feat(chess): add knight move rules (P2.10)

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

@@ -0,0 +1,87 @@
+import { describe, it, expect } from "vitest";
+import { Session } from "@paratype/rete";
+import type { EntityId } from "@paratype/rete";
+import { getLegalKnightMoves } from "./knight.js";
+
+const mkId = (n: number) => n as EntityId;
+
+function insertPiece(
+  session: Session,
+  id: number,
+  type: string,
+  color: string,
+  square: number,
+): EntityId {
+  const eid = mkId(id);
+  session.insert(eid, "PieceType", type);
+  session.insert(eid, "Color", color);
+  session.insert(eid, "Position", square);
+  return eid;
+}
+
+describe("getLegalKnightMoves", () => {
+  it("knight at e4 (28) has 8 moves when board is empty", () => {
+    const session = new Session({ autoFire: false });
+    const knight = insertPiece(session, 1, "knight", "white", 28);
+    const moves = getLegalKnightMoves(session, knight);
+    expect(moves).toHaveLength(8);
+    // All 8 L-shapes from e4: f6=45, g5=38, g3=22, f2=13, d2=11, c3=18, c5=34, d6=43
+    const targets = moves.map(m => m.to).sort((a, b) => a - b);
+    expect(targets).toEqual([11, 13, 18, 22, 34, 38, 43, 45]);
+  });
+
+  it("knight at a1 (0) has 2 moves only (board edge)", () => {
+    const session = new Session({ autoFire: false });
+    const knight = insertPiece(session, 1, "knight", "white", 0);
+    const moves = getLegalKnightMoves(session, knight);
+    expect(moves).toHaveLength(2);
+    // From a1: b3=17, c2=10
+    const targets = moves.map(m => m.to).sort((a, b) => a - b);
+    expect(targets).toEqual([10, 17]);
+  });
+
+  it("knight ignores intervening pieces (leaps)", () => {
+    const session = new Session({ autoFire: false });
+    const knight = insertPiece(session, 1, "knight", "white", 28); // e4
+    // Block all orthogonally adjacent squares — knight should still leap over them.
+    insertPiece(session, 2, "pawn", "white", 27); // d4
+    insertPiece(session, 3, "pawn", "white", 29); // f4
+    insertPiece(session, 4, "pawn", "white", 20); // e3
+    insertPiece(session, 5, "pawn", "white", 36); // e5
+    const moves = getLegalKnightMoves(session, knight);
+    // None of the blockers sits on a knight-target square, so all 8 reachable.
+    expect(moves).toHaveLength(8);
+  });
+
+  it("cannot capture ally piece", () => {
+    const session = new Session({ autoFire: false });
+    const knight = insertPiece(session, 1, "knight", "white", 28); // e4
+    insertPiece(session, 2, "rook", "white", 38); // g5 — knight target
+    const moves = getLegalKnightMoves(session, knight);
+    expect(moves.find(m => m.to === 38)).toBeUndefined();
+    expect(moves).toHaveLength(7);
+  });
+
+  it("can capture enemy piece", () => {
+    const session = new Session({ autoFire: false });
+    const knight = insertPiece(session, 1, "knight", "white", 28); // e4
+    insertPiece(session, 2, "rook", "black", 38); // g5 — knight target
+    const move = getLegalKnightMoves(session, knight).find(m => m.to === 38);
+    expect(move).toBeDefined();
+    expect(move?.isCapture).toBe(true);
+  });
+
+  it("quiet moves are marked isCapture=false", () => {
+    const session = new Session({ autoFire: false });
+    const knight = insertPiece(session, 1, "knight", "white", 28);
+    const moves = getLegalKnightMoves(session, knight);
+    for (const m of moves) {
+      expect(m.isCapture).toBe(false);
+    }
+  });
+
+  it("returns empty list when pieceId has no Position/Color", () => {
+    const session = new Session({ autoFire: false });
+    expect(getLegalKnightMoves(session, mkId(999))).toEqual([]);
+  });
+});

packages/chess/src/rules/knight.ts

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+/**
+ * Knight movement rules for @paratype/chess.
+ *
+ * Knights leap — they do NOT check for intervening pieces on the path
+ * between `from` and `to`. Only the target square matters:
+ *   - empty → legal quiet move
+ *   - enemy → legal capture
+ *   - ally  → illegal
+ */
+import type { Session, EntityId } from "@paratype/rete";
+import { knightCandidates } from "./primitives.js";
+import type { LegalMove } from "./types.js";
+import { getPiecePosition, getPieceColor, isAllyAt, isPieceAt } from "./board-queries.js";
+
+/** Get all legal knight moves (leaps — no path obstruction check). */
+export function getLegalKnightMoves(session: Session, pieceId: EntityId): LegalMove[] {
+  const from = getPiecePosition(session, pieceId);
+  const color = getPieceColor(session, pieceId);
+  if (from === null || color === null) return [];
+
+  const moves: LegalMove[] = [];
+  for (const to of knightCandidates(from)) {
+    if (isAllyAt(session, to, color)) continue;
+    const isCapture = isPieceAt(session, to);
+    moves.push({ pieceId, from, to, isCapture });
+  }
+  return moves;
+}