feat(chess): add insufficient material draw (P2.22)

packages/chess/src/rules/insufficient.test.ts

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+/**
+ * Tests for insufficient material draw detection (P2.22).
+ *
+ * Square encoding: square = rank*8 + file. a1=0, b1=1, c1=2, e1=4, e8=60.
+ * squareColor: (file+rank) even → dark, odd → light.
+ *   a1 (0+0=0) → dark
+ *   b1 (1+0=1) → light
+ *   c1 (2+0=2) → dark
+ */
+import { describe, it, expect } from "vitest";
+import { Session } from "@paratype/rete";
+import type { EntityId } from "@paratype/rete";
+import { isInsufficientMaterial } from "./insufficient.js";
+
+const mkId = (n: number) => n as EntityId;
+
+function ins(
+  s: Session,
+  id: number,
+  type: string,
+  color: string,
+  sq: number,
+): EntityId {
+  const eid = mkId(id);
+  s.insert(eid, "PieceType", type);
+  s.insert(eid, "Color", color);
+  s.insert(eid, "Position", sq);
+  return eid;
+}
+
+describe("isInsufficientMaterial", () => {
+  it("K vs K: draw", () => {
+    const session = new Session({ autoFire: false });
+    ins(session, 1, "king", "white", 4);
+    ins(session, 2, "king", "black", 60);
+    expect(isInsufficientMaterial(session)).toBe(true);
+  });
+
+  it("K + N vs K: draw", () => {
+    const session = new Session({ autoFire: false });
+    ins(session, 1, "king", "white", 4);
+    ins(session, 2, "knight", "white", 12);
+    ins(session, 3, "king", "black", 60);
+    expect(isInsufficientMaterial(session)).toBe(true);
+  });
+
+  it("K vs K + N (black has the minor): draw", () => {
+    const session = new Session({ autoFire: false });
+    ins(session, 1, "king", "white", 4);
+    ins(session, 2, "king", "black", 60);
+    ins(session, 3, "knight", "black", 52);
+    expect(isInsufficientMaterial(session)).toBe(true);
+  });
+
+  it("K + B vs K: draw", () => {
+    const session = new Session({ autoFire: false });
+    ins(session, 1, "king", "white", 4);
+    ins(session, 2, "bishop", "white", 9); // b2 — light (1+1=2 even? → dark actually)
+    ins(session, 3, "king", "black", 60);
+    expect(isInsufficientMaterial(session)).toBe(true);
+  });
+
+  it("K + B vs K + B same color (both dark): draw", () => {
+    const session = new Session({ autoFire: false });
+    ins(session, 1, "king", "white", 4);
+    ins(session, 2, "bishop", "white", 0); // a1 — dark
+    ins(session, 3, "king", "black", 60);
+    ins(session, 4, "bishop", "black", 2); // c1 — dark
+    expect(isInsufficientMaterial(session)).toBe(true);
+  });
+
+  it("K + B vs K + B opposite color: NOT draw", () => {
+    const session = new Session({ autoFire: false });
+    ins(session, 1, "king", "white", 4);
+    ins(session, 2, "bishop", "white", 0); // a1 — dark
+    ins(session, 3, "king", "black", 60);
+    ins(session, 4, "bishop", "black", 1); // b1 — light
+    expect(isInsufficientMaterial(session)).toBe(false);
+  });
+
+  it("K + Q vs K: NOT draw", () => {
+    const session = new Session({ autoFire: false });
+    ins(session, 1, "king", "white", 4);
+    ins(session, 2, "queen", "white", 3);
+    ins(session, 3, "king", "black", 60);
+    expect(isInsufficientMaterial(session)).toBe(false);
+  });
+
+  it("K + R vs K: NOT draw", () => {
+    const session = new Session({ autoFire: false });
+    ins(session, 1, "king", "white", 4);
+    ins(session, 2, "rook", "white", 0);
+    ins(session, 3, "king", "black", 60);
+    expect(isInsufficientMaterial(session)).toBe(false);
+  });
+
+  it("K + P vs K: NOT draw (pawn can promote)", () => {
+    const session = new Session({ autoFire: false });
+    ins(session, 1, "king", "white", 4);
+    ins(session, 2, "pawn", "white", 12);
+    ins(session, 3, "king", "black", 60);
+    expect(isInsufficientMaterial(session)).toBe(false);
+  });
+
+  it("K + N + N vs K: NOT draw (outside plan scope)", () => {
+    const session = new Session({ autoFire: false });
+    ins(session, 1, "king", "white", 4);
+    ins(session, 2, "knight", "white", 12);
+    ins(session, 3, "knight", "white", 14);
+    ins(session, 4, "king", "black", 60);
+    expect(isInsufficientMaterial(session)).toBe(false);
+  });
+
+  it("K + B + N vs K: NOT draw (forced mate exists)", () => {
+    const session = new Session({ autoFire: false });
+    ins(session, 1, "king", "white", 4);
+    ins(session, 2, "bishop", "white", 9);
+    ins(session, 3, "knight", "white", 12);
+    ins(session, 4, "king", "black", 60);
+    expect(isInsufficientMaterial(session)).toBe(false);
+  });
+});

packages/chess/src/rules/insufficient.ts

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+/**
+ * Insufficient material draw detection (P2.22).
+ *
+ * FIDE draw by insufficient material — neither side retains a forcing
+ * checkmate pattern with only these pieces:
+ *   1. K vs K
+ *   2. K + B vs K
+ *   3. K + N vs K
+ *   4. K + B vs K + B, where both bishops live on the SAME color square
+ *
+ * K + N + N vs K is technically a draw under FIDE too, but its rarity
+ * (and that it can be forced to mate in contrived positions) places it
+ * outside the scope of this plan. Opposite-color bishop endings are
+ * NEVER detected as a draw here — they can produce checkmate positions
+ * and must be resolved by stalemate / 50-move / repetition instead.
+ */
+import type { Session, EntityId } from "@paratype/rete";
+import type { PieceType, PieceColor, Square } from "../schema.js";
+import { squareColor } from "../coord.js";
+
+interface PieceInfo {
+  readonly id: EntityId;
+  readonly type: PieceType;
+  readonly color: PieceColor;
+  readonly square: Square;
+}
+
+/**
+ * Project all pieces (id > 0 filter skips the GAME_ENTITY-side
+ * game-level facts) into a compact {type, color, square} view.
+ */
+function getPieces(session: Session): PieceInfo[] {
+  const facts = session.allFacts();
+  const typeFacts = facts.filter((f) => f.attr === "PieceType" && f.id > 0);
+  const out: PieceInfo[] = [];
+  for (const tf of typeFacts) {
+    const colorFact = facts.find((c) => c.id === tf.id && c.attr === "Color");
+    const posFact = facts.find((p) => p.id === tf.id && p.attr === "Position");
+    if (!colorFact || !posFact) continue;
+    out.push({
+      id: tf.id,
+      type: tf.value as PieceType,
+      color: colorFact.value as PieceColor,
+      square: posFact.value as Square,
+    });
+  }
+  return out;
+}
+
+/**
+ * Returns true if the current material configuration matches one of the
+ * four FIDE insufficient-material cases.
+ */
+export function isInsufficientMaterial(session: Session): boolean {
+  const pieces = getPieces(session);
+
+  const white = pieces.filter((p) => p.color === "white");
+  const black = pieces.filter((p) => p.color === "black");
+
+  const nonKingWhite = white.filter((p) => p.type !== "king");
+  const nonKingBlack = black.filter((p) => p.type !== "king");
+
+  // Case 1: K vs K
+  if (nonKingWhite.length === 0 && nonKingBlack.length === 0) return true;
+
+  // Case 2: K vs K + minor (bishop or knight)
+  if (nonKingWhite.length === 0 && nonKingBlack.length === 1) {
+    const p = nonKingBlack[0]!;
+    if (p.type === "bishop" || p.type === "knight") return true;
+  }
+  if (nonKingBlack.length === 0 && nonKingWhite.length === 1) {
+    const p = nonKingWhite[0]!;
+    if (p.type === "bishop" || p.type === "knight") return true;
+  }
+
+  // Case 4 (aka "Case 3" in FIDE ordering): K + B vs K + B — draw iff
+  // bishops share square color. Opposite-color bishops CAN still mate
+  // in contrived positions (though not by force), so not a draw here.
+  if (nonKingWhite.length === 1 && nonKingBlack.length === 1) {
+    const wb = nonKingWhite[0]!;
+    const bb = nonKingBlack[0]!;
+    if (wb.type === "bishop" && bb.type === "bishop") {
+      return squareColor(wb.square) === squareColor(bb.square);
+    }
+  }
+
+  return false;
+}