160** (`case _ => None` in extractPromotion) - Regex `=([QRBN])` only matches those 4 characters; fallback case can never execute
+2. **GameHistory.scala:29** (`addMove$default$4` compiler-generated method) - Method overload 3 without defaults shadows the 4-param version, making promotionPiece default accessor unreachable
+3. **GameEngine.scala:201-202** (`case _` in completePromotion) - GameController.completePromotion always returns one of 4 expected MoveResult types; catch-all is defensive code
+
+**Attempted Fixes:**
+1. Added comprehensive PGN parsing tests (all 4 promotion types) - PgnParser improved from 95.8% to 99.4%
+2. Added GameHistory tests using named parameters - hit `addMove$default$3` (castleSide) but not `$default$4` (promotionPiece)
+3. Named parameter approach: `addMove(from=..., to=..., promotionPiece=...)` triggers 4-param with castleSide default ✓
+4. Positional approach: `addMove(f, t, None, None)` requires all 4 args (explicit, no defaults used) - doesn't hit $default$4
+5. Root issue: Scala's overload resolution prefers more-specific non-default overloads (2-param, 3-param) over the 4-param with defaults
+
+**Recommendation:** 99.6% (1029/1033) is maximum achievable without refactoring method overloads. Unreachable code design patterns:
+- **Pattern 1 (unreachable regex fallback):** Defensive pattern match against exhaustive regex
+- **Pattern 2 (overshadowed defaults):** Method overloads shadow default parameters in parent signature
+- **Pattern 3 (defensive catch-all):** Error handling for impossible external API returns

jacoco-reporter/jacoco_coverage_gaps.py

@@ -1,411 +0,0 @@
-#!/usr/bin/env python3
-"""
-JaCoCo Coverage Gap Reporter
-Parses a JaCoCo XML report and outputs missing line & branch (conditional)
-coverage in a structured format that Claude Code agents can act on directly.
-
-Usage:
-    python jacoco_coverage_gaps.py <jacoco-report.xml> [--min-coverage 80]
-    python jacoco_coverage_gaps.py <jacoco-report.xml> --output json
-    python jacoco_coverage_gaps.py <jacoco-report.xml> --output markdown
-    python jacoco_coverage_gaps.py <jacoco-report.xml> --output agent   (default)
-"""
-
-import xml.etree.ElementTree as ET
-import sys
-import argparse
-import json
-from pathlib import Path
-from dataclasses import dataclass, field
-from typing import Optional
-
-
-# ---------------------------------------------------------------------------
-# Data classes
-# ---------------------------------------------------------------------------
-
-@dataclass
-class LineCoverage:
-    line_number: int
-    hits: int                   # 0 = not executed
-    branch_total: int = 0       # 0 = not a branch point
-    branch_covered: int = 0
-
-    @property
-    def is_uncovered(self) -> bool:
-        return self.hits == 0
-
-    @property
-    def is_partial_branch(self) -> bool:
-        return self.branch_total > 0 and self.branch_covered < self.branch_total
-
-
-@dataclass
-class MethodCoverage:
-    name: str
-    descriptor: str
-    first_line: Optional[int]
-    missed_instructions: int
-    covered_instructions: int
-    missed_branches: int
-    covered_branches: int
-    uncovered_lines: list[int] = field(default_factory=list)
-    partial_branch_lines: list[int] = field(default_factory=list)
-
-    @property
-    def total_branches(self) -> int:
-        return self.missed_branches + self.covered_branches
-
-    @property
-    def is_fully_covered(self) -> bool:
-        return self.missed_instructions == 0 and self.missed_branches == 0
-
-    @property
-    def branch_coverage_pct(self) -> float:
-        total = self.total_branches
-        return 100.0 * self.covered_branches / total if total else 100.0
-
-    @property
-    def line_coverage_pct(self) -> float:
-        total = self.missed_instructions + self.covered_instructions
-        return 100.0 * self.covered_instructions / total if total else 100.0
-
-
-@dataclass
-class ClassCoverage:
-    class_name: str             # e.g. com/example/Foo
-    source_file: Optional[str]
-    methods: list[MethodCoverage] = field(default_factory=list)
-    all_lines: list[LineCoverage] = field(default_factory=list)
-
-    @property
-    def java_class_name(self) -> str:
-        return self.class_name.replace("/", ".")
-
-    @property
-    def source_path(self) -> Optional[str]:
-        """Best-guess relative source path."""
-        if self.source_file:
-            package = "/".join(self.class_name.split("/")[:-1])
-            return f"src/main/java/{package}/{self.source_file}" if package else f"src/main/java/{self.source_file}"
-        return None
-
-    @property
-    def uncovered_lines(self) -> list[int]:
-        return sorted({l.line_number for l in self.all_lines if l.is_uncovered})
-
-    @property
-    def partial_branch_lines(self) -> list[int]:
-        return sorted({l.line_number for l in self.all_lines if l.is_partial_branch})
-
-    @property
-    def missed_branches(self) -> int:
-        return sum(max(l.branch_total - l.branch_covered, 0) for l in self.all_lines)
-
-    @property
-    def total_branches(self) -> int:
-        return sum(l.branch_total for l in self.all_lines)
-
-    @property
-    def covered_branches(self) -> int:
-        return self.total_branches - self.missed_branches
-
-    @property
-    def missed_lines(self) -> int:
-        return len(self.uncovered_lines)
-
-    @property
-    def total_lines(self) -> int:
-        return len(self.all_lines)
-
-    @property
-    def covered_lines(self) -> int:
-        return self.total_lines - self.missed_lines
-
-    @property
-    def has_gaps(self) -> bool:
-        return bool(self.uncovered_lines or self.partial_branch_lines)
-
-
-# ---------------------------------------------------------------------------
-# Parser
-# ---------------------------------------------------------------------------
-
-def parse_jacoco_xml(xml_path: str) -> list[ClassCoverage]:
-    """Parse a JaCoCo XML report into ClassCoverage objects."""
-    tree = ET.parse(xml_path)
-    root = tree.getroot()
-
-    results: list[ClassCoverage] = []
-
-    for package in root.iter("package"):
-        for cls_elem in package.findall("class"):
-            class_name = cls_elem.get("name", "")
-            source_file = cls_elem.get("sourcefilename")
-
-            # Build method map from <method> children
-            methods: list[MethodCoverage] = []
-            for m in cls_elem.findall("method"):
-                counters = {c.get("type"): c for c in m.findall("counter")}
-
-                def _missed(t): return int(counters[t].get("missed", 0)) if t in counters else 0
-                def _covered(t): return int(counters[t].get("covered", 0)) if t in counters else 0
-
-                methods.append(MethodCoverage(
-                    name=m.get("name", ""),
-                    descriptor=m.get("desc", ""),
-                    first_line=int(m.get("line")) if m.get("line") else None,
-                    missed_instructions=_missed("INSTRUCTION"),
-                    covered_instructions=_covered("INSTRUCTION"),
-                    missed_branches=_missed("BRANCH"),
-                    covered_branches=_covered("BRANCH"),
-                ))
-
-            cc = ClassCoverage(
-                class_name=class_name,
-                source_file=source_file,
-                methods=methods,
-            )
-
-            # Per-line data lives in the matching <sourcefile> element
-            source_file_elem = package.find(f"sourcefile[@name='{source_file}']") if source_file else None
-            if source_file_elem is not None:
-                for line_elem in source_file_elem.findall("line"):
-                    nr = int(line_elem.get("nr", 0))
-                    mi = int(line_elem.get("mi", 0))   # missed instructions
-                    ci = int(line_elem.get("ci", 0))   # covered instructions
-                    mb = int(line_elem.get("mb", 0))   # missed branches
-                    cb = int(line_elem.get("cb", 0))   # covered branches
-                    hits = ci  # ci > 0 means line was executed at least once
-                    cc.all_lines.append(LineCoverage(
-                        line_number=nr,
-                        hits=hits,
-                        branch_total=mb + cb,
-                        branch_covered=cb,
-                    ))
-
-            if cc.has_gaps:
-                results.append(cc)
-
-    return results
-
-
-# ---------------------------------------------------------------------------
-# Formatters
-# ---------------------------------------------------------------------------
-
-def _compact_ranges(numbers: list[int]) -> str:
-    """Turn [1,2,3,5,7,8,9] -> '1-3, 5, 7-9'"""
-    if not numbers:
-        return ""
-    ranges = []
-    start = prev = numbers[0]
-    for n in numbers[1:]:
-        if n == prev + 1:
-            prev = n
-        else:
-            ranges.append(f"{start}-{prev}" if start != prev else str(start))
-            start = prev = n
-    ranges.append(f"{start}-{prev}" if start != prev else str(start))
-    return ", ".join(ranges)
-
-
-def format_agent(classes: list[ClassCoverage]) -> str:
-    """
-    Output optimised for Claude Code agents:
-    – structured, machine-readable yet human-legible
-    – uses file paths and line numbers agents can act on
-    – groups by file, sorts by severity (most gaps first)
-    """
-    lines: list[str] = []
-    lines.append("# JaCoCo Coverage Gaps — Agent Action Report")
-    lines.append("")
-    lines.append("## Summary")
-    total_uncovered = sum(c.missed_lines for c in classes)
-    total_partial = sum(len(c.partial_branch_lines) for c in classes)
-    total_missed_branches = sum(c.missed_branches for c in classes)
-    lines.append(f"- Files with gaps : {len(classes)}")
-    lines.append(f"- Uncovered lines : {total_uncovered}")
-    lines.append(f"- Partial branches: {total_partial} lines affected")
-    lines.append(f"- Missed branches : {total_missed_branches} branch paths")
-    lines.append("")
-    lines.append("---")
-    lines.append("")
-    lines.append("## Files Requiring Tests")
-    lines.append("")
-    lines.append("> Each entry lists the SOURCE FILE PATH, the LINE NUMBERS that need")
-    lines.append("> coverage, and the METHODS that contain those gaps.")
-    lines.append("> Write or extend unit/integration tests to exercise these paths.")
-    lines.append("")
-
-    # Sort: most uncovered lines first
-    sorted_classes = sorted(classes, key=lambda c: -(c.missed_lines + len(c.partial_branch_lines)))
-
-    for cls in sorted_classes:
-        source = cls.source_path or f"(source unknown) {cls.java_class_name}"
-        lines.append(f"### `{source}`")
-        lines.append(f"**Class**: `{cls.java_class_name}`")
-        lines.append("")
-
-        if cls.uncovered_lines:
-            lines.append(f"#### ❌ Uncovered Lines")
-            lines.append(f"Lines not executed at all: `{_compact_ranges(cls.uncovered_lines)}`")
-            lines.append("")
-            lines.append("**Methods with uncovered lines:**")
-            for method in cls.methods:
-                uncov = [l for l in cls.uncovered_lines
-                         if method.first_line and l >= method.first_line]
-                # heuristic: only attribute if there are uncovered lines near the method start
-                if method.missed_instructions > 0:
-                    sig = f"`{method.name}{method.descriptor}`"
-                    pct = method.line_coverage_pct
-                    lines.append(f"  - {sig} — {pct:.0f}% instruction coverage")
-            lines.append("")
-
-        if cls.partial_branch_lines:
-            lines.append(f"#### ⚠️  Partial Branch Coverage (Missing Conditional Paths)")
-            lines.append(f"Lines where not all branches are taken: `{_compact_ranges(cls.partial_branch_lines)}`")
-            lines.append("")
-            lines.append("**Methods with branch gaps:**")
-            for method in cls.methods:
-                if method.missed_branches > 0:
-                    sig = f"`{method.name}{method.descriptor}`"
-                    pct = method.branch_coverage_pct
-                    missing = method.missed_branches
-                    lines.append(f"  - {sig} — {pct:.0f}% branch coverage ({missing} branch path(s) never taken)")
-            lines.append("")
-
-        lines.append("**Action**: Add tests that exercise the above lines/branches.")
-        lines.append("")
-        lines.append("---")
-        lines.append("")
-
-    lines.append("## Quick Reference: All Uncovered Locations")
-    lines.append("")
-    lines.append("Copy-paste friendly list for IDE navigation or grep:")
-    lines.append("")
-    lines.append("```")
-    for cls in sorted_classes:
-        src = cls.source_path or cls.java_class_name
-        if cls.uncovered_lines:
-            for ln in cls.uncovered_lines:
-                lines.append(f"{src}:{ln}  # uncovered line")
-        if cls.partial_branch_lines:
-            for ln in cls.partial_branch_lines:
-                lines.append(f"{src}:{ln}  # partial branch")
-    lines.append("```")
-
-    return "\n".join(lines)
-
-
-def format_json(classes: list[ClassCoverage]) -> str:
-    out = []
-    for cls in classes:
-        out.append({
-            "class": cls.java_class_name,
-            "source_path": cls.source_path,
-            "uncovered_lines": cls.uncovered_lines,
-            "partial_branch_lines": cls.partial_branch_lines,
-            "missed_branches": cls.missed_branches,
-            "methods": [
-                {
-                    "name": m.name,
-                    "descriptor": m.descriptor,
-                    "first_line": m.first_line,
-                    "line_coverage_pct": round(m.line_coverage_pct, 1),
-                    "branch_coverage_pct": round(m.branch_coverage_pct, 1),
-                    "missed_branches": m.missed_branches,
-                    "missed_instructions": m.missed_instructions,
-                }
-                for m in cls.methods
-                if not m.is_fully_covered
-            ],
-        })
-    return json.dumps(out, indent=2)
-
-
-def format_markdown(classes: list[ClassCoverage]) -> str:
-    lines: list[str] = []
-    lines.append("# JaCoCo Missing Coverage Report\n")
-    for cls in sorted(classes, key=lambda c: cls.java_class_name):
-        lines.append(f"## {cls.java_class_name}")
-        if cls.source_path:
-            lines.append(f"**File**: `{cls.source_path}`\n")
-        if cls.uncovered_lines:
-            lines.append(f"**Uncovered lines**: {_compact_ranges(cls.uncovered_lines)}\n")
-        if cls.partial_branch_lines:
-            lines.append(f"**Partial branches at lines**: {_compact_ranges(cls.partial_branch_lines)}\n")
-        lines.append("| Method | Line Coverage | Branch Coverage | Missed Branches |")
-        lines.append("|--------|--------------|-----------------|-----------------|")
-        for m in cls.methods:
-            if not m.is_fully_covered:
-                lines.append(
-                    f"| `{m.name}` | {m.line_coverage_pct:.0f}% | "
-                    f"{m.branch_coverage_pct:.0f}% | {m.missed_branches} |"
-                )
-        lines.append("")
-    return "\n".join(lines)
-
-
-# ---------------------------------------------------------------------------
-# Entry point
-# ---------------------------------------------------------------------------
-
-def main() -> None:
-    parser = argparse.ArgumentParser(
-        description="Report missing line & branch coverage from a JaCoCo XML report."
-    )
-    parser.add_argument("xml_file", help="Path to jacoco.xml report file")
-    parser.add_argument(
-        "--output", "-o",
-        choices=["agent", "json", "markdown"],
-        default="json",
-        help="Output format (default: agent)",
-    )
-    parser.add_argument(
-        "--min-coverage",
-        type=float,
-        default=0.0,
-        help="Only report classes below this %% line coverage (0 = report all gaps)",
-    )
-    parser.add_argument(
-        "--package-filter", "-p",
-        default=None,
-        help="Only report classes in this package prefix (e.g. com/example/service)",
-    )
-    args = parser.parse_args()
-
-    xml_path = Path(args.xml_file)
-    if not xml_path.exists():
-        print(f"ERROR: File not found: {xml_path}", file=sys.stderr)
-        sys.exit(1)
-
-    classes = parse_jacoco_xml(str(xml_path))
-
-    # Apply package filter
-    if args.package_filter:
-        prefix = args.package_filter.replace(".", "/")
-        classes = [c for c in classes if c.class_name.startswith(prefix)]
-
-    # Apply min-coverage filter
-    if args.min_coverage > 0:
-        def _line_pct(c: ClassCoverage) -> float:
-            total = c.total_lines
-            return 100.0 * c.covered_lines / total if total else 100.0
-
-        classes = [c for c in classes if _line_pct(c) < args.min_coverage]
-
-    if not classes:
-        print("✅ No coverage gaps found matching the given filters.")
-        return
-
-    if args.output == "agent":
-        print(format_agent(classes))
-    elif args.output == "json":
-        print(format_json(classes))
-    elif args.output == "markdown":
-        print(format_markdown(classes))
-
-
-if __name__ == "__main__":
-    main()

jacoco-reporter/scoverage_coverage_gaps.py

@@ -19,6 +19,9 @@ Usage:
     python scoverage_coverage_gaps.py <scoverage.xml> --output agent   (default)
     python scoverage_coverage_gaps.py <scoverage.xml> --package-filter de.nowchess.chess.controller
     python scoverage_coverage_gaps.py <scoverage.xml> --min-coverage 80
+    python scoverage_coverage_gaps.py                                 (default: scans ./modules)
+    python scoverage_coverage_gaps.py --modules-dir ./services
+    python scoverage_coverage_gaps.py <scoverage.xml>
 """
 
 import xml.etree.ElementTree as ET
@@ -26,7 +29,8 @@ import sys
 import argparse
 import json
 import re
-from pathlib import Path, PureWindowsPath
+import glob
+from pathlib import Path
 from dataclasses import dataclass, field
 from typing import Optional
 
@@ -112,7 +116,6 @@ class ClassGap:
     @property
     def uncovered_branch_lines(self) -> list[int]:
         """Lines that are branch points and have at least one uncovered branch statement."""
-        # Group branch statements by line; a line is "partial" if some covered, some not
         from collections import defaultdict
         by_line: dict[int, list[Statement]] = defaultdict(list)
         for s in self.statements:
@@ -120,10 +123,7 @@ class ClassGap:
                 by_line[s.line].append(s)
         partial = []
         for line, stmts in by_line.items():
-            has_covered = any(s.is_covered for s in stmts)
-            has_uncovered = any(s.is_uncovered for s in stmts)
-            # Report line if any branch arm is uncovered
-            if has_uncovered:
+            if any(s.is_uncovered for s in stmts):
                 partial.append(line)
         return sorted(partial)
 
@@ -169,20 +169,10 @@ class ClassGap:
 # ---------------------------------------------------------------------------
 
 def _normalise_source(raw: str) -> str:
-    """
-    Convert an absolute Windows or Unix source path from the XML into a
-    relative src/main/scala/… path for agent consumption.
-
-    Strategy:
-      1. Replace Windows backslashes.
-      2. Find the 'src/' anchor and take everything from there.
-      3. Fall back to the package-derived path if no anchor found.
-    """
     normalised = raw.replace("\\", "/")
     match = re.search(r"(src/(?:main|test)/scala/.+)", normalised)
     if match:
         return match.group(1)
-    # Fallback: just the filename portion
     return normalised.split("/")[-1]
 
 
@@ -190,11 +180,10 @@ def _normalise_source(raw: str) -> str:
 # Parser
 # ---------------------------------------------------------------------------
 
-def parse_scoverage_xml(xml_path: str) -> list[ClassGap]:
+def parse_scoverage_xml(xml_path: str) -> tuple[dict, list[ClassGap]]:
     tree = ET.parse(xml_path)
     root = tree.getroot()
 
-    # ── Authoritative project-level totals from <scoverage> root element ──────
     project_stats = {
         "total_statements":    int(root.get("statement-count", 0)),
         "covered_statements":  int(root.get("statements-invoked", 0)),
@@ -202,17 +191,16 @@ def parse_scoverage_xml(xml_path: str) -> list[ClassGap]:
         "branch_coverage_pct": float(root.get("branch-rate", 0.0)),
     }
     project_stats["missed_statements"] = (
-        project_stats["total_statements"] - project_stats["covered_statements"]
+            project_stats["total_statements"] - project_stats["covered_statements"]
     )
 
-    class_map: dict[str, ClassGap] = {}   # full-class-name → ClassGap
+    class_map: dict[str, ClassGap] = {}
 
     for package in root.findall("packages/package"):
         for cls_elem in package.findall("classes/class"):
             class_name = cls_elem.get("name", "")
             filename   = cls_elem.get("filename", "")
 
-            # Authoritative per-class totals from <class> attributes
             cls_total     = int(cls_elem.get("statement-count", 0))
             cls_invoked   = int(cls_elem.get("statements-invoked", 0))
             cls_stmt_rate = float(cls_elem.get("statement-rate", 0.0))
@@ -221,11 +209,8 @@ def parse_scoverage_xml(xml_path: str) -> list[ClassGap]:
             for method_elem in cls_elem.findall("methods/method"):
                 method_name = method_elem.get("name", "")
 
-                # Authoritative per-method totals from <method> attributes
-                m_total     = int(method_elem.get("statement-count", 0))
-                m_invoked   = int(method_elem.get("statements-invoked", 0))
-                m_stmt_rate = float(method_elem.get("statement-rate", 0.0))
-                m_br_rate   = float(method_elem.get("branch-rate", 0.0))
+                m_total   = int(method_elem.get("statement-count", 0))
+                m_invoked = int(method_elem.get("statements-invoked", 0))
 
                 for stmt_elem in method_elem.findall("statements/statement"):
                     raw_source = stmt_elem.get("source", filename)
@@ -257,7 +242,6 @@ def parse_scoverage_xml(xml_path: str) -> list[ClassGap]:
                         method=method_name,
                     ))
 
-                # Register method-level gap using authoritative XML stats
                 cg = next(
                     (v for v in class_map.values() if v.class_name == class_name),
                     None,
@@ -268,7 +252,6 @@ def parse_scoverage_xml(xml_path: str) -> list[ClassGap]:
                 uncov_lines        = sorted({s.line for s in active if s.is_uncovered})
                 uncov_branch_lines = sorted({s.line for s in active if s.is_branch and s.is_uncovered})
                 if uncov_lines or uncov_branch_lines:
-                    # Count branches from statement-level data (not in method XML attrs)
                     total_b = sum(1 for s in active if s.is_branch)
                     cov_b   = sum(1 for s in active if s.is_branch and s.is_covered)
                     mg = MethodGap(
@@ -282,7 +265,6 @@ def parse_scoverage_xml(xml_path: str) -> list[ClassGap]:
                     )
                     cg.method_gaps.append(mg)
 
-    # ── Project stats injected so formatters never recount from statements ────
     return project_stats, [cg for cg in class_map.values() if cg.has_gaps]
 
 
@@ -310,103 +292,60 @@ def _compact_ranges(numbers: list[int]) -> str:
 # Formatters
 # ---------------------------------------------------------------------------
 
-def _pct_bar(pct: float, width: int = 20) -> str:
-    """Render a compact ASCII progress bar, e.g. [████░░░░░░░░░░░░░░░░] 23.5%"""
-    filled = round(pct / 100 * width)
-    bar = "█" * filled + "░" * (width - filled)
-    return f"[{bar}] {pct:.1f}%"
-
-
 def format_agent(project_stats: dict, classes: list[ClassGap]) -> str:
+    """
+    Compact agent format — optimised for low token count.
+    Emits only actionable gaps: file path, uncovered lines, branch-gap lines,
+    and a per-method breakdown. No ASCII bars, no redundant tables.
+    """
     lines: list[str] = []
-    lines.append("# scoverage Coverage Gaps — Agent Action Report")
-    lines.append("")
 
-    # ---- Project-level totals (authoritative from <scoverage> root element) ----
-    total_stmts      = project_stats["total_statements"]
-    covered_stmts    = project_stats["covered_statements"]
-    missed_stmts     = project_stats["missed_statements"]
+    total_stmts        = project_stats["total_statements"]
+    covered_stmts      = project_stats["covered_statements"]
+    missed_stmts       = project_stats["missed_statements"]
     overall_stmt_pct   = project_stats["stmt_coverage_pct"]
     overall_branch_pct = project_stats["branch_coverage_pct"]
-    total_branch_lines = sum(len(c.uncovered_branch_lines) for c in classes)
-    # Branch totals: count from statement data (scoverage root has no branch count attr)
-    total_branches   = sum(c.total_branches for c in classes)
-    covered_branches = sum(c.covered_branches for c in classes)
-    missed_branches  = sum(c.missed_branches for c in classes)
+    total_branches     = sum(c.total_branches for c in classes)
+    covered_branches   = sum(c.covered_branches for c in classes)
+    missed_branches    = total_branches - covered_branches
 
-    lines.append("## Project Coverage Summary")
-    lines.append("")
-    lines.append(f"| Metric            | Covered | Total | Missed | Coverage |")
-    lines.append(f"|-------------------|---------|-------|--------|----------|")
-    lines.append(f"| Statements        | {covered_stmts:>7} | {total_stmts:>5} | {missed_stmts:>6} | {_pct_bar(overall_stmt_pct)} |")
-    lines.append(f"| Branch paths      | {covered_branches:>7} | {total_branches:>5} | {missed_branches:>6} | {_pct_bar(overall_branch_pct)} |")
-    lines.append(f"| Files with gaps   | {'—':>7} | {len(classes):>5} | {'—':>6} | {'—'} |")
-    lines.append(f"| Lines w/ br. gaps | {'—':>7} | {total_branch_lines:>5} | {'—':>6} | {'—'} |")
-    lines.append("")
-    lines.append("---")
-    lines.append("")
-    lines.append("## Files Requiring Tests")
-    lines.append("")
-    lines.append("> Each entry lists the SOURCE FILE PATH, uncovered LINE NUMBERS,")
-    lines.append("> and the METHODS that contain those gaps.")
-    lines.append("> Write or extend unit/integration tests to exercise these paths.")
+    lines.append("# scoverage Coverage Gaps")
+    lines.append(
+        f"stmt: {overall_stmt_pct:.1f}% ({missed_stmts}/{total_stmts} missed) | "
+        f"branches: {overall_branch_pct:.1f}% ({missed_branches}/{total_branches} missed) | "
+        f"files with gaps: {len(classes)}"
+    )
     lines.append("")
 
     sorted_classes = sorted(classes, key=lambda c: -(c.missed_statements + c.missed_branches))
 
     for cls in sorted_classes:
-        lines.append(f"### `{cls.source_path}`")
-        lines.append(f"**Class**: `{cls.class_name}`")
-        lines.append("")
-        lines.append(f"| Metric       | Covered | Total | Missed | Coverage |")
-        lines.append(f"|--------------|---------|-------|--------|----------|")
-        lines.append(f"| Statements   | {cls.covered_statements:>7} | {cls.total_statements:>5} | {cls.missed_statements:>6} | {_pct_bar(cls.stmt_coverage_pct)} |")
-        if cls.total_branches:
-            lines.append(f"| Branch paths | {cls.covered_branches:>7} | {cls.total_branches:>5} | {cls.missed_branches:>6} | {_pct_bar(cls.branch_coverage_pct)} |")
-        lines.append("")
-
-        uncov = cls.all_uncovered_lines
-        if uncov:
-            lines.append("#### ❌ Uncovered Statements")
-            lines.append(f"Lines never executed: `{_compact_ranges(uncov)}`")
-            lines.append("")
-
+        uncov        = cls.all_uncovered_lines
         branch_lines = cls.uncovered_branch_lines
-        if branch_lines:
-            lines.append("#### ⚠️  Missing Branch Coverage (Conditional Paths)")
-            lines.append(f"Lines where not all conditional paths are taken: `{_compact_ranges(branch_lines)}`")
-            lines.append("")
+
+        lines.append(f"## {cls.source_path}")
+        lines.append(
+            f"stmt: {cls.stmt_coverage_pct:.1f}% ({cls.missed_statements} missed)"
+            + (f" | branches: {cls.branch_coverage_pct:.1f}% ({cls.missed_branches} missed)"
+               if cls.total_branches else "")
+        )
+        if uncov:
+            lines.append(f"uncovered lines: {_compact_ranges(uncov)}")
+        only_branch = [l for l in branch_lines if l not in cls.all_uncovered_lines]
+        if only_branch:
+            lines.append(f"partial branches: {_compact_ranges(only_branch)}")
 
         if cls.method_gaps:
-            lines.append("#### Methods with Gaps")
-            lines.append("")
-            lines.append("| Method | Stmt Coverage | Branch Coverage | Uncovered Lines | Branch Gap Lines |")
-            lines.append("|--------|--------------|-----------------|-----------------|------------------|")
+            lines.append("methods:")
             for mg in cls.method_gaps:
-                stmt_cell   = f"{_pct_bar(mg.stmt_coverage_pct, 10)} ({mg.total_statements - mg.covered_statements}/{mg.total_statements} missed)"
-                branch_cell = f"{_pct_bar(mg.branch_coverage_pct, 10)} ({mg.missed_branches}/{mg.total_branches} missed)" if mg.total_branches else "n/a"
-                uncov_cell  = f"`{_compact_ranges(mg.uncovered_lines)}`"        if mg.uncovered_lines        else "—"
-                br_cell     = f"`{_compact_ranges(mg.uncovered_branch_lines)}`" if mg.uncovered_branch_lines else "—"
-                lines.append(f"| `{mg.short_name}` | {stmt_cell} | {branch_cell} | {uncov_cell} | {br_cell} |")
-            lines.append("")
+                parts = [f"  {mg.short_name}"]
+                if mg.uncovered_lines:
+                    parts.append(f"lines={_compact_ranges(mg.uncovered_lines)}")
+                if mg.uncovered_branch_lines:
+                    parts.append(f"branches={_compact_ranges(mg.uncovered_branch_lines)}")
+                lines.append(" ".join(parts))
 
-        lines.append("**Action**: Add tests that exercise the lines/branches listed above.")
         lines.append("")
-        lines.append("---")
-        lines.append("")
-
-    lines.append("## Quick Reference: All Uncovered Locations")
-    lines.append("")
-    lines.append("Copy-paste friendly list for IDE navigation or grep:")
-    lines.append("")
-    lines.append("```")
-    for cls in sorted_classes:
-        for ln in cls.all_uncovered_lines:
-            lines.append(f"{cls.source_path}:{ln}  # uncovered statement")
-        for ln in cls.uncovered_branch_lines:
-            if ln not in cls.all_uncovered_lines:
-                lines.append(f"{cls.source_path}:{ln}  # partial branch")
-    lines.append("```")
 
     return "\n".join(lines)
 
@@ -511,6 +450,87 @@ def format_markdown(project_stats: dict, classes: list[ClassGap]) -> str:
     return "\n".join(lines)
 
 
+# ---------------------------------------------------------------------------
+# Scan-modules mode
+# ---------------------------------------------------------------------------
+
+# Candidate sub-paths within a module directory where scoverage.xml may live.
+_SCOVERAGE_SUBPATHS = [
+    # Gradle / default layout
+    "build/reports/scoverageTest/scoverage.xml",
+    # sbt default (scala version wildcard resolved via glob)
+    "target/scala-*/scoverage-report/scoverage.xml",
+    # Maven / flat layout
+    "target/scoverage-report/scoverage.xml",
+    # Already at root of module
+    "scoverage.xml",
+]
+
+
+def _find_scoverage_xml(module_dir: Path) -> Optional[Path]:
+    """Return the first scoverage.xml found inside *module_dir*, or None."""
+    for pattern in _SCOVERAGE_SUBPATHS:
+        hits = sorted(module_dir.glob(pattern))
+        if hits:
+            return hits[0]
+    return None
+
+
+def format_module_gaps(module_name: str, classes: list[ClassGap], stmt_pct: float) -> str:
+    """
+    One summary line per module. If coverage is not 100%, append an agent hint.
+    """
+    if not classes:
+        return f"[{module_name}] stmt: {stmt_pct:.1f}% ✅"
+
+    line = f"[{module_name}] stmt: {stmt_pct:.1f}%  files_with_gaps: {len(classes)}"
+    if stmt_pct < 100.0:
+        line += f"  # hint: run ./coverage {module_name} for details"
+    return line
+
+
+def run_scan_modules(modules_dir: str, package_filter: Optional[str], min_coverage: float) -> None:
+    base = Path(modules_dir)
+    if not base.is_dir():
+        print(f"ERROR: modules directory not found: {base}", file=sys.stderr)
+        sys.exit(1)
+
+    module_dirs = sorted(p for p in base.iterdir() if p.is_dir())
+    if not module_dirs:
+        print(f"No sub-directories found in {base}", file=sys.stderr)
+        sys.exit(1)
+
+    results: list[str] = []
+    missing: list[str] = []
+
+    for mod_dir in module_dirs:
+        if mod_dir.name.startswith("build"):
+            continue
+        xml_path = _find_scoverage_xml(mod_dir)
+        if xml_path is None:
+            missing.append(mod_dir.name)
+            continue
+
+        project_stats, classes = parse_scoverage_xml(str(xml_path))
+
+        if package_filter:
+            classes = [c for c in classes if c.class_name.startswith(package_filter)]
+        if min_coverage > 0:
+            classes = [c for c in classes if c.stmt_coverage_pct < min_coverage]
+
+        results.append(
+            format_module_gaps(mod_dir.name, classes, project_stats["stmt_coverage_pct"])
+        )
+
+    print("\n".join(results))
+
+    if missing:
+        print(
+            f"\n# Modules without scoverage.xml: {', '.join(missing)}",
+            file=sys.stderr,
+        )
+
+
 # ---------------------------------------------------------------------------
 # Entry point
 # ---------------------------------------------------------------------------
@@ -519,7 +539,13 @@ def main() -> None:
     parser = argparse.ArgumentParser(
         description="Report missing statement & branch coverage from a scoverage XML report."
     )
-    parser.add_argument("xml_file", help="Path to scoverage.xml report file")
+
+    # Positional xml_file is optional when --scan-modules is used
+    parser.add_argument(
+        "xml_file",
+        nargs="?",
+        help="Path to scoverage.xml report file (not required with --scan-modules)",
+    )
     parser.add_argument(
         "--output", "-o",
         choices=["agent", "json", "markdown"],
@@ -537,8 +563,30 @@ def main() -> None:
         default=None,
         help="Only report classes in this package prefix (e.g. de.nowchess.chess.controller)",
     )
+    # ── Scan-modules mode ──────────────────────────────────────────────────
+    parser.add_argument(
+        "--scan-modules",
+        action="store_true",
+        help=(
+            "Scan every sub-directory of --modules-dir for a scoverage.xml "
+            "and print a compact coverage-gaps summary per module."
+        ),
+    )
+    parser.add_argument(
+        "--modules-dir",
+        default="./modules",
+        help="Root directory that contains one sub-directory per module (default: ./modules)",
+    )
+
     args = parser.parse_args()
 
+    # ── Scan-modules path (explicit flag, or default when no xml_file given) ──
+    if args.scan_modules or not args.xml_file:
+        run_scan_modules(args.modules_dir, args.package_filter, args.min_coverage)
+        return
+
+    # ── Single-file path ──────────────────────────────────────────────────
+
     xml_path = Path(args.xml_file)
     if not xml_path.exists():
         print(f"ERROR: File not found: {xml_path}", file=sys.stderr)
@@ -565,4 +613,4 @@ def main() -> None:
 
 
 if __name__ == "__main__":
-    main()
+    main()

jacoco-reporter/test_gaps.py

@@ -0,0 +1,288 @@
+#!/usr/bin/env python3
+"""
+Test Gap Reporter
+Scans JUnit XML test results under modules/*/build/test-results/*.xml and
+outputs a minimal summary optimised for agent consumption.
+
+Usage:
+    python test_gaps.py                        # scan all modules (default)
+    python test_gaps.py --module chess         # single module
+    python test_gaps.py --module all           # explicit all
+    python test_gaps.py --modules-dir ./modules
+    python test_gaps.py --results-subdir build/test-results
+"""
+
+import xml.etree.ElementTree as ET
+import sys
+import argparse
+from pathlib import Path
+from dataclasses import dataclass, field
+from typing import Optional
+
+
+# ---------------------------------------------------------------------------
+# Data classes
+# ---------------------------------------------------------------------------
+
+@dataclass
+class TestCase:
+    classname: str
+    name: str
+    time: float
+    failure: Optional[str] = None   # message if failed
+    error: Optional[str]   = None   # message if errored
+    skipped: bool          = False
+
+    @property
+    def short_class(self) -> str:
+        return self.classname.split(".")[-1]
+
+    @property
+    def status(self) -> str:
+        if self.failure is not None:
+            return "FAIL"
+        if self.error is not None:
+            return "ERROR"
+        if self.skipped:
+            return "SKIP"
+        return "OK"
+
+
+@dataclass
+class SuiteResult:
+    name: str
+    total: int
+    failures: int
+    errors: int
+    skipped: int
+    time: float
+    cases: list[TestCase] = field(default_factory=list)
+
+    @property
+    def passed(self) -> int:
+        return self.total - self.failures - self.errors - self.skipped
+
+    @property
+    def is_clean(self) -> bool:
+        return self.failures == 0 and self.errors == 0
+
+    @property
+    def bad_cases(self) -> list[TestCase]:
+        return [c for c in self.cases if c.status in ("FAIL", "ERROR")]
+
+    @property
+    def skipped_cases(self) -> list[TestCase]:
+        return [c for c in self.cases if c.skipped]
+
+
+@dataclass
+class ModuleResult:
+    name: str
+    suites: list[SuiteResult] = field(default_factory=list)
+
+    @property
+    def total(self)    -> int: return sum(s.total    for s in self.suites)
+    @property
+    def failures(self) -> int: return sum(s.failures for s in self.suites)
+    @property
+    def errors(self)   -> int: return sum(s.errors   for s in self.suites)
+    @property
+    def skipped(self)  -> int: return sum(s.skipped  for s in self.suites)
+    @property
+    def passed(self)   -> int: return sum(s.passed   for s in self.suites)
+    @property
+    def is_clean(self) -> bool: return self.failures == 0 and self.errors == 0
+
+    @property
+    def bad_cases(self) -> list[TestCase]:
+        return [c for s in self.suites for c in s.bad_cases]
+
+    @property
+    def skipped_cases(self) -> list[TestCase]:
+        return [c for s in self.suites for c in s.skipped_cases]
+
+
+# ---------------------------------------------------------------------------
+# Parser
+# ---------------------------------------------------------------------------
+
+def parse_suite_xml(xml_path: Path) -> SuiteResult:
+    tree = ET.parse(xml_path)
+    root = tree.getroot()
+
+    # Handle both <testsuite> root and <testsuites> wrapper
+    suites = [root] if root.tag == "testsuite" else root.findall("testsuite")
+
+    # Merge multiple suites from one file into a single SuiteResult
+    total = failures = errors = skipped = 0
+    elapsed = 0.0
+    name = xml_path.stem
+    cases: list[TestCase] = []
+
+    for suite in suites:
+        total    += int(suite.get("tests",    0))
+        failures += int(suite.get("failures", 0))
+        errors   += int(suite.get("errors",   0))
+        skipped  += int(suite.get("skipped",  0))
+        elapsed  += float(suite.get("time",   0.0))
+        if suite.get("name"):
+            name = suite.get("name")
+
+        for tc in suite.findall("testcase"):
+            fail_el = tc.find("failure")
+            err_el  = tc.find("error")
+            skip_el = tc.find("skipped")
+            cases.append(TestCase(
+                classname=tc.get("classname", ""),
+                name=tc.get("name", ""),
+                time=float(tc.get("time", 0.0)),
+                failure=fail_el.get("message", fail_el.text or "") if fail_el is not None else None,
+                error=err_el.get("message",  err_el.text  or "") if err_el  is not None else None,
+                skipped=skip_el is not None,
+            ))
+
+    return SuiteResult(
+        name=name, total=total, failures=failures,
+        errors=errors, skipped=skipped, time=elapsed, cases=cases,
+    )
+
+
+def load_module(module_dir: Path, results_subdir: str) -> Optional[ModuleResult]:
+    results_dir = module_dir / results_subdir
+    if not results_dir.is_dir():
+        return None
+
+    xml_files = sorted(results_dir.glob("*.xml"))
+    if not xml_files:
+        return None
+
+    mod = ModuleResult(name=module_dir.name)
+    for xml_path in xml_files:
+        try:
+            mod.suites.append(parse_suite_xml(xml_path))
+        except ET.ParseError:
+            pass  # skip malformed files silently
+    return mod if mod.suites else None
+
+
+# ---------------------------------------------------------------------------
+# Formatter
+# ---------------------------------------------------------------------------
+
+def _truncate(text: str, max_len: int = 120) -> str:
+    text = " ".join(text.split())        # collapse whitespace
+    return text[:max_len] + "…" if len(text) > max_len else text
+
+
+def format_module(mod: ModuleResult) -> str:
+    parts = [f"[{mod.name}]"]
+
+    if mod.is_clean and mod.skipped == 0:
+        parts.append(f"tests: {mod.total} ✅")
+        return " ".join(parts)
+
+    parts.append(f"tests: {mod.total}")
+    if mod.failures:  parts.append(f"failed: {mod.failures}")
+    if mod.errors:    parts.append(f"errors: {mod.errors}")
+    if mod.skipped:   parts.append(f"skipped: {mod.skipped}")
+
+    # Agent hint only when there are actual failures/errors
+    if not mod.is_clean:
+        parts.append(f" # hint: run ./test {mod.name} for details")
+
+    lines = [" ".join(parts)]
+
+    # List each failed/errored test — this IS the actionable info
+    for tc in mod.bad_cases:
+        msg = tc.failure if tc.failure is not None else tc.error
+        label = f"  {tc.status}: {tc.short_class} > {tc.name}"
+        if msg:
+            label += f"  [{_truncate(msg, 80)}]"
+        lines.append(label)
+
+    # Skipped: compact, one line total
+    if mod.skipped_cases:
+        skipped_names = ", ".join(
+            f"{c.short_class}.{c.name}" for c in mod.skipped_cases[:5]
+        )
+        if len(mod.skipped_cases) > 5:
+            skipped_names += f" (+{len(mod.skipped_cases) - 5} more)"
+        lines.append(f"  SKIP: {skipped_names}")
+
+    return "\n".join(lines)
+
+
+# ---------------------------------------------------------------------------
+# Runner
+# ---------------------------------------------------------------------------
+
+def run(modules_dir: str, results_subdir: str, module_filter: Optional[str]) -> None:
+    base = Path(modules_dir)
+    if not base.is_dir():
+        print(f"ERROR: modules directory not found: {base}", file=sys.stderr)
+        sys.exit(1)
+
+    # Resolve which module dirs to scan
+    if module_filter and module_filter != "all":
+        mod_dir = base / module_filter
+        if not mod_dir.is_dir():
+            print(f"ERROR: module not found: {mod_dir}", file=sys.stderr)
+            sys.exit(1)
+        candidates = [mod_dir]
+    else:
+        candidates = sorted(p for p in base.iterdir() if p.is_dir())
+
+    results: list[str] = []
+    missing: list[str] = []
+
+    for mod_dir in candidates:
+        if mod_dir.name.startswith("build"):
+            continue
+        mod = load_module(mod_dir, results_subdir)
+        if mod is None:
+            missing.append(mod_dir.name)
+            continue
+        results.append(format_module(mod))
+
+    print("\n".join(results))
+
+    if missing:
+        print(
+            f"\n# Modules without test results: {', '.join(missing)}",
+            file=sys.stderr,
+        )
+
+
+# ---------------------------------------------------------------------------
+# Entry point
+# ---------------------------------------------------------------------------
+
+def main() -> None:
+    parser = argparse.ArgumentParser(
+        description="Minimal test-gap reporter for JUnit XML results across modules."
+    )
+    parser.add_argument(
+        "--module", "-m",
+        nargs="?",
+        const="all",
+        default="all",
+        help="Module name to scan, or 'all' (default: all)",
+    )
+    parser.add_argument(
+        "--modules-dir",
+        default="./modules",
+        help="Root directory containing one sub-directory per module (default: ./modules)",
+    )
+    parser.add_argument(
+        "--results-subdir",
+        default="build/test-results/test",
+        help="Sub-path inside each module dir where *.xml files live (default: build/test-results/test)",
+    )
+    args = parser.parse_args()
+
+    filter_ = None if args.module == "all" else args.module
+    run(args.modules_dir, args.results_subdir, filter_)
+
+
+if __name__ == "__main__":
+    main()

modules/api/CHANGELOG.md

@@ -0,0 +1,8 @@
+##  (2026-03-27)
+##  (2026-03-28)
+##  (2026-03-28)
+##  (2026-03-29)
+##  (2026-03-31)
+##  (2026-04-01)
+##  (2026-04-01)
+##  (2026-04-01)

modules/api/build.gradle.kts

@@ -49,16 +49,17 @@ dependencies {
 
     testImplementation(platform("org.junit:junit-bom:5.13.4"))
     testImplementation("org.junit.jupiter:junit-jupiter")
-    testRuntimeOnly("org.junit.platform:junit-platform-launcher")
     testImplementation("org.scalatest:scalatest_3:${versions["SCALATEST"]!!}")
     testImplementation("co.helmethair:scalatest-junit-runner:${versions["SCALATEST_JUNIT"]!!}")
+
+    testRuntimeOnly("org.junit.platform:junit-platform-launcher")
 }
 
 tasks.test {
     useJUnitPlatform {
         includeEngines("scalatest")
         testLogging {
-            events("passed", "skipped", "failed")
+            events("skipped", "failed")
         }
     }
     finalizedBy(tasks.reportScoverage)

modules/api/src/main/scala/de/nowchess/api/board/Board.scala

@@ -8,10 +8,12 @@ object Board:
 
   extension (b: Board)
     def pieceAt(sq: Square): Option[Piece] = b.get(sq)
+    def updated(sq: Square, piece: Piece): Board = b.updated(sq, piece)
+    def removed(sq: Square): Board = b.removed(sq)
     def withMove(from: Square, to: Square): (Board, Option[Piece]) =
       val captured = b.get(to)
-      val updated  = b.removed(from).updated(to, b(from))
-      (updated, captured)
+      val updatedBoard = b.removed(from).updated(to, b(from))
+      (updatedBoard, captured)
     def pieces: Map[Square, Piece] = b
 
   val initial: Board =

modules/api/src/test/scala/de/nowchess/api/board/BoardTest.scala

@@ -0,0 +1,122 @@
+package de.nowchess.api.board
+
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class BoardTest extends AnyFunSuite with Matchers:
+
+  private val e2 = Square(File.E, Rank.R2)
+  private val e4 = Square(File.E, Rank.R4)
+  private val d7 = Square(File.D, Rank.R7)
+
+  test("pieceAt returns Some for occupied square") {
+    Board.initial.pieceAt(e2) shouldBe Some(Piece.WhitePawn)
+  }
+
+  test("pieceAt returns None for empty square") {
+    Board.initial.pieceAt(e4) shouldBe None
+  }
+
+  test("withMove moves piece and vacates origin") {
+    val (board, captured) = Board.initial.withMove(e2, e4)
+    captured shouldBe None
+    board.pieceAt(e4) shouldBe Some(Piece.WhitePawn)
+    board.pieceAt(e2) shouldBe None
+  }
+
+  test("withMove returns captured piece when destination is occupied") {
+    val from = Square(File.A, Rank.R1)
+    val to   = Square(File.A, Rank.R8)
+    val b    = Board(Map(from -> Piece.WhiteRook, to -> Piece.BlackRook))
+    val (board, captured) = b.withMove(from, to)
+    captured shouldBe Some(Piece.BlackRook)
+    board.pieceAt(to) shouldBe Some(Piece.WhiteRook)
+    board.pieceAt(from) shouldBe None
+  }
+
+  test("pieces returns the underlying map") {
+    val map = Map(e2 -> Piece.WhitePawn)
+    val b   = Board(map)
+    b.pieces shouldBe map
+  }
+
+  test("Board.apply constructs board from map") {
+    val map = Map(e2 -> Piece.WhitePawn)
+    val b   = Board(map)
+    b.pieceAt(e2) shouldBe Some(Piece.WhitePawn)
+  }
+
+  test("initial board has 32 pieces") {
+    Board.initial.pieces should have size 32
+  }
+
+  test("initial board has 16 white pieces") {
+    Board.initial.pieces.values.count(_.color == Color.White) shouldBe 16
+  }
+
+  test("initial board has 16 black pieces") {
+    Board.initial.pieces.values.count(_.color == Color.Black) shouldBe 16
+  }
+
+  test("initial board white pawns on rank 2") {
+    File.values.foreach { file =>
+      Board.initial.pieceAt(Square(file, Rank.R2)) shouldBe Some(Piece.WhitePawn)
+    }
+  }
+
+  test("initial board black pawns on rank 7") {
+    File.values.foreach { file =>
+      Board.initial.pieceAt(Square(file, Rank.R7)) shouldBe Some(Piece.BlackPawn)
+    }
+  }
+
+  test("initial board white back rank") {
+    val expectedBackRank = Vector(
+      PieceType.Rook, PieceType.Knight, PieceType.Bishop, PieceType.Queen,
+      PieceType.King, PieceType.Bishop, PieceType.Knight, PieceType.Rook
+    )
+    File.values.zipWithIndex.foreach { (file, i) =>
+      Board.initial.pieceAt(Square(file, Rank.R1)) shouldBe
+        Some(Piece(Color.White, expectedBackRank(i)))
+    }
+  }
+
+  test("initial board black back rank") {
+    val expectedBackRank = Vector(
+      PieceType.Rook, PieceType.Knight, PieceType.Bishop, PieceType.Queen,
+      PieceType.King, PieceType.Bishop, PieceType.Knight, PieceType.Rook
+    )
+    File.values.zipWithIndex.foreach { (file, i) =>
+      Board.initial.pieceAt(Square(file, Rank.R8)) shouldBe
+        Some(Piece(Color.Black, expectedBackRank(i)))
+    }
+  }
+
+  test("ranks 3-6 are empty on initial board") {
+    val emptyRanks = Seq(Rank.R3, Rank.R4, Rank.R5, Rank.R6)
+    for
+      rank <- emptyRanks
+      file <- File.values
+    do
+      Board.initial.pieceAt(Square(file, rank)) shouldBe None
+  }
+
+  test("updated adds or replaces piece at square") {
+    val b = Board(Map(e2 -> Piece.WhitePawn))
+    val updated = b.updated(e4, Piece.WhiteKnight)
+    updated.pieceAt(e2) shouldBe Some(Piece.WhitePawn)
+    updated.pieceAt(e4) shouldBe Some(Piece.WhiteKnight)
+  }
+
+  test("updated replaces existing piece") {
+    val b = Board(Map(e2 -> Piece.WhitePawn))
+    val updated = b.updated(e2, Piece.WhiteKnight)
+    updated.pieceAt(e2) shouldBe Some(Piece.WhiteKnight)
+  }
+
+  test("removed deletes piece from board") {
+    val b = Board(Map(e2 -> Piece.WhitePawn, e4 -> Piece.WhiteKnight))
+    val removed = b.removed(e2)
+    removed.pieceAt(e2) shouldBe None
+    removed.pieceAt(e4) shouldBe Some(Piece.WhiteKnight)
+  }

modules/api/src/test/scala/de/nowchess/api/board/ColorTest.scala

@@ -0,0 +1,22 @@
+package de.nowchess.api.board
+
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class ColorTest extends AnyFunSuite with Matchers:
+
+  test("White.opposite returns Black") {
+    Color.White.opposite shouldBe Color.Black
+  }
+
+  test("Black.opposite returns White") {
+    Color.Black.opposite shouldBe Color.White
+  }
+
+  test("White.label returns 'White'") {
+    Color.White.label shouldBe "White"
+  }
+
+  test("Black.label returns 'Black'") {
+    Color.Black.label shouldBe "Black"
+  }

modules/api/src/test/scala/de/nowchess/api/board/PieceTest.scala

@@ -0,0 +1,60 @@
+package de.nowchess.api.board
+
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class PieceTest extends AnyFunSuite with Matchers:
+
+  test("Piece holds color and pieceType") {
+    val p = Piece(Color.White, PieceType.Queen)
+    p.color    shouldBe Color.White
+    p.pieceType shouldBe PieceType.Queen
+  }
+
+  test("WhitePawn convenience constant") {
+    Piece.WhitePawn shouldBe Piece(Color.White, PieceType.Pawn)
+  }
+
+  test("WhiteKnight convenience constant") {
+    Piece.WhiteKnight shouldBe Piece(Color.White, PieceType.Knight)
+  }
+
+  test("WhiteBishop convenience constant") {
+    Piece.WhiteBishop shouldBe Piece(Color.White, PieceType.Bishop)
+  }
+
+  test("WhiteRook convenience constant") {
+    Piece.WhiteRook shouldBe Piece(Color.White, PieceType.Rook)
+  }
+
+  test("WhiteQueen convenience constant") {
+    Piece.WhiteQueen shouldBe Piece(Color.White, PieceType.Queen)
+  }
+
+  test("WhiteKing convenience constant") {
+    Piece.WhiteKing shouldBe Piece(Color.White, PieceType.King)
+  }
+
+  test("BlackPawn convenience constant") {
+    Piece.BlackPawn shouldBe Piece(Color.Black, PieceType.Pawn)
+  }
+
+  test("BlackKnight convenience constant") {
+    Piece.BlackKnight shouldBe Piece(Color.Black, PieceType.Knight)
+  }
+
+  test("BlackBishop convenience constant") {
+    Piece.BlackBishop shouldBe Piece(Color.Black, PieceType.Bishop)
+  }
+
+  test("BlackRook convenience constant") {
+    Piece.BlackRook shouldBe Piece(Color.Black, PieceType.Rook)
+  }
+
+  test("BlackQueen convenience constant") {
+    Piece.BlackQueen shouldBe Piece(Color.Black, PieceType.Queen)
+  }
+
+  test("BlackKing convenience constant") {
+    Piece.BlackKing shouldBe Piece(Color.Black, PieceType.King)
+  }

modules/api/src/test/scala/de/nowchess/api/board/PieceTypeTest.scala

@@ -0,0 +1,30 @@
+package de.nowchess.api.board
+
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class PieceTypeTest extends AnyFunSuite with Matchers:
+
+  test("Pawn.label returns 'Pawn'") {
+    PieceType.Pawn.label shouldBe "Pawn"
+  }
+
+  test("Knight.label returns 'Knight'") {
+    PieceType.Knight.label shouldBe "Knight"
+  }
+
+  test("Bishop.label returns 'Bishop'") {
+    PieceType.Bishop.label shouldBe "Bishop"
+  }
+
+  test("Rook.label returns 'Rook'") {
+    PieceType.Rook.label shouldBe "Rook"
+  }
+
+  test("Queen.label returns 'Queen'") {
+    PieceType.Queen.label shouldBe "Queen"
+  }
+
+  test("King.label returns 'King'") {
+    PieceType.King.label shouldBe "King"
+  }

modules/api/src/test/scala/de/nowchess/api/board/SquareTest.scala

@@ -0,0 +1,62 @@
+package de.nowchess.api.board
+
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class SquareTest extends AnyFunSuite with Matchers:
+
+  test("Square.toString produces lowercase file and rank number") {
+    Square(File.E, Rank.R4).toString shouldBe "e4"
+  }
+
+  test("Square.toString for a1") {
+    Square(File.A, Rank.R1).toString shouldBe "a1"
+  }
+
+  test("Square.toString for h8") {
+    Square(File.H, Rank.R8).toString shouldBe "h8"
+  }
+
+  test("fromAlgebraic parses valid square e4") {
+    Square.fromAlgebraic("e4") shouldBe Some(Square(File.E, Rank.R4))
+  }
+
+  test("fromAlgebraic parses valid square a1") {
+    Square.fromAlgebraic("a1") shouldBe Some(Square(File.A, Rank.R1))
+  }
+
+  test("fromAlgebraic parses valid square h8") {
+    Square.fromAlgebraic("h8") shouldBe Some(Square(File.H, Rank.R8))
+  }
+
+  test("fromAlgebraic is case-insensitive for file") {
+    Square.fromAlgebraic("E4") shouldBe Some(Square(File.E, Rank.R4))
+  }
+
+  test("fromAlgebraic returns None for empty string") {
+    Square.fromAlgebraic("") shouldBe None
+  }
+
+  test("fromAlgebraic returns None for string too short") {
+    Square.fromAlgebraic("e") shouldBe None
+  }
+
+  test("fromAlgebraic returns None for string too long") {
+    Square.fromAlgebraic("e42") shouldBe None
+  }
+
+  test("fromAlgebraic returns None for invalid file character") {
+    Square.fromAlgebraic("z4") shouldBe None
+  }
+
+  test("fromAlgebraic returns None for non-digit rank") {
+    Square.fromAlgebraic("ex") shouldBe None
+  }
+
+  test("fromAlgebraic returns None for rank 0") {
+    Square.fromAlgebraic("e0") shouldBe None
+  }
+
+  test("fromAlgebraic returns None for rank 9") {
+    Square.fromAlgebraic("e9") shouldBe None
+  }

modules/api/src/test/scala/de/nowchess/api/game/GameStateTest.scala

@@ -0,0 +1,77 @@
+package de.nowchess.api.game
+
+import de.nowchess.api.board.Color
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class GameStateTest extends AnyFunSuite with Matchers:
+
+  test("CastlingRights.None has both flags false") {
+    CastlingRights.None.kingSide  shouldBe false
+    CastlingRights.None.queenSide shouldBe false
+  }
+
+  test("CastlingRights.Both has both flags true") {
+    CastlingRights.Both.kingSide  shouldBe true
+    CastlingRights.Both.queenSide shouldBe true
+  }
+
+  test("CastlingRights constructor sets fields") {
+    val cr = CastlingRights(kingSide = true, queenSide = false)
+    cr.kingSide  shouldBe true
+    cr.queenSide shouldBe false
+  }
+
+  test("GameResult cases exist") {
+    GameResult.WhiteWins shouldBe GameResult.WhiteWins
+    GameResult.BlackWins shouldBe GameResult.BlackWins
+    GameResult.Draw      shouldBe GameResult.Draw
+  }
+
+  test("GameStatus.NotStarted") {
+    GameStatus.NotStarted shouldBe GameStatus.NotStarted
+  }
+
+  test("GameStatus.InProgress") {
+    GameStatus.InProgress shouldBe GameStatus.InProgress
+  }
+
+  test("GameStatus.Finished carries result") {
+    val status = GameStatus.Finished(GameResult.Draw)
+    status shouldBe GameStatus.Finished(GameResult.Draw)
+    status match
+      case GameStatus.Finished(r) => r shouldBe GameResult.Draw
+      case _                      => fail("expected Finished")
+  }
+
+  test("GameState.initial has standard FEN piece placement") {
+    GameState.initial.piecePlacement shouldBe "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR"
+  }
+
+  test("GameState.initial active color is White") {
+    GameState.initial.activeColor shouldBe Color.White
+  }
+
+  test("GameState.initial white has full castling rights") {
+    GameState.initial.castlingWhite shouldBe CastlingRights.Both
+  }
+
+  test("GameState.initial black has full castling rights") {
+    GameState.initial.castlingBlack shouldBe CastlingRights.Both
+  }
+
+  test("GameState.initial en-passant target is None") {
+    GameState.initial.enPassantTarget shouldBe None
+  }
+
+  test("GameState.initial half-move clock is 0") {
+    GameState.initial.halfMoveClock shouldBe 0
+  }
+
+  test("GameState.initial full-move number is 1") {
+    GameState.initial.fullMoveNumber shouldBe 1
+  }
+
+  test("GameState.initial status is InProgress") {
+    GameState.initial.status shouldBe GameStatus.InProgress
+  }

modules/api/src/test/scala/de/nowchess/api/move/MoveTest.scala

@@ -0,0 +1,56 @@
+package de.nowchess.api.move
+
+import de.nowchess.api.board.{File, Rank, Square}
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class MoveTest extends AnyFunSuite with Matchers:
+
+  private val e2 = Square(File.E, Rank.R2)
+  private val e4 = Square(File.E, Rank.R4)
+
+  test("Move defaults moveType to Normal") {
+    val m = Move(e2, e4)
+    m.moveType shouldBe MoveType.Normal
+  }
+
+  test("Move stores from and to squares") {
+    val m = Move(e2, e4)
+    m.from shouldBe e2
+    m.to   shouldBe e4
+  }
+
+  test("Move with CastleKingside moveType") {
+    val m = Move(e2, e4, MoveType.CastleKingside)
+    m.moveType shouldBe MoveType.CastleKingside
+  }
+
+  test("Move with CastleQueenside moveType") {
+    val m = Move(e2, e4, MoveType.CastleQueenside)
+    m.moveType shouldBe MoveType.CastleQueenside
+  }
+
+  test("Move with EnPassant moveType") {
+    val m = Move(e2, e4, MoveType.EnPassant)
+    m.moveType shouldBe MoveType.EnPassant
+  }
+
+  test("Move with Promotion to Queen") {
+    val m = Move(e2, e4, MoveType.Promotion(PromotionPiece.Queen))
+    m.moveType shouldBe MoveType.Promotion(PromotionPiece.Queen)
+  }
+
+  test("Move with Promotion to Knight") {
+    val m = Move(e2, e4, MoveType.Promotion(PromotionPiece.Knight))
+    m.moveType shouldBe MoveType.Promotion(PromotionPiece.Knight)
+  }
+
+  test("Move with Promotion to Bishop") {
+    val m = Move(e2, e4, MoveType.Promotion(PromotionPiece.Bishop))
+    m.moveType shouldBe MoveType.Promotion(PromotionPiece.Bishop)
+  }
+
+  test("Move with Promotion to Rook") {
+    val m = Move(e2, e4, MoveType.Promotion(PromotionPiece.Rook))
+    m.moveType shouldBe MoveType.Promotion(PromotionPiece.Rook)
+  }

modules/api/src/test/scala/de/nowchess/api/player/PlayerInfoTest.scala

@@ -0,0 +1,23 @@
+package de.nowchess.api.player
+
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class PlayerInfoTest extends AnyFunSuite with Matchers:
+
+  test("PlayerId.apply wraps a string") {
+    val id = PlayerId("player-123")
+    id.value shouldBe "player-123"
+  }
+
+  test("PlayerId.value unwraps to original string") {
+    val raw = "abc-456"
+    PlayerId(raw).value shouldBe raw
+  }
+
+  test("PlayerInfo holds id and displayName") {
+    val id   = PlayerId("p1")
+    val info = PlayerInfo(id, "Magnus")
+    info.id.value      shouldBe "p1"
+    info.displayName   shouldBe "Magnus"
+  }

modules/api/src/test/scala/de/nowchess/api/response/ApiResponseTest.scala

@@ -0,0 +1,62 @@
+package de.nowchess.api.response
+
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class ApiResponseTest extends AnyFunSuite with Matchers:
+
+  test("ApiResponse.Success carries data") {
+    val r = ApiResponse.Success(42)
+    r.data shouldBe 42
+  }
+
+  test("ApiResponse.Failure carries error list") {
+    val err = ApiError("CODE", "msg")
+    val r   = ApiResponse.Failure(List(err))
+    r.errors shouldBe List(err)
+  }
+
+  test("ApiResponse.error creates single-error Failure") {
+    val err = ApiError("NOT_FOUND", "not found")
+    val f   = ApiResponse.error(err)
+    f shouldBe ApiResponse.Failure(List(err))
+  }
+
+  test("ApiError holds code and message") {
+    val e = ApiError("CODE", "message")
+    e.code    shouldBe "CODE"
+    e.message shouldBe "message"
+    e.field   shouldBe None
+  }
+
+  test("ApiError holds optional field") {
+    val e = ApiError("INVALID", "bad value", Some("email"))
+    e.field shouldBe Some("email")
+  }
+
+  test("Pagination.totalPages with exact division") {
+    Pagination(page = 0, pageSize = 10, totalItems = 30).totalPages shouldBe 3
+  }
+
+  test("Pagination.totalPages rounds up") {
+    Pagination(page = 0, pageSize = 10, totalItems = 25).totalPages shouldBe 3
+  }
+
+  test("Pagination.totalPages is 0 when totalItems is 0") {
+    Pagination(page = 0, pageSize = 10, totalItems = 0).totalPages shouldBe 0
+  }
+
+  test("Pagination.totalPages is 0 when pageSize is 0") {
+    Pagination(page = 0, pageSize = 0, totalItems = 100).totalPages shouldBe 0
+  }
+
+  test("Pagination.totalPages is 0 when pageSize is negative") {
+    Pagination(page = 0, pageSize = -1, totalItems = 100).totalPages shouldBe 0
+  }
+
+  test("PagedResponse holds items and pagination") {
+    val pagination = Pagination(page = 1, pageSize = 5, totalItems = 20)
+    val pr         = PagedResponse(List("a", "b"), pagination)
+    pr.items      shouldBe List("a", "b")
+    pr.pagination shouldBe pagination
+  }

modules/api/versions.env

@@ -0,0 +1,3 @@
+MAJOR=0
+MINOR=0
+PATCH=8

modules/core/CHANGELOG.md

@@ -0,0 +1,144 @@
+##  (2026-03-27)
+
+### Features
+
+* add GameRules stub with PositionStatus enum ([76d4168](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/76d4168038de23e5d6083d4e8f0504fbf31d15a3))
+* add MovedInCheck/Checkmate/Stalemate MoveResult variants (stub dispatch) ([8b7ec57](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/8b7ec57e5ea6ee1615a1883848a426dc07d26364))
+* implement GameRules with isInCheck, legalMoves, gameStatus ([94a02ff](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/94a02ff6849436d9496c70a0f16c21666dae8e4e))
+* implement legal castling ([#1](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/1)) ([00d326c](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/00d326c1ba67711fbe180f04e1100c3f01dd0254))
+* wire check/checkmate/stalemate into processMove and gameLoop ([5264a22](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/5264a225418b885c5e6ea6411b96f85e38837f6c))
+
+### Bug Fixes
+
+* add missing kings to gameLoop capture test board ([aedd787](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/aedd787b77203c2af934751dba7b784eaf165032))
+* correct test board positions and captureOutput/withInput interaction ([f0481e2](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/f0481e2561b779df00925b46ee281dc36a795150))
+* update main class path in build configuration and adjust VCS directory mapping ([7b1f8b1](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/7b1f8b117623d327232a1a92a8a44d18582e0189))
+##  (2026-03-28)
+
+### Features
+
+* add GameRules stub with PositionStatus enum ([76d4168](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/76d4168038de23e5d6083d4e8f0504fbf31d15a3))
+* add MovedInCheck/Checkmate/Stalemate MoveResult variants (stub dispatch) ([8b7ec57](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/8b7ec57e5ea6ee1615a1883848a426dc07d26364))
+* implement GameRules with isInCheck, legalMoves, gameStatus ([94a02ff](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/94a02ff6849436d9496c70a0f16c21666dae8e4e))
+* implement legal castling ([#1](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/1)) ([00d326c](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/00d326c1ba67711fbe180f04e1100c3f01dd0254))
+* wire check/checkmate/stalemate into processMove and gameLoop ([5264a22](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/5264a225418b885c5e6ea6411b96f85e38837f6c))
+
+### Bug Fixes
+
+* add missing kings to gameLoop capture test board ([aedd787](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/aedd787b77203c2af934751dba7b784eaf165032))
+* correct test board positions and captureOutput/withInput interaction ([f0481e2](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/f0481e2561b779df00925b46ee281dc36a795150))
+* update main class path in build configuration and adjust VCS directory mapping ([7b1f8b1](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/7b1f8b117623d327232a1a92a8a44d18582e0189))
+##  (2026-03-29)
+
+### Features
+
+* add GameRules stub with PositionStatus enum ([76d4168](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/76d4168038de23e5d6083d4e8f0504fbf31d15a3))
+* add MovedInCheck/Checkmate/Stalemate MoveResult variants (stub dispatch) ([8b7ec57](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/8b7ec57e5ea6ee1615a1883848a426dc07d26364))
+* implement GameRules with isInCheck, legalMoves, gameStatus ([94a02ff](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/94a02ff6849436d9496c70a0f16c21666dae8e4e))
+* implement legal castling ([#1](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/1)) ([00d326c](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/00d326c1ba67711fbe180f04e1100c3f01dd0254))
+* NCS-6 Implementing FEN & PGN ([#7](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/7)) ([f28e69d](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/f28e69dc181416aa2f221fdc4b45c2cda5efbf07))
+* wire check/checkmate/stalemate into processMove and gameLoop ([5264a22](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/5264a225418b885c5e6ea6411b96f85e38837f6c))
+
+### Bug Fixes
+
+* add missing kings to gameLoop capture test board ([aedd787](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/aedd787b77203c2af934751dba7b784eaf165032))
+* correct test board positions and captureOutput/withInput interaction ([f0481e2](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/f0481e2561b779df00925b46ee281dc36a795150))
+* update main class path in build configuration and adjust VCS directory mapping ([7b1f8b1](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/7b1f8b117623d327232a1a92a8a44d18582e0189))
+##  (2026-03-29)
+
+### Features
+
+* add GameRules stub with PositionStatus enum ([76d4168](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/76d4168038de23e5d6083d4e8f0504fbf31d15a3))
+* add MovedInCheck/Checkmate/Stalemate MoveResult variants (stub dispatch) ([8b7ec57](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/8b7ec57e5ea6ee1615a1883848a426dc07d26364))
+* implement GameRules with isInCheck, legalMoves, gameStatus ([94a02ff](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/94a02ff6849436d9496c70a0f16c21666dae8e4e))
+* implement legal castling ([#1](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/1)) ([00d326c](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/00d326c1ba67711fbe180f04e1100c3f01dd0254))
+* NCS-6 Implementing FEN & PGN ([#7](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/7)) ([f28e69d](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/f28e69dc181416aa2f221fdc4b45c2cda5efbf07))
+* NCS-9 En passant implementation ([#8](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/8)) ([919beb3](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/919beb3b4bfa8caf2f90976a415fe9b19b7e9747))
+* wire check/checkmate/stalemate into processMove and gameLoop ([5264a22](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/5264a225418b885c5e6ea6411b96f85e38837f6c))
+
+### Bug Fixes
+
+* add missing kings to gameLoop capture test board ([aedd787](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/aedd787b77203c2af934751dba7b784eaf165032))
+* correct test board positions and captureOutput/withInput interaction ([f0481e2](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/f0481e2561b779df00925b46ee281dc36a795150))
+* update main class path in build configuration and adjust VCS directory mapping ([7b1f8b1](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/7b1f8b117623d327232a1a92a8a44d18582e0189))
+##  (2026-03-31)
+
+### Features
+
+* add GameRules stub with PositionStatus enum ([76d4168](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/76d4168038de23e5d6083d4e8f0504fbf31d15a3))
+* add MovedInCheck/Checkmate/Stalemate MoveResult variants (stub dispatch) ([8b7ec57](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/8b7ec57e5ea6ee1615a1883848a426dc07d26364))
+* implement GameRules with isInCheck, legalMoves, gameStatus ([94a02ff](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/94a02ff6849436d9496c70a0f16c21666dae8e4e))
+* implement legal castling ([#1](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/1)) ([00d326c](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/00d326c1ba67711fbe180f04e1100c3f01dd0254))
+* NCS-16 Core Separation via Patterns ([#10](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/10)) ([1361dfc](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/1361dfc89553b146864fb8ff3526cf12cf3f293a))
+* NCS-6 Implementing FEN & PGN ([#7](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/7)) ([f28e69d](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/f28e69dc181416aa2f221fdc4b45c2cda5efbf07))
+* NCS-9 En passant implementation ([#8](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/8)) ([919beb3](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/919beb3b4bfa8caf2f90976a415fe9b19b7e9747))
+* wire check/checkmate/stalemate into processMove and gameLoop ([5264a22](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/5264a225418b885c5e6ea6411b96f85e38837f6c))
+
+### Bug Fixes
+
+* add missing kings to gameLoop capture test board ([aedd787](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/aedd787b77203c2af934751dba7b784eaf165032))
+* correct test board positions and captureOutput/withInput interaction ([f0481e2](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/f0481e2561b779df00925b46ee281dc36a795150))
+* update main class path in build configuration and adjust VCS directory mapping ([7b1f8b1](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/7b1f8b117623d327232a1a92a8a44d18582e0189))
+##  (2026-04-01)
+
+### Features
+
+* add GameRules stub with PositionStatus enum ([76d4168](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/76d4168038de23e5d6083d4e8f0504fbf31d15a3))
+* add MovedInCheck/Checkmate/Stalemate MoveResult variants (stub dispatch) ([8b7ec57](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/8b7ec57e5ea6ee1615a1883848a426dc07d26364))
+* implement GameRules with isInCheck, legalMoves, gameStatus ([94a02ff](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/94a02ff6849436d9496c70a0f16c21666dae8e4e))
+* implement legal castling ([#1](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/1)) ([00d326c](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/00d326c1ba67711fbe180f04e1100c3f01dd0254))
+* NCS-10 Implement Pawn Promotion ([#12](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/12)) ([13bfc16](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/13bfc16cfe25db78ec607db523ca6d993c13430c))
+* NCS-16 Core Separation via Patterns ([#10](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/10)) ([1361dfc](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/1361dfc89553b146864fb8ff3526cf12cf3f293a))
+* NCS-6 Implementing FEN & PGN ([#7](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/7)) ([f28e69d](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/f28e69dc181416aa2f221fdc4b45c2cda5efbf07))
+* NCS-9 En passant implementation ([#8](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/8)) ([919beb3](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/919beb3b4bfa8caf2f90976a415fe9b19b7e9747))
+* wire check/checkmate/stalemate into processMove and gameLoop ([5264a22](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/5264a225418b885c5e6ea6411b96f85e38837f6c))
+
+### Bug Fixes
+
+* add missing kings to gameLoop capture test board ([aedd787](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/aedd787b77203c2af934751dba7b784eaf165032))
+* correct test board positions and captureOutput/withInput interaction ([f0481e2](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/f0481e2561b779df00925b46ee281dc36a795150))
+* update main class path in build configuration and adjust VCS directory mapping ([7b1f8b1](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/7b1f8b117623d327232a1a92a8a44d18582e0189))
+* update move validation to check for king safety ([#13](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/13)) ([e5e20c5](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/e5e20c566e368b12ca1dc59680c34e9112bf6762))
+##  (2026-04-01)
+
+### Features
+
+* add GameRules stub with PositionStatus enum ([76d4168](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/76d4168038de23e5d6083d4e8f0504fbf31d15a3))
+* add MovedInCheck/Checkmate/Stalemate MoveResult variants (stub dispatch) ([8b7ec57](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/8b7ec57e5ea6ee1615a1883848a426dc07d26364))
+* implement GameRules with isInCheck, legalMoves, gameStatus ([94a02ff](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/94a02ff6849436d9496c70a0f16c21666dae8e4e))
+* implement legal castling ([#1](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/1)) ([00d326c](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/00d326c1ba67711fbe180f04e1100c3f01dd0254))
+* NCS-10 Implement Pawn Promotion ([#12](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/12)) ([13bfc16](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/13bfc16cfe25db78ec607db523ca6d993c13430c))
+* NCS-11 50-move rule ([#9](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/9)) ([412ed98](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/412ed986a95703a3b282276540153480ceed229d))
+* NCS-16 Core Separation via Patterns ([#10](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/10)) ([1361dfc](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/1361dfc89553b146864fb8ff3526cf12cf3f293a))
+* NCS-6 Implementing FEN & PGN ([#7](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/7)) ([f28e69d](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/f28e69dc181416aa2f221fdc4b45c2cda5efbf07))
+* NCS-9 En passant implementation ([#8](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/8)) ([919beb3](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/919beb3b4bfa8caf2f90976a415fe9b19b7e9747))
+* wire check/checkmate/stalemate into processMove and gameLoop ([5264a22](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/5264a225418b885c5e6ea6411b96f85e38837f6c))
+
+### Bug Fixes
+
+* add missing kings to gameLoop capture test board ([aedd787](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/aedd787b77203c2af934751dba7b784eaf165032))
+* correct test board positions and captureOutput/withInput interaction ([f0481e2](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/f0481e2561b779df00925b46ee281dc36a795150))
+* update main class path in build configuration and adjust VCS directory mapping ([7b1f8b1](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/7b1f8b117623d327232a1a92a8a44d18582e0189))
+* update move validation to check for king safety ([#13](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/13)) ([e5e20c5](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/e5e20c566e368b12ca1dc59680c34e9112bf6762))
+##  (2026-04-01)
+
+### Features
+
+* add GameRules stub with PositionStatus enum ([76d4168](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/76d4168038de23e5d6083d4e8f0504fbf31d15a3))
+* add MovedInCheck/Checkmate/Stalemate MoveResult variants (stub dispatch) ([8b7ec57](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/8b7ec57e5ea6ee1615a1883848a426dc07d26364))
+* implement GameRules with isInCheck, legalMoves, gameStatus ([94a02ff](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/94a02ff6849436d9496c70a0f16c21666dae8e4e))
+* implement legal castling ([#1](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/1)) ([00d326c](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/00d326c1ba67711fbe180f04e1100c3f01dd0254))
+* NCS-10 Implement Pawn Promotion ([#12](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/12)) ([13bfc16](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/13bfc16cfe25db78ec607db523ca6d993c13430c))
+* NCS-11 50-move rule ([#9](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/9)) ([412ed98](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/412ed986a95703a3b282276540153480ceed229d))
+* NCS-16 Core Separation via Patterns ([#10](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/10)) ([1361dfc](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/1361dfc89553b146864fb8ff3526cf12cf3f293a))
+* NCS-17 Implement basic ScalaFX UI ([#14](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/14)) ([3ff8031](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/3ff80318b4f16c59733a46498581a5c27f048287))
+* NCS-6 Implementing FEN & PGN ([#7](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/7)) ([f28e69d](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/f28e69dc181416aa2f221fdc4b45c2cda5efbf07))
+* NCS-9 En passant implementation ([#8](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/8)) ([919beb3](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/919beb3b4bfa8caf2f90976a415fe9b19b7e9747))
+* wire check/checkmate/stalemate into processMove and gameLoop ([5264a22](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/5264a225418b885c5e6ea6411b96f85e38837f6c))
+
+### Bug Fixes
+
+* add missing kings to gameLoop capture test board ([aedd787](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/aedd787b77203c2af934751dba7b784eaf165032))
+* correct test board positions and captureOutput/withInput interaction ([f0481e2](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/f0481e2561b779df00925b46ee281dc36a795150))
+* update main class path in build configuration and adjust VCS directory mapping ([7b1f8b1](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/7b1f8b117623d327232a1a92a8a44d18582e0189))
+* update move validation to check for king safety ([#13](https://git.janis-eccarius.de/NowChess/NowChessSystems/issues/13)) ([e5e20c5](https://git.janis-eccarius.de/NowChess/NowChessSystems/commit/e5e20c566e368b12ca1dc59680c34e9112bf6762))

modules/core/build.gradle.kts

@@ -52,16 +52,17 @@ dependencies {
 
     testImplementation(platform("org.junit:junit-bom:5.13.4"))
     testImplementation("org.junit.jupiter:junit-jupiter")
-    testRuntimeOnly("org.junit.platform:junit-platform-launcher")
     testImplementation("org.scalatest:scalatest_3:${versions["SCALATEST"]!!}")
     testImplementation("co.helmethair:scalatest-junit-runner:${versions["SCALATEST_JUNIT"]!!}")
+
+    testRuntimeOnly("org.junit.platform:junit-platform-launcher")
 }
 
 tasks.test {
     useJUnitPlatform {
         includeEngines("scalatest")
         testLogging {
-            events("passed", "skipped", "failed")
+            events("skipped", "failed")
         }
     }
     finalizedBy(tasks.reportScoverage)

modules/core/src/main/scala/de/nowchess/chess/Main.scala

@@ -1,11 +0,0 @@
-package de.nowchess.chess
-
-import de.nowchess.api.board.Color
-import de.nowchess.chess.controller.GameController
-import de.nowchess.chess.logic.GameContext
-
-object Main {
-  def main(args: Array[String]): Unit =
-    println("NowChess TUI — type moves in coordinate notation (e.g. e2e4). Type 'quit' to exit.")
-    GameController.gameLoop(GameContext.initial, Color.White)
-}

modules/core/src/main/scala/de/nowchess/chess/command/Command.scala

@@ -0,0 +1,64 @@
+package de.nowchess.chess.command
+
+import de.nowchess.api.board.{Square, Board, Color, Piece}
+import de.nowchess.chess.logic.GameHistory
+
+/** Marker trait for all commands that can be executed and undone.
+ *  Commands encapsulate user actions and game state transitions.
+ */
+trait Command:
+  /** Execute the command and return true if successful, false otherwise. */
+  def execute(): Boolean
+
+  /** Undo the command and return true if successful, false otherwise. */
+  def undo(): Boolean
+
+  /** A human-readable description of this command. */
+  def description: String
+
+/** Command to move a piece from one square to another.
+ *  Stores the move result so undo can restore previous state.
+ */
+case class MoveCommand(
+  from: Square,
+  to: Square,
+  moveResult: Option[MoveResult] = None,
+  previousBoard: Option[Board] = None,
+  previousHistory: Option[GameHistory] = None,
+  previousTurn: Option[Color] = None
+) extends Command:
+
+  override def execute(): Boolean =
+    moveResult.isDefined
+
+  override def undo(): Boolean =
+    previousBoard.isDefined && previousHistory.isDefined && previousTurn.isDefined
+
+  override def description: String = s"Move from $from to $to"
+
+// Sealed hierarchy of move outcomes (for tracking state changes)
+sealed trait MoveResult
+object MoveResult:
+  case class Successful(newBoard: Board, newHistory: GameHistory, newTurn: Color, captured: Option[Piece]) extends MoveResult
+  case object InvalidFormat extends MoveResult
+  case object InvalidMove extends MoveResult
+
+/** Command to quit the game. */
+case class QuitCommand() extends Command:
+  override def execute(): Boolean = true
+  override def undo(): Boolean = false
+  override def description: String = "Quit game"
+
+/** Command to reset the board to initial position. */
+case class ResetCommand(
+  previousBoard: Option[Board] = None,
+  previousHistory: Option[GameHistory] = None,
+  previousTurn: Option[Color] = None
+) extends Command:
+
+  override def execute(): Boolean = true
+
+  override def undo(): Boolean =
+    previousBoard.isDefined && previousHistory.isDefined && previousTurn.isDefined
+
+  override def description: String = "Reset board"

modules/core/src/main/scala/de/nowchess/chess/command/CommandInvoker.scala

@@ -0,0 +1,73 @@
+package de.nowchess.chess.command
+
+/** Manages command execution and history for undo/redo support. */
+class CommandInvoker:
+  private val executedCommands = scala.collection.mutable.ListBuffer[Command]()
+  private var currentIndex = -1
+
+  /** Execute a command and add it to history.
+   *  Discards any redo history if not at the end of the stack.
+   */
+  def execute(command: Command): Boolean = synchronized {
+    if command.execute() then
+      // Remove any commands after current index (redo stack is discarded)
+      while currentIndex < executedCommands.size - 1 do
+        executedCommands.remove(executedCommands.size - 1)
+      executedCommands += command
+      currentIndex += 1
+      true
+    else
+      false
+  }
+
+  /** Undo the last executed command if possible. */
+  def undo(): Boolean = synchronized {
+    if currentIndex >= 0 && currentIndex < executedCommands.size then
+      val command = executedCommands(currentIndex)
+      if command.undo() then
+        currentIndex -= 1
+        true
+      else
+        false
+    else
+      false
+  }
+
+  /** Redo the next command in history if available. */
+  def redo(): Boolean = synchronized {
+    if currentIndex + 1 < executedCommands.size then
+      val command = executedCommands(currentIndex + 1)
+      if command.execute() then
+        currentIndex += 1
+        true
+      else
+        false
+    else
+      false
+  }
+
+  /** Get the history of all executed commands. */
+  def history: List[Command] = synchronized {
+    executedCommands.toList
+  }
+
+  /** Get the current position in command history. */
+  def getCurrentIndex: Int = synchronized {
+    currentIndex
+  }
+
+  /** Clear all command history. */
+  def clear(): Unit = synchronized {
+    executedCommands.clear()
+    currentIndex = -1
+  }
+
+  /** Check if undo is available. */
+  def canUndo: Boolean = synchronized {
+    currentIndex >= 0
+  }
+
+  /** Check if redo is available. */
+  def canRedo: Boolean = synchronized {
+    currentIndex + 1 < executedCommands.size
+  }

modules/core/src/main/scala/de/nowchess/chess/controller/GameController.scala

@@ -1,10 +1,8 @@
 package de.nowchess.chess.controller
 
-import scala.io.StdIn
-import de.nowchess.api.board.{Board, Color, File, Piece, Rank, Square}
-import de.nowchess.api.game.CastlingRights
-import de.nowchess.chess.logic.{GameContext, MoveValidator, GameRules, PositionStatus, CastleSide, withCastle}
-import de.nowchess.chess.view.Renderer
+import de.nowchess.api.board.{Board, Color, File, Piece, PieceType, Rank, Square}
+import de.nowchess.api.move.PromotionPiece
+import de.nowchess.chess.logic.*
 
 // ---------------------------------------------------------------------------
 // Result ADT returned by the pure processMove function
@@ -17,8 +15,16 @@ object MoveResult:
   case object NoPiece                                                                   extends MoveResult
   case object WrongColor                                                                extends MoveResult
   case object IllegalMove                                                               extends MoveResult
-  case class  Moved(newCtx: GameContext, captured: Option[Piece], newTurn: Color)      extends MoveResult
-  case class  MovedInCheck(newCtx: GameContext, captured: Option[Piece], newTurn: Color) extends MoveResult
+  case class  PromotionRequired(
+    from: Square,
+    to: Square,
+    boardBefore: Board,
+    historyBefore: GameHistory,
+    captured: Option[Piece],
+    turn: Color
+  ) extends MoveResult
+  case class  Moved(newBoard: Board, newHistory: GameHistory, captured: Option[Piece], newTurn: Color)      extends MoveResult
+  case class  MovedInCheck(newBoard: Board, newHistory: GameHistory, captured: Option[Piece], newTurn: Color) extends MoveResult
   case class  Checkmate(winner: Color)                                                  extends MoveResult
   case object Stalemate                                                                 extends MoveResult
 
@@ -31,106 +37,70 @@ object GameController:
   /** Pure function: interprets one raw input line against the current game context.
    *  Has no I/O side effects — all output must be handled by the caller.
    */
-  def processMove(ctx: GameContext, turn: Color, raw: String): MoveResult =
+  def processMove(board: Board, history: GameHistory, turn: Color, raw: String): MoveResult =
     raw.trim match
-      case "quit" | "q" =>
-        MoveResult.Quit
+      case "quit" | "q" => MoveResult.Quit
       case trimmed =>
         Parser.parseMove(trimmed) match
-          case None =>
-            MoveResult.InvalidFormat(trimmed)
-          case Some((from, to)) =>
-            ctx.board.pieceAt(from) match
-              case None =>
-                MoveResult.NoPiece
-              case Some(piece) if piece.color != turn =>
-                MoveResult.WrongColor
-              case Some(_) =>
-                if !MoveValidator.isLegal(ctx, from, to) then
-                  MoveResult.IllegalMove
-                else
-                  val castleOpt = if MoveValidator.isCastle(ctx.board, from, to)
-                                  then Some(MoveValidator.castleSide(from, to))
-                                  else None
-                  val (newBoard, captured) = castleOpt match
-                    case Some(side) => (ctx.board.withCastle(turn, side), None)
-                    case None       => ctx.board.withMove(from, to)
-                  val newCtx = applyRightsRevocation(
-                    ctx.copy(board = newBoard), turn, from, to, castleOpt
-                  )
-                  GameRules.gameStatus(newCtx, turn.opposite) match
-                    case PositionStatus.Normal  => MoveResult.Moved(newCtx, captured, turn.opposite)
-                    case PositionStatus.InCheck => MoveResult.MovedInCheck(newCtx, captured, turn.opposite)
-                    case PositionStatus.Mated   => MoveResult.Checkmate(turn)
-                    case PositionStatus.Drawn   => MoveResult.Stalemate
+          case None              => MoveResult.InvalidFormat(trimmed)
+          case Some((from, to))  => validateAndApply(board, history, turn, from, to)
 
-  private def applyRightsRevocation(
-    ctx: GameContext,
-    turn: Color,
+  /** Apply a previously detected promotion move with the chosen piece.
+   *  Called after processMove returned PromotionRequired.
+   */
+  def completePromotion(
+    board: Board,
+    history: GameHistory,
     from: Square,
     to: Square,
-    castle: Option[CastleSide]
-  ): GameContext =
-    // Step 1: Revoke all rights for a castling move (idempotent with step 2)
-    val ctx0 = castle.fold(ctx)(_ => ctx.withUpdatedRights(turn, CastlingRights.None))
+    piece: PromotionPiece,
+    turn: Color
+  ): MoveResult =
+    val (boardAfterMove, captured) = board.withMove(from, to)
+    val promotedPieceType = piece match
+      case PromotionPiece.Queen  => PieceType.Queen
+      case PromotionPiece.Rook   => PieceType.Rook
+      case PromotionPiece.Bishop => PieceType.Bishop
+      case PromotionPiece.Knight => PieceType.Knight
+    val newBoard    = boardAfterMove.updated(to, Piece(turn, promotedPieceType))
+    // Promotion is always a pawn move → clock resets
+    val newHistory = history.addMove(from, to, None, Some(piece), wasPawnMove = true)
+    toMoveResult(newBoard, newHistory, captured, turn)
 
-    // Step 2: Source-square revocation
-    val ctx1 = from match
-      case Square(File.E, Rank.R1) => ctx0.withUpdatedRights(Color.White, CastlingRights.None)
-      case Square(File.E, Rank.R8) => ctx0.withUpdatedRights(Color.Black, CastlingRights.None)
-      case Square(File.A, Rank.R1) => ctx0.withUpdatedRights(Color.White, ctx0.whiteCastling.copy(queenSide = false))
-      case Square(File.H, Rank.R1) => ctx0.withUpdatedRights(Color.White, ctx0.whiteCastling.copy(kingSide  = false))
-      case Square(File.A, Rank.R8) => ctx0.withUpdatedRights(Color.Black, ctx0.blackCastling.copy(queenSide = false))
-      case Square(File.H, Rank.R8) => ctx0.withUpdatedRights(Color.Black, ctx0.blackCastling.copy(kingSide  = false))
-      case _                       => ctx0
+  // ---------------------------------------------------------------------------
+  // Private helpers
+  // ---------------------------------------------------------------------------
 
-    // Step 3: Destination-square revocation (enemy captures a rook on its home square)
-    to match
-      case Square(File.A, Rank.R1) => ctx1.withUpdatedRights(Color.White, ctx1.whiteCastling.copy(queenSide = false))
-      case Square(File.H, Rank.R1) => ctx1.withUpdatedRights(Color.White, ctx1.whiteCastling.copy(kingSide  = false))
-      case Square(File.A, Rank.R8) => ctx1.withUpdatedRights(Color.Black, ctx1.blackCastling.copy(queenSide = false))
-      case Square(File.H, Rank.R8) => ctx1.withUpdatedRights(Color.Black, ctx1.blackCastling.copy(kingSide  = false))
-      case _                       => ctx1
+  private def validateAndApply(board: Board, history: GameHistory, turn: Color, from: Square, to: Square): MoveResult =
+    board.pieceAt(from) match
+      case None                              => MoveResult.NoPiece
+      case Some(piece) if piece.color != turn => MoveResult.WrongColor
+      case Some(_) =>
+        if !GameRules.legalMoves(board, history, turn).contains(from -> to) then MoveResult.IllegalMove
+        else if MoveValidator.isPromotionMove(board, from, to) then
+          MoveResult.PromotionRequired(from, to, board, history, board.pieceAt(to), turn)
+        else applyNormalMove(board, history, turn, from, to)
 
-  /** Thin I/O shell: renders the board, reads a line, delegates to processMove,
-   *  prints the outcome, and recurses until the game ends.
-   */
-  def gameLoop(ctx: GameContext, turn: Color): Unit =
-    println()
-    print(Renderer.render(ctx.board))
-    println(s"${turn.label}'s turn. Enter move: ")
-    val input = Option(StdIn.readLine()).getOrElse("quit").trim
-    processMove(ctx, turn, input) match
-      case MoveResult.Quit =>
-        println("Game over. Goodbye!")
-      case MoveResult.InvalidFormat(raw) =>
-        println(s"Invalid move format '$raw'. Use coordinate notation, e.g. e2e4.")
-        gameLoop(ctx, turn)
-      case MoveResult.NoPiece =>
-        println(s"No piece on ${Parser.parseMove(input).map(_._1).fold("?")(_.toString)}.")
-        gameLoop(ctx, turn)
-      case MoveResult.WrongColor =>
-        println(s"That is not your piece.")
-        gameLoop(ctx, turn)
-      case MoveResult.IllegalMove =>
-        println(s"Illegal move.")
-        gameLoop(ctx, turn)
-      case MoveResult.Moved(newCtx, captured, newTurn) =>
-        val prevTurn = newTurn.opposite
-        captured.foreach: cap =>
-          val toSq = Parser.parseMove(input).map(_._2).fold("?")(_.toString)
-          println(s"${prevTurn.label} captures ${cap.color.label} ${cap.pieceType.label} on $toSq")
-        gameLoop(newCtx, newTurn)
-      case MoveResult.MovedInCheck(newCtx, captured, newTurn) =>
-        val prevTurn = newTurn.opposite
-        captured.foreach: cap =>
-          val toSq = Parser.parseMove(input).map(_._2).fold("?")(_.toString)
-          println(s"${prevTurn.label} captures ${cap.color.label} ${cap.pieceType.label} on $toSq")
-        println(s"${newTurn.label} is in check!")
-        gameLoop(newCtx, newTurn)
-      case MoveResult.Checkmate(winner) =>
-        println(s"Checkmate! ${winner.label} wins.")
-        gameLoop(GameContext.initial, Color.White)
-      case MoveResult.Stalemate =>
-        println("Stalemate! The game is a draw.")
-        gameLoop(GameContext.initial, Color.White)
+  private def applyNormalMove(board: Board, history: GameHistory, turn: Color, from: Square, to: Square): MoveResult =
+    val castleOpt              = Option.when(MoveValidator.isCastle(board, from, to))(MoveValidator.castleSide(from, to))
+    val isEP                   = EnPassantCalculator.isEnPassant(board, history, from, to)
+    val (newBoard, captured)   = castleOpt match
+      case Some(side) => (board.withCastle(turn, side), None)
+      case None =>
+        val (b, cap) = board.withMove(from, to)
+        if isEP then
+          val capturedSq = EnPassantCalculator.capturedPawnSquare(to, turn)
+          (b.removed(capturedSq), board.pieceAt(capturedSq))
+        else (b, cap)
+    val pieceType   = board.pieceAt(from).map(_.pieceType).getOrElse(PieceType.Pawn)
+    val wasPawnMove = pieceType == PieceType.Pawn
+    val wasCapture  = captured.isDefined
+    val newHistory  = history.addMove(from, to, castleOpt, wasPawnMove = wasPawnMove, wasCapture = wasCapture, pieceType = pieceType)
+    toMoveResult(newBoard, newHistory, captured, turn)
+
+  private def toMoveResult(newBoard: Board, newHistory: GameHistory, captured: Option[Piece], turn: Color): MoveResult =
+    GameRules.gameStatus(newBoard, newHistory, turn.opposite) match
+      case PositionStatus.Normal  => MoveResult.Moved(newBoard, newHistory, captured, turn.opposite)
+      case PositionStatus.InCheck => MoveResult.MovedInCheck(newBoard, newHistory, captured, turn.opposite)
+      case PositionStatus.Mated   => MoveResult.Checkmate(turn)
+      case PositionStatus.Drawn   => MoveResult.Stalemate

modules/core/src/main/scala/de/nowchess/chess/engine/GameEngine.scala

@@ -0,0 +1,352 @@
+package de.nowchess.chess.engine
+
+import de.nowchess.api.board.{Board, Color, Piece, Square}
+import de.nowchess.api.move.PromotionPiece
+import de.nowchess.chess.logic.{GameHistory, GameRules, PositionStatus}
+import de.nowchess.chess.controller.{GameController, Parser, MoveResult}
+import de.nowchess.chess.observer.*
+import de.nowchess.chess.command.{CommandInvoker, MoveCommand}
+import de.nowchess.chess.notation.{PgnExporter, PgnParser}
+
+/** Pure game engine that manages game state and notifies observers of state changes.
+ *  This class is the single source of truth for the game state.
+ *  All user interactions must go through this engine via Commands, and all state changes
+ *  are communicated to observers via GameEvent notifications.
+ */
+class GameEngine(
+  initialBoard: Board = Board.initial,
+  initialHistory: GameHistory = GameHistory.empty,
+  initialTurn: Color = Color.White,
+  completePromotionFn: (Board, GameHistory, Square, Square, PromotionPiece, Color) => MoveResult =
+    GameController.completePromotion
+) extends Observable:
+  private var currentBoard: Board = initialBoard
+  private var currentHistory: GameHistory = initialHistory
+  private var currentTurn: Color = initialTurn
+  private val invoker = new CommandInvoker()
+
+  /** Inner class for tracking pending promotion state */
+  private case class PendingPromotion(
+    from: Square, to: Square,
+    boardBefore: Board, historyBefore: GameHistory,
+    turn: Color
+  )
+
+  /** Current pending promotion, if any */
+  private var pendingPromotion: Option[PendingPromotion] = None
+
+  /** True if a pawn promotion move is pending and needs a piece choice. */
+  def isPendingPromotion: Boolean = synchronized { pendingPromotion.isDefined }
+
+  // Synchronized accessors for current state
+  def board: Board = synchronized { currentBoard }
+  def history: GameHistory = synchronized { currentHistory }
+  def turn: Color = synchronized { currentTurn }
+
+  /** Check if undo is available. */
+  def canUndo: Boolean = synchronized { invoker.canUndo }
+
+  /** Check if redo is available. */
+  def canRedo: Boolean = synchronized { invoker.canRedo }
+
+  /** Get the command history for inspection (testing/debugging). */
+  def commandHistory: List[de.nowchess.chess.command.Command] = synchronized { invoker.history }
+
+  /** Process a raw move input string and update game state if valid.
+   *  Notifies all observers of the outcome via GameEvent.
+   */
+  def processUserInput(rawInput: String): Unit = synchronized {
+    val trimmed = rawInput.trim.toLowerCase
+    trimmed match
+      case "quit" | "q" =>
+        // Client should handle quit logic; we just return
+        ()
+
+      case "undo" =>
+        performUndo()
+
+      case "redo" =>
+        performRedo()
+
+      case "draw" =>
+        if currentHistory.halfMoveClock >= 100 then
+          currentBoard   = Board.initial
+          currentHistory = GameHistory.empty
+          currentTurn    = Color.White
+          invoker.clear()
+          notifyObservers(DrawClaimedEvent(currentBoard, currentHistory, currentTurn))
+        else
+          notifyObservers(InvalidMoveEvent(
+            currentBoard, currentHistory, currentTurn,
+            "Draw cannot be claimed: the 50-move rule has not been triggered."
+          ))
+
+      case "" =>
+        val event = InvalidMoveEvent(
+          currentBoard,
+          currentHistory,
+          currentTurn,
+          "Please enter a valid move or command."
+        )
+        notifyObservers(event)
+
+      case moveInput =>
+        Parser.parseMove(moveInput) match
+          case None =>
+            notifyObservers(InvalidMoveEvent(
+              currentBoard, currentHistory, currentTurn,
+              s"Invalid move format '$moveInput'. Use coordinate notation, e.g. e2e4."
+            ))
+          case Some((from, to)) =>
+            handleParsedMove(from, to, moveInput)
+  }
+
+  private def handleParsedMove(from: Square, to: Square, moveInput: String): Unit =
+    val cmd = MoveCommand(
+      from = from,
+      to = to,
+      previousBoard = Some(currentBoard),
+      previousHistory = Some(currentHistory),
+      previousTurn = Some(currentTurn)
+    )
+    GameController.processMove(currentBoard, currentHistory, currentTurn, moveInput) match
+      case MoveResult.InvalidFormat(_) | MoveResult.NoPiece | MoveResult.WrongColor | MoveResult.IllegalMove | MoveResult.Quit =>
+        handleFailedMove(moveInput)
+
+      case MoveResult.Moved(newBoard, newHistory, captured, newTurn) =>
+        val updatedCmd = cmd.copy(moveResult = Some(de.nowchess.chess.command.MoveResult.Successful(newBoard, newHistory, newTurn, captured)))
+        invoker.execute(updatedCmd)
+        updateGameState(newBoard, newHistory, newTurn)
+        emitMoveEvent(from.toString, to.toString, captured, newTurn)
+        if currentHistory.halfMoveClock >= 100 then
+          notifyObservers(FiftyMoveRuleAvailableEvent(currentBoard, currentHistory, currentTurn))
+
+      case MoveResult.MovedInCheck(newBoard, newHistory, captured, newTurn) =>
+        val updatedCmd = cmd.copy(moveResult = Some(de.nowchess.chess.command.MoveResult.Successful(newBoard, newHistory, newTurn, captured)))
+        invoker.execute(updatedCmd)
+        updateGameState(newBoard, newHistory, newTurn)
+        emitMoveEvent(from.toString, to.toString, captured, newTurn)
+        notifyObservers(CheckDetectedEvent(currentBoard, currentHistory, currentTurn))
+        if currentHistory.halfMoveClock >= 100 then
+          notifyObservers(FiftyMoveRuleAvailableEvent(currentBoard, currentHistory, currentTurn))
+
+      case MoveResult.Checkmate(winner) =>
+        val updatedCmd = cmd.copy(moveResult = Some(de.nowchess.chess.command.MoveResult.Successful(Board.initial, GameHistory.empty, Color.White, None)))
+        invoker.execute(updatedCmd)
+        currentBoard = Board.initial
+        currentHistory = GameHistory.empty
+        currentTurn = Color.White
+        notifyObservers(CheckmateEvent(currentBoard, currentHistory, currentTurn, winner))
+
+      case MoveResult.Stalemate =>
+        val updatedCmd = cmd.copy(moveResult = Some(de.nowchess.chess.command.MoveResult.Successful(Board.initial, GameHistory.empty, Color.White, None)))
+        invoker.execute(updatedCmd)
+        currentBoard = Board.initial
+        currentHistory = GameHistory.empty
+        currentTurn = Color.White
+        notifyObservers(StalemateEvent(currentBoard, currentHistory, currentTurn))
+
+      case MoveResult.PromotionRequired(promFrom, promTo, boardBefore, histBefore, _, promotingTurn) =>
+        pendingPromotion = Some(PendingPromotion(promFrom, promTo, boardBefore, histBefore, promotingTurn))
+        notifyObservers(PromotionRequiredEvent(currentBoard, currentHistory, currentTurn, promFrom, promTo))
+
+  /** Undo the last move. */
+  def undo(): Unit = synchronized {
+    performUndo()
+  }
+
+  /** Redo the last undone move. */
+  def redo(): Unit = synchronized {
+    performRedo()
+  }
+
+  /** Apply a player's promotion piece choice.
+   *  Must only be called when isPendingPromotion is true.
+   */
+  def completePromotion(piece: PromotionPiece): Unit = synchronized {
+    pendingPromotion match
+      case None =>
+        notifyObservers(InvalidMoveEvent(currentBoard, currentHistory, currentTurn, "No promotion pending."))
+      case Some(pending) =>
+        pendingPromotion = None
+        val cmd = MoveCommand(
+          from = pending.from,
+          to = pending.to,
+          previousBoard = Some(pending.boardBefore),
+          previousHistory = Some(pending.historyBefore),
+          previousTurn = Some(pending.turn)
+        )
+        completePromotionFn(
+          pending.boardBefore, pending.historyBefore,
+          pending.from, pending.to, piece, pending.turn
+        ) match
+          case MoveResult.Moved(newBoard, newHistory, captured, newTurn) =>
+            val updatedCmd = cmd.copy(moveResult = Some(de.nowchess.chess.command.MoveResult.Successful(newBoard, newHistory, newTurn, captured)))
+            invoker.execute(updatedCmd)
+            updateGameState(newBoard, newHistory, newTurn)
+            emitMoveEvent(pending.from.toString, pending.to.toString, captured, newTurn)
+
+          case MoveResult.MovedInCheck(newBoard, newHistory, captured, newTurn) =>
+            val updatedCmd = cmd.copy(moveResult = Some(de.nowchess.chess.command.MoveResult.Successful(newBoard, newHistory, newTurn, captured)))
+            invoker.execute(updatedCmd)
+            updateGameState(newBoard, newHistory, newTurn)
+            emitMoveEvent(pending.from.toString, pending.to.toString, captured, newTurn)
+            notifyObservers(CheckDetectedEvent(currentBoard, currentHistory, currentTurn))
+
+          case MoveResult.Checkmate(winner) =>
+            val updatedCmd = cmd.copy(moveResult = Some(de.nowchess.chess.command.MoveResult.Successful(Board.initial, GameHistory.empty, Color.White, None)))
+            invoker.execute(updatedCmd)
+            currentBoard = Board.initial
+            currentHistory = GameHistory.empty
+            currentTurn = Color.White
+            notifyObservers(CheckmateEvent(currentBoard, currentHistory, currentTurn, winner))
+
+          case MoveResult.Stalemate =>
+            val updatedCmd = cmd.copy(moveResult = Some(de.nowchess.chess.command.MoveResult.Successful(Board.initial, GameHistory.empty, Color.White, None)))
+            invoker.execute(updatedCmd)
+            currentBoard = Board.initial
+            currentHistory = GameHistory.empty
+            currentTurn = Color.White
+            notifyObservers(StalemateEvent(currentBoard, currentHistory, currentTurn))
+
+          case _ =>
+            notifyObservers(InvalidMoveEvent(currentBoard, currentHistory, currentTurn, "Error completing promotion."))
+  }
+
+  /** Validate and load a PGN string.
+   *  Each move is replayed through the command system so undo/redo is available after loading.
+   *  Returns Right(()) on success; Left(error) if any move is illegal or the position impossible. */
+  def loadPgn(pgn: String): Either[String, Unit] = synchronized {
+    PgnParser.validatePgn(pgn) match
+      case Left(err) =>
+        Left(err)
+      case Right(game) =>
+        val initialBoardBeforeLoad = currentBoard
+        val initialHistoryBeforeLoad = currentHistory
+        val initialTurnBeforeLoad = currentTurn
+        
+        currentBoard = Board.initial
+        currentHistory = GameHistory.empty
+        currentTurn = Color.White
+        pendingPromotion = None
+        invoker.clear()
+
+        var error: Option[String] = None
+        import scala.util.control.Breaks._
+        breakable {
+          game.moves.foreach { move =>
+            handleParsedMove(move.from, move.to, s"${move.from}${move.to}")
+            move.promotionPiece.foreach(completePromotion)
+            
+            // If the move failed to execute properly, stop and report
+            // (validatePgn should have caught this, but we're being safe)
+            if pendingPromotion.isDefined && move.promotionPiece.isEmpty then
+              error = Some(s"Promotion required for move ${move.from}${move.to}")
+              break()
+          }
+        }
+        
+        error match
+          case Some(err) =>
+            currentBoard = initialBoardBeforeLoad
+            currentHistory = initialHistoryBeforeLoad
+            currentTurn = initialTurnBeforeLoad
+            Left(err)
+          case None =>
+            notifyObservers(PgnLoadedEvent(currentBoard, currentHistory, currentTurn))
+            Right(())
+  }
+
+  /** Load an arbitrary board position, clearing all history and undo/redo state. */
+  def loadPosition(board: Board, history: GameHistory, turn: Color): Unit = synchronized {
+    currentBoard = board
+    currentHistory = history
+    currentTurn = turn
+    pendingPromotion = None
+    invoker.clear()
+    notifyObservers(BoardResetEvent(currentBoard, currentHistory, currentTurn))
+  }
+
+  /** Reset the board to initial position. */
+  def reset(): Unit = synchronized {
+    currentBoard = Board.initial
+    currentHistory = GameHistory.empty
+    currentTurn = Color.White
+    invoker.clear()
+    notifyObservers(BoardResetEvent(
+      currentBoard,
+      currentHistory,
+      currentTurn
+    ))
+  }
+
+  // ──── Private Helpers ────
+
+  private def performUndo(): Unit =
+    if invoker.canUndo then
+      val cmd = invoker.history(invoker.getCurrentIndex)
+      (cmd: @unchecked) match
+        case moveCmd: MoveCommand =>
+          val notation = currentHistory.moves.lastOption.map(PgnExporter.moveToAlgebraic).getOrElse("")
+          moveCmd.previousBoard.foreach(currentBoard = _)
+          moveCmd.previousHistory.foreach(currentHistory = _)
+          moveCmd.previousTurn.foreach(currentTurn = _)
+          invoker.undo()
+          notifyObservers(MoveUndoneEvent(currentBoard, currentHistory, currentTurn, notation))
+    else
+      notifyObservers(InvalidMoveEvent(currentBoard, currentHistory, currentTurn, "Nothing to undo."))
+
+  private def performRedo(): Unit =
+    if invoker.canRedo then
+      val cmd = invoker.history(invoker.getCurrentIndex + 1)
+      (cmd: @unchecked) match
+        case moveCmd: MoveCommand =>
+          for case de.nowchess.chess.command.MoveResult.Successful(nb, nh, nt, cap) <- moveCmd.moveResult do
+            updateGameState(nb, nh, nt)
+            invoker.redo()
+            val notation    = nh.moves.lastOption.map(PgnExporter.moveToAlgebraic).getOrElse("")
+            val capturedDesc = cap.map(c => s"${c.color.label} ${c.pieceType.label}")
+            notifyObservers(MoveRedoneEvent(currentBoard, currentHistory, currentTurn, notation, moveCmd.from.toString, moveCmd.to.toString, capturedDesc))
+    else
+      notifyObservers(InvalidMoveEvent(currentBoard, currentHistory, currentTurn, "Nothing to redo."))
+
+  private def updateGameState(newBoard: Board, newHistory: GameHistory, newTurn: Color): Unit =
+    currentBoard = newBoard
+    currentHistory = newHistory
+    currentTurn = newTurn
+
+  private def emitMoveEvent(fromSq: String, toSq: String, captured: Option[Piece], newTurn: Color): Unit =
+    val capturedDesc = captured.map(c => s"${c.color.label} ${c.pieceType.label}")
+    notifyObservers(MoveExecutedEvent(
+      currentBoard,
+      currentHistory,
+      newTurn,
+      fromSq,
+      toSq,
+      capturedDesc
+    ))
+
+  private def handleFailedMove(moveInput: String): Unit =
+    (GameController.processMove(currentBoard, currentHistory, currentTurn, moveInput): @unchecked) match
+      case MoveResult.NoPiece =>
+        notifyObservers(InvalidMoveEvent(
+          currentBoard,
+          currentHistory,
+          currentTurn,
+          "No piece on that square."
+        ))
+      case MoveResult.WrongColor =>
+        notifyObservers(InvalidMoveEvent(
+          currentBoard,
+          currentHistory,
+          currentTurn,
+          "That is not your piece."
+        ))
+      case MoveResult.IllegalMove =>
+        notifyObservers(InvalidMoveEvent(
+          currentBoard,
+          currentHistory,
+          currentTurn,
+          "Illegal move."
+        ))
+

modules/core/src/main/scala/de/nowchess/chess/logic/CastleSide.scala

@@ -0,0 +1,23 @@
+package de.nowchess.chess.logic
+
+import de.nowchess.api.board.*
+
+enum CastleSide:
+  case Kingside, Queenside
+
+extension (b: Board)
+  def withCastle(color: Color, side: CastleSide): Board =
+    val rank = if color == Color.White then Rank.R1 else Rank.R8
+    val kingFrom = Square(File.E, rank)
+    val (kingTo, rookFrom, rookTo) = side match
+      case CastleSide.Kingside =>
+        (Square(File.G, rank), Square(File.H, rank), Square(File.F, rank))
+      case CastleSide.Queenside =>
+        (Square(File.C, rank), Square(File.A, rank), Square(File.D, rank))
+
+    val king = b.pieceAt(kingFrom).get
+    val rook = b.pieceAt(rookFrom).get
+
+    b.removed(kingFrom).removed(rookFrom)
+      .updated(kingTo, king)
+      .updated(rookTo, rook)

modules/core/src/main/scala/de/nowchess/chess/logic/CastlingRightsCalculator.scala

@@ -0,0 +1,31 @@
+package de.nowchess.chess.logic
+
+import de.nowchess.api.board.{Color, File, Rank, Square}
+import de.nowchess.api.game.CastlingRights
+
+/** Derives castling rights from move history. */
+object CastlingRightsCalculator:
+
+  def deriveCastlingRights(history: GameHistory, color: Color): CastlingRights =
+    val (kingRow, kingsideRookFile, queensideRookFile) = color match
+      case Color.White => (Rank.R1, File.H, File.A)
+      case Color.Black => (Rank.R8, File.H, File.A)
+
+    // Check if king has moved
+    val kingHasMoved = history.moves.exists: move =>
+      move.from == Square(File.E, kingRow) || move.castleSide.isDefined
+
+    if kingHasMoved then
+      CastlingRights.None
+    else
+      // Check if kingside rook has moved or was captured
+      val kingsideLost = history.moves.exists: move =>
+        move.from == Square(kingsideRookFile, kingRow) ||
+        move.to == Square(kingsideRookFile, kingRow)
+
+      // Check if queenside rook has moved or was captured
+      val queensideLost = history.moves.exists: move =>
+        move.from == Square(queensideRookFile, kingRow) ||
+        move.to == Square(queensideRookFile, kingRow)
+
+      CastlingRights(kingSide = !kingsideLost, queenSide = !queensideLost)

modules/core/src/main/scala/de/nowchess/chess/logic/EnPassantCalculator.scala

@@ -0,0 +1,32 @@
+package de.nowchess.chess.logic
+
+import de.nowchess.api.board.*
+
+object EnPassantCalculator:
+
+  /** Returns the en passant target square if the last move was a double pawn push.
+   *  The target is the square the pawn passed through (e.g. e2→e4 yields e3).
+   */
+  def enPassantTarget(board: Board, history: GameHistory): Option[Square] =
+    history.moves.lastOption.flatMap: move =>
+      val rankDiff = move.to.rank.ordinal - move.from.rank.ordinal
+      val isDoublePush = math.abs(rankDiff) == 2
+      val isPawn = board.pieceAt(move.to).exists(_.pieceType == PieceType.Pawn)
+      if isDoublePush && isPawn then
+        val midRankIdx = move.from.rank.ordinal + rankDiff / 2
+        Some(Square(move.to.file, Rank.values(midRankIdx)))
+      else None
+
+  /** True if moving from→to is an en passant capture. */
+  def isEnPassant(board: Board, history: GameHistory, from: Square, to: Square): Boolean =
+    board.pieceAt(from).exists(_.pieceType == PieceType.Pawn) &&
+    enPassantTarget(board, history).contains(to) &&
+    math.abs(to.file.ordinal - from.file.ordinal) == 1
+
+  /** Returns the square of the pawn to remove when an en passant capture lands on `to`.
+   *  White captures upward → captured pawn is one rank below `to`.
+   *  Black captures downward → captured pawn is one rank above `to`.
+   */
+  def capturedPawnSquare(to: Square, color: Color): Square =
+    val capturedRankIdx = to.rank.ordinal + (if color == Color.White then -1 else 1)
+    Square(to.file, Rank.values(capturedRankIdx))

modules/core/src/main/scala/de/nowchess/chess/logic/GameContext.scala

@@ -1,47 +0,0 @@
-package de.nowchess.chess.logic
-
-import de.nowchess.api.board.{Board, Color, File, Piece, PieceType, Rank, Square}
-import de.nowchess.api.game.CastlingRights
-
-enum CastleSide:
-  case Kingside, Queenside
-
-case class GameContext(
-  board: Board,
-  whiteCastling: CastlingRights,
-  blackCastling: CastlingRights
-):
-  def castlingFor(color: Color): CastlingRights =
-    if color == Color.White then whiteCastling else blackCastling
-
-  def withUpdatedRights(color: Color, rights: CastlingRights): GameContext =
-    if color == Color.White then copy(whiteCastling = rights)
-    else copy(blackCastling = rights)
-
-object GameContext:
-  /** Convenience constructor for test boards: no castling rights on either side. */
-  def apply(board: Board): GameContext =
-    GameContext(board, CastlingRights.None, CastlingRights.None)
-
-  val initial: GameContext =
-    GameContext(Board.initial, CastlingRights.Both, CastlingRights.Both)
-
-extension (b: Board)
-  def withCastle(color: Color, side: CastleSide): Board =
-    val (kingFrom, kingTo, rookFrom, rookTo) = (color, side) match
-      case (Color.White, CastleSide.Kingside)  =>
-        (Square(File.E, Rank.R1), Square(File.G, Rank.R1),
-         Square(File.H, Rank.R1), Square(File.F, Rank.R1))
-      case (Color.White, CastleSide.Queenside) =>
-        (Square(File.E, Rank.R1), Square(File.C, Rank.R1),
-         Square(File.A, Rank.R1), Square(File.D, Rank.R1))
-      case (Color.Black, CastleSide.Kingside)  =>
-        (Square(File.E, Rank.R8), Square(File.G, Rank.R8),
-         Square(File.H, Rank.R8), Square(File.F, Rank.R8))
-      case (Color.Black, CastleSide.Queenside) =>
-        (Square(File.E, Rank.R8), Square(File.C, Rank.R8),
-         Square(File.A, Rank.R8), Square(File.D, Rank.R8))
-    val king = Piece(color, PieceType.King)
-    val rook = Piece(color, PieceType.Rook)
-    Board(b.pieces.removed(kingFrom).removed(rookFrom)
-                  .updated(kingTo, king).updated(rookTo, rook))

modules/core/src/main/scala/de/nowchess/chess/logic/GameHistory.scala

@@ -0,0 +1,49 @@
+package de.nowchess.chess.logic
+
+import de.nowchess.api.board.{PieceType, Square}
+import de.nowchess.api.move.PromotionPiece
+
+/** A single move recorded in the game history. Distinct from api.move.Move which represents user intent. */
+case class HistoryMove(
+  from: Square,
+  to: Square,
+  castleSide: Option[CastleSide],
+  promotionPiece: Option[PromotionPiece] = None,
+  pieceType: PieceType = PieceType.Pawn,
+  isCapture: Boolean = false
+)
+
+/** Complete game history: ordered list of moves plus the half-move clock for the 50-move rule.
+ *
+ *  @param moves         moves played so far, oldest first
+ *  @param halfMoveClock plies since the last pawn move or capture (FIDE 50-move rule counter)
+ */
+case class GameHistory(moves: List[HistoryMove] = List.empty, halfMoveClock: Int = 0):
+
+  /** Add a raw HistoryMove record. Clock increments by 1.
+   *  Use the coordinate overload when you know whether the move is a pawn move or capture.
+   */
+  def addMove(move: HistoryMove): GameHistory =
+    GameHistory(moves :+ move, halfMoveClock + 1)
+
+  /** Add a move by coordinates.
+   *
+   *  @param wasPawnMove true when the moving piece is a pawn — resets the clock to 0
+   *  @param wasCapture  true when a piece was captured (including en passant) — resets the clock to 0
+   *
+   *  If neither flag is set the clock increments by 1.
+   */
+  def addMove(
+    from: Square,
+    to: Square,
+    castleSide: Option[CastleSide] = None,
+    promotionPiece: Option[PromotionPiece] = None,
+    wasPawnMove: Boolean = false,
+    wasCapture: Boolean = false,
+    pieceType: PieceType = PieceType.Pawn
+  ): GameHistory =
+    val newClock = if wasPawnMove || wasCapture then 0 else halfMoveClock + 1
+    GameHistory(moves :+ HistoryMove(from, to, castleSide, promotionPiece, pieceType, wasCapture), newClock)
+
+object GameHistory:
+  val empty: GameHistory = GameHistory()

modules/core/src/main/scala/de/nowchess/chess/logic/GameRules.scala

@@ -1,7 +1,7 @@
 package de.nowchess.chess.logic
 
 import de.nowchess.api.board.*
-import de.nowchess.chess.logic.GameContext
+import de.nowchess.chess.logic.GameHistory
 
 enum PositionStatus:
   case Normal, InCheck, Mated, Drawn
@@ -20,17 +20,17 @@ object GameRules:
       }
 
   /** All (from, to) moves for `color` that do not leave their own king in check. */
-  def legalMoves(ctx: GameContext, color: Color): Set[(Square, Square)] =
-    ctx.board.pieces
+  def legalMoves(board: Board, history: GameHistory, color: Color): Set[(Square, Square)] =
+    board.pieces
       .collect { case (from, piece) if piece.color == color => from }
       .flatMap { from =>
-        MoveValidator.legalTargets(ctx, from)           // context-aware: includes castling
+        MoveValidator.legalTargets(board, history, from)           // context-aware: includes castling
           .filter { to =>
             val newBoard =
-              if MoveValidator.isCastle(ctx.board, from, to) then
-                ctx.board.withCastle(color, MoveValidator.castleSide(from, to))
+              if MoveValidator.isCastle(board, from, to) then
+                board.withCastle(color, MoveValidator.castleSide(from, to))
               else
-                ctx.board.withMove(from, to)._1
+                board.withMove(from, to)._1
             !isInCheck(newBoard, color)
           }
           .map(to => from -> to)
@@ -38,9 +38,9 @@ object GameRules:
       .toSet
 
   /** Position status for the side whose turn it is (`color`). */
-  def gameStatus(ctx: GameContext, color: Color): PositionStatus =
-    val moves   = legalMoves(ctx, color)
-    val inCheck = isInCheck(ctx.board, color)
+  def gameStatus(board: Board, history: GameHistory, color: Color): PositionStatus =
+    val moves   = legalMoves(board, history, color)
+    val inCheck = isInCheck(board, color)
     if moves.isEmpty && inCheck then PositionStatus.Mated
     else if moves.isEmpty       then PositionStatus.Drawn
     else if inCheck             then PositionStatus.InCheck

modules/core/src/main/scala/de/nowchess/chess/logic/MoveValidator.scala

@@ -1,7 +1,7 @@
 package de.nowchess.chess.logic
 
 import de.nowchess.api.board.*
-import de.nowchess.chess.logic.{GameContext, CastleSide}
+import de.nowchess.chess.logic.{CastleSide, GameHistory}
 
 object MoveValidator:
 
@@ -73,7 +73,7 @@ object MoveValidator:
     val fi       = from.file.ordinal
     val ri       = from.rank.ordinal
     val dir      = if color == Color.White then 1 else -1
-    val startRank = if color == Color.White then 1 else 6  // R2 = ordinal 1, R7 = ordinal 6
+    val startRank = if color == Color.White then Rank.R2.ordinal else Rank.R7.ordinal
 
     val oneStep = squareAt(fi, ri + dir)
 
@@ -126,42 +126,58 @@ object MoveValidator:
   def castleSide(from: Square, to: Square): CastleSide =
     if to.file.ordinal > from.file.ordinal then CastleSide.Kingside else CastleSide.Queenside
 
-  def castlingTargets(ctx: GameContext, color: Color): Set[Square] =
-    val rights = ctx.castlingFor(color)
+  def castlingTargets(board: Board, history: GameHistory, color: Color): Set[Square] =
+    val rights = CastlingRightsCalculator.deriveCastlingRights(history, color)
     val rank   = if color == Color.White then Rank.R1 else Rank.R8
     val kingSq = Square(File.E, rank)
     val enemy  = color.opposite
 
-    if ctx.board.pieceAt(kingSq) != Some(Piece(color, PieceType.King)) then return Set.empty
-    if GameRules.isInCheck(ctx.board, color) then return Set.empty
+    if !board.pieceAt(kingSq).contains(Piece(color, PieceType.King)) ||
+       GameRules.isInCheck(board, color) then Set.empty
+    else
+      val kingsideSq = Option.when(
+        rights.kingSide &&
+        board.pieceAt(Square(File.H, rank)).contains(Piece(color, PieceType.Rook)) &&
+        List(Square(File.F, rank), Square(File.G, rank)).forall(s => board.pieceAt(s).isEmpty) &&
+        !List(Square(File.F, rank), Square(File.G, rank)).exists(s => isAttackedBy(board, s, enemy))
+      )(Square(File.G, rank))
 
-    var result = Set.empty[Square]
+      val queensideSq = Option.when(
+        rights.queenSide &&
+        board.pieceAt(Square(File.A, rank)).contains(Piece(color, PieceType.Rook)) &&
+        List(Square(File.B, rank), Square(File.C, rank), Square(File.D, rank)).forall(s => board.pieceAt(s).isEmpty) &&
+        !List(Square(File.D, rank), Square(File.C, rank)).exists(s => isAttackedBy(board, s, enemy))
+      )(Square(File.C, rank))
 
-    if rights.kingSide then
-      val rookSq  = Square(File.H, rank)
-      val transit = List(Square(File.F, rank), Square(File.G, rank))
-      if ctx.board.pieceAt(rookSq).contains(Piece(color, PieceType.Rook)) &&
-         transit.forall(s => ctx.board.pieceAt(s).isEmpty) &&
-         !transit.exists(s => isAttackedBy(ctx.board, s, enemy)) then
-        result += Square(File.G, rank)
+      kingsideSq.toSet ++ queensideSq.toSet
 
-    if rights.queenSide then
-      val rookSq       = Square(File.A, rank)
-      val emptySquares = List(Square(File.B, rank), Square(File.C, rank), Square(File.D, rank))
-      val transitSqs   = List(Square(File.D, rank), Square(File.C, rank))
-      if ctx.board.pieceAt(rookSq).contains(Piece(color, PieceType.Rook)) &&
-         emptySquares.forall(s => ctx.board.pieceAt(s).isEmpty) &&
-         !transitSqs.exists(s => isAttackedBy(ctx.board, s, enemy)) then
-        result += Square(File.C, rank)
-
-    result
-
-  def legalTargets(ctx: GameContext, from: Square): Set[Square] =
-    ctx.board.pieceAt(from) match
+  def legalTargets(board: Board, history: GameHistory, from: Square): Set[Square] =
+    board.pieceAt(from) match
       case Some(piece) if piece.pieceType == PieceType.King =>
-        legalTargets(ctx.board, from) ++ castlingTargets(ctx, piece.color)
+        legalTargets(board, from) ++ castlingTargets(board, history, piece.color)
+      case Some(piece) if piece.pieceType == PieceType.Pawn =>
+        pawnTargets(board, history, from, piece.color)
       case _ =>
-        legalTargets(ctx.board, from)
+        legalTargets(board, from)
 
-  def isLegal(ctx: GameContext, from: Square, to: Square): Boolean =
-    legalTargets(ctx, from).contains(to)
+  private def pawnTargets(board: Board, history: GameHistory, from: Square, color: Color): Set[Square] =
+    val existing = pawnTargets(board, from, color)
+    val fi  = from.file.ordinal
+    val ri  = from.rank.ordinal
+    val dir = if color == Color.White then 1 else -1
+    val epCapture: Set[Square] =
+      EnPassantCalculator.enPassantTarget(board, history).filter: target =>
+        squareAt(fi - 1, ri + dir).contains(target) || squareAt(fi + 1, ri + dir).contains(target)
+      .toSet
+    existing ++ epCapture
+
+  def isLegal(board: Board, history: GameHistory, from: Square, to: Square): Boolean =
+    legalTargets(board, history, from).contains(to)
+
+  /** Returns true if the piece on `from` is a pawn moving to its back rank (promotion). */
+  def isPromotionMove(board: Board, from: Square, to: Square): Boolean =
+    board.pieceAt(from) match
+      case Some(Piece(_, PieceType.Pawn)) =>
+        (from.rank == Rank.R7 && to.rank == Rank.R8) ||
+        (from.rank == Rank.R2 && to.rank == Rank.R1)
+      case _ => false

modules/core/src/main/scala/de/nowchess/chess/notation/FenExporter.scala

@@ -0,0 +1,60 @@
+package de.nowchess.chess.notation
+
+import de.nowchess.api.board.*
+import de.nowchess.api.game.{CastlingRights, GameState}
+import de.nowchess.api.board.Color
+
+object FenExporter:
+
+  /** Convert a Board to FEN piece-placement string (rank 8 to rank 1, separated by '/'). */
+  def boardToFen(board: Board): String =
+    Rank.values.reverse
+      .map(rank => buildRankString(board, rank))
+      .mkString("/")
+
+  /** Build the FEN representation for a single rank. */
+  private def buildRankString(board: Board, rank: Rank): String =
+    val rankSquares = File.values.map(file => Square(file, rank))
+    val rankChars = scala.collection.mutable.ListBuffer[Char]()
+    var emptyCount = 0
+
+    for square <- rankSquares do
+      board.pieceAt(square) match
+        case Some(piece) =>
+          if emptyCount > 0 then
+            rankChars += emptyCount.toString.charAt(0)
+            emptyCount = 0
+          rankChars += pieceToPgnChar(piece)
+        case None =>
+          emptyCount += 1
+
+    if emptyCount > 0 then rankChars += emptyCount.toString.charAt(0)
+    rankChars.mkString
+
+  /** Convert a GameState to a complete FEN string. */
+  def gameStateToFen(state: GameState): String =
+    val piecePlacement = state.piecePlacement
+    val activeColor = if state.activeColor == Color.White then "w" else "b"
+    val castling = castlingString(state.castlingWhite, state.castlingBlack)
+    val enPassant = state.enPassantTarget.map(_.toString).getOrElse("-")
+    s"$piecePlacement $activeColor $castling $enPassant ${state.halfMoveClock} ${state.fullMoveNumber}"
+
+  /** Convert castling rights to FEN notation. */
+  private def castlingString(white: CastlingRights, black: CastlingRights): String =
+    val wk = if white.kingSide then "K" else ""
+    val wq = if white.queenSide then "Q" else ""
+    val bk = if black.kingSide then "k" else ""
+    val bq = if black.queenSide then "q" else ""
+    val result = s"$wk$wq$bk$bq"
+    if result.isEmpty then "-" else result
+
+  /** Convert a Piece to its FEN character (uppercase = White, lowercase = Black). */
+  private def pieceToPgnChar(piece: Piece): Char =
+    val base = piece.pieceType match
+      case PieceType.Pawn   => 'p'
+      case PieceType.Knight => 'n'
+      case PieceType.Bishop => 'b'
+      case PieceType.Rook   => 'r'
+      case PieceType.Queen  => 'q'
+      case PieceType.King   => 'k'
+    if piece.color == Color.White then base.toUpper else base

modules/core/src/main/scala/de/nowchess/chess/notation/FenParser.scala

@@ -0,0 +1,103 @@
+package de.nowchess.chess.notation
+
+import de.nowchess.api.board.*
+import de.nowchess.api.game.{CastlingRights, GameState, GameStatus}
+
+object FenParser:
+
+  /** Parse a complete FEN string into a GameState.
+   *  Returns None if the format is invalid. */
+  def parseFen(fen: String): Option[GameState] =
+    val parts = fen.trim.split("\\s+")
+    Option.when(parts.length == 6)(parts).flatMap: parts =>
+      for
+        _ <- parseBoard(parts(0))
+        activeColor <- parseColor(parts(1))
+        castlingRights <- parseCastling(parts(2))
+        enPassant <- parseEnPassant(parts(3))
+        halfMoveClock <- parts(4).toIntOption
+        fullMoveNumber <- parts(5).toIntOption
+        if halfMoveClock >= 0 && fullMoveNumber >= 1
+      yield GameState(
+        piecePlacement = parts(0),
+        activeColor = activeColor,
+        castlingWhite = castlingRights._1,
+        castlingBlack = castlingRights._2,
+        enPassantTarget = enPassant,
+        halfMoveClock = halfMoveClock,
+        fullMoveNumber = fullMoveNumber,
+        status = GameStatus.InProgress
+      )
+
+  /** Parse active color ("w" or "b"). */
+  private def parseColor(s: String): Option[Color] =
+    if s == "w" then Some(Color.White)
+    else if s == "b" then Some(Color.Black)
+    else None
+
+  /** Parse castling rights string (e.g. "KQkq", "K", "-") into rights for White and Black. */
+  private def parseCastling(s: String): Option[(CastlingRights, CastlingRights)] =
+    if s == "-" then
+      Some((CastlingRights.None, CastlingRights.None))
+    else if s.length <= 4 && s.forall(c => "KQkq".contains(c)) then
+      val white = CastlingRights(kingSide = s.contains('K'), queenSide = s.contains('Q'))
+      val black = CastlingRights(kingSide = s.contains('k'), queenSide = s.contains('q'))
+      Some((white, black))
+    else
+      None
+
+  /** Parse en passant target square ("-" for none, or algebraic like "e3"). */
+  private def parseEnPassant(s: String): Option[Option[Square]] =
+    if s == "-" then Some(None)
+    else Square.fromAlgebraic(s).map(Some(_))
+
+  /** Parses a FEN piece-placement string (rank 8 to rank 1, separated by '/') into a Board.
+   *  Returns None if the format is invalid. */
+  def parseBoard(fen: String): Option[Board] =
+    val rankStrings = fen.split("/", -1)
+    if rankStrings.length != 8 then None
+    else
+      // Parse each rank, collecting all (Square, Piece) pairs or failing on the first error
+      val parsedRanks: Option[List[List[(Square, Piece)]]] =
+        rankStrings.zipWithIndex.foldLeft(Option(List.empty[List[(Square, Piece)]])):
+          case (None, _) => None
+          case (Some(acc), (rankStr, rankIdx)) =>
+            val rank = Rank.values(7 - rankIdx) // ranks go 8→1, so reverse
+            parsePieceRank(rankStr, rank).map(squares => acc :+ squares)
+      parsedRanks.map(ranks => Board(ranks.flatten.toMap))
+
+  /** Parse a single rank string (e.g. "rnbqkbnr" or "p3p3") into a list of (Square, Piece) pairs.
+   *  Returns None if the rank string contains invalid characters or the wrong number of files. */
+  private def parsePieceRank(rankStr: String, rank: Rank): Option[List[(Square, Piece)]] =
+    var fileIdx = 0
+    val squares = scala.collection.mutable.ListBuffer[(Square, Piece)]()
+    var failed = false
+
+    for c <- rankStr if !failed do
+      if fileIdx > 7 then
+        failed = true
+      else if c.isDigit then
+        fileIdx += c.asDigit
+      else
+        charToPiece(c) match
+          case None => failed = true
+          case Some(piece) =>
+            val file = File.values(fileIdx)
+            squares += (Square(file, rank) -> piece)
+            fileIdx += 1
+
+    if failed || fileIdx != 8 then None
+    else Some(squares.toList)
+
+  /** Convert a FEN piece character to a Piece. Uppercase = White, lowercase = Black. */
+  private def charToPiece(c: Char): Option[Piece] =
+    val color = if Character.isUpperCase(c) then Color.White else Color.Black
+    val pieceTypeOpt = c.toLower match
+      case 'p' => Some(PieceType.Pawn)
+      case 'n' => Some(PieceType.Knight)
+      case 'b' => Some(PieceType.Bishop)
+      case 'r' => Some(PieceType.Rook)
+      case 'q' => Some(PieceType.Queen)
+      case 'k' => Some(PieceType.King)
+      case _   => None
+    pieceTypeOpt.map(pt => Piece(color, pt))

modules/core/src/main/scala/de/nowchess/chess/notation/PgnExporter.scala

@@ -0,0 +1,54 @@
+package de.nowchess.chess.notation
+
+import de.nowchess.api.board.{PieceType, *}
+import de.nowchess.api.move.PromotionPiece
+import de.nowchess.chess.logic.{CastleSide, GameHistory, HistoryMove}
+
+object PgnExporter:
+
+  /** Export a game with headers and history to PGN format. */
+  def exportGame(headers: Map[String, String], history: GameHistory): String =
+    val headerLines = headers.map { case (key, value) =>
+      s"""[$key "$value"]"""
+    }.mkString("\n")
+
+    val moveText = if history.moves.isEmpty then ""
+    else
+      val groupedMoves = history.moves.zipWithIndex.groupBy(_._2 / 2)
+      val moveLines = for (moveNumber, movePairs) <- groupedMoves.toList.sortBy(_._1) yield
+        val moveNum = moveNumber + 1
+        val whiteMoveStr = movePairs.find(_._2 % 2 == 0).map(p => moveToAlgebraic(p._1)).getOrElse("")
+        val blackMoveStr = movePairs.find(_._2 % 2 == 1).map(p => moveToAlgebraic(p._1)).getOrElse("")
+        if blackMoveStr.isEmpty then s"$moveNum. $whiteMoveStr"
+        else s"$moveNum. $whiteMoveStr $blackMoveStr"
+
+      val termination = headers.getOrElse("Result", "*")
+      moveLines.mkString(" ") + s" $termination"
+
+    if headerLines.isEmpty then moveText
+    else if moveText.isEmpty then headerLines
+    else s"$headerLines\n\n$moveText"
+
+  /** Convert a HistoryMove to Standard Algebraic Notation. */
+  def moveToAlgebraic(move: HistoryMove): String =
+    move.castleSide match
+      case Some(CastleSide.Kingside)  => "O-O"
+      case Some(CastleSide.Queenside) => "O-O-O"
+      case None =>
+        val dest       = move.to.toString
+        val capStr     = if move.isCapture then "x" else ""
+        val promSuffix = move.promotionPiece match
+          case Some(PromotionPiece.Queen)  => "=Q"
+          case Some(PromotionPiece.Rook)   => "=R"
+          case Some(PromotionPiece.Bishop) => "=B"
+          case Some(PromotionPiece.Knight) => "=N"
+          case None                        => ""
+        move.pieceType match
+          case PieceType.Pawn =>
+            if move.isCapture then s"${move.from.file.toString.toLowerCase}x$dest$promSuffix"
+            else s"$dest$promSuffix"
+          case PieceType.Knight => s"N$capStr$dest$promSuffix"
+          case PieceType.Bishop => s"B$capStr$dest$promSuffix"
+          case PieceType.Rook   => s"R$capStr$dest$promSuffix"
+          case PieceType.Queen  => s"Q$capStr$dest$promSuffix"
+          case PieceType.King   => s"K$capStr$dest$promSuffix"

modules/core/src/main/scala/de/nowchess/chess/notation/PgnParser.scala

@@ -0,0 +1,267 @@
+package de.nowchess.chess.notation
+
+import de.nowchess.api.board.*
+import de.nowchess.api.move.PromotionPiece
+import de.nowchess.chess.logic.{CastleSide, GameHistory, HistoryMove, GameRules, MoveValidator, withCastle}
+
+/** A parsed PGN game containing headers and the resolved move list. */
+case class PgnGame(
+  headers: Map[String, String],
+  moves: List[HistoryMove]
+)
+
+object PgnParser:
+
+  /** Strictly validate a PGN text.
+   *  Returns Right(PgnGame) if every move token is a legal move in the evolving position.
+   *  Returns Left(error message) on the first illegal or impossible move, or any unrecognised token. */
+  def validatePgn(pgn: String): Either[String, PgnGame] =
+    val lines = pgn.split("\n").map(_.trim)
+    val (headerLines, rest) = lines.span(_.startsWith("["))
+    val headers  = parseHeaders(headerLines)
+    val moveText = rest.mkString(" ")
+    validateMovesText(moveText).map(moves => PgnGame(headers, moves))
+
+  /** Parse a complete PGN text into a PgnGame with headers and moves.
+   *  Always succeeds (returns Some); malformed tokens are silently skipped. */
+  def parsePgn(pgn: String): Option[PgnGame] =
+    val lines = pgn.split("\n").map(_.trim)
+    val (headerLines, rest) = lines.span(_.startsWith("["))
+
+    val headers  = parseHeaders(headerLines)
+    val moveText = rest.mkString(" ")
+    val moves    = parseMovesText(moveText)
+
+    Some(PgnGame(headers, moves))
+
+  /** Parse PGN header lines of the form [Key "Value"]. */
+  private def parseHeaders(lines: Array[String]): Map[String, String] =
+    val pattern = """^\[(\w+)\s+"([^"]*)"\s*\]$""".r
+    lines.flatMap(line => pattern.findFirstMatchIn(line).map(m => m.group(1) -> m.group(2))).toMap
+
+  /** Parse the move-text section (e.g. "1. e4 e5 2. Nf3") into resolved HistoryMoves. */
+  private def parseMovesText(moveText: String): List[HistoryMove] =
+    val tokens = moveText.split("\\s+").filter(_.nonEmpty)
+
+    // Fold over tokens, threading (board, history, currentColor, accumulator)
+    val (_, _, _, moves) = tokens.foldLeft(
+      (Board.initial, GameHistory.empty, Color.White, List.empty[HistoryMove])
+    ):
+      case (state @ (board, history, color, acc), token) =>
+        // Skip move-number markers (e.g. "1.", "2.") and result tokens
+        if isMoveNumberOrResult(token) then state
+        else
+          parseAlgebraicMove(token, board, history, color) match
+            case None       => state  // unrecognised token — skip silently
+            case Some(move) =>
+              val newBoard   = applyMoveToBoard(board, move, color)
+              val newHistory = history.addMove(move)
+              (newBoard, newHistory, color.opposite, acc :+ move)
+
+    moves
+
+  /** Apply a single HistoryMove to a Board, handling castling and promotion. */
+  private def applyMoveToBoard(board: Board, move: HistoryMove, color: Color): Board =
+    move.castleSide match
+      case Some(side) => board.withCastle(color, side)
+      case None =>
+        val (boardAfterMove, _) = board.withMove(move.from, move.to)
+        move.promotionPiece match
+          case Some(pp) =>
+            val pieceType = pp match
+              case PromotionPiece.Queen  => PieceType.Queen
+              case PromotionPiece.Rook   => PieceType.Rook
+              case PromotionPiece.Bishop => PieceType.Bishop
+              case PromotionPiece.Knight => PieceType.Knight
+            boardAfterMove.updated(move.to, Piece(color, pieceType))
+          case None => boardAfterMove
+
+  /** True for move-number tokens ("1.", "12.") and PGN result tokens. */
+  private def isMoveNumberOrResult(token: String): Boolean =
+    token.matches("""\d+\.""") ||
+    token == "*"               ||
+    token == "1-0"             ||
+    token == "0-1"             ||
+    token == "1/2-1/2"
+
+  /** Parse a single algebraic notation token into a HistoryMove, given the current board state. */
+  def parseAlgebraicMove(notation: String, board: Board, history: GameHistory, color: Color): Option[HistoryMove] =
+    notation match
+      case "O-O" | "O-O+" | "O-O#" =>
+        val rank = if color == Color.White then Rank.R1 else Rank.R8
+        Some(HistoryMove(Square(File.E, rank), Square(File.G, rank), Some(CastleSide.Kingside), pieceType = PieceType.King))
+
+      case "O-O-O" | "O-O-O+" | "O-O-O#" =>
+        val rank = if color == Color.White then Rank.R1 else Rank.R8
+        Some(HistoryMove(Square(File.E, rank), Square(File.C, rank), Some(CastleSide.Queenside), pieceType = PieceType.King))
+
+      case _ =>
+        parseRegularMove(notation, board, history, color)
+
+  /** Parse regular algebraic notation (pawn moves, piece moves, captures, disambiguation). */
+  private def parseRegularMove(notation: String, board: Board, history: GameHistory, color: Color): Option[HistoryMove] =
+    // Strip check/mate/capture indicators and promotion suffix (e.g. =Q)
+    val clean = notation
+      .replace("+", "")
+      .replace("#", "")
+      .replace("x", "")
+      .replaceAll("=[NBRQ]$", "")
+
+    // The destination square is always the last two characters
+    if clean.length < 2 then None
+    else
+      val destStr = clean.takeRight(2)
+      Square.fromAlgebraic(destStr).flatMap: toSquare =>
+        val disambig = clean.dropRight(2) // "" | "N"|"B"|"R"|"Q"|"K" | file | rank | file+rank
+
+        // Determine required piece type: upper-case first char = piece letter; else pawn
+        val requiredPieceType: Option[PieceType] =
+          if disambig.nonEmpty && disambig.head.isUpper then charToPieceType(disambig.head)
+          else if clean.head.isUpper then charToPieceType(clean.head)
+          else Some(PieceType.Pawn)
+
+        // Collect the disambiguation hint that remains after stripping the piece letter
+        val hint =
+          if disambig.nonEmpty && disambig.head.isUpper then disambig.tail
+          else disambig  // hint is file/rank info or empty
+
+        // Candidate source squares: pieces of `color` that can geometrically reach `toSquare`.
+        // We prefer pieces that can actually reach the target; if none can (positionally illegal
+        // PGN input), fall back to any piece of the matching type belonging to `color`.
+        val reachable: Set[Square] =
+          board.pieces.collect {
+            case (from, piece) if piece.color == color &&
+              MoveValidator.legalTargets(board, from).contains(toSquare) => from
+          }.toSet
+
+        val candidates: Set[Square] =
+          if reachable.nonEmpty then reachable
+          else
+            // Fallback for positionally-illegal but syntactically valid PGN notation:
+            // find any piece of `color` with the correct piece type on the board.
+            board.pieces.collect {
+              case (from, piece) if piece.color == color => from
+            }.toSet
+
+        // Filter by required piece type
+        val byPiece = candidates.filter(from =>
+          requiredPieceType.forall(pt => board.pieceAt(from).exists(_.pieceType == pt))
+        )
+
+        // Apply disambiguation hint (file letter or rank digit)
+        val disambiguated =
+          if hint.isEmpty then byPiece
+          else byPiece.filter(from => matchesHint(from, hint))
+
+        val promotion     = extractPromotion(notation)
+        val movePieceType = requiredPieceType.getOrElse(PieceType.Pawn)
+        val moveIsCapture = notation.contains('x')
+        disambiguated.headOption.map(from => HistoryMove(from, toSquare, None, promotion, movePieceType, moveIsCapture))
+
+  /** True if `sq` matches a disambiguation hint (file letter, rank digit, or both). */
+  private def matchesHint(sq: Square, hint: String): Boolean =
+    hint.forall(c => if c >= 'a' && c <= 'h' then sq.file.toString.equalsIgnoreCase(c.toString)
+    else if c >= '1' && c <= '8' then sq.rank.ordinal == (c - '1')
+    else true)
+
+  /** Extract a promotion piece from a notation string containing =Q/=R/=B/=N. */
+  private[notation] def extractPromotion(notation: String): Option[PromotionPiece] =
+    val promotionPattern = """=([A-Z])""".r
+    promotionPattern.findFirstMatchIn(notation).flatMap { m =>
+      m.group(1) match
+        case "Q" => Some(PromotionPiece.Queen)
+        case "R" => Some(PromotionPiece.Rook)
+        case "B" => Some(PromotionPiece.Bishop)
+        case "N" => Some(PromotionPiece.Knight)
+        case _   => None
+    }
+
+  /** Convert a piece-letter character to a PieceType. */
+  private def charToPieceType(c: Char): Option[PieceType] =
+    c match
+      case 'N' => Some(PieceType.Knight)
+      case 'B' => Some(PieceType.Bishop)
+      case 'R' => Some(PieceType.Rook)
+      case 'Q' => Some(PieceType.Queen)
+      case 'K' => Some(PieceType.King)
+      case _   => None
+
+  // ── Strict validation helpers ─────────────────────────────────────────────
+
+  /** Walk all move tokens, failing immediately on any unresolvable or illegal move. */
+  private def validateMovesText(moveText: String): Either[String, List[HistoryMove]] =
+    val tokens = moveText.split("\\s+").filter(_.nonEmpty)
+    tokens.foldLeft(Right((Board.initial, GameHistory.empty, Color.White, List.empty[HistoryMove])): Either[String, (Board, GameHistory, Color, List[HistoryMove])]) {
+      case (acc, token) =>
+        acc.flatMap { case (board, history, color, moves) =>
+          if isMoveNumberOrResult(token) then Right((board, history, color, moves))
+          else
+            strictParseAlgebraicMove(token, board, history, color) match
+              case None       => Left(s"Illegal or impossible move: '$token'")
+              case Some(move) =>
+                val newBoard   = applyMoveToBoard(board, move, color)
+                val newHistory = history.addMove(move)
+                Right((newBoard, newHistory, color.opposite, moves :+ move))
+        }
+    }.map(_._4)
+
+  /** Strict algebraic move parse — no fallback to positionally-illegal moves. */
+  private def strictParseAlgebraicMove(notation: String, board: Board, history: GameHistory, color: Color): Option[HistoryMove] =
+    val rank = if color == Color.White then Rank.R1 else Rank.R8
+    notation match
+      case "O-O" | "O-O+" | "O-O#" =>
+        val dest = Square(File.G, rank)
+        Option.when(MoveValidator.castlingTargets(board, history, color).contains(dest))(
+          HistoryMove(Square(File.E, rank), dest, Some(CastleSide.Kingside), pieceType = PieceType.King)
+        )
+      case "O-O-O" | "O-O-O+" | "O-O-O#" =>
+        val dest = Square(File.C, rank)
+        Option.when(MoveValidator.castlingTargets(board, history, color).contains(dest))(
+          HistoryMove(Square(File.E, rank), dest, Some(CastleSide.Queenside), pieceType = PieceType.King)
+        )
+      case _ =>
+        strictParseRegularMove(notation, board, history, color)
+
+  /** Strict regular move parse — uses only legally reachable squares, no fallback. */
+  private def strictParseRegularMove(notation: String, board: Board, history: GameHistory, color: Color): Option[HistoryMove] =
+    val clean = notation
+      .replace("+", "")
+      .replace("#", "")
+      .replace("x", "")
+      .replaceAll("=[NBRQ]$", "")
+
+    if clean.length < 2 then None
+    else
+      val destStr = clean.takeRight(2)
+      Square.fromAlgebraic(destStr).flatMap { toSquare =>
+        val disambig = clean.dropRight(2)
+
+        val requiredPieceType: Option[PieceType] =
+          if disambig.nonEmpty && disambig.head.isUpper then charToPieceType(disambig.head)
+          else if clean.head.isUpper then charToPieceType(clean.head)
+          else Some(PieceType.Pawn)
+
+        val hint =
+          if disambig.nonEmpty && disambig.head.isUpper then disambig.tail
+          else disambig
+
+        // Strict: only squares from which a legal move (including en passant/castling awareness) exists.
+        val reachable: Set[Square] =
+          board.pieces.collect {
+            case (from, piece) if piece.color == color &&
+              MoveValidator.legalTargets(board, history, from).contains(toSquare) => from
+          }.toSet
+
+        val byPiece = reachable.filter(from =>
+          requiredPieceType.forall(pt => board.pieceAt(from).exists(_.pieceType == pt))
+        )
+
+        val disambiguated =
+          if hint.isEmpty then byPiece
+          else byPiece.filter(from => matchesHint(from, hint))
+
+        val promotion     = extractPromotion(notation)
+        val movePieceType = requiredPieceType.getOrElse(PieceType.Pawn)
+        val moveIsCapture = notation.contains('x')
+        disambiguated.headOption.map(from => HistoryMove(from, toSquare, None, promotion, movePieceType, moveIsCapture))
+      }

modules/core/src/main/scala/de/nowchess/chess/observer/Observer.scala

@@ -0,0 +1,136 @@
+package de.nowchess.chess.observer
+
+import de.nowchess.api.board.{Board, Color, Square}
+import de.nowchess.chess.logic.GameHistory
+
+/** Base trait for all game state events.
+ *  Events are immutable snapshots of game state changes.
+ */
+sealed trait GameEvent:
+  def board: Board
+  def history: GameHistory
+  def turn: Color
+
+/** Fired when a move is successfully executed. */
+case class MoveExecutedEvent(
+  board: Board,
+  history: GameHistory,
+  turn: Color,
+  fromSquare: String,
+  toSquare: String,
+  capturedPiece: Option[String]
+) extends GameEvent
+
+/** Fired when the current player is in check. */
+case class CheckDetectedEvent(
+  board: Board,
+  history: GameHistory,
+  turn: Color
+) extends GameEvent
+
+/** Fired when the game reaches checkmate. */
+case class CheckmateEvent(
+  board: Board,
+  history: GameHistory,
+  turn: Color,
+  winner: Color
+) extends GameEvent
+
+/** Fired when the game reaches stalemate. */
+case class StalemateEvent(
+  board: Board,
+  history: GameHistory,
+  turn: Color
+) extends GameEvent
+
+/** Fired when a move is invalid. */
+case class InvalidMoveEvent(
+  board: Board,
+  history: GameHistory,
+  turn: Color,
+  reason: String
+) extends GameEvent
+
+/** Fired when a pawn reaches the back rank and the player must choose a promotion piece. */
+case class PromotionRequiredEvent(
+  board: Board,
+  history: GameHistory,
+  turn: Color,
+  from: Square,
+  to: Square
+) extends GameEvent
+
+/** Fired when the board is reset. */
+case class BoardResetEvent(
+  board: Board,
+  history: GameHistory,
+  turn: Color
+) extends GameEvent
+
+/** Fired after any move where the half-move clock reaches 100 — the 50-move rule is now claimable. */
+case class FiftyMoveRuleAvailableEvent(
+  board: Board,
+  history: GameHistory,
+  turn: Color
+) extends GameEvent
+
+/** Fired when a player successfully claims a draw under the 50-move rule. */
+case class DrawClaimedEvent(
+  board: Board,
+  history: GameHistory,
+  turn: Color
+) extends GameEvent
+
+/** Fired when a move is undone, carrying PGN notation of the reversed move. */
+case class MoveUndoneEvent(
+  board: Board,
+  history: GameHistory,
+  turn: Color,
+  pgnNotation: String
+) extends GameEvent
+
+/** Fired when a previously undone move is redone, carrying PGN notation of the replayed move. */
+case class MoveRedoneEvent(
+  board: Board,
+  history: GameHistory,
+  turn: Color,
+  pgnNotation: String,
+  fromSquare: String,
+  toSquare: String,
+  capturedPiece: Option[String]
+) extends GameEvent
+
+/** Fired after a PGN string is successfully loaded and all moves are replayed into history. */
+case class PgnLoadedEvent(
+  board: Board,
+  history: GameHistory,
+  turn: Color
+) extends GameEvent
+
+/** Observer trait: implement to receive game state updates. */
+trait Observer:
+  def onGameEvent(event: GameEvent): Unit
+
+/** Observable trait: manages observers and notifies them of events. */
+trait Observable:
+  private val observers = scala.collection.mutable.Set[Observer]()
+
+  /** Register an observer to receive game events. */
+  def subscribe(observer: Observer): Unit = synchronized {
+    observers += observer
+  }
+
+  /** Unregister an observer. */
+  def unsubscribe(observer: Observer): Unit = synchronized {
+    observers -= observer
+  }
+
+  /** Notify all observers of a game event. */
+  protected def notifyObservers(event: GameEvent): Unit = synchronized {
+    observers.foreach(_.onGameEvent(event))
+  }
+
+  /** Return current list of observers (for testing). */
+  def observerCount: Int = synchronized {
+    observers.size
+  }

modules/core/src/main/scala/de/nowchess/chess/view/Renderer.scala

@@ -11,21 +11,18 @@ object Renderer:
   private val AnsiBlackPiece  = "\u001b[30m"         // black text
 
   def render(board: Board): String =
-    val sb = new StringBuilder
-    sb.append("  a  b  c  d  e  f  g  h\n")
-    for rank <- (0 until 8).reverse do
-      sb.append(s"${rank + 1} ")
-      for file <- 0 until 8 do
-        val sq          = Square(File.values(file), Rank.values(rank))
-        val isLightSq   = (file + rank) % 2 != 0
-        val bgColor     = if isLightSq then AnsiLightSquare else AnsiDarkSquare
-        val cellContent = board.pieceAt(sq) match
+    val rows = (0 until 8).reverse.map { rank =>
+      val cells = (0 until 8).map { file =>
+        val sq        = Square(File.values(file), Rank.values(rank))
+        val isLightSq = (file + rank) % 2 != 0
+        val bgColor   = if isLightSq then AnsiLightSquare else AnsiDarkSquare
+        board.pieceAt(sq) match
           case Some(piece) =>
             val fgColor = if piece.color == Color.White then AnsiWhitePiece else AnsiBlackPiece
             s"$bgColor$fgColor ${piece.unicode} $AnsiReset"
           case None =>
             s"$bgColor   $AnsiReset"
-        sb.append(cellContent)
-      sb.append(s" ${rank + 1}\n")
-    sb.append("  a  b  c  d  e  f  g  h\n")
-    sb.toString
+      }.mkString
+      s"${rank + 1} $cells ${rank + 1}"
+    }.mkString("\n")
+    s"  a  b  c  d  e  f  g  h\n$rows\n  a  b  c  d  e  f  g  h\n"

modules/core/src/test/scala/de/nowchess/chess/command/CommandInvokerBranchTest.scala

@@ -0,0 +1,216 @@
+package de.nowchess.chess.command
+
+import de.nowchess.api.board.{Square, File, Rank, Board, Color}
+import de.nowchess.chess.logic.GameHistory
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class CommandInvokerBranchTest extends AnyFunSuite with Matchers:
+  
+  private def sq(f: File, r: Rank): Square = Square(f, r)
+
+  // ──── Helper: Command that always fails ────
+  private case class FailingCommand() extends Command:
+    override def execute(): Boolean = false
+    override def undo(): Boolean = false
+    override def description: String = "Failing command"
+
+  // ──── Helper: Command that conditionally fails on undo or execute ────
+  private case class ConditionalFailCommand(var shouldFailOnUndo: Boolean = false, var shouldFailOnExecute: Boolean = false) extends Command:
+    override def execute(): Boolean = !shouldFailOnExecute
+    override def undo(): Boolean = !shouldFailOnUndo
+    override def description: String = "Conditional fail"
+
+  private def createMoveCommand(from: Square, to: Square, executeSucceeds: Boolean = true): MoveCommand =
+    val cmd = MoveCommand(
+      from = from,
+      to = to,
+      moveResult = if executeSucceeds then Some(MoveResult.Successful(Board.initial, GameHistory.empty, Color.White, None)) else None,
+      previousBoard = Some(Board.initial),
+      previousHistory = Some(GameHistory.empty),
+      previousTurn = Some(Color.White)
+    )
+    cmd
+
+  // ──── BRANCH: execute() returns false ────
+  test("CommandInvoker.execute() with failing command returns false"):
+    val invoker = new CommandInvoker()
+    val cmd = FailingCommand()
+    invoker.execute(cmd) shouldBe false
+    invoker.history.size shouldBe 0
+    invoker.getCurrentIndex shouldBe -1
+
+  test("CommandInvoker.execute() does not add failed command to history"):
+    val invoker = new CommandInvoker()
+    val failingCmd = FailingCommand()
+    val successCmd = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    
+    invoker.execute(failingCmd) shouldBe false
+    invoker.history.size shouldBe 0
+    
+    invoker.execute(successCmd) shouldBe true
+    invoker.history.size shouldBe 1
+    invoker.history(0) shouldBe successCmd
+
+  // ──── BRANCH: undo() with invalid index (currentIndex < 0) ────
+  test("CommandInvoker.undo() returns false when currentIndex < 0"):
+    val invoker = new CommandInvoker()
+    // currentIndex starts at -1
+    invoker.undo() shouldBe false
+
+  test("CommandInvoker.undo() returns false when empty history"):
+    val invoker = new CommandInvoker()
+    invoker.canUndo shouldBe false
+    invoker.undo() shouldBe false
+
+  // ──── BRANCH: undo() with invalid index (currentIndex >= size) ────
+  test("CommandInvoker.undo() returns false when currentIndex >= history size"):
+    val invoker = new CommandInvoker()
+    val cmd1 = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    val cmd2 = createMoveCommand(sq(File.E, Rank.R7), sq(File.E, Rank.R5))
+    
+    invoker.execute(cmd1)
+    invoker.execute(cmd2)
+    // currentIndex now = 1, history.size = 2
+    
+    invoker.undo()  // currentIndex becomes 0
+    invoker.undo()  // currentIndex becomes -1
+    invoker.undo()  // currentIndex still -1, should fail
+
+  // ──── BRANCH: undo() command returns false ────
+  test("CommandInvoker.undo() returns false when command.undo() fails"):
+    val invoker = new CommandInvoker()
+    val failingCmd = ConditionalFailCommand(shouldFailOnUndo = true)
+    
+    invoker.execute(failingCmd) shouldBe true
+    invoker.canUndo shouldBe true
+    
+    invoker.undo() shouldBe false
+    // Index should not change when undo fails
+    invoker.getCurrentIndex shouldBe 0
+
+  test("CommandInvoker.undo() returns true when command.undo() succeeds"):
+    val invoker = new CommandInvoker()
+    val successCmd = ConditionalFailCommand(shouldFailOnUndo = false)
+    
+    invoker.execute(successCmd) shouldBe true
+    invoker.undo() shouldBe true
+    invoker.getCurrentIndex shouldBe -1
+
+  // ──── BRANCH: redo() with invalid index (currentIndex + 1 >= size) ────
+  test("CommandInvoker.redo() returns false when nothing to redo"):
+    val invoker = new CommandInvoker()
+    invoker.redo() shouldBe false
+
+  test("CommandInvoker.redo() returns false when at end of history"):
+    val invoker = new CommandInvoker()
+    val cmd = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    
+    invoker.execute(cmd)
+    // currentIndex = 0, history.size = 1
+    invoker.canRedo shouldBe false
+    invoker.redo() shouldBe false
+
+  test("CommandInvoker.redo() returns false when currentIndex + 1 >= size"):
+    val invoker = new CommandInvoker()
+    val cmd1 = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    val cmd2 = createMoveCommand(sq(File.E, Rank.R7), sq(File.E, Rank.R5))
+    
+    invoker.execute(cmd1)
+    invoker.execute(cmd2)
+    // currentIndex = 1, size = 2, currentIndex + 1 = 2, so 2 < 2 is false
+    invoker.canRedo shouldBe false
+    invoker.redo() shouldBe false
+
+  // ──── BRANCH: redo() command returns false ────
+  test("CommandInvoker.redo() returns false when command.execute() fails"):
+    val invoker = new CommandInvoker()
+    val cmd1 = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    val redoFailCmd = ConditionalFailCommand(shouldFailOnExecute = false)  // Succeeds on first execute
+    
+    invoker.execute(cmd1)
+    invoker.execute(redoFailCmd)  // Succeeds and added to history
+    
+    invoker.undo()
+    // currentIndex = 0, redoFailCmd is at index 1
+    invoker.canRedo shouldBe true
+    
+    // Now modify to fail on next execute (redo)
+    redoFailCmd.shouldFailOnExecute = true
+    invoker.redo() shouldBe false
+    // currentIndex should not change
+    invoker.getCurrentIndex shouldBe 0
+
+  test("CommandInvoker.redo() returns true when command.execute() succeeds"):
+    val invoker = new CommandInvoker()
+    val cmd = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    
+    invoker.execute(cmd) shouldBe true
+    invoker.undo() shouldBe true
+    invoker.redo() shouldBe true
+    invoker.getCurrentIndex shouldBe 0
+
+  // ──── BRANCH: execute() with redo history discarding (while loop) ────
+  test("CommandInvoker.execute() discards redo history via while loop"):
+    val invoker = new CommandInvoker()
+    val cmd1 = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    val cmd2 = createMoveCommand(sq(File.E, Rank.R7), sq(File.E, Rank.R5))
+    val cmd3 = createMoveCommand(sq(File.D, Rank.R2), sq(File.D, Rank.R4))
+    
+    invoker.execute(cmd1)
+    invoker.execute(cmd2)
+    // currentIndex = 1, size = 2
+    
+    invoker.undo()
+    // currentIndex = 0, size = 2
+    // Redo history exists: cmd2 is at index 1
+    invoker.canRedo shouldBe true
+    
+    invoker.execute(cmd3)
+    // while loop should discard cmd2
+    invoker.canRedo shouldBe false
+    invoker.history.size shouldBe 2
+    invoker.history(1) shouldBe cmd3
+
+  test("CommandInvoker.execute() discards multiple redo commands"):
+    val invoker = new CommandInvoker()
+    val cmd1 = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    val cmd2 = createMoveCommand(sq(File.E, Rank.R7), sq(File.E, Rank.R5))
+    val cmd3 = createMoveCommand(sq(File.G, Rank.R1), sq(File.F, Rank.R3))
+    val cmd4 = createMoveCommand(sq(File.D, Rank.R2), sq(File.D, Rank.R4))
+    
+    invoker.execute(cmd1)
+    invoker.execute(cmd2)
+    invoker.execute(cmd3)
+    invoker.execute(cmd4)
+    // currentIndex = 3, size = 4
+    
+    invoker.undo()
+    invoker.undo()
+    // currentIndex = 1, size = 4
+    // Redo history: cmd3 (idx 2), cmd4 (idx 3)
+    invoker.canRedo shouldBe true
+    
+    val newCmd = createMoveCommand(sq(File.B, Rank.R2), sq(File.B, Rank.R4))
+    invoker.execute(newCmd)
+    // While loop should discard indices 2 and 3 (cmd3 and cmd4)
+    invoker.history.size shouldBe 3
+    invoker.canRedo shouldBe false
+
+  // ──── BRANCH: execute() with no redo history to discard ────
+  test("CommandInvoker.execute() with no redo history (while condition false)"):
+    val invoker = new CommandInvoker()
+    val cmd1 = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    val cmd2 = createMoveCommand(sq(File.E, Rank.R7), sq(File.E, Rank.R5))
+    
+    invoker.execute(cmd1)
+    invoker.execute(cmd2)
+    // currentIndex = 1, size = 2
+    // currentIndex < size - 1 is 1 < 1 which is false, so while loop doesn't run
+    
+    invoker.canRedo shouldBe false
+    
+    val cmd3 = createMoveCommand(sq(File.D, Rank.R2), sq(File.D, Rank.R4))
+    invoker.execute(cmd3)  // While loop condition should be false, no iterations
+    invoker.history.size shouldBe 3
+

modules/core/src/test/scala/de/nowchess/chess/command/CommandInvokerTest.scala

@@ -0,0 +1,123 @@
+package de.nowchess.chess.command
+
+import de.nowchess.api.board.{Square, File, Rank, Board, Color}
+import de.nowchess.chess.logic.GameHistory
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class CommandInvokerTest extends AnyFunSuite with Matchers:
+
+  private def sq(f: File, r: Rank): Square = Square(f, r)
+  
+  private def createMoveCommand(from: Square, to: Square): MoveCommand =
+    MoveCommand(
+      from = from,
+      to = to,
+      moveResult = Some(MoveResult.Successful(Board.initial, GameHistory.empty, Color.White, None)),
+      previousBoard = Some(Board.initial),
+      previousHistory = Some(GameHistory.empty),
+      previousTurn = Some(Color.White)
+    )
+
+  test("CommandInvoker executes a command and adds it to history"):
+    val invoker = new CommandInvoker()
+    val cmd = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    invoker.execute(cmd) shouldBe true
+    invoker.history.size shouldBe 1
+    invoker.getCurrentIndex shouldBe 0
+
+  test("CommandInvoker executes multiple commands in sequence"):
+    val invoker = new CommandInvoker()
+    val cmd1 = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    val cmd2 = createMoveCommand(sq(File.E, Rank.R7), sq(File.E, Rank.R5))
+    invoker.execute(cmd1) shouldBe true
+    invoker.execute(cmd2) shouldBe true
+    invoker.history.size shouldBe 2
+    invoker.getCurrentIndex shouldBe 1
+
+  test("CommandInvoker.canUndo returns false when empty"):
+    val invoker = new CommandInvoker()
+    invoker.canUndo shouldBe false
+
+  test("CommandInvoker.canUndo returns true after execution"):
+    val invoker = new CommandInvoker()
+    val cmd = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    invoker.execute(cmd)
+    invoker.canUndo shouldBe true
+
+  test("CommandInvoker.undo decrements current index"):
+    val invoker = new CommandInvoker()
+    val cmd = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    invoker.execute(cmd)
+    invoker.getCurrentIndex shouldBe 0
+    invoker.undo() shouldBe true
+    invoker.getCurrentIndex shouldBe -1
+
+  test("CommandInvoker.canRedo returns true after undo"):
+    val invoker = new CommandInvoker()
+    val cmd = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    invoker.execute(cmd)
+    invoker.undo()
+    invoker.canRedo shouldBe true
+
+  test("CommandInvoker.redo re-executes a command"):
+    val invoker = new CommandInvoker()
+    val cmd = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    invoker.execute(cmd)
+    invoker.undo() shouldBe true
+    invoker.redo() shouldBe true
+    invoker.getCurrentIndex shouldBe 0
+
+  test("CommandInvoker.canUndo returns false when at beginning"):
+    val invoker = new CommandInvoker()
+    val cmd = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    invoker.execute(cmd)
+    invoker.undo()
+    invoker.canUndo shouldBe false
+
+  test("CommandInvoker clear removes all history"):
+    val invoker = new CommandInvoker()
+    val cmd = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    invoker.execute(cmd)
+    invoker.clear()
+    invoker.history.size shouldBe 0
+    invoker.getCurrentIndex shouldBe -1
+
+  test("CommandInvoker discards all history when executing after undoing all"):
+    val invoker = new CommandInvoker()
+    val cmd1 = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    val cmd2 = createMoveCommand(sq(File.E, Rank.R7), sq(File.E, Rank.R5))
+    val cmd3 = createMoveCommand(sq(File.D, Rank.R2), sq(File.D, Rank.R4))
+    invoker.execute(cmd1)
+    invoker.execute(cmd2)
+    invoker.undo()
+    invoker.undo()
+    // After undoing twice, we're at the beginning (before any commands)
+    invoker.getCurrentIndex shouldBe -1
+    invoker.canRedo shouldBe true
+    // Executing a new command from the beginning discards all redo history
+    invoker.execute(cmd3)
+    invoker.canRedo shouldBe false
+    invoker.history.size shouldBe 1
+    invoker.history(0) shouldBe cmd3
+    invoker.getCurrentIndex shouldBe 0
+
+  test("CommandInvoker discards redo history when executing mid-history"):
+    val invoker = new CommandInvoker()
+    val cmd1 = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    val cmd2 = createMoveCommand(sq(File.E, Rank.R7), sq(File.E, Rank.R5))
+    val cmd3 = createMoveCommand(sq(File.D, Rank.R2), sq(File.D, Rank.R4))
+    invoker.execute(cmd1)
+    invoker.execute(cmd2)
+    invoker.undo()
+    // After one undo, we're at the end of cmd1
+    invoker.getCurrentIndex shouldBe 0
+    invoker.canRedo shouldBe true
+    // Executing a new command discards cmd2 (the redo history)
+    invoker.execute(cmd3)
+    invoker.canRedo shouldBe false
+    invoker.history.size shouldBe 2
+    invoker.history(0) shouldBe cmd1
+    invoker.history(1) shouldBe cmd3
+    invoker.getCurrentIndex shouldBe 1
+

modules/core/src/test/scala/de/nowchess/chess/command/CommandInvokerThreadSafetyTest.scala

@@ -0,0 +1,131 @@
+package de.nowchess.chess.command
+
+import de.nowchess.api.board.{Square, File, Rank, Board, Color}
+import de.nowchess.chess.logic.GameHistory
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+import scala.collection.mutable
+
+class CommandInvokerThreadSafetyTest extends AnyFunSuite with Matchers:
+
+  private def sq(f: File, r: Rank): Square = Square(f, r)
+
+  private def createMoveCommand(from: Square, to: Square): MoveCommand =
+    MoveCommand(
+      from = from,
+      to = to,
+      moveResult = Some(MoveResult.Successful(Board.initial, GameHistory.empty, Color.White, None)),
+      previousBoard = Some(Board.initial),
+      previousHistory = Some(GameHistory.empty),
+      previousTurn = Some(Color.White)
+    )
+
+  test("CommandInvoker is thread-safe for concurrent execute and history reads"):
+    val invoker = new CommandInvoker()
+    @volatile var raceDetected = false
+    val exceptions = mutable.ListBuffer[Exception]()
+
+    // Thread 1: executes commands
+    val executorThread = new Thread(new Runnable {
+      def run(): Unit = {
+        try {
+          for i <- 1 to 1000 do
+            val cmd = createMoveCommand(
+              sq(File.E, Rank.R2),
+              sq(File.E, Rank.R4)
+            )
+            invoker.execute(cmd)
+        } catch {
+          case e: Exception =>
+            exceptions += e
+            raceDetected = true
+        }
+      }
+    })
+
+    // Thread 2: reads history during execution
+    val readerThread = new Thread(new Runnable {
+      def run(): Unit = {
+        try {
+          for _ <- 1 to 1000 do
+            val _ = invoker.history
+            val _ = invoker.getCurrentIndex
+            Thread.sleep(0) // Yield to increase contention
+        } catch {
+          case e: Exception =>
+            exceptions += e
+            raceDetected = true
+        }
+      }
+    })
+
+    executorThread.start()
+    readerThread.start()
+    executorThread.join()
+    readerThread.join()
+
+    exceptions.isEmpty shouldBe true
+    raceDetected shouldBe false
+
+  test("CommandInvoker is thread-safe for concurrent execute, undo, and redo"):
+    val invoker = new CommandInvoker()
+    @volatile var raceDetected = false
+    val exceptions = mutable.ListBuffer[Exception]()
+
+    // Pre-populate with some commands
+    for _ <- 1 to 5 do
+      invoker.execute(createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4)))
+
+    // Thread 1: executes new commands
+    val executorThread = new Thread(new Runnable {
+      def run(): Unit = {
+        try {
+          for _ <- 1 to 500 do
+            invoker.execute(createMoveCommand(sq(File.D, Rank.R2), sq(File.D, Rank.R4)))
+        } catch {
+          case e: Exception =>
+            exceptions += e
+            raceDetected = true
+        }
+      }
+    })
+
+    // Thread 2: undoes commands
+    val undoThread = new Thread(new Runnable {
+      def run(): Unit = {
+        try {
+          for _ <- 1 to 500 do
+            if invoker.canUndo then
+              invoker.undo()
+        } catch {
+          case e: Exception =>
+            exceptions += e
+            raceDetected = true
+        }
+      }
+    })
+
+    // Thread 3: redoes commands
+    val redoThread = new Thread(new Runnable {
+      def run(): Unit = {
+        try {
+          for _ <- 1 to 500 do
+            if invoker.canRedo then
+              invoker.redo()
+        } catch {
+          case e: Exception =>
+            exceptions += e
+            raceDetected = true
+        }
+      }
+    })
+
+    executorThread.start()
+    undoThread.start()
+    redoThread.start()
+    executorThread.join()
+    undoThread.join()
+    redoThread.join()
+
+    exceptions.isEmpty shouldBe true
+    raceDetected shouldBe false

modules/core/src/test/scala/de/nowchess/chess/command/CommandTest.scala

@@ -0,0 +1,52 @@
+package de.nowchess.chess.command
+
+import de.nowchess.api.board.{Board, Color}
+import de.nowchess.chess.logic.GameHistory
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class CommandTest extends AnyFunSuite with Matchers:
+
+  test("QuitCommand can be created"):
+    val cmd = QuitCommand()
+    cmd shouldNot be(null)
+
+  test("QuitCommand execute returns true"):
+    val cmd = QuitCommand()
+    cmd.execute() shouldBe true
+
+  test("QuitCommand undo returns false (cannot undo quit)"):
+    val cmd = QuitCommand()
+    cmd.undo() shouldBe false
+
+  test("QuitCommand description"):
+    val cmd = QuitCommand()
+    cmd.description shouldBe "Quit game"
+
+  test("ResetCommand with no prior state"):
+    val cmd = ResetCommand()
+    cmd.execute() shouldBe true
+    cmd.undo() shouldBe false
+
+  test("ResetCommand with prior state can undo"):
+    val cmd = ResetCommand(
+      previousBoard = Some(Board.initial),
+      previousHistory = Some(GameHistory.empty),
+      previousTurn = Some(Color.White)
+    )
+    cmd.execute() shouldBe true
+    cmd.undo() shouldBe true
+
+  test("ResetCommand with partial state cannot undo"):
+    val cmd = ResetCommand(
+      previousBoard = Some(Board.initial),
+      previousHistory = None,  // missing
+      previousTurn = Some(Color.White)
+    )
+    cmd.execute() shouldBe true
+    cmd.undo() shouldBe false
+
+  test("ResetCommand description"):
+    val cmd = ResetCommand()
+    cmd.description shouldBe "Reset board"
+