feat(chess): add capture resolution (P2.14)

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

@@ -0,0 +1,63 @@
+import { describe, it, expect } from "vitest";
+import { Session } from "@paratype/rete";
+import { applyCapture, getCapturablePieces, canCapture } from "./capture.js";
+import type { EntityId } from "@paratype/rete";
+
+const mkId = (n: number) => n as EntityId;
+const ins = (s: Session, id: number, type: string, color: string, sq: number) => {
+  const eid = mkId(id);
+  s.insert(eid, "PieceType", type); s.insert(eid, "Color", color); s.insert(eid, "Position", sq);
+  return eid;
+};
+
+describe("applyCapture", () => {
+  it("removes all facts for the captured piece", () => {
+    const session = new Session({ autoFire: false });
+    const black = ins(session, 2, "knight", "black", 28);
+    applyCapture(session, black);
+    expect(session.contains(black, "PieceType")).toBe(false);
+    expect(session.contains(black, "Color")).toBe(false);
+    expect(session.contains(black, "Position")).toBe(false);
+  });
+
+  it("does not affect other pieces", () => {
+    const session = new Session({ autoFire: false });
+    const white = ins(session, 1, "pawn", "white", 20);
+    const black = ins(session, 2, "knight", "black", 28);
+    applyCapture(session, black);
+    expect(session.contains(white, "Position")).toBe(true);
+  });
+});
+
+describe("getCapturablePieces", () => {
+  it("returns all enemy piece ids", () => {
+    const session = new Session({ autoFire: false });
+    ins(session, 1, "pawn", "white", 20);
+    ins(session, 2, "pawn", "white", 21);
+    ins(session, 3, "rook", "black", 50);
+    ins(session, 4, "bishop", "black", 51);
+    const capturable = getCapturablePieces(session, "white");
+    expect(capturable).toContain(mkId(3));
+    expect(capturable).toContain(mkId(4));
+    expect(capturable).not.toContain(mkId(1));
+  });
+});
+
+describe("canCapture", () => {
+  it("returns true when enemy on square", () => {
+    const session = new Session({ autoFire: false });
+    ins(session, 1, "rook", "black", 28);
+    expect(canCapture(session, 28, "white")).toBe(true);
+  });
+
+  it("returns false when own piece on square", () => {
+    const session = new Session({ autoFire: false });
+    ins(session, 1, "rook", "white", 28);
+    expect(canCapture(session, 28, "white")).toBe(false);
+  });
+
+  it("returns false when square is empty", () => {
+    const session = new Session({ autoFire: false });
+    expect(canCapture(session, 28, "white")).toBe(false);
+  });
+});

packages/chess/src/rules/capture.ts

@@ -0,0 +1,45 @@
+/**
+ * Capture resolution for @paratype/chess.
+ * When a capture occurs, the captured piece's facts are retracted from WM.
+ */
+import type { Session, EntityId } from "@paratype/rete";
+import type { PieceColor } from "../schema.js";
+
+/** All known chess fact attributes for a piece entity. */
+const PIECE_ATTRS = ["PieceType", "Color", "Position", "HasMoved", "Hp"] as const;
+
+/**
+ * Apply a capture: retract all known facts for the captured piece.
+ * Called after confirming the capture is legal.
+ */
+export function applyCapture(session: Session, capturedId: EntityId): void {
+  for (const attr of PIECE_ATTRS) {
+    if (session.contains(capturedId, attr)) {
+      session.retract(capturedId, attr);
+    }
+  }
+}
+
+/**
+ * Get all enemy piece entity IDs that a given color could potentially capture.
+ * Returns the IDs of all pieces belonging to the opposite color.
+ */
+export function getCapturablePieces(session: Session, attackingColor: PieceColor): EntityId[] {
+  const enemyColor: PieceColor = attackingColor === "white" ? "black" : "white";
+  const facts = session.allFacts();
+  return facts
+    .filter(f => f.attr === "Color" && f.value === enemyColor && f.id > 0)
+    .map(f => f.id as EntityId);
+}
+
+/**
+ * Check if capturing the piece at the given square is legal
+ * (i.e., it's an enemy piece, not an ally).
+ */
+export function canCapture(session: Session, square: number, attackingColor: PieceColor): boolean {
+  const facts = session.allFacts();
+  const positionFact = facts.find(f => f.attr === "Position" && f.value === square);
+  if (!positionFact) return false; // no piece at square
+  const colorFact = facts.find(f => f.id === positionFact.id && f.attr === "Color");
+  return colorFact?.value !== undefined && colorFact.value !== attackingColor;
+}