# NCS-22: Module Refactoring with Interface Abstraction

> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) to implement this plan task-by-task.

**Goal:** Split `modules/core` into clean layers (api → rule → core) with RuleSet as single source of truth for chess rules.

**Architecture:** Three-layer model with immutable GameContext bundling all game state. RuleSet interface abstracts all rule decisions. GameEngine calls RuleSet directly; GameController removed.

**Tech Stack:** Scala 3, Gradle, scoverage (100% coverage required)

---

### Task 1: Create GameContext immutable type in modules/api

**Files:**
- Create: `modules/api/src/main/scala/de/nowchess/api/game/GameContext.scala`

**Dependency:** modules/api depends only on itself (no other modules)

- [ ] **Step 1: Write GameContext case class with all game state**

```scala
package de.nowchess.api.game

import de.nowchess.api.board.{Board, Color, Square}
import de.nowchess.api.move.Move

/** Immutable bundle of complete game state.
 *  All state changes produce new GameContext instances.
 */
case class GameContext(
  board: Board,
  turn: Color,
  castlingRights: CastlingRights,
  enPassantSquare: Option[Square],
  halfMoveClock: Int,
  moves: List[Move]
):
  /** Create new context with updated board. */
  def withBoard(newBoard: Board): GameContext = copy(board = newBoard)
  
  /** Create new context with updated turn. */
  def withTurn(newTurn: Color): GameContext = copy(turn = newTurn)
  
  /** Create new context with updated castling rights. */
  def withCastlingRights(newRights: CastlingRights): GameContext = copy(castlingRights = newRights)
  
  /** Create new context with updated en passant square. */
  def withEnPassantSquare(newSq: Option[Square]): GameContext = copy(enPassantSquare = newSq)
  
  /** Create new context with updated half-move clock. */
  def withHalfMoveClock(newClock: Int): GameContext = copy(halfMoveClock = newClock)
  
  /** Create new context with move appended to history. */
  def withMove(move: Move): GameContext = copy(moves = moves :+ move)

object GameContext:
  /** Initial position: white to move, all castling rights, no en passant. */
  def initial: GameContext = GameContext(
    board = Board.initial,
    turn = Color.White,
    castlingRights = CastlingRights.initial,
    enPassantSquare = None,
    halfMoveClock = 0,
    moves = List.empty
  )
```

- [ ] **Step 2: Create CastlingRights type in modules/api**

Create `modules/api/src/main/scala/de/nowchess/api/board/CastlingRights.scala`:

```scala
package de.nowchess.api.board

case class CastlingRights(
  whiteKingSide: Boolean,
  whiteQueenSide: Boolean,
  blackKingSide: Boolean,
  blackQueenSide: Boolean
):
  def removeWhiteKingSide: CastlingRights = copy(whiteKingSide = false)
  def removeWhiteQueenSide: CastlingRights = copy(whiteQueenSide = false)
  def removeBlackKingSide: CastlingRights = copy(blackKingSide = false)
  def removeBlackQueenSide: CastlingRights = copy(blackQueenSide = false)

object CastlingRights:
  def initial: CastlingRights = CastlingRights(true, true, true, true)
```

- [ ] **Step 3: Verify GameContext compiles**

Run: `./gradlew :modules:api:compileScala`

Expected: SUCCESS

- [ ] **Step 4: Commit**

```bash
git add modules/api/src/main/scala/de/nowchess/api/game/GameContext.scala
git add modules/api/src/main/scala/de/nowchess/api/board/CastlingRights.scala
git commit -m "feat(api): add immutable GameContext type"
```

---

### Task 2: Refactor RuleSet interface in modules/rule

**Files:**
- Modify: `modules/rule/src/main/scala/de/nowchess/rules/RuleSet.scala`

- [ ] **Step 1: Replace RuleSet with GameContext-based interface**

```scala
package de.nowchess.rules

import de.nowchess.api.game.GameContext
import de.nowchess.api.board.Square
import de.nowchess.api.move.Move

/** Extension point for chess rule variants (standard, Chess960, etc.).
 *  All rule queries are stateless: given a GameContext, return the answer.
 */
trait RuleSet:
  /** All pseudo-legal moves for the piece on `square` (ignores check). */
  def candidateMoves(context: GameContext, square: Square): List[Move]

  /** Legal moves for `square`: candidates that don't leave own king in check. */
  def legalMoves(context: GameContext, square: Square): List[Move]

  /** All legal moves for the side to move. */
  def allLegalMoves(context: GameContext): List[Move]

  /** True if the side to move's king is in check. */
  def isCheck(context: GameContext): Boolean

  /** True if the side to move is in check and has no legal moves. */
  def isCheckmate(context: GameContext): Boolean

  /** True if the side to move is not in check and has no legal moves. */
  def isStalemate(context: GameContext): Boolean

  /** True if neither side has enough material to checkmate. */
  def isInsufficientMaterial(context: GameContext): Boolean

  /** True if halfMoveClock >= 100 (50-move rule). */
  def isFiftyMoveRule(context: GameContext): Boolean
```

- [ ] **Step 2: Verify RuleSet compiles**

Run: `./gradlew :modules:rule:compileScala`

Expected: SUCCESS

- [ ] **Step 3: Commit**

```bash
git add modules/rule/src/main/scala/de/nowchess/rules/RuleSet.scala
git commit -m "refactor(rule): update RuleSet to use GameContext"
```

---

### Task 3: Implement StandardRules move generation engine

**Files:**
- Modify: `modules/rule/src/main/scala/de/nowchess/rules/StandardRules.scala`

Complete rewrite of StandardRules to implement all move generation logic using GameContext and NowChess types.

- [ ] **Step 1: Rewrite StandardRules with full implementation**

See plan file for complete StandardRules code. Includes:
- Direction vectors and helpers
- Public API (all RuleSet methods)
- Move generation (pawns, knights, sliding pieces, kings, castling)
- Check/checkmate/stalemate detection
- Insufficient material detection

- [ ] **Step 2: Verify StandardRules compiles**

Run: `./gradlew :modules:rule:compileScala`

Expected: SUCCESS

- [ ] **Step 3: Commit**

```bash
git add modules/rule/src/main/scala/de/nowchess/rules/StandardRules.scala
git commit -m "refactor(rule): implement StandardRules with GameContext"
```

---

### Task 4: Configure module dependencies

**Files:**
- Create: `modules/rule/build.gradle.kts`
- Modify: `modules/core/build.gradle.kts`

- [ ] **Step 1: Create modules/rule/build.gradle.kts**

See plan file for full gradle config (standard Scala module setup with api dependency).

- [ ] **Step 2: Modify modules/core/build.gradle.kts**

Add `implementation(project(":modules:rule"))` to dependencies.

- [ ] **Step 3: Verify gradle build configuration**

Run: `./gradlew :modules:rule:compileScala :modules:core:compileScala`

Expected: SUCCESS

- [ ] **Step 4: Commit**

```bash
git add modules/rule/build.gradle.kts
git add modules/core/build.gradle.kts
git commit -m "build: configure rule module and add dependency"
```

---

### Task 5: Refactor GameEngine to use RuleSet directly

**Files:**
- Modify: `modules/core/src/main/scala/de/nowchess/chess/engine/GameEngine.scala`

Major refactoring: remove GameController calls, use RuleSet for all validation, replace GameHistory with GameContext.

- [ ] **Step 1: Update GameEngine constructor and imports**

Inject RuleSet, replace GameHistory with GameContext, update field names.

- [ ] **Step 2: Replace processUserInput and handleParsedMove**

Use `ruleSet.legalMoves()` for validation, apply moves with RuleSet checks.

- [ ] **Step 3: Update undo/redo to use GameContext**

Use MoveCommand with previousContext instead of previousBoard/previousHistory/previousTurn.

- [ ] **Step 4: Update reset and load methods**

Replace GameHistory references with GameContext.

- [ ] **Step 5: Verify GameEngine compiles**

Run: `./gradlew :modules:core:compileScala`

Expected: SUCCESS

- [ ] **Step 6: Commit**

```bash
git add modules/core/src/main/scala/de/nowchess/chess/engine/GameEngine.scala
git commit -m "refactor(core): update GameEngine to use RuleSet, remove GameController calls"
```

---

### Task 6: Update observer events to use GameContext

**Files:**
- Modify: All GameEvent files in `modules/core/src/main/scala/de/nowchess/chess/observer/`

Replace (board, history, turn) parameters with GameContext in all event types.

- [ ] **Step 1: Update all GameEvent case classes**

For each event:
- Replace: `(board: Board, history: GameHistory, turn: Color)`
- With: `(context: GameContext)`

Affected events:
- MoveExecutedEvent
- CheckDetectedEvent
- CheckmateEvent
- StalemateEvent
- MoveUndoneEvent
- MoveRedoneEvent
- FiftyMoveRuleAvailableEvent
- BoardResetEvent
- InvalidMoveEvent
- DrawClaimedEvent
- Others as needed

- [ ] **Step 2: Verify compilation**

Run: `./gradlew :modules:core:compileScala`

Expected: SUCCESS

- [ ] **Step 3: Commit**

```bash
git add modules/core/src/main/scala/de/nowchess/chess/observer/
git commit -m "refactor(observer): update GameEvent types to use GameContext"
```

---

### Task 7: Delete GameController and move logic files from core

**Files:**
- Delete: `modules/core/src/main/scala/de/nowchess/chess/controller/GameController.scala`
- Delete: Logic files from `modules/core/src/main/scala/de/nowchess/chess/logic/`

- [ ] **Step 1: Delete GameController**

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

- [ ] **Step 2: Delete logic files (moved to rule module)**

```bash
rm modules/core/src/main/scala/de/nowchess/chess/logic/MoveValidator.scala
rm modules/core/src/main/scala/de/nowchess/chess/logic/GameRules.scala
rm modules/core/src/main/scala/de/nowchess/chess/logic/CastlingRightsCalculator.scala
rm modules/core/src/main/scala/de/nowchess/chess/logic/EnPassantCalculator.scala
# Delete any other logic files that are now in StandardRules
```

- [ ] **Step 3: Commit deletion**

```bash
git add -u modules/core/src/main/scala/de/nowchess/chess/controller/
git add -u modules/core/src/main/scala/de/nowchess/chess/logic/
git commit -m "refactor(core): remove GameController and moved logic files"
```

---

### Task 8: Verify full build and green state

**Files:**
- None (validation only)

- [ ] **Step 1: Clean and build all modules**

Run: `./gradlew clean build`

Expected: SUCCESS

- [ ] **Step 2: Run core tests**

Run: `./gradlew :modules:core:test`

Expected: Tests may fail (expected; tests need refactoring per spec)

- [ ] **Step 3: Run rule tests**

Run: `./gradlew :modules:rule:test`

Expected: No tests yet (we'll write FEN/PGN-based tests separately)

- [ ] **Step 4: Commit successful build state**

```bash
git commit --allow-empty -m "build: full build succeeds post-refactoring"
```