[vchess.git] / public / javascripts / base_rules.js

class PiPo //Piece+Position
{
    // o: {piece[p], color[c], posX[x], posY[y]}
    constructor(o)
    {
        this.p = o.p;
        this.c = o.c;
        this.x = o.x;
        this.y = o.y;
    }
}

class Move
{
    // o: {appear, vanish, [start,] [end,]}
    // appear,vanish = arrays of PiPo
    // start,end = coordinates to apply to trigger move visually (think castle)
    constructor(o)
    {
        this.appear = o.appear;
        this.vanish = o.vanish;
        this.start = !!o.start ? o.start : {x:o.vanish[0].x, y:o.vanish[0].y};
        this.end = !!o.end ? o.end : {x:o.appear[0].x, y:o.appear[0].y};
    }
}

// NOTE: x coords = top to bottom; y = left to right (from white player perspective)
class ChessRules
{
    // Path to pieces
    static getPpath(b)
    {
        return b; //usual pieces in pieces/ folder
    }
    // Turn "wb" into "B" (for FEN)
    static board2fen(b)
    {
        return b[0]=='w' ? b[1].toUpperCase() : b[1];
    }
    // Turn "p" into "bp" (for board)
    static fen2board(f)
    {
        return f.charCodeAt()<=90 ? "w"+f.toLowerCase() : "b"+f;
    }

    /////////////////
    // INITIALIZATION

    // fen == "position flags"
    constructor(fen, moves)
    {
        this.moves = moves;
        // Use fen string to initialize variables, flags and board
        this.board = VariantRules.GetBoard(fen);
        this.setFlags(fen);
        this.initVariables(fen);
    }

    initVariables(fen)
    {
        this.INIT_COL_KING = {'w':-1, 'b':-1};
        this.INIT_COL_ROOK = {'w':[-1,-1], 'b':[-1,-1]};
        this.kingPos = {'w':[-1,-1], 'b':[-1,-1]}; //respective squares of white and black king
        const fenParts = fen.split(" ");
        const position = fenParts[0].split("/");
        for (let i=0; i<position.length; i++)
        {
            let k = 0; //column index on board
            for (let j=0; j<position[i].length; j++)
            {
                switch (position[i].charAt(j))
                {
                    case 'k':
                        this.kingPos['b'] = [i,k];
                        this.INIT_COL_KING['b'] = k;
                        break;
                    case 'K':
                        this.kingPos['w'] = [i,k];
                        this.INIT_COL_KING['w'] = k;
                        break;
                    case 'r':
                        if (this.INIT_COL_ROOK['b'][0] < 0)
                            this.INIT_COL_ROOK['b'][0] = k;
                        else
                            this.INIT_COL_ROOK['b'][1] = k;
                        break;
                    case 'R':
                        if (this.INIT_COL_ROOK['w'][0] < 0)
                            this.INIT_COL_ROOK['w'][0] = k;
                        else
                            this.INIT_COL_ROOK['w'][1] = k;
                        break;
                    default:
                        let num = parseInt(position[i].charAt(j));
                        if (!isNaN(num))
                            k += (num-1);
                }
                k++;
            }
        }
        const epSq = this.moves.length > 0 ? this.getEpSquare(this.lastMove) : undefined;
        this.epSquares = [ epSq ];
    }

    // Turn diagram fen into double array ["wb","wp","bk",...]
    static GetBoard(fen)
    {
        let rows = fen.split(" ")[0].split("/");
        const [sizeX,sizeY] = VariantRules.size;
        let board = doubleArray(sizeX, sizeY, "");
        for (let i=0; i<rows.length; i++)
        {
            let j = 0;
            for (let indexInRow = 0; indexInRow < rows[i].length; indexInRow++)
            {
                let character = rows[i][indexInRow];
                let num = parseInt(character);
                if (!isNaN(num))
                    j += num; //just shift j
                else //something at position i,j
                    board[i][j++] = VariantRules.fen2board(character);
            }
        }
        return board;
    }

    // Overridable: flags can change a lot
    setFlags(fen)
    {
        // white a-castle, h-castle, black a-castle, h-castle
        this.castleFlags = {'w': new Array(2), 'b': new Array(2)};
        let flags = fen.split(" ")[1]; //flags right after position
        for (let i=0; i<4; i++)
            this.castleFlags[i < 2 ? 'w' : 'b'][i%2] = (flags.charAt(i) == '1');
    }

    ///////////////////
    // GETTERS, SETTERS

    // Simple useful getters
    static get size() { return [8,8]; }
    // Two next functions return 'undefined' if called on empty square
    getColor(i,j) { return this.board[i][j].charAt(0); }
    getPiece(i,j) { return this.board[i][j].charAt(1); }

    // Color
    getOppCol(color) { return color=="w" ? "b" : "w"; }

    get lastMove() {
        const L = this.moves.length;
        return L>0 ? this.moves[L-1] : null;
    }
    get turn() {
        return this.moves.length%2==0 ? 'w' : 'b';
    }

    // Pieces codes
    static get PAWN() { return 'p'; }
    static get ROOK() { return 'r'; }
    static get KNIGHT() { return 'n'; }
    static get BISHOP() { return 'b'; }
    static get QUEEN() { return 'q'; }
    static get KING() { return 'k'; }

    // Empty square
    static get EMPTY() { return ''; }

    // Some pieces movements
    static get steps() {
        return {
            'r': [ [-1,0],[1,0],[0,-1],[0,1] ],
            'n': [ [-1,-2],[-1,2],[1,-2],[1,2],[-2,-1],[-2,1],[2,-1],[2,1] ],
            'b': [ [-1,-1],[-1,1],[1,-1],[1,1] ],
            'q': [ [-1,0],[1,0],[0,-1],[0,1],[-1,-1],[-1,1],[1,-1],[1,1] ]
        };
    }

    // Aggregates flags into one object
    get flags() {
        return this.castleFlags;
    }

    // Reverse operation
    parseFlags(flags)
    {
        this.castleFlags = flags;
    }

    // En-passant square, if any
    getEpSquare(move)
    {
        const [sx,sy,ex] = [move.start.x,move.start.y,move.end.x];
        if (this.getPiece(sx,sy) == VariantRules.PAWN && Math.abs(sx - ex) == 2)
        {
            return {
                x: (sx + ex)/2,
                y: sy
            };
        }
        return undefined; //default
    }

    // can thing on square1 take thing on square2
    canTake([x1,y1], [x2,y2])
    {
        return this.getColor(x1,y1) != this.getColor(x2,y2);
    }

    ///////////////////
    // MOVES GENERATION

    // All possible moves from selected square (assumption: color is OK)
    getPotentialMovesFrom([x,y])
    {
        switch (this.getPiece(x,y))
        {
            case VariantRules.PAWN:
                return this.getPotentialPawnMoves([x,y]);
            case VariantRules.ROOK:
                return this.getPotentialRookMoves([x,y]);
            case VariantRules.KNIGHT:
                return this.getPotentialKnightMoves([x,y]);
            case VariantRules.BISHOP:
                return this.getPotentialBishopMoves([x,y]);
            case VariantRules.QUEEN:
                return this.getPotentialQueenMoves([x,y]);
            case VariantRules.KING:
                return this.getPotentialKingMoves([x,y]);
        }
    }

    // Build a regular move from its initial and destination squares; tr: transformation
    getBasicMove([sx,sy], [ex,ey], tr)
    {
        let mv = new Move({
            appear: [
                new PiPo({
                    x: ex,
                    y: ey,
                    c: !!tr ? tr.c : this.getColor(sx,sy),
                    p: !!tr ? tr.p : this.getPiece(sx,sy)
                })
            ],
            vanish: [
                new PiPo({
                    x: sx,
                    y: sy,
                    c: this.getColor(sx,sy),
                    p: this.getPiece(sx,sy)
                })
            ]
        });

        // The opponent piece disappears if we take it
        if (this.board[ex][ey] != VariantRules.EMPTY)
        {
            mv.vanish.push(
                new PiPo({
                    x: ex,
                    y: ey,
                    c: this.getColor(ex,ey),
                    p: this.getPiece(ex,ey)
                })
            );
        }
        return mv;
    }

    // Generic method to find possible moves of non-pawn pieces ("sliding or jumping")
    getSlideNJumpMoves([x,y], steps, oneStep)
    {
        const color = this.getColor(x,y);
        let moves = [];
        const [sizeX,sizeY] = VariantRules.size;
        outerLoop:
        for (let step of steps)
        {
            let i = x + step[0];
            let j = y + step[1];
            while (i>=0 && i<sizeX && j>=0 && j<sizeY && this.board[i][j] == VariantRules.EMPTY)
            {
                moves.push(this.getBasicMove([x,y], [i,j]));
                if (oneStep !== undefined)
                    continue outerLoop;
                i += step[0];
                j += step[1];
            }
            if (i>=0 && i<8 && j>=0 && j<8 && this.canTake([x,y], [i,j]))
                moves.push(this.getBasicMove([x,y], [i,j]));
        }
        return moves;
    }

    // What are the pawn moves from square x,y considering color "color" ?
    getPotentialPawnMoves([x,y])
    {
        const color = this.turn;
        let moves = [];
        const V = VariantRules;
        const [sizeX,sizeY] = VariantRules.size;
        const shift = (color == "w" ? -1 : 1);
        const firstRank = (color == 'w' ? sizeY-1 : 0);
        const startRank = (color == "w" ? sizeY-2 : 1);
        const lastRank = (color == "w" ? 0 : sizeY-1);

        if (x+shift >= 0 && x+shift < sizeX && x+shift != lastRank)
        {
            // Normal moves
            if (this.board[x+shift][y] == V.EMPTY)
            {
                moves.push(this.getBasicMove([x,y], [x+shift,y]));
                // Next condition because variants with pawns on 1st rank generally allow them to jump
                if ([startRank,firstRank].includes(x) && this.board[x+2*shift][y] == V.EMPTY)
                {
                    // Two squares jump
                    moves.push(this.getBasicMove([x,y], [x+2*shift,y]));
                }
            }
            // Captures
            if (y>0 && this.canTake([x,y], [x+shift,y-1]) && this.board[x+shift][y-1] != V.EMPTY)
                moves.push(this.getBasicMove([x,y], [x+shift,y-1]));
            if (y<sizeY-1 && this.canTake([x,y], [x+shift,y+1]) && this.board[x+shift][y+1] != V.EMPTY)
                moves.push(this.getBasicMove([x,y], [x+shift,y+1]));
        }

        if (x+shift == lastRank)
        {
            // Promotion
            let promotionPieces = [V.ROOK,V.KNIGHT,V.BISHOP,V.QUEEN];
            promotionPieces.forEach(p => {
                // Normal move
                if (this.board[x+shift][y] == V.EMPTY)
                    moves.push(this.getBasicMove([x,y], [x+shift,y], {c:color,p:p}));
                // Captures
                if (y>0 && this.canTake([x,y], [x+shift,y-1]) && this.board[x+shift][y-1] != V.EMPTY)
                    moves.push(this.getBasicMove([x,y], [x+shift,y-1], {c:color,p:p}));
                if (y<sizeY-1 && this.canTake([x,y], [x+shift,y+1]) && this.board[x+shift][y+1] != V.EMPTY)
                    moves.push(this.getBasicMove([x,y], [x+shift,y+1], {c:color,p:p}));
            });
        }

        // En passant
        const Lep = this.epSquares.length;
        const epSquare = Lep>0 ? this.epSquares[Lep-1] : undefined;
        if (!!epSquare && epSquare.x == x+shift && Math.abs(epSquare.y - y) == 1)
        {
            let epStep = epSquare.y - y;
            var enpassantMove = this.getBasicMove([x,y], [x+shift,y+epStep]);
            enpassantMove.vanish.push({
                x: x,
                y: y+epStep,
                p: 'p',
                c: this.getColor(x,y+epStep)
            });
            moves.push(enpassantMove);
        }

        return moves;
    }

    // What are the rook moves from square x,y ?
    getPotentialRookMoves(sq)
    {
        return this.getSlideNJumpMoves(sq, VariantRules.steps[VariantRules.ROOK]);
    }

    // What are the knight moves from square x,y ?
    getPotentialKnightMoves(sq)
    {
        return this.getSlideNJumpMoves(sq, VariantRules.steps[VariantRules.KNIGHT], "oneStep");
    }

    // What are the bishop moves from square x,y ?
    getPotentialBishopMoves(sq)
    {
        return this.getSlideNJumpMoves(sq, VariantRules.steps[VariantRules.BISHOP]);
    }

    // What are the queen moves from square x,y ?
    getPotentialQueenMoves(sq)
    {
        return this.getSlideNJumpMoves(sq, VariantRules.steps[VariantRules.QUEEN]);
    }

    // What are the king moves from square x,y ?
    getPotentialKingMoves(sq)
    {
        // Initialize with normal moves
        let moves = this.getSlideNJumpMoves(sq, VariantRules.steps[VariantRules.QUEEN], "oneStep");
        return moves.concat(this.getCastleMoves(sq));
    }

    getCastleMoves([x,y])
    {
        const c = this.getColor(x,y);
        if (x != (c=="w" ? 7 : 0) || y != this.INIT_COL_KING[c])
            return []; //x isn't first rank, or king has moved (shortcut)

        const V = VariantRules;

        // Castling ?
        const oppCol = this.getOppCol(c);
        let moves = [];
        let i = 0;
        const finalSquares = [ [2,3], [6,5] ]; //king, then rook
        castlingCheck:
        for (let castleSide=0; castleSide < 2; castleSide++) //large, then small
        {
            if (!this.castleFlags[c][castleSide])
                continue;
            // If this code is reached, rooks and king are on initial position

            // Nothing on the path of the king (and no checks; OK also if y==finalSquare)?
            let step = finalSquares[castleSide][0] < y ? -1 : 1;
            for (i=y; i!=finalSquares[castleSide][0]; i+=step)
            {
                if (this.isAttacked([x,i], oppCol) || (this.board[x][i] != V.EMPTY &&
                    // NOTE: next check is enough, because of chessboard constraints
                    (this.getColor(x,i) != c || ![V.KING,V.ROOK].includes(this.getPiece(x,i)))))
                {
                    continue castlingCheck;
                }
            }

            // Nothing on the path to the rook?
            step = castleSide == 0 ? -1 : 1;
            for (i = y + step; i != this.INIT_COL_ROOK[c][castleSide]; i += step)
            {
                if (this.board[x][i] != V.EMPTY)
                    continue castlingCheck;
            }
            const rookPos = this.INIT_COL_ROOK[c][castleSide];

            // Nothing on final squares, except maybe king and castling rook?
            for (i=0; i<2; i++)
            {
                if (this.board[x][finalSquares[castleSide][i]] != V.EMPTY &&
                    this.getPiece(x,finalSquares[castleSide][i]) != V.KING &&
                    finalSquares[castleSide][i] != rookPos)
                {
                    continue castlingCheck;
                }
            }

            // If this code is reached, castle is valid
            moves.push( new Move({
                appear: [
                    new PiPo({x:x,y:finalSquares[castleSide][0],p:V.KING,c:c}),
                    new PiPo({x:x,y:finalSquares[castleSide][1],p:V.ROOK,c:c})],
                vanish: [
                    new PiPo({x:x,y:y,p:V.KING,c:c}),
                    new PiPo({x:x,y:rookPos,p:V.ROOK,c:c})],
                end: Math.abs(y - rookPos) <= 2
                    ? {x:x, y:rookPos}
                    : {x:x, y:y + 2 * (castleSide==0 ? -1 : 1)}
            }) );
        }

        return moves;
    }

    ///////////////////
    // MOVES VALIDATION

    canIplay(side, [x,y])
    {
        return ((side=='w' && this.moves.length%2==0) || (side=='b' && this.moves.length%2==1))
            && this.getColor(x,y) == side;
    }

    getPossibleMovesFrom(sq)
    {
        // Assuming color is right (already checked)
        return this.filterValid( this.getPotentialMovesFrom(sq) );
    }

    // TODO: once a promotion is filtered, the others results are same: useless computations
    filterValid(moves)
    {
        if (moves.length == 0)
            return [];
        return moves.filter(m => { return !this.underCheck(m); });
    }

    // Search for all valid moves considering current turn (for engine and game end)
    getAllValidMoves()
    {
        const color = this.turn;
        const oppCol = this.getOppCol(color);
        var potentialMoves = [];
        let [sizeX,sizeY] = VariantRules.size;
        for (var i=0; i<sizeX; i++)
        {
            for (var j=0; j<sizeY; j++)
            {
                // Next condition ... != oppCol is a little HACK to work with checkered variant
                if (this.board[i][j] != VariantRules.EMPTY && this.getColor(i,j) != oppCol)
                    Array.prototype.push.apply(potentialMoves, this.getPotentialMovesFrom([i,j]));
            }
        }
        // NOTE: prefer lazy undercheck tests, letting the king being taken?
        // No: if happen on last 1/2 move, could lead to forbidden moves, wrong evals
        return this.filterValid(potentialMoves);
    }

    // Stop at the first move found
    atLeastOneMove()
    {
        const color = this.turn;
        const oppCol = this.getOppCol(color);
        let [sizeX,sizeY] = VariantRules.size;
        for (let i=0; i<sizeX; i++)
        {
            for (let j=0; j<sizeY; j++)
            {
                if (this.board[i][j] != VariantRules.EMPTY && this.getColor(i,j) != oppCol)
                {
                    const moves = this.getPotentialMovesFrom([i,j]);
                    if (moves.length > 0)
                    {
                        for (let k=0; k<moves.length; k++)
                        {
                            if (this.filterValid([moves[k]]).length > 0)
                                return true;
                        }
                    }
                }
            }
        }
        return false;
    }

    // Check if pieces of color 'colors' are attacking square x,y
    isAttacked(sq, colors)
    {
        return (this.isAttackedByPawn(sq, colors)
            || this.isAttackedByRook(sq, colors)
            || this.isAttackedByKnight(sq, colors)
            || this.isAttackedByBishop(sq, colors)
            || this.isAttackedByQueen(sq, colors)
            || this.isAttackedByKing(sq, colors));
    }

    // Is square x,y attacked by pawns of color c ?
    isAttackedByPawn([x,y], colors)
    {
        for (let c of colors)
        {
            let pawnShift = (c=="w" ? 1 : -1);
            if (x+pawnShift>=0 && x+pawnShift<8)
            {
                for (let i of [-1,1])
                {
                    if (y+i>=0 && y+i<8 && this.getPiece(x+pawnShift,y+i)==VariantRules.PAWN
                        && this.getColor(x+pawnShift,y+i)==c)
                    {
                        return true;
                    }
                }
            }
        }
        return false;
    }

    // Is square x,y attacked by rooks of color c ?
    isAttackedByRook(sq, colors)
    {
        return this.isAttackedBySlideNJump(sq, colors,
            VariantRules.ROOK, VariantRules.steps[VariantRules.ROOK]);
    }

    // Is square x,y attacked by knights of color c ?
    isAttackedByKnight(sq, colors)
    {
        return this.isAttackedBySlideNJump(sq, colors,
            VariantRules.KNIGHT, VariantRules.steps[VariantRules.KNIGHT], "oneStep");
    }

    // Is square x,y attacked by bishops of color c ?
    isAttackedByBishop(sq, colors)
    {
        return this.isAttackedBySlideNJump(sq, colors,
            VariantRules.BISHOP, VariantRules.steps[VariantRules.BISHOP]);
    }

    // Is square x,y attacked by queens of color c ?
    isAttackedByQueen(sq, colors)
    {
        return this.isAttackedBySlideNJump(sq, colors,
            VariantRules.QUEEN, VariantRules.steps[VariantRules.QUEEN]);
    }

    // Is square x,y attacked by king of color c ?
    isAttackedByKing(sq, colors)
    {
        return this.isAttackedBySlideNJump(sq, colors,
            VariantRules.KING, VariantRules.steps[VariantRules.QUEEN], "oneStep");
    }

    // Generic method for non-pawn pieces ("sliding or jumping"): is x,y attacked by piece != color ?
    isAttackedBySlideNJump([x,y], colors, piece, steps, oneStep)
    {
        for (let step of steps)
        {
            let rx = x+step[0], ry = y+step[1];
            while (rx>=0 && rx<8 && ry>=0 && ry<8 && this.board[rx][ry] == VariantRules.EMPTY
                && !oneStep)
            {
                rx += step[0];
                ry += step[1];
            }
            if (rx>=0 && rx<8 && ry>=0 && ry<8 && this.board[rx][ry] != VariantRules.EMPTY
                && this.getPiece(rx,ry) == piece && colors.includes(this.getColor(rx,ry)))
            {
                return true;
            }
        }
        return false;
    }

    // Is color c under check after move ?
    underCheck(move)
    {
        const color = this.turn;
        this.play(move);
        let res = this.isAttacked(this.kingPos[color], this.getOppCol(color));
        this.undo(move);
        return res;
    }

    // On which squares is color c under check (after move) ?
    getCheckSquares(move)
    {
        this.play(move);
        const color = this.turn; //opponent
        let res = this.isAttacked(this.kingPos[color], this.getOppCol(color))
            ? [ JSON.parse(JSON.stringify(this.kingPos[color])) ] //need to duplicate!
            : [ ];
        this.undo(move);
        return res;
    }

    // Apply a move on board
    static PlayOnBoard(board, move)
    {
        for (let psq of move.vanish)
            board[psq.x][psq.y] = VariantRules.EMPTY;
        for (let psq of move.appear)
            board[psq.x][psq.y] = psq.c + psq.p;
    }
    // Un-apply the played move
    static UndoOnBoard(board, move)
    {
        for (let psq of move.appear)
            board[psq.x][psq.y] = VariantRules.EMPTY;
        for (let psq of move.vanish)
            board[psq.x][psq.y] = psq.c + psq.p;
    }

    // Before move is played, update variables + flags
    updateVariables(move)
    {
        const piece = this.getPiece(move.start.x,move.start.y);
        const c = this.getColor(move.start.x,move.start.y);
        const firstRank = (c == "w" ? 7 : 0);

        // Update king position + flags
        if (piece == VariantRules.KING && move.appear.length > 0)
        {
            this.kingPos[c][0] = move.appear[0].x;
            this.kingPos[c][1] = move.appear[0].y;
            this.castleFlags[c] = [false,false];
            return;
        }
        const oppCol = this.getOppCol(c);
        const oppFirstRank = 7 - firstRank;
        if (move.start.x == firstRank //our rook moves?
            && this.INIT_COL_ROOK[c].includes(move.start.y))
        {
            const flagIdx = (move.start.y == this.INIT_COL_ROOK[c][0] ? 0 : 1);
            this.castleFlags[c][flagIdx] = false;
        }
        else if (move.end.x == oppFirstRank //we took opponent rook?
            && this.INIT_COL_ROOK[oppCol].includes(move.end.y))
        {
            const flagIdx = (move.end.y == this.INIT_COL_ROOK[oppCol][0] ? 0 : 1);
            this.castleFlags[oppCol][flagIdx] = false;
        }
    }

    unupdateVariables(move)
    {
        // (Potentially) Reset king position
        const c = this.getColor(move.start.x,move.start.y);
        if (this.getPiece(move.start.x,move.start.y) == VariantRules.KING)
            this.kingPos[c] = [move.start.x, move.start.y];
    }

    play(move, ingame)
    {
        // DEBUG:
//      if (!this.states) this.states = [];
//      if (!ingame) this.states.push(JSON.stringify(this.board));

        if (!!ingame)
            move.notation = this.getNotation(move);

        move.flags = JSON.stringify(this.flags); //save flags (for undo)
        this.updateVariables(move);
        this.moves.push(move);
        this.epSquares.push( this.getEpSquare(move) );
        VariantRules.PlayOnBoard(this.board, move);
    }

    undo(move)
    {
        VariantRules.UndoOnBoard(this.board, move);
        this.epSquares.pop();
        this.moves.pop();
        this.unupdateVariables(move);
        this.parseFlags(JSON.parse(move.flags));

        // DEBUG:
//      let state = this.states.pop();
//      if (JSON.stringify(this.board) != state)
//          debugger;
    }

    //////////////
    // END OF GAME

    checkRepetition()
    {
        // Check for 3 repetitions
        if (this.moves.length >= 8)
        {
            // NOTE: crude detection, only moves repetition
            const L = this.moves.length;
            if (_.isEqual(this.moves[L-1], this.moves[L-5]) &&
                _.isEqual(this.moves[L-2], this.moves[L-6]) &&
                _.isEqual(this.moves[L-3], this.moves[L-7]) &&
                _.isEqual(this.moves[L-4], this.moves[L-8]))
            {
                return true;
            }
        }
        return false;
    }

    checkGameOver()
    {
        if (this.checkRepetition())
            return "1/2";

        if (this.atLeastOneMove()) // game not over
            return "*";

        // Game over
        return this.checkGameEnd();
    }

    // No moves are possible: compute score
    checkGameEnd()
    {
        const color = this.turn;
        // No valid move: stalemate or checkmate?
        if (!this.isAttacked(this.kingPos[color], this.getOppCol(color)))
            return "1/2";
        // OK, checkmate
        return color == "w" ? "0-1" : "1-0";
    }

    ////////
    //ENGINE

    // Pieces values
    static get VALUES() {
        return {
            'p': 1,
            'r': 5,
            'n': 3,
            'b': 3,
            'q': 9,
            'k': 1000
        };
    }

    static get INFINITY() {
        return 9999; //"checkmate" (unreachable eval)
    }

    static get THRESHOLD_MATE() {
        // At this value or above, the game is over
        return VariantRules.INFINITY;
    }

    // Assumption: at least one legal move
    getComputerMove(moves1) //moves1 might be precomputed (Magnetic chess)
    {
        const maxeval = VariantRules.INFINITY;
        const color = this.turn;
        if (!moves1)
            moves1 = this.getAllValidMoves();

        // Rank moves using a min-max at depth 2
        for (let i=0; i<moves1.length; i++)
        {
            moves1[i].eval = (color=="w" ? -1 : 1) * maxeval; //very low, I'm checkmated
            let eval2 = (color=="w" ? 1 : -1) * maxeval; //initialized with checkmate value
            this.play(moves1[i]);
            // Second half-move:
            let moves2 = this.getAllValidMoves();
            // If no possible moves AND underCheck, eval2 is correct.
            // If !underCheck, eval2 is 0 (stalemate).
            if (moves2.length == 0 && this.checkGameEnd() == "1/2")
                eval2 = 0;
            for (let j=0; j<moves2.length; j++)
            {
                this.play(moves2[j]);
                let evalPos = this.evalPosition();
                if ((color == "w" && evalPos < eval2) || (color=="b" && evalPos > eval2))
                    eval2 = evalPos;
                this.undo(moves2[j]);
            }
            if ((color=="w" && eval2 > moves1[i].eval) || (color=="b" && eval2 < moves1[i].eval))
                moves1[i].eval = eval2;
            this.undo(moves1[i]);
        }
        moves1.sort( (a,b) => { return (color=="w" ? 1 : -1) * (b.eval - a.eval); });

        // Skip depth 3 if we found a checkmate (or if we are checkmated in 1...)
        if (Math.abs(moves1[0].eval) < VariantRules.THRESHOLD_MATE)
        {
            // TODO: show current analyzed move for depth 3, allow stopping eval (return moves1[0])
            for (let i=0; i<moves1.length; i++)
            {
                this.play(moves1[i]);
                // 0.1 * oldEval : heuristic to avoid some bad moves (not all...)
                moves1[i].eval = 0.1*moves1[i].eval + this.alphabeta(2, -maxeval, maxeval);
                this.undo(moves1[i]);
            }
            moves1.sort( (a,b) => { return (color=="w" ? 1 : -1) * (b.eval - a.eval); });
        }

        let candidates = [0]; //indices of candidates moves
        for (let j=1; j<moves1.length && moves1[j].eval == moves1[0].eval; j++)
            candidates.push(j);
//      console.log(moves1.map(m => { return [this.getNotation(m), m.eval]; }));
        return moves1[_.sample(candidates, 1)];
    }

    alphabeta(depth, alpha, beta)
  {
        const maxeval = VariantRules.INFINITY;
        const color = this.turn;
        if (!this.atLeastOneMove())
        {
            switch (this.checkGameEnd())
            {
                case "1/2": return 0;
                default: return color=="w" ? -maxeval : maxeval;
            }
        }
        if (depth == 0)
      return this.evalPosition();
        const moves = this.getAllValidMoves();
    let v = color=="w" ? -maxeval : maxeval;
        if (color == "w")
        {
            for (let i=0; i<moves.length; i++)
      {
                this.play(moves[i]);
                v = Math.max(v, this.alphabeta(depth-1, alpha, beta));
                this.undo(moves[i]);
                alpha = Math.max(alpha, v);
                if (alpha >= beta)
                    break; //beta cutoff
            }
        }
        else //color=="b"
        {
            for (let i=0; i<moves.length; i++)
            {
                this.play(moves[i]);
                v = Math.min(v, this.alphabeta(depth-1, alpha, beta));
                this.undo(moves[i]);
                beta = Math.min(beta, v);
                if (alpha >= beta)
                    break; //alpha cutoff
            }
        }
        return v;
    }

    evalPosition()
    {
        const [sizeX,sizeY] = VariantRules.size;
        let evaluation = 0;
        //Just count material for now
        for (let i=0; i<sizeX; i++)
        {
            for (let j=0; j<sizeY; j++)
            {
                if (this.board[i][j] != VariantRules.EMPTY)
                {
                    const sign = this.getColor(i,j) == "w" ? 1 : -1;
                    evaluation += sign * VariantRules.VALUES[this.getPiece(i,j)];
                }
            }
        }
        return evaluation;
    }

    ////////////
    // FEN utils

    // Overridable..
    static GenRandInitFen()
    {
        let pieces = [new Array(8), new Array(8)];
        // Shuffle pieces on first and last rank
        for (let c = 0; c <= 1; c++)
        {
            let positions = _.range(8);

            // Get random squares for bishops
            let randIndex = 2 * _.random(3);
            let bishop1Pos = positions[randIndex];
            // The second bishop must be on a square of different color
            let randIndex_tmp = 2 * _.random(3) + 1;
            let bishop2Pos = positions[randIndex_tmp];
            // Remove chosen squares
            positions.splice(Math.max(randIndex,randIndex_tmp), 1);
            positions.splice(Math.min(randIndex,randIndex_tmp), 1);

            // Get random squares for knights
            randIndex = _.random(5);
            let knight1Pos = positions[randIndex];
            positions.splice(randIndex, 1);
            randIndex = _.random(4);
            let knight2Pos = positions[randIndex];
            positions.splice(randIndex, 1);

            // Get random square for queen
            randIndex = _.random(3);
            let queenPos = positions[randIndex];
            positions.splice(randIndex, 1);

            // Rooks and king positions are now fixed, because of the ordering rook-king-rook
            let rook1Pos = positions[0];
            let kingPos = positions[1];
            let rook2Pos = positions[2];

            // Finally put the shuffled pieces in the board array
            pieces[c][rook1Pos] = 'r';
            pieces[c][knight1Pos] = 'n';
            pieces[c][bishop1Pos] = 'b';
            pieces[c][queenPos] = 'q';
            pieces[c][kingPos] = 'k';
            pieces[c][bishop2Pos] = 'b';
            pieces[c][knight2Pos] = 'n';
            pieces[c][rook2Pos] = 'r';
        }
        let fen = pieces[0].join("") +
            "/pppppppp/8/8/8/8/PPPPPPPP/" +
            pieces[1].join("").toUpperCase() +
            " 1111"; //add flags
        return fen;
    }

    // Return current fen according to pieces+colors state
    getFen()
    {
        return this.getBaseFen() + " " + this.getFlagsFen();
    }

    getBaseFen()
    {
        let fen = "";
        let [sizeX,sizeY] = VariantRules.size;
        for (let i=0; i<sizeX; i++)
        {
            let emptyCount = 0;
            for (let j=0; j<sizeY; j++)
            {
                if (this.board[i][j] == VariantRules.EMPTY)
                    emptyCount++;
                else
                {
                    if (emptyCount > 0)
                    {
                        // Add empty squares in-between
                        fen += emptyCount;
                        emptyCount = 0;
                    }
                    fen += VariantRules.board2fen(this.board[i][j]);
                }
            }
            if (emptyCount > 0)
            {
                // "Flush remainder"
                fen += emptyCount;
            }
            if (i < sizeX - 1)
                fen += "/"; //separate rows
        }
        return fen;
    }

    // Overridable..
    getFlagsFen()
    {
        let fen = "";
        // Add castling flags
        for (let i of ['w','b'])
        {
            for (let j=0; j<2; j++)
                fen += this.castleFlags[i][j] ? '1' : '0';
        }
        return fen;
    }

    // Context: just before move is played, turn hasn't changed
    getNotation(move)
    {
        if (move.appear.length == 2 && move.appear[0].p == VariantRules.KING)
        {
            // Castle
            if (move.end.y < move.start.y)
                return "0-0-0";
            else
                return "0-0";
        }

        // Translate final square
        const finalSquare =
            String.fromCharCode(97 + move.end.y) + (VariantRules.size[0]-move.end.x);

        const piece = this.getPiece(move.start.x, move.start.y);
        if (piece == VariantRules.PAWN)
        {
            // Pawn move
            let notation = "";
            if (move.vanish.length > move.appear.length)
            {
                // Capture
                const startColumn = String.fromCharCode(97 + move.start.y);
                notation = startColumn + "x" + finalSquare;
            }
            else //no capture
                notation = finalSquare;
            if (move.appear.length > 0 && piece != move.appear[0].p) //promotion
                notation += "=" + move.appear[0].p.toUpperCase();
            return notation;
        }

        else
        {
            // Piece movement
            return piece.toUpperCase() +
                (move.vanish.length > move.appear.length ? "x" : "") + finalSquare;
        }
    }

    // The score is already computed when calling this function
    getPGN(mycolor, score, fenStart, mode)
    {
        let pgn = "";
        pgn += '[Site "vchess.club"]<br>';
        const d = new Date();
        const opponent = mode=="human" ? "Anonymous" : "Computer";
        pgn += '[Variant "' + variant + '"]<br>';
        pgn += '[Date "' + d.getFullYear() + '-' + (d.getMonth()+1) + '-' + d.getDate() + '"]<br>';
        pgn += '[White "' + (mycolor=='w'?'Myself':opponent) + '"]<br>';
        pgn += '[Black "' + (mycolor=='b'?'Myself':opponent) + '"]<br>';
        pgn += '[Fen "' + fenStart + '"]<br>';
        pgn += '[Result "' + score + '"]<br><br>';

        for (let i=0; i<this.moves.length; i++)
        {
            if (i % 2 == 0)
                pgn += ((i/2)+1) + ".";
            pgn += this.moves[i].notation + " ";
        }

        pgn += score;
        return pgn;
    }
}