An attempt to more correctly solve pinning.

engine/src/moves.test.ts

@@ -1070,4 +1070,31 @@ describe("castling", () => {
       },
     ]);
   });
+
+  it("pinned pieces", () => {
+    const generator = make("K7/8/8/8/8/5Q2/8/7q");
+
+    assert.sameDeepMembers(generator.generateMoves("f3"), [
+      {
+        from: "f3",
+        to: "e4",
+        type: "normal",
+      },
+      {
+        from: "f3",
+        to: "d5",
+        type: "normal",
+      },
+      {
+        from: "f3",
+        to: "c6",
+        type: "normal",
+      },
+      {
+        from: "f3",
+        to: "b7",
+        type: "normal",
+      },
+    ]);
+  });
 });

engine/src/moves.ts

@@ -13,7 +13,7 @@ import {
   UP_LEFT,
   UP_RIGHT,
 } from "./offsets";
-import { pins } from "./pins";
+import { pins, TAllowedMoves } from "./pins";
 import {
   getRank,
   getSide,
@@ -54,7 +54,7 @@ export class MoveGenerator {
     const side = getSide(piece);
 
     // TODO(aryann): Remove this check once all test boards have kings on them.
-    let pinnedPieces: TSquare[] = [];
+    let pinnedPieces: Map<TSquare, TAllowedMoves> = new Map();
     try {
       const kingSquare = this.board.getKingSquare(side);
       pinnedPieces = pins(this.board, kingSquare);
@@ -63,10 +63,9 @@ export class MoveGenerator {
     const moves: TMove[] = [];
 
     for (const move of this.generatePseudoLegalMoves(from)) {
-      if (pinnedPieces.includes(move.from)) {
+      const pinnedPiece = pinnedPieces.get(move.from);
+      if (pinnedPiece && !pinnedPiece.includes(move.to)) {
         // Pinned pieces cannot be moved.
-
-        // TODO(aryann): Allow pinned piece moves that are aligned with the king.
         continue;
       }
 

engine/src/offsets.ts

@@ -1,4 +1,4 @@
-import { NUM_FILES, NUM_RANKS } from "./types";
+import { NUM_FILES, NUM_RANKS, TPiece } from "./types";
 
 export type Offset = { file: number; rank: number };
 
@@ -11,13 +11,19 @@ export const DOWN_LEFT: Offset = { file: -1, rank: -1 };
 export const LEFT: Offset = { file: -1, rank: 0 };
 export const UP_LEFT: Offset = { file: -1, rank: 1 };
 
-export const SLIDING_PIECE_OFFSETS = {
+export const SLIDING_PIECE_OFFSETS: { [key in TPiece]: Offset[] } = {
   Q: [UP, UP_RIGHT, RIGHT, DOWN_RIGHT, DOWN, DOWN_LEFT, LEFT, UP_LEFT],
   q: [UP, UP_RIGHT, RIGHT, DOWN_RIGHT, DOWN, DOWN_LEFT, LEFT, UP_LEFT],
   R: [UP, RIGHT, DOWN, LEFT],
   r: [UP, RIGHT, DOWN, LEFT],
   B: [UP_RIGHT, DOWN_RIGHT, DOWN_LEFT, UP_LEFT],
   b: [UP_RIGHT, DOWN_RIGHT, DOWN_LEFT, UP_LEFT],
+  p: [],
+  P: [],
+  n: [],
+  N: [],
+  k: [],
+  K: [],
 };
 
 export const KNIGHT_OFFSETS: Offset[] = [

engine/src/pins.test.ts

@@ -1,53 +1,147 @@
 import { assert, describe, it } from "vitest";
 import { BoardState } from "./board";
-import { pins } from "./pins";
+import { pins, TAllowedMoves } from "./pins";
+import { TSquare } from "./types";
 
 describe("pins", () => {
   it("initial state", () => {
     const board = new BoardState();
 
-    assert.deepEqual(pins(board, "e1"), []);
+    assert.deepEqual(pins(board, "e1"), new Map<TSquare, TAllowedMoves>([]));
   });
 
   it("adjacent rooks", () => {
     const board = new BoardState("8/3r4/3R4/3K4/3R4/3r4/8/8 w KQkq - 0 1");
 
-    assert.deepEqual(pins(board, "d5"), ["d4", "d6"]);
+    assert.deepEqual(
+      pins(board, "d5"),
+      new Map<TSquare, TAllowedMoves>([
+        ["d4", []],
+        ["d6", []],
+      ])
+    );
   });
 
-  it("far rooks", () => {
+  it("far enemy rooks", () => {
     const board = new BoardState("3r4/8/3R4/3K4/3R4/8/8/3r4 w KQkq - 0 1");
 
-    assert.deepEqual(pins(board, "d5"), ["d4", "d6"]);
+    assert.deepEqual(
+      pins(board, "d5"),
+      new Map<TSquare, TAllowedMoves>([
+        ["d4", []],
+        ["d6", []],
+      ])
+    );
+  });
+
+  it("far friendly rooks", () => {
+    const board = new BoardState("3r4/3R4/8/3K4/8/8/3R4/3r4 w KQkq - 0 1");
+
+    assert.deepEqual(
+      pins(board, "d5"),
+      new Map<TSquare, TAllowedMoves>([
+        ["d2", ["d4", "d3"]],
+        ["d7", ["d6"]],
+      ])
+    );
   });
 
   it("adjacent bishops", () => {
     const board = new BoardState("8/1b6/2R5/3K4/4R3/5b2/8/8 w KQkq - 0 1");
 
-    assert.deepEqual(pins(board, "d5"), ["e4", "c6"]);
+    assert.deepEqual(
+      pins(board, "d5"),
+      new Map<TSquare, TAllowedMoves>([
+        ["e4", []],
+        ["c6", []],
+      ])
+    );
   });
 
-  it("far bishops", () => {
+  it("far enemy bishop", () => {
     const board = new BoardState("K7/1R6/8/8/8/8/8/7b w KQkq - 0 1");
 
-    assert.deepEqual(pins(board, "a8"), ["b7"]);
+    assert.deepEqual(
+      pins(board, "a8"),
+      new Map<TSquare, TAllowedMoves>([["b7", []]])
+    );
+  });
+
+  it("far friendly bishop with bishop pin", () => {
+    const board = new BoardState("K7/8/8/8/8/8/6B1/7b w KQkq - 0 1");
+
+    assert.deepEqual(
+      pins(board, "a8"),
+      new Map<TSquare, TAllowedMoves>([["g2", ["b7", "c6", "d5", "e4", "f3"]]])
+    );
+  });
+
+  it("far friendly rook with bishop pin", () => {
+    const board = new BoardState("K7/8/8/8/8/8/6R1/7b w KQkq - 0 1");
+
+    assert.deepEqual(
+      pins(board, "a8"),
+      new Map<TSquare, TAllowedMoves>([["g2", []]])
+    );
   });
 
   it("adjacent queens", () => {
     const board = new BoardState("8/3q4/3R4/3K4/3R4/3q4/8/8 w KQkq - 0 1");
 
-    assert.deepEqual(pins(board, "d5"), ["d4", "d6"]);
+    assert.deepEqual(
+      pins(board, "d5"),
+      new Map<TSquare, TAllowedMoves>([
+        ["d4", []],
+        ["d6", []],
+      ])
+    );
   });
 
-  it("far queen", () => {
+  it("far enemy queen with rook pin", () => {
     const board = new BoardState("K7/1R6/8/8/8/8/8/7q w KQkq - 0 1");
 
-    assert.deepEqual(pins(board, "a8"), ["b7"]);
+    assert.deepEqual(
+      pins(board, "a8"),
+      new Map<TSquare, TAllowedMoves>([["b7", []]])
+    );
+  });
+
+  it("far enemy queen with bishop pin", () => {
+    const board = new BoardState("K7/8/8/8/8/8/6B1/7q w KQkq - 0 1");
+
+    assert.deepEqual(
+      pins(board, "a8"),
+      new Map<TSquare, TAllowedMoves>([["g2", ["b7", "c6", "d5", "e4", "f3"]]])
+    );
+  });
+
+  it("far enemy queen with rook pin", () => {
+    const board = new BoardState("K7/8/8/8/8/8/6R1/7q w KQkq - 0 1");
+
+    assert.deepEqual(
+      pins(board, "a8"),
+      new Map<TSquare, TAllowedMoves>([["g2", []]])
+    );
+  });
+
+  it("far enemy queen with queen pin", () => {
+    const board = new BoardState("K7/8/8/8/8/5Q2/8/7q w KQkq - 0 1");
+
+    assert.deepEqual(
+      pins(board, "a8"),
+      new Map<TSquare, TAllowedMoves>([["f3", ["b7", "c6", "d5", "e4"]]])
+    );
   });
 
   it("multiple defenders", () => {
     const board = new BoardState("3r4/3P4/3R4/3K4/3R4/3P4/8/3r4 w KQkq - 0 1");
 
-    assert.deepEqual(pins(board, "d5"), ["d4", "d6"]);
+    assert.deepEqual(
+      pins(board, "d5"),
+      new Map<TSquare, TAllowedMoves>([
+        ["d4", []],
+        ["d6", []],
+      ])
+    );
   });
 });

engine/src/pins.ts

@@ -5,13 +5,14 @@ import {
   DOWN_RIGHT,
   isInRange,
   LEFT,
+  Offset,
   RIGHT,
   SLIDING_PIECE_OFFSETS,
   UP,
   UP_LEFT,
   UP_RIGHT,
 } from "./offsets";
-import { getSide, NUM_FILES, SQUARES, TSquare } from "./types";
+import { getSide, NUM_FILES, SQUARES, TPiece, TSquare } from "./types";
 
 const OFFSETS = [
   UP,
@@ -24,11 +25,17 @@ const OFFSETS = [
   UP_LEFT,
 ];
 
-export const pins = (board: BoardState, square: TSquare): TSquare[] => {
-  const pins: TSquare[] = [];
+export type TAllowedMoves = TSquare[];
+
+export const pins = (
+  board: BoardState,
+  square: TSquare
+): Map<TSquare, TAllowedMoves> => {
+  const pins = new Map<TSquare, TAllowedMoves>();
+
   const piece = board.get(square);
   if (!piece) {
-    return [];
+    return pins;
   }
 
   const side = getSide(piece);
@@ -43,7 +50,10 @@ export const pins = (board: BoardState, square: TSquare): TSquare[] => {
     let newFile = file;
     let newRank = rank;
 
-    let pinCandidate: TSquare | undefined = undefined;
+    let pinCandidate: { square: TSquare; piece: TPiece } | undefined =
+      undefined;
+    let allowedMoves: TSquare[] = [];
+
     for (;;) {
       newFile += offset.file;
       newRank += offset.rank;
@@ -55,12 +65,16 @@ export const pins = (board: BoardState, square: TSquare): TSquare[] => {
       const currentSquare = SQUARES[newRank * NUM_FILES + newFile];
       const currentPiece = board.get(currentSquare);
       if (!currentPiece) {
+        if (!pinCandidate) {
+          allowedMoves.push(currentSquare);
+        }
+
         continue;
       }
 
       if (!pinCandidate) {
         if (getSide(currentPiece) === side) {
-          pinCandidate = currentSquare;
+          pinCandidate = { square: currentSquare, piece: currentPiece };
           continue;
         } else {
           // The closest sliding piece along this direction is from the other
@@ -70,29 +84,40 @@ export const pins = (board: BoardState, square: TSquare): TSquare[] => {
       }
 
       if (getSide(currentPiece) !== side) {
-        if (
-          currentPiece != "Q" &&
-          currentPiece != "q" &&
-          currentPiece != "R" &&
-          currentPiece != "r" &&
-          currentPiece != "B" &&
-          currentPiece != "b"
-        ) {
-          break;
-        }
-
-        for (const pieceOffset of SLIDING_PIECE_OFFSETS[currentPiece]) {
+        for (const attackerOffset of SLIDING_PIECE_OFFSETS[currentPiece]) {
           if (
-            pieceOffset.file === offset.file &&
-            pieceOffset.rank === offset.rank
+            attackerOffset.file !== offset.file ||
+            attackerOffset.rank !== offset.rank
           ) {
-            pins.push(pinCandidate);
-            break;
+            continue;
           }
+
+          if (movesInSameDirection(pinCandidate.piece, attackerOffset)) {
+            // The
+            allowedMoves.push(currentSquare);
+          } else {
+            allowedMoves = [];
+          }
+
+          pins.set(pinCandidate.square, allowedMoves);
+
+          break;
         }
       }
     }
   }
 
   return pins;
 };
+
+const movesInSameDirection = (piece: TPiece, targetOffset: Offset): boolean => {
+  for (const offset of SLIDING_PIECE_OFFSETS[piece]) {
+    if (
+      offset.file === targetOffset.file &&
+      offset.rank === targetOffset.rank
+    ) {
+      return true;
+    }
+  }
+  return false;
+};