feat(engine): shouldAdvanceTurn hook + HalfMovesThisTurn fact

Phase A.3 of the rule-variants epic — preset-controlled turn flip
gating. Enables double-move (white plays 2, then black plays 2) and
monster (scope-aware: white plays 2, black plays 1) without engine
changes at the preset layer.

- Adds HalfMovesThisTurn: number to ChessAttrMap. Seeded to 0 in
  applyLayout. Distinct from HalfmoveClock (FIDE 50-move rule) —
  this counter is within-turn and resets on flip.
- Adds TurnAdvanceContext + shouldAdvanceTurn hook to PresetDef.
  Hook receives post-increment count via ctx.halfMovesThisTurn so
  'play N half-moves before flipping' reads as the predicate
  'halfMovesThisTurn < N -> false'.
- engine.applyMove: increments HalfMovesThisTurn BEFORE polling
  shouldAdvanceTurn; first false wins; on flip resets to 0 and
  increments FullmoveNumber after black; onTurnStart is gated on
  shouldAdvance (vetoed flips don't fire turn-start hooks, so
  mid-turn stamina regen bugs are prevented by construction).
- starting-position.test.ts updated with HalfMovesThisTurn=0 seed
  assertion.
- New turn-advance.test.ts (7 tests): seeding, baseline unchanged,
  never-flip veto, flip-after-two (double-move surface), onTurnStart
  gating across vetoed + non-vetoed flips, FullmoveNumber increments
  only on actual flips after black.

Tests: 1441 passing (was 1434, +7 new). Blocks: unblocks B.2
(double-move), B.3 (monster-rules), B.4 (first-promotion-wins).

.sisyphus/notepads/rule-variants/learnings.md

@@ -153,3 +153,44 @@ section).
   would be a visible diff. Keeping enforcement out for now
   because the 1 call site per legal-move request is hot-path-
   adjacent and a `Set`-based prune would show up.
+
+## [2026-04-20 20:13] Task: A.3 — shouldAdvanceTurn hook + HalfMovesThisTurn fact
+
+**Shipped**:
+- `HalfMovesThisTurn: number` added to `ChessAttrMap`
+  (`packages/chess/src/schema.ts`). Seeded to 0 alongside `Turn` in
+  `applyLayout` (`packages/chess/src/starting-position.ts`).
+- `TurnAdvanceContext` type + `shouldAdvanceTurn` hook on
+  `PresetDef`. Hook receives post-increment count. First `false`
+  wins; iteration stops.
+- `engine.applyMove`:
+  1. Increments `HalfMovesThisTurn` BEFORE polling (so hooks see
+     count including the move just committed).
+  2. Polls every active preset's `shouldAdvanceTurn`; first `false`
+     cancels the flip.
+  3. On flip: updates `Turn`, resets `HalfMovesThisTurn` to 0,
+     increments `FullmoveNumber` after black.
+  4. On skip: leaves `Turn` and `HalfMovesThisTurn` alone (count
+     accumulates).
+  5. `onTurnStart` is GATED on `shouldAdvance === true`. Vetoed
+     flips don't fire turn-start (so double-move stamina doesn't
+     regen mid-turn).
+- `starting-position.test.ts` updated to assert the new fact seed.
+- New `turn-advance.test.ts` — 7 tests: seeding, baseline unchanged,
+  never-flip veto accumulates count, flip-after-two double-move
+  surface, `onTurnStart` gating against vetoes, FullmoveNumber only
+  on real flips.
+
+**Verification**: `bun run check` green. 1441 tests (was 1434, +7).
+
+**Gotchas**:
+- Two DISTINCT half-move counters now exist. `HalfmoveClock`
+  (FIDE 50-move rule) vs `HalfMovesThisTurn` (this epic's
+  within-turn counter). Documented in schema comment; hook docs
+  emphasize the distinction.
+- `tickAfterMove(color)` runs on the mover's color regardless of
+  flip decision — correct: a 3-turn buff ticks per actual move
+  made, not per Turn flip. Matches existing semantics.
+- `session.allFacts()` automatically carries the new fact across
+  serialization boundaries (snapshot/restore) without protocol
+  changes — no server-side work needed for the A.3 addition.

packages/chess/src/engine.ts

@@ -58,6 +58,7 @@ import type {
   PieceSpawnReason,
   RoyalContext,
   SelfCheckFilterContext,
+  TurnAdvanceContext,
   TurnStartContext,
 } from "./presets/registry.js";
 import type { ChessAttrKey } from "./schema.js";
@@ -1176,18 +1177,63 @@ export class ChessEngine {
       clearEnPassantTarget(this.session);
     }
 
-    // Update halfmove clock
+    // Update halfmove clock (50-move rule). This is the FIDE clock —
+    // distinct from the rule-variants `HalfMovesThisTurn` counter
+    // below, which tracks within-turn move count for presets like
+    // double-move.
     updateHalfmoveClock(this.session, move, movingType === "pawn");
 
-    // Switch turn
-    const nextColor: PieceColor = color === "white" ? "black" : "white";
-    this.session.insert(GAME_ENTITY, "Turn", nextColor);
+    // Phase A.3: increment HalfMovesThisTurn BEFORE polling
+    // `shouldAdvanceTurn`. The hook sees the POST-increment count so
+    // a "play N half-moves before flipping" predicate reads as
+    // `ctx.halfMovesThisTurn < N → false`.
+    const prevHalfMovesThisTurn =
+      (this.session.get(GAME_ENTITY, "HalfMovesThisTurn") as number) ?? 0;
+    const nextHalfMovesThisTurn = prevHalfMovesThisTurn + 1;
+    this.session.insert(
+      GAME_ENTITY,
+      "HalfMovesThisTurn",
+      nextHalfMovesThisTurn,
+    );
 
-    // Increment fullmove number after black's move
-    if (color === "black") {
-      const fn =
-        ((this.session.get(GAME_ENTITY, "FullmoveNumber") as number) ?? 1) + 1;
-      this.session.insert(GAME_ENTITY, "FullmoveNumber", fn);
+    // Poll every active preset's shouldAdvanceTurn hook. FIRST FALSE
+    // WINS — as soon as one veto lands, the engine stops polling and
+    // skips the turn flip. Scope-unaware: a `scope=white` preset that
+    // vetoes on black's moves is a bug the preset must guard against
+    // (check `ctx.mover`).
+    const turnAdvanceCtx: TurnAdvanceContext = {
+      engine: this,
+      mover: color,
+      halfMovesThisTurn: nextHalfMovesThisTurn,
+    };
+    let shouldAdvance = true;
+    for (const entry of this.activePresets.list()) {
+      const def = PRESET_REGISTRY.get(entry.id);
+      const verdict = def?.shouldAdvanceTurn?.(turnAdvanceCtx);
+      if (verdict === false) {
+        shouldAdvance = false;
+        break;
+      }
+    }
+
+    // Switch turn iff no preset vetoed. When skipped, `Turn` and
+    // `HalfMovesThisTurn` are left as-is (HalfMovesThisTurn was
+    // already incremented above; the next move's poll sees it grow).
+    const nextColor: PieceColor = shouldAdvance
+      ? (color === "white" ? "black" : "white")
+      : color;
+
+    if (shouldAdvance) {
+      this.session.insert(GAME_ENTITY, "Turn", nextColor);
+      // Reset within-turn counter on every actual flip.
+      this.session.insert(GAME_ENTITY, "HalfMovesThisTurn", 0);
+
+      // Increment fullmove number after black's completed turn.
+      if (color === "black") {
+        const fn =
+          ((this.session.get(GAME_ENTITY, "FullmoveNumber") as number) ?? 1) + 1;
+        this.session.insert(GAME_ENTITY, "FullmoveNumber", fn);
+      }
     }
 
     // Record position for threefold repetition
@@ -1223,9 +1269,15 @@ export class ChessEngine {
     // cooldown ticks — runs BEFORE the player asks for their legal
     // moves, so regenerated stamina / refreshed cooldowns are
     // reflected in the first legal-move query.
-    const turnStartCtx: TurnStartContext = { engine: this, turn: nextColor };
-    for (const preset of this.activePresets.getForColor(nextColor)) {
-      preset.onTurnStart?.(turnStartCtx);
+    //
+    // Skipped when `shouldAdvanceTurn` vetoed the flip — no new turn
+    // began, so "start of turn" hooks must NOT fire (otherwise
+    // stamina regens mid-turn for double-move, etc.).
+    if (shouldAdvance) {
+      const turnStartCtx: TurnStartContext = { engine: this, turn: nextColor };
+      for (const preset of this.activePresets.getForColor(nextColor)) {
+        preset.onTurnStart?.(turnStartCtx);
+      }
     }
 
     // Determine terminal state BEFORE we log, so the MoveRecord

packages/chess/src/presets/registry.ts

@@ -251,6 +251,27 @@ export interface FilterLegalMovesContext extends HookContext {
   readonly moves: readonly LegalMove[];
 }
 
+/**
+ * Context passed to `shouldAdvanceTurn`. Fires from `engine.applyMove`
+ * AFTER the move is committed AND AFTER `HalfMovesThisTurn` has been
+ * incremented, but BEFORE the engine decides whether to flip `Turn`.
+ *
+ * - `mover` is the color that just moved.
+ * - `halfMovesThisTurn` is the post-increment count (i.e. how many
+ *   consecutive half-moves this side has played in the CURRENT turn,
+ *   INCLUDING the one that just committed). First-move queries see
+ *   `1`, second-move queries see `2`.
+ *
+ * Phase A.3 (rule-variants epic): lets presets like Double-Move
+ * (white plays 2, black plays 1) and Monster (white plays 2, black
+ * plays 1 — scope-aware white-only variant) gate the turn flip
+ * without touching the engine.
+ */
+export interface TurnAdvanceContext extends HookContext {
+  readonly mover: "white" | "black";
+  readonly halfMovesThisTurn: number;
+}
+
 /**
  * Why a piece is being spawned. Used by `onPieceSpawn` hooks to
  * decide whether to participate (e.g., a preset that resurrects
@@ -546,6 +567,36 @@ export interface PresetDef {
     ctx: FilterLegalMovesContext,
   ) => readonly LegalMove[];
 
+  /**
+   * Decide whether the current turn should flip after this move.
+   *
+   * Called from `engine.applyMove` AFTER all mutations (move applied,
+   * captures resolved, damage dispatched, en-passant/castling side
+   * effects landed) and AFTER `HalfMovesThisTurn` has been
+   * incremented. The engine iterates every active preset's hook; if
+   * ANY returns `false`, the turn flip is SKIPPED (the mover plays
+   * again). `undefined` / `true` = no opinion, let subsequent
+   * presets / the default flip proceed.
+   *
+   * "First false wins" semantics:
+   *   - As soon as one preset vetoes the flip, the engine stops
+   *     polling and skips the flip.
+   *   - `HalfMovesThisTurn` is NOT reset when the flip is skipped,
+   *     so the next move's poll sees an incremented count.
+   *   - `HalfMovesThisTurn` IS reset to 0 when the flip proceeds.
+   *
+   * The engine reads `ctx.halfMovesThisTurn` AFTER the increment, so
+   * a "play N half-moves before flipping" preset uses the predicate
+   * `ctx.halfMovesThisTurn < N → return false`.
+   *
+   * Canonical users: `double-move` (flip after 2 for both colors),
+   * `monster-rules` (flip after 2 for white, 1 for black — scope
+   * aware via `mover` inspection).
+   */
+  readonly shouldAdvanceTurn?: (
+    ctx: TurnAdvanceContext,
+  ) => boolean | undefined;
+
   /**
    * Phase hook: fires AFTER the move has been confirmed legal but
    * BEFORE any state mutation. Return `{ cancel: true, reason }` to

packages/chess/src/presets/turn-advance.test.ts

@@ -0,0 +1,221 @@
+/**
+ * Tests for the `shouldAdvanceTurn` hook + `HalfMovesThisTurn` game
+ * fact (Phase A.3 of the rule-variants epic).
+ *
+ * Semantics under test:
+ *   - `HalfMovesThisTurn` is seeded to 0 by `applyLayout`.
+ *   - `engine.applyMove` increments `HalfMovesThisTurn` BEFORE polling
+ *     `shouldAdvanceTurn`. Hooks see the post-increment count.
+ *   - FIRST FALSE from any active preset vetoes the flip. When vetoed
+ *     the mover plays again; `Turn` stays; `HalfMovesThisTurn` is
+ *     NOT reset.
+ *   - When the flip proceeds, `HalfMovesThisTurn` resets to 0 and
+ *     `FullmoveNumber` increments after black.
+ *   - `onTurnStart` only fires when the flip actually happened — no
+ *     mid-turn regen for double-move stamina etc.
+ *
+ * Coverage:
+ *   - Baseline (no preset with the hook): every move flips, count
+ *     always 0 after applyMove. Protects the 1434-test regression.
+ *   - "Never flip" prototype: white keeps moving; `Turn` stays white;
+ *     count accumulates.
+ *   - "Flip after 2" prototype (the double-move surface): white plays
+ *     2, then black plays 2, then white plays 2, …
+ *   - Skip-flip does NOT fire `onTurnStart`.
+ *   - `FullmoveNumber` only increments on actual flips after black's
+ *     turn.
+ */
+import { describe, it, expect, beforeEach } from "vitest";
+import "./index.js";
+import { ChessEngine } from "../engine.js";
+import { PRESET_REGISTRY } from "./registry.js";
+import { GAME_ENTITY } from "../schema.js";
+
+const NEVER_FLIP_ID = "test-never-flip";
+const FLIP_AFTER_TWO_ID = "test-flip-after-two";
+const TURN_START_COUNTER_ID = "test-turn-start-counter";
+
+// Use module-level counters captured by closure so the hook can
+// observe how many times onTurnStart was invoked.
+let turnStartInvocations = 0;
+
+beforeEach(() => {
+  turnStartInvocations = 0;
+
+  PRESET_REGISTRY.register({
+    id: NEVER_FLIP_ID,
+    name: "Never flip (test)",
+    description: "Always veto the turn flip — mover plays forever.",
+    incompatibleWith: [],
+    requires: [],
+    shouldAdvanceTurn() {
+      return false;
+    },
+  });
+
+  PRESET_REGISTRY.register({
+    id: FLIP_AFTER_TWO_ID,
+    name: "Flip after two (test)",
+    description: "Flip only after both colors have played 2 half-moves.",
+    incompatibleWith: [],
+    requires: [],
+    shouldAdvanceTurn({ halfMovesThisTurn }) {
+      // halfMovesThisTurn is POST-increment: seeing 1 means "mover
+      // just played their first move in this turn"; seeing 2 means
+      // "mover just played their second — OK to flip now".
+      if (halfMovesThisTurn < 2) return false;
+      return undefined; // no opinion — let defaults flip
+    },
+  });
+
+  PRESET_REGISTRY.register({
+    id: TURN_START_COUNTER_ID,
+    name: "Turn start counter (test)",
+    description: "Counts how many times onTurnStart fires.",
+    incompatibleWith: [],
+    requires: [],
+    onTurnStart() {
+      turnStartInvocations++;
+    },
+  });
+});
+
+function firstLegalMove(engine: ChessEngine) {
+  const moves = engine.getAllLegalMoves();
+  if (moves.length === 0) throw new Error("no legal moves available");
+  return moves[0]!;
+}
+
+describe("HalfMovesThisTurn — seeding", () => {
+  it("seeded to 0 by applyLayout", () => {
+    const engine = new ChessEngine();
+    expect(engine.session.get(GAME_ENTITY, "HalfMovesThisTurn")).toBe(0);
+  });
+});
+
+describe("shouldAdvanceTurn — baseline (no hook)", () => {
+  it("every move flips and resets HalfMovesThisTurn to 0", () => {
+    const engine = new ChessEngine();
+
+    expect(engine.getCurrentTurn()).toBe("white");
+    engine.applyMove(firstLegalMove(engine));
+    expect(engine.getCurrentTurn()).toBe("black");
+    expect(engine.session.get(GAME_ENTITY, "HalfMovesThisTurn")).toBe(0);
+
+    engine.applyMove(firstLegalMove(engine));
+    expect(engine.getCurrentTurn()).toBe("white");
+    expect(engine.session.get(GAME_ENTITY, "HalfMovesThisTurn")).toBe(0);
+
+    // FullmoveNumber went from 1 → 2 after black's move.
+    expect(engine.session.get(GAME_ENTITY, "FullmoveNumber")).toBe(2);
+  });
+});
+
+describe("shouldAdvanceTurn — never-flip veto", () => {
+  it("white plays forever; Turn never flips; count accumulates", () => {
+    const engine = new ChessEngine();
+    engine.setActivePresets([
+      { id: NEVER_FLIP_ID, scope: "both", turnsRemaining: null },
+    ]);
+
+    expect(engine.getCurrentTurn()).toBe("white");
+    engine.applyMove(firstLegalMove(engine));
+    expect(engine.getCurrentTurn()).toBe("white");
+    expect(engine.session.get(GAME_ENTITY, "HalfMovesThisTurn")).toBe(1);
+
+    engine.applyMove(firstLegalMove(engine));
+    expect(engine.getCurrentTurn()).toBe("white");
+    expect(engine.session.get(GAME_ENTITY, "HalfMovesThisTurn")).toBe(2);
+
+    engine.applyMove(firstLegalMove(engine));
+    expect(engine.getCurrentTurn()).toBe("white");
+    expect(engine.session.get(GAME_ENTITY, "HalfMovesThisTurn")).toBe(3);
+
+    // FullmoveNumber never moves — it only increments on actual
+    // flips after black.
+    expect(engine.session.get(GAME_ENTITY, "FullmoveNumber")).toBe(1);
+  });
+});
+
+describe("shouldAdvanceTurn — flip-after-two prototype (double-move surface)", () => {
+  it("each side plays 2 half-moves before the flip", () => {
+    const engine = new ChessEngine();
+    engine.setActivePresets([
+      { id: FLIP_AFTER_TWO_ID, scope: "both", turnsRemaining: null },
+    ]);
+
+    // Move 1: white plays → count=1, veto. Still white's turn.
+    engine.applyMove(firstLegalMove(engine));
+    expect(engine.getCurrentTurn()).toBe("white");
+    expect(engine.session.get(GAME_ENTITY, "HalfMovesThisTurn")).toBe(1);
+
+    // Move 2: white plays → count=2, no veto. Flip to black.
+    engine.applyMove(firstLegalMove(engine));
+    expect(engine.getCurrentTurn()).toBe("black");
+    expect(engine.session.get(GAME_ENTITY, "HalfMovesThisTurn")).toBe(0);
+
+    // Move 3: black plays → count=1, veto. Still black's turn.
+    engine.applyMove(firstLegalMove(engine));
+    expect(engine.getCurrentTurn()).toBe("black");
+    expect(engine.session.get(GAME_ENTITY, "HalfMovesThisTurn")).toBe(1);
+
+    // Move 4: black plays → count=2, no veto. Flip to white.
+    engine.applyMove(firstLegalMove(engine));
+    expect(engine.getCurrentTurn()).toBe("white");
+    expect(engine.session.get(GAME_ENTITY, "HalfMovesThisTurn")).toBe(0);
+
+    // FullmoveNumber incremented exactly once (after black's full
+    // 2-half-move turn completed at move 4).
+    expect(engine.session.get(GAME_ENTITY, "FullmoveNumber")).toBe(2);
+  });
+});
+
+describe("shouldAdvanceTurn — onTurnStart gating", () => {
+  it("onTurnStart does NOT fire when flip is vetoed", () => {
+    const engine = new ChessEngine();
+    engine.setActivePresets([
+      { id: NEVER_FLIP_ID, scope: "both", turnsRemaining: null },
+      { id: TURN_START_COUNTER_ID, scope: "both", turnsRemaining: null },
+    ]);
+
+    // Baseline: no move played → no turnStart fires.
+    expect(turnStartInvocations).toBe(0);
+
+    // Every move is vetoed → onTurnStart never fires.
+    engine.applyMove(firstLegalMove(engine));
+    engine.applyMove(firstLegalMove(engine));
+    engine.applyMove(firstLegalMove(engine));
+    expect(turnStartInvocations).toBe(0);
+  });
+
+  it("onTurnStart fires once per actual flip (no veto)", () => {
+    const engine = new ChessEngine();
+    engine.setActivePresets([
+      { id: TURN_START_COUNTER_ID, scope: "both", turnsRemaining: null },
+    ]);
+
+    expect(turnStartInvocations).toBe(0);
+    engine.applyMove(firstLegalMove(engine)); // flip → +1
+    expect(turnStartInvocations).toBe(1);
+    engine.applyMove(firstLegalMove(engine)); // flip → +1
+    expect(turnStartInvocations).toBe(2);
+  });
+
+  it("onTurnStart fires once per flip even with flip-after-two", () => {
+    const engine = new ChessEngine();
+    engine.setActivePresets([
+      { id: FLIP_AFTER_TWO_ID, scope: "both", turnsRemaining: null },
+      { id: TURN_START_COUNTER_ID, scope: "both", turnsRemaining: null },
+    ]);
+
+    expect(turnStartInvocations).toBe(0);
+    engine.applyMove(firstLegalMove(engine)); // white move 1 — veto, no fire
+    expect(turnStartInvocations).toBe(0);
+    engine.applyMove(firstLegalMove(engine)); // white move 2 — flip, fire
+    expect(turnStartInvocations).toBe(1);
+    engine.applyMove(firstLegalMove(engine)); // black move 1 — veto, no fire
+    expect(turnStartInvocations).toBe(1);
+    engine.applyMove(firstLegalMove(engine)); // black move 2 — flip, fire
+    expect(turnStartInvocations).toBe(2);
+  });
+});

packages/chess/src/schema.ts

@@ -55,6 +55,18 @@ export interface ChessAttrMap {
   Turn: PieceColor;
   HalfmoveClock: number;
   FullmoveNumber: number;
+  /**
+   * Number of half-moves (single player moves) completed within the
+   * CURRENT turn. Reset to 0 whenever `Turn` flips. Incremented in
+   * `engine.applyMove` BEFORE the turn-flip decision is polled via
+   * `shouldAdvanceTurn`, so preset hooks see the count AFTER their
+   * current move has been accounted for.
+   *
+   * FIDE chess never exceeds 1 (every applyMove flips). Variants like
+   * Double-Move (white plays 2, black plays 1) and Monster (white
+   * plays 2, black plays 1) use this counter to gate the flip.
+   */
+  HalfMovesThisTurn: number;
   EnPassantTarget: Square | null;
   GameStatus: GameStatus;
   Winner: GameResult | null;

packages/chess/src/starting-position.test.ts

@@ -119,6 +119,7 @@ describe("generateStartingPosition()", () => {
     expect(session.get(GAME_ENTITY, "Turn")).toBe("white");
     expect(session.get(GAME_ENTITY, "HalfmoveClock")).toBe(0);
     expect(session.get(GAME_ENTITY, "FullmoveNumber")).toBe(1);
+    expect(session.get(GAME_ENTITY, "HalfMovesThisTurn")).toBe(0);
     expect(session.get(GAME_ENTITY, "EnPassantTarget")).toBe(null);
     expect(session.get(GAME_ENTITY, "GameStatus")).toBe("active");
     expect(session.get(GAME_ENTITY, "Winner")).toBe(null);

packages/chess/src/starting-position.ts

@@ -182,6 +182,11 @@ export function applyLayout(
   session.insert(GAME_ENTITY, "Turn", "white");
   session.insert(GAME_ENTITY, "HalfmoveClock", 0);
   session.insert(GAME_ENTITY, "FullmoveNumber", 1);
+  // `HalfMovesThisTurn` counts the number of half-moves played since
+  // the last `Turn` flip. Seeded alongside `Turn` so both start in
+  // the same committed-state. Incremented in `engine.applyMove`
+  // BEFORE the turn-flip decision; reset to 0 on flip.
+  session.insert(GAME_ENTITY, "HalfMovesThisTurn", 0);
   session.insert(GAME_ENTITY, "EnPassantTarget", null);
   session.insert(GAME_ENTITY, "GameStatus", "active");
   session.insert(GAME_ENTITY, "Winner", null);