From 4278f2d19e054ddd67d14db1decd53303a3b6a78 Mon Sep 17 00:00:00 2001
From: "claude (duplicate_chess)" <claude@sethpc.xyz>
Date: Tue, 19 May 2026 00:56:38 -0400
Subject: [PATCH] feat(engine): endgame detection with provisional rules

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
---
 src/engine/endgame.test.ts | 61 ++++++++++++++++++++++++++++++++++++++
 src/engine/endgame.ts      | 51 +++++++++++++++++++++++++++++++
 2 files changed, 112 insertions(+)
 create mode 100644 src/engine/endgame.test.ts
 create mode 100644 src/engine/endgame.ts

diff --git a/src/engine/endgame.test.ts b/src/engine/endgame.test.ts
new file mode 100644
index 0000000..af51e68
--- /dev/null
+++ b/src/engine/endgame.test.ts
@@ -0,0 +1,61 @@
+import { describe, it, expect } from 'vitest';
+import { DuplicateGame } from './game';
+import { playSymmetric } from './test-helpers';
+import { evaluateStatus } from './endgame';
+
+describe('evaluateStatus', () => {
+  it('reports an ongoing game at the start', () => {
+    const s = evaluateStatus(new DuplicateGame());
+    expect(s.state).toBe('playing');
+    expect(s.checks).toEqual([]);
+  });
+
+  it('detects a double-board checkmate (Fool\'s mate, played symmetrically)', () => {
+    const g = new DuplicateGame();
+    playSymmetric(g, [
+      [{ from: 'f2', to: 'f3' }, { from: 'e7', to: 'e5' }],
+      [{ from: 'g2', to: 'g4' }, { from: 'd8', to: 'h4' }],
+    ]);
+    expect(g.currentPlayer).toBe('N'); // North (White) is mated
+    const s = evaluateStatus(g);
+    expect(s.state).toBe('checkmate');
+    expect(s.checks.sort()).toEqual(['NE', 'NW']);
+    expect(s.result).toEqual({ N: 'loss', S: 'draw', E: 'win', W: 'win' });
+  });
+
+  it('detects threefold repetition of the whole system', () => {
+    const g = new DuplicateGame();
+    const cycle: Array<[{ from: string; to: string }, { from: string; to: string }]> = [
+      [{ from: 'g1', to: 'f3' }, { from: 'g8', to: 'f6' }],
+      [{ from: 'f3', to: 'g1' }, { from: 'f6', to: 'g8' }],
+    ];
+    playSymmetric(g, cycle); // back to start (occurrence 2)
+    playSymmetric(g, cycle); // back to start (occurrence 3)
+    const s = evaluateStatus(g);
+    expect(s.state).toBe('draw');
+    expect(s.reason).toBe('threefold');
+    expect(s.result).toEqual({ N: 'draw', S: 'draw', E: 'draw', W: 'draw' });
+  });
+
+  it('detects a stalemate as an all-draw game end (provisional rule)', () => {
+    const g = new DuplicateGame();
+    // The known fastest stalemate, played symmetrically on all four boards.
+    playSymmetric(g, [
+      [{ from: 'e2', to: 'e3' }, { from: 'a7', to: 'a5' }],
+      [{ from: 'd1', to: 'h5' }, { from: 'a8', to: 'a6' }],
+      [{ from: 'h5', to: 'a5' }, { from: 'h7', to: 'h5' }],
+      [{ from: 'a5', to: 'c7' }, { from: 'a6', to: 'h6' }],
+      [{ from: 'h2', to: 'h4' }, { from: 'f7', to: 'f6' }],
+      [{ from: 'c7', to: 'd7' }, { from: 'e8', to: 'f7' }],
+      [{ from: 'd7', to: 'b7' }, { from: 'd8', to: 'd3' }],
+      [{ from: 'b7', to: 'b8' }, { from: 'd3', to: 'h7' }],
+      [{ from: 'b8', to: 'c8' }, { from: 'f7', to: 'g6' }],
+      [{ from: 'c8', to: 'e6' }], // no black reply — Black is stalemated
+    ]);
+    expect(g.currentPlayer).toBe('E'); // a Black player, with no move
+    const s = evaluateStatus(g);
+    expect(s.state).toBe('stalemate');
+    expect(s.reason).toBe('stalemate');
+    expect(s.result).toEqual({ N: 'draw', S: 'draw', E: 'draw', W: 'draw' });
+  });
+});
diff --git a/src/engine/endgame.ts b/src/engine/endgame.ts
new file mode 100644
index 0000000..d2bc710
--- /dev/null
+++ b/src/engine/endgame.ts
@@ -0,0 +1,51 @@
+import type { DuplicateGame } from './game';
+import type { GameStatus, GameResult, BoardId } from './types';
+import { PLAYERS, PLAYER_BOARDS, BOARD_PLAYERS } from './boards';
+import { legalSyncedMoves } from './legality';
+
+/** PROVISIONAL (spec §6): the 50-move rule fires after this many rounds. */
+const FIFTY_MOVE_ROUNDS = 50;
+const FIFTY_MOVE_PLIES = FIFTY_MOVE_ROUNDS * 4;
+
+function allDraw(): GameResult {
+  return { N: 'draw', S: 'draw', E: 'draw', W: 'draw' };
+}
+
+/** Evaluate the game from the perspective of the player to move. */
+export function evaluateStatus(game: DuplicateGame): GameStatus {
+  const player = game.currentPlayer;
+  const [a, b] = PLAYER_BOARDS[player];
+
+  const checks: BoardId[] = [];
+  if (game.boards[a].inCheck()) checks.push(a);
+  if (game.boards[b].inCheck()) checks.push(b);
+
+  const synced = legalSyncedMoves(game);
+
+  if (synced.length === 0) {
+    if (checks.length > 0) {
+      // Checkmate. PROVISIONAL (spec §6): every opponent delivering a check wins.
+      const winners = checks.map((board) =>
+        BOARD_PLAYERS[board].w === player
+          ? BOARD_PLAYERS[board].b
+          : BOARD_PLAYERS[board].w,
+      );
+      const result = {} as GameResult;
+      for (const p of PLAYERS) {
+        result[p] = p === player ? 'loss' : winners.includes(p) ? 'win' : 'draw';
+      }
+      return { state: 'checkmate', result, checks };
+    }
+    // PROVISIONAL (spec §6): a no-synchronized-move stalemate ends the game, all draw.
+    return { state: 'stalemate', result: allDraw(), reason: 'stalemate', checks };
+  }
+
+  if (game.repetitionCount() >= 3) {
+    return { state: 'draw', result: allDraw(), reason: 'threefold', checks };
+  }
+  if (game.pliesSinceProgress >= FIFTY_MOVE_PLIES) {
+    return { state: 'draw', result: allDraw(), reason: 'fifty-move', checks };
+  }
+
+  return { state: 'playing', checks };
+}