feat(engine): coordinate notation and JSON save/load

src/engine/notation.test.ts

@@ -0,0 +1,23 @@
+import { describe, it, expect } from 'vitest';
+import { toCoordinate, serialize, deserialize } from './notation';
+import type { HistoryEntry } from './types';
+
+describe('notation', () => {
+  it('renders a move as a coordinate token', () => {
+    expect(toCoordinate({ from: 'e2', to: 'e4' })).toBe('e2e4');
+    expect(toCoordinate({ from: 'e7', to: 'e8', promotion: 'q' })).toBe('e7e8q');
+  });
+
+  it('round-trips a game through serialize/deserialize', () => {
+    const history: HistoryEntry[] = [
+      { player: 'N', from: 'e2', to: 'e4' },
+      { player: 'S', from: 'e2', to: 'e4' },
+    ];
+    const restored = deserialize(serialize(history));
+    expect(restored).toEqual(history);
+  });
+
+  it('rejects a file that is not a duplicate-chess save', () => {
+    expect(() => deserialize('{"variant":"chess","version":1,"moves":[]}')).toThrow();
+  });
+});

src/engine/notation.ts

@@ -0,0 +1,25 @@
+import type { SyncMove, HistoryEntry } from './types';
+
+/** Coordinate notation, e.g. "e2e4" or "e7e8q". */
+export function toCoordinate(move: SyncMove): string {
+  return `${move.from}${move.to}${move.promotion ?? ''}`;
+}
+
+export interface SavedGame {
+  variant: 'duplicate-chess';
+  version: 1;
+  moves: HistoryEntry[];
+}
+
+export function serialize(history: HistoryEntry[]): string {
+  const data: SavedGame = { variant: 'duplicate-chess', version: 1, moves: history };
+  return JSON.stringify(data, null, 2);
+}
+
+export function deserialize(json: string): HistoryEntry[] {
+  const data = JSON.parse(json) as Partial<SavedGame>;
+  if (data.variant !== 'duplicate-chess' || !Array.isArray(data.moves)) {
+    throw new Error('Not a duplicate-chess save file');
+  }
+  return data.moves;
+}