From 83d99778db84fc3b843ded68312b0268da06862b Mon Sep 17 00:00:00 2001
From: Joey Yakimowich-Payne <jyapayne@pm.me>
Date: Thu, 16 Apr 2026 14:52:51 -0600
Subject: [PATCH] feat(chess): add king basic move rules (P2.12)

---
 packages/chess/src/rules/king.test.ts | 74 +++++++++++++++++++++++++++
 packages/chess/src/rules/king.ts      | 32 ++++++++++++
 2 files changed, 106 insertions(+)
 create mode 100644 packages/chess/src/rules/king.test.ts
 create mode 100644 packages/chess/src/rules/king.ts

diff --git a/packages/chess/src/rules/king.test.ts b/packages/chess/src/rules/king.test.ts
new file mode 100644
index 0000000..cc00b18
--- /dev/null
+++ b/packages/chess/src/rules/king.test.ts
@@ -0,0 +1,74 @@
+import { describe, it, expect } from "vitest";
+import { Session } from "@paratype/rete";
+import type { EntityId } from "@paratype/rete";
+import { getLegalKingMoves } from "./king.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("getLegalKingMoves", () => {
+  it("king at e4 (28) has 8 moves on empty board", () => {
+    const session = new Session({ autoFire: false });
+    const king = insertPiece(session, 1, "king", "white", 28);
+    expect(getLegalKingMoves(session, king)).toHaveLength(8);
+  });
+
+  it("king at a1 (0) has 3 moves on empty board", () => {
+    const session = new Session({ autoFire: false });
+    const king = insertPiece(session, 1, "king", "white", 0);
+    expect(getLegalKingMoves(session, king)).toHaveLength(3);
+  });
+
+  it("king at h8 (63) has 3 moves on empty board", () => {
+    const session = new Session({ autoFire: false });
+    const king = insertPiece(session, 1, "king", "black", 63);
+    expect(getLegalKingMoves(session, king)).toHaveLength(3);
+  });
+
+  it("king blocked by own pieces", () => {
+    const session = new Session({ autoFire: false });
+    const king = insertPiece(session, 1, "king", "white", 28); // e4
+    insertPiece(session, 2, "pawn", "white", 27); // d4
+    insertPiece(session, 3, "pawn", "white", 29); // f4
+    const moves = getLegalKingMoves(session, king);
+    expect(moves.find(m => m.to === 27)).toBeUndefined();
+    expect(moves.find(m => m.to === 29)).toBeUndefined();
+    expect(moves).toHaveLength(6);
+  });
+
+  it("king can capture enemy piece", () => {
+    const session = new Session({ autoFire: false });
+    const king = insertPiece(session, 1, "king", "white", 28); // e4
+    insertPiece(session, 2, "pawn", "black", 27); // d4
+    const capture = getLegalKingMoves(session, king).find(m => m.to === 27);
+    expect(capture).toBeDefined();
+    expect(capture?.isCapture).toBe(true);
+  });
+
+  it("quiet moves are marked isCapture=false", () => {
+    const session = new Session({ autoFire: false });
+    const king = insertPiece(session, 1, "king", "white", 28);
+    const moves = getLegalKingMoves(session, king);
+    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(getLegalKingMoves(session, mkId(999))).toEqual([]);
+  });
+});
diff --git a/packages/chess/src/rules/king.ts b/packages/chess/src/rules/king.ts
new file mode 100644
index 0000000..cfc7565
--- /dev/null
+++ b/packages/chess/src/rules/king.ts
@@ -0,0 +1,32 @@
+/**
+ * King movement rules for @paratype/chess.
+ *
+ * Basic king moves: 1 square in any of 8 directions.
+ * Excludes:
+ *   - Castling (P2.15)
+ *   - Check-aware rejection / king-in-check filter (P2.18)
+ *
+ * Rules:
+ *   - empty target square → legal quiet move
+ *   - enemy on target     → legal capture
+ *   - ally on target      → illegal
+ */
+import type { Session, EntityId } from "@paratype/rete";
+import { kingCandidates } from "./primitives.js";
+import type { LegalMove } from "./types.js";
+import { getPiecePosition, getPieceColor, isAllyAt, isPieceAt } from "./board-queries.js";
+
+/** Get all legal basic king moves (1 square, any direction). */
+export function getLegalKingMoves(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 kingCandidates(from)) {
+    if (isAllyAt(session, to, color)) continue;
+    const isCapture = isPieceAt(session, to);
+    moves.push({ pieceId, from, to, isCapture });
+  }
+  return moves;
+}