chessgroundxx/src/fen.ts

lichess.org chess ui

import { pos2key, invRanks } from "./util.js";
import * as cg from "./types.js";

export const initial: cg.FEN =
  "rnbakabnr/9/1c5c1/p1p1p1p1p/9/9/P1P1P1P1P/1C5C1/9/RNBAKABNR w";
const roles: { [letter: string]: cg.Role } = {
  p: "pawn",
  r: "rook",
  n: "knight",
  b: "bishop",
  a: "advisor",
  c: "cannon",
  k: "king",
};

const letters = {
  pawn: "p",
  rook: "r",
  knight: "n",
  bishop: "b",
  advisor: "a",
  cannon: "c",
  king: "k",
};

export function read(fen: cg.FEN): cg.Pieces {
  if (fen === "start") fen = initial;
  if (fen.indexOf("[") !== -1) fen = fen.slice(0, fen.indexOf("["));
  const pieces: cg.Pieces = new Map();
  let row: number = fen.split("/").length;
  let col: number = 0;

  for (const c of fen) {
    switch (c) {
      case " ":
        return pieces;
      case "/":
        --row;
        if (row === 0) return pieces;
        col = 0;
        break;
      case "~":
        // const piece = pieces.get(pos2key([col, row]));
        break;
      default:
        const nb = c.charCodeAt(0);

        if (nb < 58) {
          col += c === "0" ? 9 : nb - 48;
        } else {
          ++col;
          const role = c.toLowerCase();

          let piece = {
            role: roles[role],
            color: (c === role ? "black" : "white") as cg.Color,
          } as cg.Piece;

          pieces.set(pos2key([col, row]), piece);
        }
    }
  }
  return pieces;
}

export function write(pieces: cg.Pieces): cg.FEN {
  return invRanks
    .map((y) =>
      cg.files
        .map((x) => {
          const piece = pieces.get((x + y) as cg.Key);
          if (piece) {
            const letter = letters[piece.role];
            return piece.color === "white" ? letter.toUpperCase() : letter;
          } else return "1";
        })
        .join("")
    )
    .join("/")
    .replace(/1{2,}/g, (s) => s.length.toString());
}