From 7473cc69b36ceb419c376e96c677905aa7cd8cea Mon Sep 17 00:00:00 2001
From: "claude (duplicate_chess)" <claude@sethpc.xyz>
Date: Tue, 19 May 2026 00:50:51 -0400
Subject: [PATCH] feat(engine): synchronized-move intersection and selection
 highlight

---
 src/engine/legality.test.ts | 51 +++++++++++++++++++++++++++++++
 src/engine/legality.ts      | 61 +++++++++++++++++++++++++++++++++++++
 2 files changed, 112 insertions(+)
 create mode 100644 src/engine/legality.test.ts
 create mode 100644 src/engine/legality.ts

diff --git a/src/engine/legality.test.ts b/src/engine/legality.test.ts
new file mode 100644
index 0000000..818bd1d
--- /dev/null
+++ b/src/engine/legality.test.ts
@@ -0,0 +1,51 @@
+import { describe, it, expect } from 'vitest';
+import { DuplicateGame } from './game';
+import { legalSyncedMoves, selectionHighlight } from './legality';
+
+describe('legalSyncedMoves', () => {
+  it('returns all 20 white opening moves when a player\'s two boards are identical', () => {
+    const g = new DuplicateGame();
+    expect(legalSyncedMoves(g)).toHaveLength(20);
+  });
+
+  it('excludes a move legal on only one of the player\'s boards', () => {
+    // N e2e4, S e2e4, W e7e5, E d7d5  -> NW black has pe5, NE black has pd5.
+    const g = new DuplicateGame([
+      { player: 'N', from: 'e2', to: 'e4' },
+      { player: 'S', from: 'e2', to: 'e4' },
+      { player: 'E', from: 'd7', to: 'd5' },
+      { player: 'W', from: 'e7', to: 'e5' },
+    ]);
+    expect(g.currentPlayer).toBe('N');
+    const keys = legalSyncedMoves(g).map((m) => `${m.from}${m.to}`);
+    // e4-d5 is a capture on NE but illegal on NW (d5 empty there): not synced.
+    expect(keys).not.toContain('e4d5');
+    // e4-e5 is legal on NE but blocked on NW (black pawn on e5): not synced.
+    expect(keys).not.toContain('e4e5');
+  });
+});
+
+describe('selectionHighlight', () => {
+  it('marks every destination playable when the two boards agree', () => {
+    const g = new DuplicateGame();
+    const h = selectionHighlight(g, 'e2');
+    expect(h.playable.sort()).toEqual(['e3', 'e4']);
+    expect(h.onlyA).toEqual([]);
+    expect(h.onlyB).toEqual([]);
+  });
+
+  it('splits destinations into playable vs board-local-only on divergence', () => {
+    const g = new DuplicateGame([
+      { player: 'N', from: 'e2', to: 'e4' },
+      { player: 'S', from: 'e2', to: 'e4' },
+      { player: 'E', from: 'd7', to: 'd5' },
+      { player: 'W', from: 'e7', to: 'e5' },
+    ]);
+    const h = selectionHighlight(g, 'e4'); // North's e4 pawn
+    expect(h.boardA).toBe('NW');
+    expect(h.boardB).toBe('NE');
+    expect(h.playable).toEqual([]);          // nothing legal on both
+    expect(h.onlyA).toEqual([]);             // e4 is blocked on NW
+    expect(h.onlyB.sort()).toEqual(['d5', 'e5']); // capture + advance on NE only
+  });
+});
diff --git a/src/engine/legality.ts b/src/engine/legality.ts
new file mode 100644
index 0000000..0784447
--- /dev/null
+++ b/src/engine/legality.ts
@@ -0,0 +1,61 @@
+import type { DuplicateGame } from './game';
+import type { SyncMove, Square, BoardId, PromotionPiece } from './types';
+import { PLAYER_BOARDS } from './boards';
+
+function key(m: { from: string; to: string; promotion?: string }): string {
+  return `${m.from}${m.to}${m.promotion ?? ''}`;
+}
+
+/** Every synchronized-legal move for the player to move (the intersection). */
+export function legalSyncedMoves(game: DuplicateGame): SyncMove[] {
+  const [a, b] = PLAYER_BOARDS[game.currentPlayer];
+  const movesA = game.boards[a].moves({ verbose: true });
+  const keysB = new Set(game.boards[b].moves({ verbose: true }).map(key));
+  const seen = new Set<string>();
+  const result: SyncMove[] = [];
+  for (const m of movesA) {
+    const k = key(m);
+    if (keysB.has(k) && !seen.has(k)) {
+      seen.add(k);
+      result.push({
+        from: m.from,
+        to: m.to,
+        promotion: (m.promotion as PromotionPiece) || undefined,
+      });
+    }
+  }
+  return result;
+}
+
+export interface SelectionHighlight {
+  /** The current player's first board. */
+  boardA: BoardId;
+  /** The current player's second board. */
+  boardB: BoardId;
+  /** Destinations legal on BOTH boards (actually playable). */
+  playable: Square[];
+  /** Destinations legal on board A only. */
+  onlyA: Square[];
+  /** Destinations legal on board B only. */
+  onlyB: Square[];
+}
+
+/** Triple-highlight data for the current player's piece grabbed at `from`. */
+export function selectionHighlight(
+  game: DuplicateGame,
+  from: Square,
+): SelectionHighlight {
+  const [a, b] = PLAYER_BOARDS[game.currentPlayer];
+  const destA = new Set(
+    game.boards[a].moves({ verbose: true }).filter((m) => m.from === from).map((m) => m.to),
+  );
+  const destB = new Set(
+    game.boards[b].moves({ verbose: true }).filter((m) => m.from === from).map((m) => m.to),
+  );
+  const playable: Square[] = [];
+  const onlyA: Square[] = [];
+  const onlyB: Square[] = [];
+  for (const sq of destA) (destB.has(sq) ? playable : onlyA).push(sq);
+  for (const sq of destB) if (!destA.has(sq)) onlyB.push(sq);
+  return { boardA: a, boardB: b, playable, onlyA, onlyB };
+}