feat: get FEN of current position

server/src/chess/engine.ts

@@ -38,14 +38,14 @@ export type Piece = {
 
 // prettier-ignore
 export const SQUARES = Object.freeze([
-  'a1', 'b1', 'c1', 'd1', 'e1', 'f1', 'g1', 'h1',
-  'a2', 'b2', 'c2', 'd2', 'e2', 'f2', 'g2', 'h2',
-  'a3', 'b3', 'c3', 'd3', 'e3', 'f3', 'g3', 'h3',
-  'a4', 'b4', 'c4', 'd4', 'e4', 'f4', 'g4', 'h4',
-  'a5', 'b5', 'c5', 'd5', 'e5', 'f5', 'g5', 'h5',
-  'a6', 'b6', 'c6', 'd6', 'e6', 'f6', 'g6', 'h6',
-  'a7', 'b7', 'c7', 'd7', 'e7', 'f7', 'g7', 'h7',
   'a8', 'b8', 'c8', 'd8', 'e8', 'f8', 'g8', 'h8',
+  'a7', 'b7', 'c7', 'd7', 'e7', 'f7', 'g7', 'h7',
+  'a6', 'b6', 'c6', 'd6', 'e6', 'f6', 'g6', 'h6',
+  'a5', 'b5', 'c5', 'd5', 'e5', 'f5', 'g5', 'h5',
+  'a4', 'b4', 'c4', 'd4', 'e4', 'f4', 'g4', 'h4',
+  'a3', 'b3', 'c3', 'd3', 'e3', 'f3', 'g3', 'h3',
+  'a2', 'b2', 'c2', 'd2', 'e2', 'f2', 'g2', 'h2',
+  'a1', 'b1', 'c1', 'd1', 'e1', 'f1', 'g1', 'h1',
 ] as const);
 
 function isSquareValid(string: string): boolean {
@@ -362,12 +362,62 @@ export class Chess {
     this._fullMoves = parseInt(fields[5]);
   }
 
+  getFEN() {
+    let position = "";
+
+    let emptySquares = 0;
+
+    for (let i = 0; i < 64; i++) {
+      if (i % 8 == 0) {
+        if (emptySquares) position += `${emptySquares}`;
+
+        position += "/";
+
+        emptySquares = 0;
+      }
+
+      const piece = this.getPiece(i);
+
+      if (piece == null) {
+        emptySquares++;
+        continue;
+      }
+
+      if (emptySquares) position += `${emptySquares}`;
+
+      position +=
+        piece.color == COLOR.WHITE
+          ? piece.type.toUpperCase()
+          : piece.type.toLowerCase();
+
+      emptySquares = 0;
+    }
+
+    let castling = "";
+
+    if (this._castling.w & MOVE_FLAGS.K_CASTLE) castling += "K";
+    if (this._castling.w & MOVE_FLAGS.Q_CASTLE) castling += "Q";
+    if (this._castling.b & MOVE_FLAGS.K_CASTLE) castling += "k";
+    if (this._castling.b & MOVE_FLAGS.Q_CASTLE) castling += "q";
+
+    return [
+      position.substring(1),
+      this._turn,
+      castling,
+      this._enPassant,
+      this._halfMoves,
+      this._fullMoves,
+    ].join(" ");
+  }
+
+  getPiece(square: number) {
+    return this._board[square];
+  }
+
   getMoves() {}
 
   getMovesForSquare(square: Square) {}
 
-  getFEN() {}
-
   makeMove() {}
 
   isSquareAttacked() {}