Fixing computer play

[vchess.git] / client / src / base_rules.js

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

import { ArrayFun } from "@/utils/array";
import { random, sample, shuffle } from "@/utils/alea";

export const PiPo = 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...
export const Move = 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)
export const ChessRules = 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 last for Checkered
    if (move.appear.length > 0 && Math.abs(sx - ex) == 2
      && move.appear[0].p == V.PAWN && ["w","b"].includes(move.appear[0].c))
    {
      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 = ArrayFun.range(8);

      // Get random squares for bishops
      let randIndex = 2 * random(4);
      const bishop1Pos = positions[randIndex];
      // The second bishop must be on a square of different color
      let randIndex_tmp = 2 * random(4) + 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(6);
      const knight1Pos = positions[randIndex];
      positions.splice(randIndex, 1);
      randIndex = random(5);
      const knight2Pos = positions[randIndex];
      positions.splice(randIndex, 1);

      // Get random square for queen
      randIndex = random(4);
      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 = ArrayFun.init(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(ArrayFun.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)];

    // 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)];
  }

  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;
    }
  }
}