piece.js

class Piece {
    constructor(color, unit) {
        this.unit = unit.toUpperCase();
        this.color = color;
        this.frozen = false;
        this.promoted = false;
        this.original = false;
    }
}

function isEmpty(piece) {
    return piece.color == "";
}

function clearPiece(piece) {
    piece.color = "";
    piece.unit = "";
    piece.frozen = false;
    piece.promoted = false;
    piece.original = false;
}

function copyPiece(dst, src) {
    dst.unit = src.unit;
    dst.color = src.color;
    dst.frozen = src.frozen;
    dst.promoted = src.promoted;
    dst.original = src.original;
}

function getDetailedUnitType(file, rank, unit) {
    // returns unit unless it's a bishop, when we append L or D for light or dark bishop.
    if (unit == 'B') {
        if ((file + rank) % 2 == 1) return "BL"; else return "BD";
    } else return unit;
}

function placePieceAt(file, rank, piece) {
    if (board[file][rank].unit != "") {
        const detailedUnitType = getDetailedUnitType(file, rank, board[file][rank].unit);
        positionData.detailedUnitCount[board[file][rank].color + detailedUnitType]--;
    }

    // during cage verification, it is possible for one or both kings to be missing.
    if (positionData.detailedUnitCount[board[file][rank].color + 'K'] == 0) {
        positionData.kingPosition[board[file][rank].color] = null;
    }

    if (piece.unit != "") {
        const detailedUnitType = getDetailedUnitType(file, rank, piece.unit);
        positionData.detailedUnitCount[piece.color + detailedUnitType]++;
    }

    if (piece.unit == 'K') {
        positionData.kingPosition[piece.color] = new Square(file, rank);
    }
    copyPiece(board[file][rank], piece);
}

function emptyPieceAt(file, rank) {
    placePieceAt(file, rank, new Piece("", ""));
}

function setDefaultPieceParameters(file, rank, piece) {
    // set default original and frozen values sensibly

    if (piece.color == "" || piece.unit == "") {
        piece.unit = "";
        piece.color = "";
    }
    if (piece.unit == 'P') {
        assert(rank > 0 && rank < 7, 'Tried to add a pawn on rank ' + rank);
        piece.original = true;
        if (piece.color == "w") {
            piece.frozen = (rank == 1);
        }
        if (piece.color == "b") {
            piece.frozen = (rank == 6);
        }
    }
    if (piece.unit == 'K') {
        piece.original = true;
    }
    if (piece.frozen) {
        piece.original = true;
    }
}