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

// (Orthodox) Chess rules are defined in ChessRules class.
// Variants generally inherit from it, and modify some parts.

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;
    }
}

// TODO: for animation, moves should contains "moving" and "fading" maybe...
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
{
    //////////////
    // MISC UTILS

    static get HasFlags() { return true; } //some variants don't have flags

    static get HasEnpassant() { return true; } //some variants don't have ep.

    // 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;
    }

    // Check if FEN describe a position
    static IsGoodFen(fen)
    {
        const fenParsed = V.ParseFen(fen);
        // 1) Check position
        if (!V.IsGoodPosition(fenParsed.position))
            return false;
        // 2) Check turn
        if (!fenParsed.turn || !V.IsGoodTurn(fenParsed.turn))
            return false;
        // 3) Check moves count
        if (!fenParsed.movesCount || !(parseInt(fenParsed.movesCount) >= 0))
            return false;
        // 4) Check flags
        if (V.HasFlags && (!fenParsed.flags || !V.IsGoodFlags(fenParsed.flags)))
            return false;
        // 5) Check enpassant
        if (V.HasEnpassant &&
            (!fenParsed.enpassant || !V.IsGoodEnpassant(fenParsed.enpassant)))
        {
            return false;
        }
        return true;
    }

    // Is position part of the FEN a priori correct?
    static IsGoodPosition(position)
    {
        if (position.length == 0)
            return false;
        const rows = position.split("/");
        if (rows.length != V.size.x)
            return false;
        for (let row of rows)
        {
            let sumElts = 0;
            for (let i=0; i<row.length; i++)
            {
                if (V.PIECES.includes(row[i].toLowerCase()))
                    sumElts++;
                else
                {
                    const num = parseInt(row[i]);
                    if (isNaN(num))
                        return false;
                    sumElts += num;
                }
            }
            if (sumElts != V.size.y)
                return false;
        }
        return true;
    }

    // For FEN checking
    static IsGoodTurn(turn)
    {
        return ["w","b"].includes(turn);
    }

    // For FEN checking
    static IsGoodFlags(flags)
    {
        return !!flags.match(/^[01]{4,4}$/);
    }

    static IsGoodEnpassant(enpassant)
    {
        if (enpassant != "-")
        {
            const ep = V.SquareToCoords(fenParsed.enpassant);
            if (isNaN(ep.x) || !V.OnBoard(ep))
                return false;
        }
        return true;
    }

    // 3 --> d (column number to letter)
    static CoordToColumn(colnum)
    {
        return String.fromCharCode(97 + colnum);
    }

    // d --> 3 (column letter to number)
    static ColumnToCoord(column)
    {
        return column.charCodeAt(0) - 97;
    }

    // a4 --> {x:3,y:0}
    static SquareToCoords(sq)
    {
        return {
            // NOTE: column is always one char => max 26 columns
            // row is counted from black side => subtraction
            x: V.size.x - parseInt(sq.substr(1)),
            y: sq[0].charCodeAt() - 97
        };
    }

    // {x:0,y:4} --> e8
    static CoordsToSquare(coords)
    {
        return V.CoordToColumn(coords.y) + (V.size.x - coords.x);
    }

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

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

    // En-passant square, if any
    getEpSquare(moveOrSquare)
    {
        if (!moveOrSquare)
            return undefined;
        if (typeof moveOrSquare === "string")
        {
            const square = moveOrSquare;
            if (square == "-")
                return undefined;
            return V.SquareToCoords(square);
        }
        // Argument is a move:
        const move = moveOrSquare;
        const [sx,sy,ex] = [move.start.x,move.start.y,move.end.x];
        // TODO: next conditions are first for Atomic, and third for Checkered
        if (move.appear.length > 0 && move.appear[0].p == V.PAWN && ["w","b"].includes(move.appear[0].c) && 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);
    }

    // Is (x,y) on the chessboard?
    static OnBoard(x,y)
    {
        return (x>=0 && x<V.size.x && y>=0 && y<V.size.y);
    }

    // Used in interface: 'side' arg == player color
    canIplay(side, [x,y])
    {
        return (this.turn == side && this.getColor(x,y) == side);
    }

    // On which squares is color under check ? (for interface)
    getCheckSquares(color)
    {
        return this.isAttacked(this.kingPos[color], [V.GetOppCol(color)])
            ? [JSON.parse(JSON.stringify(this.kingPos[color]))] //need to duplicate!
            : [];
    }

    /////////////
    // FEN UTILS

    // Setup the initial random (assymetric) position
    static GenRandInitFen()
    {
        let pieces = { "w": new Array(8), "b": new Array(8) };
        // Shuffle pieces on first and last rank
        for (let c of ["w","b"])
        {
            let positions = _.range(8);

            // Get random squares for bishops
            let randIndex = 2 * _.random(3);
            const bishop1Pos = positions[randIndex];
            // The second bishop must be on a square of different color
            let randIndex_tmp = 2 * _.random(3) + 1;
            const 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);
            const knight1Pos = positions[randIndex];
            positions.splice(randIndex, 1);
            randIndex = _.random(4);
            const knight2Pos = positions[randIndex];
            positions.splice(randIndex, 1);

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

            // Rooks and king positions are now fixed,
            // because of the ordering rook-king-rook
            const rook1Pos = positions[0];
            const kingPos = positions[1];
            const 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';
        }
        return pieces["b"].join("") +
            "/pppppppp/8/8/8/8/PPPPPPPP/" +
            pieces["w"].join("").toUpperCase() +
            " w 0 1111 -"; //add turn + flags + enpassant
    }

    // "Parse" FEN: just return untransformed string data
    static ParseFen(fen)
    {
        const fenParts = fen.split(" ");
        let res =
        {
            position: fenParts[0],
            turn: fenParts[1],
            movesCount: fenParts[2],
        };
        let nextIdx = 3;
        if (V.HasFlags)
            Object.assign(res, {flags: fenParts[nextIdx++]});
        if (V.HasEnpassant)
            Object.assign(res, {enpassant: fenParts[nextIdx]});
        return res;
    }

    // Return current fen (game state)
    getFen()
    {
        return this.getBaseFen() + " " +
            this.getTurnFen() + " " + this.movesCount +
            (V.HasFlags ? (" " + this.getFlagsFen()) : "") +
            (V.HasEnpassant ? (" " + this.getEnpassantFen()) : "");
    }

    // Position part of the FEN string
    getBaseFen()
    {
        let position = "";
        for (let i=0; i<V.size.x; i++)
        {
            let emptyCount = 0;
            for (let j=0; j<V.size.y; j++)
            {
                if (this.board[i][j] == V.EMPTY)
                    emptyCount++;
                else
                {
                    if (emptyCount > 0)
                    {
                        // Add empty squares in-between
                        position += emptyCount;
                        emptyCount = 0;
                    }
                    position += V.board2fen(this.board[i][j]);
                }
            }
            if (emptyCount > 0)
            {
                // "Flush remainder"
                position += emptyCount;
            }
            if (i < V.size.x - 1)
                position += "/"; //separate rows
        }
        return position;
    }

    getTurnFen()
    {
        return this.turn;
    }

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

    // Enpassant part of the FEN string
    getEnpassantFen()
    {
        const L = this.epSquares.length;
        if (!this.epSquares[L-1])
            return "-"; //no en-passant
        return V.CoordsToSquare(this.epSquares[L-1]);
    }

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

    // Extract (relevant) flags from fen
    setFlags(fenflags)
    {
        // white a-castle, h-castle, black a-castle, h-castle
        this.castleFlags = {'w': [true,true], 'b': [true,true]};
        if (!fenflags)
            return;
        for (let i=0; i<4; i++)
            this.castleFlags[i < 2 ? 'w' : 'b'][i%2] = (fenflags.charAt(i) == '1');
    }

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

    // Fen string fully describes the game state
    constructor(fen)
    {
        const fenParsed = V.ParseFen(fen);
        this.board = V.GetBoard(fenParsed.position);
        this.turn = fenParsed.turn[0]; //[0] to work with MarseilleRules
        this.movesCount = parseInt(fenParsed.movesCount);
        this.setOtherVariables(fen);
    }

    // Scan board for kings and rooks positions
    scanKingsRooks(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]}; //squares of white and black king
        const fenRows = V.ParseFen(fen).position.split("/");
        for (let i=0; i<fenRows.length; i++)
        {
            let k = 0; //column index on board
            for (let j=0; j<fenRows[i].length; j++)
            {
                switch (fenRows[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:
                        const num = parseInt(fenRows[i].charAt(j));
                        if (!isNaN(num))
                            k += (num-1);
                }
                k++;
            }
        }
    }

    // Some additional variables from FEN (variant dependant)
    setOtherVariables(fen)
    {
        // Set flags and enpassant:
        const parsedFen = V.ParseFen(fen);
        if (V.HasFlags)
            this.setFlags(parsedFen.flags);
        if (V.HasEnpassant)
        {
            const epSq = parsedFen.enpassant != "-"
                ? V.SquareToCoords(parsedFen.enpassant)
                : undefined;
            this.epSquares = [ epSq ];
        }
        // Search for king and rooks positions:
        this.scanKingsRooks(fen);
    }

    /////////////////////
    // GETTERS & SETTERS

    static get size()
    {
        return {x:8, y:8};
    }

    // Color of thing on suqare (i,j). 'undefined' if square is empty
    getColor(i,j)
    {
        return this.board[i][j].charAt(0);
    }

    // Piece type on square (i,j). 'undefined' if square is empty
    getPiece(i,j)
    {
        return this.board[i][j].charAt(1);
    }

    // Get opponent color
    static GetOppCol(color)
    {
        return (color=="w" ? "b" : "w");
    }

    // Get next color (for compatibility with 3 and 4 players games)
    static GetNextCol(color)
    {
        return V.GetOppCol(color);
    }

    // Pieces codes (for a clearer code)
    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'; }

    // For FEN checking:
    static get PIECES()
    {
        return [V.PAWN,V.ROOK,V.KNIGHT,V.BISHOP,V.QUEEN,V.KING];
    }

    // 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] ],
        };
    }

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

    // All possible moves from selected square (assumption: color is OK)
    getPotentialMovesFrom([x,y])
    {
        switch (this.getPiece(x,y))
        {
            case V.PAWN:
                return this.getPotentialPawnMoves([x,y]);
            case V.ROOK:
                return this.getPotentialRookMoves([x,y]);
            case V.KNIGHT:
                return this.getPotentialKnightMoves([x,y]);
            case V.BISHOP:
                return this.getPotentialBishopMoves([x,y]);
            case V.QUEEN:
                return this.getPotentialQueenMoves([x,y]);
            case V.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] != V.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 = [];
        outerLoop:
        for (let step of steps)
        {
            let i = x + step[0];
            let j = y + step[1];
            while (V.OnBoard(i,j) && this.board[i][j] == V.EMPTY)
            {
                moves.push(this.getBasicMove([x,y], [i,j]));
                if (oneStep !== undefined)
                    continue outerLoop;
                i += step[0];
                j += step[1];
            }
            if (V.OnBoard(i,j) && 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 ?
    getPotentialPawnMoves([x,y])
    {
        const color = this.turn;
        let moves = [];
        const [sizeX,sizeY] = [V.size.x,V.size.y];
        const shiftX = (color == "w" ? -1 : 1);
        const firstRank = (color == 'w' ? sizeX-1 : 0);
        const startRank = (color == "w" ? sizeX-2 : 1);
        const lastRank = (color == "w" ? 0 : sizeX-1);
        const pawnColor = this.getColor(x,y); //can be different for checkered

        // NOTE: next condition is generally true (no pawn on last rank)
        if (x+shiftX >= 0 && x+shiftX < sizeX)
        {
            const finalPieces = x + shiftX == lastRank
                ? [V.ROOK,V.KNIGHT,V.BISHOP,V.QUEEN]
                : [V.PAWN]
            // One square forward
            if (this.board[x+shiftX][y] == V.EMPTY)
            {
                for (let piece of finalPieces)
                {
                    moves.push(this.getBasicMove([x,y], [x+shiftX,y],
                        {c:pawnColor,p:piece}));
                }
                // Next condition because pawns on 1st rank can generally jump
                if ([startRank,firstRank].includes(x)
                    && this.board[x+2*shiftX][y] == V.EMPTY)
                {
                    // Two squares jump
                    moves.push(this.getBasicMove([x,y], [x+2*shiftX,y]));
                }
            }
            // Captures
            for (let shiftY of [-1,1])
            {
                if (y + shiftY >= 0 && y + shiftY < sizeY
                    && this.board[x+shiftX][y+shiftY] != V.EMPTY
                    && this.canTake([x,y], [x+shiftX,y+shiftY]))
                {
                    for (let piece of finalPieces)
                    {
                        moves.push(this.getBasicMove([x,y], [x+shiftX,y+shiftY],
                            {c:pawnColor,p:piece}));
                    }
                }
            }
        }

        if (V.HasEnpassant)
        {
            // En passant
            const Lep = this.epSquares.length;
            const epSquare = this.epSquares[Lep-1]; //always at least one element
            if (!!epSquare && epSquare.x == x+shiftX && Math.abs(epSquare.y - y) == 1)
            {
                let enpassantMove = this.getBasicMove([x,y], [epSquare.x,epSquare.y]);
                enpassantMove.vanish.push({
                    x: x,
                    y: epSquare.y,
                    p: 'p',
                    c: this.getColor(x,epSquare.y)
                });
                moves.push(enpassantMove);
            }
        }

        return moves;
    }

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

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

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

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

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

    getCastleMoves([x,y])
    {
        const c = this.getColor(x,y);
        if (x != (c=="w" ? V.size.x-1 : 0) || y != this.INIT_COL_KING[c])
            return []; //x isn't first rank, or king has moved (shortcut)

        // Castling ?
        const oppCol = V.GetOppCol(c);
        let moves = [];
        let i = 0;
        const finalSquares = [ [2,3], [V.size.y-2,V.size.y-3] ]; //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

    // For the interface: possible moves for the current turn from square sq
    getPossibleMovesFrom(sq)
    {
        return this.filterValid( this.getPotentialMovesFrom(sq) );
    }

    // TODO: promotions (into R,B,N,Q) should be filtered only once
    filterValid(moves)
    {
        if (moves.length == 0)
            return [];
        const color = this.turn;
        return moves.filter(m => {
            this.play(m);
            const res = !this.underCheck(color);
            this.undo(m);
            return res;
        });
    }

    // Search for all valid moves considering current turn
    // (for engine and game end)
    getAllValidMoves()
    {
        const color = this.turn;
        const oppCol = V.GetOppCol(color);
        let potentialMoves = [];
        for (let i=0; i<V.size.x; i++)
        {
            for (let j=0; j<V.size.y; j++)
            {
                // Next condition "!= oppCol" to work with checkered variant
                if (this.board[i][j] != V.EMPTY && this.getColor(i,j) != oppCol)
                {
                    Array.prototype.push.apply(potentialMoves,
                        this.getPotentialMovesFrom([i,j]));
                }
            }
        }
        return this.filterValid(potentialMoves);
    }

    // Stop at the first move found
    atLeastOneMove()
    {
        const color = this.turn;
        const oppCol = V.GetOppCol(color);
        for (let i=0; i<V.size.x; i++)
        {
            for (let j=0; j<V.size.y; j++)
            {
                if (this.board[i][j] != V.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 in 'colors' are attacking (king) on 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 'colors' pawns ?
    isAttackedByPawn([x,y], colors)
    {
        for (let c of colors)
        {
            let pawnShift = (c=="w" ? 1 : -1);
            if (x+pawnShift>=0 && x+pawnShift<V.size.x)
            {
                for (let i of [-1,1])
                {
                    if (y+i>=0 && y+i<V.size.y && this.getPiece(x+pawnShift,y+i)==V.PAWN
                        && this.getColor(x+pawnShift,y+i)==c)
                    {
                        return true;
                    }
                }
            }
        }
        return false;
    }

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

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

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

    // Is square x,y attacked by 'colors' queens ?
    isAttackedByQueen(sq, colors)
    {
        return this.isAttackedBySlideNJump(sq, colors, V.QUEEN,
            V.steps[V.ROOK].concat(V.steps[V.BISHOP]));
    }

    // Is square x,y attacked by 'colors' king(s) ?
    isAttackedByKing(sq, colors)
    {
        return this.isAttackedBySlideNJump(sq, colors, V.KING,
            V.steps[V.ROOK].concat(V.steps[V.BISHOP]), "oneStep");
    }

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

    // Is color under check after his move ?
    underCheck(color)
    {
        return this.isAttacked(this.kingPos[color], [V.GetOppCol(color)]);
    }

    /////////////////
    // MOVES PLAYING

    // Apply a move on board
    static PlayOnBoard(board, move)
    {
        for (let psq of move.vanish)
            board[psq.x][psq.y] = V.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] = V.EMPTY;
        for (let psq of move.vanish)
            board[psq.x][psq.y] = psq.c + psq.p;
    }

    // After move is played, update variables + flags
    updateVariables(move)
    {
        let piece = undefined;
        let c = undefined;
        if (move.vanish.length >= 1)
        {
            // Usual case, something is moved
            piece = move.vanish[0].p;
            c = move.vanish[0].c;
        }
        else
        {
            // Crazyhouse-like variants
            piece = move.appear[0].p;
            c = move.appear[0].c;
        }
        if (c == "c") //if (!["w","b"].includes(c))
        {
            // 'c = move.vanish[0].c' doesn't work for Checkered
            c = V.GetOppCol(this.turn);
        }
        const firstRank = (c == "w" ? V.size.x-1 : 0);

        // Update king position + flags
        if (piece == V.KING && move.appear.length > 0)
        {
            this.kingPos[c][0] = move.appear[0].x;
            this.kingPos[c][1] = move.appear[0].y;
            if (V.HasFlags)
                this.castleFlags[c] = [false,false];
            return;
        }
        if (V.HasFlags)
        {
            // Update castling flags if rooks are moved
            const oppCol = V.GetOppCol(c);
            const oppFirstRank = (V.size.x-1) - 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;
            }
        }
    }

    // After move is undo-ed *and flags resetted*, un-update other variables
    // TODO: more symmetry, by storing flags increment in move (?!)
    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) == V.KING)
            this.kingPos[c] = [move.start.x, move.start.y];
    }

    play(move)
    {
        // DEBUG:
//      if (!this.states) this.states = [];
//      const stateFen = this.getBaseFen() + this.getTurnFen() + this.getFlagsFen();
//      this.states.push(stateFen);

        if (V.HasFlags)
            move.flags = JSON.stringify(this.aggregateFlags()); //save flags (for undo)
        if (V.HasEnpassant)
            this.epSquares.push( this.getEpSquare(move) );
        if (!move.color)
            move.color = this.turn; //for interface
        V.PlayOnBoard(this.board, move);
        this.turn = V.GetOppCol(this.turn);
        this.movesCount++;
        this.updateVariables(move);
    }

    undo(move)
    {
        if (V.HasEnpassant)
            this.epSquares.pop();
        if (V.HasFlags)
            this.disaggregateFlags(JSON.parse(move.flags));
        V.UndoOnBoard(this.board, move);
        this.turn = V.GetOppCol(this.turn);
        this.movesCount--;
        this.unupdateVariables(move);

        // DEBUG:
//      const stateFen = this.getBaseFen() + this.getTurnFen() + this.getFlagsFen();
//      if (stateFen != this.states[this.states.length-1]) debugger;
//      this.states.pop();
    }

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

    // What is the score ? (Interesting if game is over)
    getCurrentScore()
    {
        if (this.atLeastOneMove()) // game not over
            return "*";

        // Game over
        const color = this.turn;
        // No valid move: stalemate or checkmate?
        if (!this.isAttacked(this.kingPos[color], [V.GetOppCol(color)]))
            return "1/2";
        // OK, checkmate
        return (color == "w" ? "0-1" : "1-0");
    }

    ///////////////
    // ENGINE PLAY

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

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

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

    // Search depth: 2 for high branching factor, 4 for small (Loser chess, eg.)
    static get SEARCH_DEPTH() { return 3; }

    // Assumption: at least one legal move
    // NOTE: works also for extinction chess because depth is 3...
    getComputerMove()
    {
        const maxeval = V.INFINITY;
        const color = this.turn;
        // Some variants may show a bigger moves list to the human (Switching),
        // thus the argument "computer" below (which is generally ignored)
        let moves1 = this.getAllValidMoves("computer");

        // Can I mate in 1 ? (for Magnetic & Extinction)
        for (let i of _.shuffle(_.range(moves1.length)))
        {
            this.play(moves1[i]);
            let finish = (Math.abs(this.evalPosition()) >= V.THRESHOLD_MATE);
            if (!finish)
            {
                const score = this.getCurrentScore();
                if (["1-0","0-1"].includes(score))
                    finish = true;
            }
            this.undo(moves1[i]);
            if (finish)
                return moves1[i];
        }

        // Rank moves using a min-max at depth 2
        for (let i=0; i<moves1.length; i++)
        {
            // Initial self evaluation is very low: "I'm checkmated"
            moves1[i].eval = (color=="w" ? -1 : 1) * maxeval;
            this.play(moves1[i]);
            const score1 = this.getCurrentScore();
            let eval2 = undefined;
            if (score1 == "*")
            {
                // Initial enemy evaluation is very low too, for him
                eval2 = (color=="w" ? 1 : -1) * maxeval;
                // Second half-move:
                let moves2 = this.getAllValidMoves("computer");
                for (let j=0; j<moves2.length; j++)
                {
                    this.play(moves2[j]);
                    const score2 = this.getCurrentScore();
                    const evalPos = score2 == "*"
                        ? this.evalPosition()
                        : (score2=="1/2" ? 0 : (score2=="1-0" ? 1 : -1) * maxeval);
                    if ((color == "w" && evalPos < eval2)
                        || (color=="b" && evalPos > eval2))
                    {
                        eval2 = evalPos;
                    }
                    this.undo(moves2[j]);
                }
            }
            else
                eval2 = (score1=="1/2" ? 0 : (score1=="1-0" ? 1 : -1) * maxeval);
            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); });

        let candidates = [0]; //indices of candidates moves
        for (let j=1; j<moves1.length && moves1[j].eval == moves1[0].eval; j++)
            candidates.push(j);
        let currentBest = moves1[_.sample(candidates, 1)];

        // From here, depth >= 3: may take a while, so we control time
        const timeStart = Date.now();

        // Skip depth 3+ if we found a checkmate (or if we are checkmated in 1...)
        if (V.SEARCH_DEPTH >= 3 && Math.abs(moves1[0].eval) < V.THRESHOLD_MATE)
        {
            for (let i=0; i<moves1.length; i++)
            {
                if (Date.now()-timeStart >= 5000) //more than 5 seconds
                    return currentBest; //depth 2 at least
                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(V.SEARCH_DEPTH-1, -maxeval, maxeval);
                this.undo(moves1[i]);
            }
            moves1.sort( (a,b) => {
                return (color=="w" ? 1 : -1) * (b.eval - a.eval); });
        }
        else
            return currentBest;
//      console.log(moves1.map(m => { return [this.getNotation(m), m.eval]; }));

        candidates = [0];
        for (let j=1; j<moves1.length && moves1[j].eval == moves1[0].eval; j++)
            candidates.push(j);
        return moves1[_.sample(candidates, 1)];
    }

    alphabeta(depth, alpha, beta)
  {
        const maxeval = V.INFINITY;
        const color = this.turn;
        const score = this.getCurrentScore();
        if (score != "*")
            return (score=="1/2" ? 0 : (score=="1-0" ? 1 : -1) * maxeval);
        if (depth == 0)
      return this.evalPosition();
        const moves = this.getAllValidMoves("computer");
    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()
    {
        let evaluation = 0;
        // Just count material for now
        for (let i=0; i<V.size.x; i++)
        {
            for (let j=0; j<V.size.y; j++)
            {
                if (this.board[i][j] != V.EMPTY)
                {
                    const sign = this.getColor(i,j) == "w" ? 1 : -1;
                    evaluation += sign * V.VALUES[this.getPiece(i,j)];
                }
            }
        }
        return evaluation;
    }

    /////////////////////////
    // MOVES + GAME NOTATION
    /////////////////////////

    // Context: just before move is played, turn hasn't changed
    // TODO: un-ambiguous notation (switch on piece type, check directions...)
    getNotation(move)
    {
        if (move.appear.length == 2 && move.appear[0].p == V.KING) //castle
            return (move.end.y < move.start.y ? "0-0-0" : "0-0");

        // Translate final square
        const finalSquare = V.CoordsToSquare(move.end);

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

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