# 50-Move Rule — Design Spec
**Branch:** feat/NCS-11
**Date:** 2026-03-31

---

## Overview

Implement the FIDE 50-move rule: when 100 consecutive half-moves (plies) have been played without a pawn move or capture, the player whose turn it is may claim a draw by typing `draw`. The engine notifies observers when the threshold is reached so the UI can prompt the player.

---

## Motivation

The 50-move rule prevents games from continuing indefinitely in positions where neither side can force checkmate. Under FIDE rules it is a player-claimed draw, not automatic.

---

## Section 1: Data Model — `GameHistory`

`GameHistory` gains one new field:

```scala
case class GameHistory(moves: List[HistoryMove] = List.empty, halfMoveClock: Int = 0)
```

The default value `0` means all existing construction sites compile unchanged.

### Clock update rule

The clock resets to 0 on any pawn move or capture; otherwise it increments by 1.

The main `addMove` overload gains two optional boolean flags:

```scala
def addMove(
  from: Square,
  to: Square,
  castleSide: Option[CastleSide] = None,
  promotionPiece: Option[PromotionPiece] = None,
  wasPawnMove: Boolean = false,
  wasCapture: Boolean = false
): GameHistory =
  val newClock = if wasPawnMove || wasCapture then 0 else halfMoveClock + 1
  GameHistory(moves :+ HistoryMove(from, to, castleSide, promotionPiece), newClock)
```

The base `addMove(HistoryMove)` overload is made **private**; all public call sites route through the flagged overload above.

The no-argument overload `addMove(from, to)` used in tests and en passant history recording defaults both flags to `false` (clock increments) and remains for backward compatibility.

---

## Section 2: Clock Update in `GameController`

### `applyNormalMove`

Two flags are derived from already-available data before calling `history.addMove`:

```scala
val wasPawnMove = board.pieceAt(from).exists(_.pieceType == PieceType.Pawn)
val wasCapture  = captured.isDefined   // computed earlier in the same method
val newHistory  = history.addMove(from, to, castleOpt,
                    wasPawnMove = wasPawnMove, wasCapture = wasCapture)
```

En passant moves are pawn captures, so both flags are `true` — the clock resets.

### `completePromotion`

Pawn promotion is always a pawn move, so `wasPawnMove = true`:

```scala
val newHistory = history.addMove(from, to, None, Some(piece), wasPawnMove = true)
```

---

## Section 3: Claim Mechanism and New Events

### New events (`Observer.scala`)

```scala
/** Fired after any move where the 50-move rule threshold is reached (halfMoveClock >= 100). */
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
```

### Claim handling in `GameEngine.processUserInput`

A new `"draw"` case is added before the move-parsing fallthrough:

```scala
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."
    ))
```

The game state resets to initial (same pattern as `Checkmate` and `Stalemate`). The command invoker is cleared so undo/redo history does not survive the draw claim.

### Availability notification in `GameEngine`

After any move that results in `Moved` or `MovedInCheck`, the engine checks whether the threshold has been crossed:

```scala
if newHistory.halfMoveClock >= 100 then
  notifyObservers(FiftyMoveRuleAvailableEvent(currentBoard, currentHistory, currentTurn))
```

This fires immediately after the `MoveExecutedEvent` (or `CheckDetectedEvent`) for that move.

---

## Section 4: FEN Integration

`FenExporter.gameStateToFen` and `FenParser.parseFen` already handle `halfMoveClock` at the `GameState` level — no changes to those files are needed.

The bridge between `GameHistory.halfMoveClock` and `GameState.halfMoveClock` is a caller responsibility:

**FEN export (writing):** When constructing a `GameState` for FEN export, pass `halfMoveClock = history.halfMoveClock`. Since `GameEngine` already exposes `def history: GameHistory`, this works automatically once the field is populated:

```scala
GameState(
  piecePlacement  = FenExporter.boardToFen(engine.board),
  activeColor     = engine.turn,
  ...,
  halfMoveClock   = engine.history.halfMoveClock,
  ...
)
```

**FEN import (reading):** When loading from a parsed `GameState`, initialise the engine with a `GameHistory` carrying the parsed clock:

```scala
val gs = FenParser.parseFen(fenString).get
new GameEngine(
  initialBoard   = FenParser.parseBoard(gs.piecePlacement).get,
  initialHistory = GameHistory(halfMoveClock = gs.halfMoveClock),
  initialTurn    = gs.activeColor
)
```

A round-trip test is added to `FenExporterTest` / `FenParserTest` verifying that a non-zero clock survives export → import.

---

## Section 5: PGN Integration

`PgnExporter.exportGame` currently hardcodes `" *"` as the game termination marker. PGN standard requires the marker to match the `Result` header (`1-0`, `0-1`, `1/2-1/2`, or `*`).

### Change to `PgnExporter`

Replace the hardcoded `" *"` with the value from the `Result` header:

```scala
val termination = headers.getOrElse("Result", "*")
moveLines.mkString(" ") + s" $termination"
```

### Draw claim result

When `DrawClaimedEvent` is handled by a caller that exports PGN, it should pass:

```scala
Map("Result" -> "1/2-1/2", ...)
```

The move text will then end with `1/2-1/2`, which is correct per PGN standard for a drawn game.

A test is added to `PgnExporterTest` verifying that `exportGame` with `"Result" -> "1/2-1/2"` produces a move text ending in `1/2-1/2`.

---

## Section 6: Files Changed

| File | Change |
|------|--------|
| `modules/core/src/main/scala/de/nowchess/chess/logic/GameHistory.scala` | Add `halfMoveClock` field; extend `addMove` with `wasPawnMove`/`wasCapture` flags; make base overload private |
| `modules/core/src/main/scala/de/nowchess/chess/controller/GameController.scala` | Compute and pass flags in `applyNormalMove` and `completePromotion` |
| `modules/core/src/main/scala/de/nowchess/chess/observer/Observer.scala` | Add `FiftyMoveRuleAvailableEvent` and `DrawClaimedEvent` |
| `modules/core/src/main/scala/de/nowchess/chess/engine/GameEngine.scala` | Handle `"draw"` input; fire `FiftyMoveRuleAvailableEvent` after eligible moves |
| `modules/core/src/main/scala/de/nowchess/chess/notation/PgnExporter.scala` | Derive termination marker from `Result` header instead of hardcoding `*` |
| `modules/core/src/test/scala/de/nowchess/chess/logic/GameHistoryTest.scala` | New test suite for clock update rules |
| `modules/core/src/test/scala/de/nowchess/chess/controller/GameControllerTest.scala` | Tests for clock values in `applyNormalMove` and `completePromotion` |
| `modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineTest.scala` | Tests for `"draw"` command and `FiftyMoveRuleAvailableEvent` |
| `modules/core/src/test/scala/de/nowchess/chess/notation/PgnExporterTest.scala` | Test for `1/2-1/2` termination marker |
| `modules/core/src/test/scala/de/nowchess/chess/notation/FenExporterTest.scala` | Round-trip test: non-zero `halfMoveClock` survives FEN export → import |

`EnPassantCalculator`, `CastlingRightsCalculator`, `MoveValidator`, `GameRules`, and their tests are **not** touched.

---

## Section 7: Testing

### `GameHistoryTest`
- Clock starts at 0
- Clock increments on a normal (non-pawn, non-capture) move
- Clock resets to 0 on a pawn move (`wasPawnMove = true`)
- Clock resets to 0 on a capture (`wasCapture = true`)
- Clock resets to 0 when both flags are true (en passant)
- Clock carries correctly across multiple sequential moves

### `GameControllerTest`
- `applyNormalMove` with a non-pawn, non-capture produces `history.halfMoveClock = 1`
- `applyNormalMove` with a pawn move produces `history.halfMoveClock = 0`
- `applyNormalMove` with a capture produces `history.halfMoveClock = 0`
- `completePromotion` always produces `history.halfMoveClock = 0`

### `GameEngineTest`
- `processUserInput("draw")` fires `DrawClaimedEvent` and resets state when `halfMoveClock >= 100`
- `processUserInput("draw")` fires `InvalidMoveEvent` when `halfMoveClock < 100`
- A successful non-pawn, non-capture move that brings the clock to exactly 100 fires `FiftyMoveRuleAvailableEvent`
- A successful move that does not reach 100 does not fire `FiftyMoveRuleAvailableEvent`

### `PgnExporterTest`
- `exportGame` with `"Result" -> "1/2-1/2"` produces move text ending in `1/2-1/2`
- `exportGame` with no `Result` header still produces `*` as before (backward-compatible)

### `FenExporterTest`
- Round-trip: a `GameHistory` with `halfMoveClock = 42` exported to FEN and re-parsed yields `halfMoveClock = 42`