feat(engine): endgame detection with provisional rules

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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 };
+}