client/src/variants/Dynamo.js

   1 import { ChessRules, Move, PiPo } from "@/base_rules";
   2 import { randInt } from "@/utils/alea";
   3
   4 export class DynamoRules extends ChessRules {
   5   // TODO? later, allow to push out pawns on a and h files
   6   static get HasEnpassant() {
   7     return false;
   8   }
   9
  10   canIplay(side, [x, y]) {
  11     // Sometimes opponent's pieces can be moved directly
  12     return this.turn == side;
  13   }
  14
  15   setOtherVariables(fen) {
  16     super.setOtherVariables(fen);
  17     this.subTurn = 1;
  18     // Local stack of "action moves"
  19     this.amoves = [];
  20     const amove = V.ParseFen(fen).amove;
  21     if (amove != "-") {
  22       const amoveParts = amove.split("/");
  23       let move = {
  24         // No need for start & end
  25         appear: [],
  26         vanish: []
  27       };
  28       [0, 1].map(i => {
  29         if (amoveParts[i] != "-") {
  30           amoveParts[i].split(".").forEach(av => {
  31             // Format is "bpe3"
  32             const xy = V.SquareToCoords(av.substr(2));
  33             move[i == 0 ? "appear" : "vanish"].push(
  34               new PiPo({
  35                 x: xy.x,
  36                 y: xy.y,
  37                 c: av[0],
  38                 p: av[1]
  39               })
  40             );
  41           });
  42         }
  43       });
  44       this.amoves.push(move);
  45     }
  46     // Stack "first moves" (on subTurn 1) to merge and check opposite moves
  47     this.firstMove = [];
  48   }
  49
  50   static ParseFen(fen) {
  51     return Object.assign(
  52       ChessRules.ParseFen(fen),
  53       { amove: fen.split(" ")[4] }
  54     );
  55   }
  56
  57   static IsGoodFen(fen) {
  58     if (!ChessRules.IsGoodFen(fen)) return false;
  59     const fenParts = fen.split(" ");
  60     if (fenParts.length != 5) return false;
  61     if (fenParts[4] != "-") {
  62       // TODO: a single regexp instead.
  63       // Format is [bpa2[.wpd3]] || '-'/[bbc3[.wrd5]] || '-'
  64       const amoveParts = fenParts[4].split("/");
  65       if (amoveParts.length != 2) return false;
  66       for (let part of amoveParts) {
  67         if (part != "-") {
  68           for (let psq of part.split("."))
  69             if (!psq.match(/^[a-r]{3}[1-8]$/)) return false;
  70         }
  71       }
  72     }
  73     return true;
  74   }
  75
  76   getFen() {
  77     return super.getFen() + " " + this.getAmoveFen();
  78   }
  79
  80   getFenForRepeat() {
  81     return super.getFenForRepeat() + "_" + this.getAmoveFen();
  82   }
  83
  84   getAmoveFen() {
  85     const L = this.amoves.length;
  86     if (L == 0) return "-";
  87     return (
  88       ["appear","vanish"].map(
  89         mpart => {
  90           if (this.amoves[L-1][mpart].length == 0) return "-";
  91           return (
  92             this.amoves[L-1][mpart].map(
  93               av => {
  94                 const square = V.CoordsToSquare({ x: av.x, y: av.y });
  95                 return av.c + av.p + square;
  96               }
  97             ).join(".")
  98           );
  99         }
 100       ).join("/")
 101     );
 102   }
 103
 104   canTake() {
 105     // Captures don't occur (only pulls & pushes)
 106     return false;
 107   }
 108
 109   // Step is right, just add (push/pull) moves in this direction
 110   // Direction is assumed normalized.
 111   getMovesInDirection([x, y], [dx, dy], nbSteps) {
 112     nbSteps = nbSteps || 8; //max 8 steps anyway
 113     let [i, j] = [x + dx, y + dy];
 114     let moves = [];
 115     const color = this.getColor(x, y);
 116     const piece = this.getPiece(x, y);
 117     const lastRank = (color == 'w' ? 0 : 7);
 118     let counter = 1;
 119     while (V.OnBoard(i, j) && this.board[i][j] == V.EMPTY) {
 120       if (i == lastRank && piece == V.PAWN) {
 121         // Promotion by push or pull
 122         V.PawnSpecs.promotions.forEach(p => {
 123           let move = super.getBasicMove([x, y], [i, j], { c: color, p: p });
 124           moves.push(move);
 125         });
 126       }
 127       else moves.push(super.getBasicMove([x, y], [i, j]));
 128       if (++counter > nbSteps) break;
 129       i += dx;
 130       j += dy;
 131     }
 132     if (!V.OnBoard(i, j) && piece != V.KING) {
 133       // Add special "exit" move, by "taking king"
 134       moves.push(
 135         new Move({
 136           start: { x: x, y: y },
 137           end: { x: this.kingPos[color][0], y: this.kingPos[color][1] },
 138           appear: [],
 139           vanish: [{ x: x, y: y, c: color, p: piece }]
 140         })
 141       );
 142     }
 143     return moves;
 144   }
 145
 146   // Normalize direction to know the step
 147   getNormalizedDirection([dx, dy]) {
 148     const absDir = [Math.abs(dx), Math.abs(dy)];
 149     let divisor = 0;
 150     if (absDir[0] != 0 && absDir[1] != 0 && absDir[0] != absDir[1])
 151       // Knight
 152       divisor = Math.min(absDir[0], absDir[1]);
 153     else
 154       // Standard slider (or maybe a pawn or king: same)
 155       divisor = Math.max(absDir[0], absDir[1]);
 156     return [dx / divisor, dy / divisor];
 157   }
 158
 159   // There was something on x2,y2, maybe our color, pushed or (self)pulled
 160   isAprioriValidExit([x1, y1], [x2, y2], color2, piece2) {
 161     const color1 = this.getColor(x1, y1);
 162     const pawnShift = (color1 == 'w' ? -1 : 1);
 163     const lastRank = (color1 == 'w' ? 0 : 7);
 164     const deltaX = Math.abs(x1 - x2);
 165     const deltaY = Math.abs(y1 - y2);
 166     const checkSlider = () => {
 167       const dir = this.getNormalizedDirection([x2 - x1, y2 - y1]);
 168       let [i, j] = [x1 + dir[0], y1 + dir[1]];
 169       while (V.OnBoard(i, j) && this.board[i][j] == V.EMPTY) {
 170         i += dir[0];
 171         j += dir[1];
 172       }
 173       return !V.OnBoard(i, j);
 174     };
 175     switch (piece2 || this.getPiece(x1, y1)) {
 176       case V.PAWN:
 177         return (
 178           x1 + pawnShift == x2 &&
 179           (
 180             (color1 == color2 && x2 == lastRank && y1 == y2) ||
 181             (
 182               color1 != color2 &&
 183               deltaY == 1 &&
 184               !V.OnBoard(2 * x2 - x1, 2 * y2 - y1)
 185             )
 186           )
 187         );
 188       case V.ROOK:
 189         if (x1 != x2 && y1 != y2) return false;
 190         return checkSlider();
 191       case V.KNIGHT:
 192         return (
 193           deltaX + deltaY == 3 &&
 194           (deltaX == 1 || deltaY == 1) &&
 195           !V.OnBoard(2 * x2 - x1, 2 * y2 - y1)
 196         );
 197       case V.BISHOP:
 198         if (deltaX != deltaY) return false;
 199         return checkSlider();
 200       case V.QUEEN:
 201         if (deltaX != 0 && deltaY != 0 && deltaX != deltaY) return false;
 202         return checkSlider();
 203       case V.KING:
 204         return (
 205           deltaX <= 1 &&
 206           deltaY <= 1 &&
 207           !V.OnBoard(2 * x2 - x1, 2 * y2 - y1)
 208         );
 209     }
 210     return false;
 211   }
 212
 213   isAprioriValidVertical([x1, y1], x2) {
 214     const piece = this.getPiece(x1, y1);
 215     const deltaX = Math.abs(x1 - x2);
 216     const startRank = (this.getColor(x1, y1) == 'w' ? 6 : 1);
 217     return (
 218       [V.QUEEN, V.ROOK].includes(piece) ||
 219       (
 220         [V.KING, V.PAWN].includes(piece) &&
 221         (
 222           deltaX == 1 ||
 223           (deltaX == 2 && piece == V.PAWN && x1 == startRank)
 224         )
 225       )
 226     );
 227   }
 228
 229   // NOTE: for pushes, play the pushed piece first.
 230   //       for pulls: play the piece doing the action first
 231   // NOTE: to push a piece out of the board, make it slide until its king
 232   getPotentialMovesFrom([x, y]) {
 233     const color = this.turn;
 234     const sqCol = this.getColor(x, y);
 235     const pawnShift = (color == 'w' ? -1 : 1);
 236     const pawnStartRank = (color == 'w' ? 6 : 1);
 237     const getMoveHash = (m) => {
 238       return V.CoordsToSquare(m.start) + V.CoordsToSquare(m.end);
 239     };
 240     if (this.subTurn == 1) {
 241       const addMoves = (dir, nbSteps) => {
 242         const newMoves =
 243           this.getMovesInDirection([x, y], [-dir[0], -dir[1]], nbSteps)
 244           .filter(m => !movesHash[getMoveHash(m)]);
 245         newMoves.forEach(m => { movesHash[getMoveHash(m)] = true; });
 246         Array.prototype.push.apply(moves, newMoves);
 247       };
 248       // Free to play any move (if piece of my color):
 249       let moves =
 250         sqCol == color
 251           ? super.getPotentialMovesFrom([x, y])
 252           : [];
 253       // There may be several suicide moves: keep only one
 254       let hasExit = false;
 255       moves = moves.filter(m => {
 256         const suicide = (m.appear.length == 0);
 257         if (suicide) {
 258           if (hasExit) return false;
 259           hasExit = true;
 260         }
 261         return true;
 262       });
 263       // Structure to avoid adding moves twice (can be action & move)
 264       let movesHash = {};
 265       moves.forEach(m => { movesHash[getMoveHash(m)] = true; });
 266       // [x, y] is pushed by 'color'
 267       for (let step of V.steps[V.KNIGHT]) {
 268         const [i, j] = [x + step[0], y + step[1]];
 269         if (
 270           V.OnBoard(i, j) &&
 271           this.board[i][j] != V.EMPTY &&
 272           this.getColor(i, j) == color &&
 273           this.getPiece(i, j) == V.KNIGHT
 274         ) {
 275           addMoves(step, 1);
 276         }
 277       }
 278       for (let step of V.steps[V.ROOK].concat(V.steps[V.BISHOP])) {
 279         let [i, j] = [x + step[0], y + step[1]];
 280         while (V.OnBoard(i, j) && this.board[i][j] == V.EMPTY) {
 281           i += step[0];
 282           j += step[1];
 283         }
 284         if (
 285           V.OnBoard(i, j) &&
 286           this.board[i][j] != V.EMPTY &&
 287           this.getColor(i, j) == color
 288         ) {
 289           const deltaX = Math.abs(i - x);
 290           const deltaY = Math.abs(j - y);
 291           switch (this.getPiece(i, j)) {
 292             case V.PAWN:
 293               if (
 294                 (x - i) / deltaX == pawnShift &&
 295                 deltaX <= 2 &&
 296                 deltaY <= 1
 297               ) {
 298                 if (sqCol == color && deltaY == 0) {
 299                   // Pushed forward
 300                   const maxSteps = (i == pawnStartRank && deltaX == 1 ? 2 : 1);
 301                   addMoves(step, maxSteps);
 302                 }
 303                 else if (sqCol != color && deltaY == 1 && deltaX == 1)
 304                   // Pushed diagonally
 305                   addMoves(step, 1);
 306               }
 307               break;
 308             case V.ROOK:
 309               if (deltaX == 0 || deltaY == 0) addMoves(step);
 310               break;
 311             case V.BISHOP:
 312               if (deltaX == deltaY) addMoves(step);
 313               break;
 314             case V.QUEEN:
 315               // All steps are valid for a queen:
 316               addMoves(step);
 317               break;
 318             case V.KING:
 319               if (deltaX <= 1 && deltaY <= 1) addMoves(step, 1);
 320               break;
 321           }
 322         }
 323       }
 324       return moves;
 325     }
 326     // If subTurn == 2 then we should have a first move,
 327     // which restrict what we can play now: only in the first move direction
 328     const L = this.firstMove.length;
 329     const fm = this.firstMove[L-1];
 330     if (
 331       (fm.appear.length == 2 && fm.vanish.length == 2) ||
 332       (fm.vanish[0].c == sqCol && sqCol != color)
 333     ) {
 334       // Castle or again opponent color: no move playable then.
 335       return [];
 336     }
 337     const piece = this.getPiece(x, y);
 338     const getPushExit = () => {
 339       // Piece at subTurn 1 exited: can I have caused the exit?
 340       if (
 341         this.isAprioriValidExit(
 342           [x, y],
 343           [fm.start.x, fm.start.y],
 344           fm.vanish[0].c
 345         )
 346       ) {
 347         // Seems so:
 348         const dir = this.getNormalizedDirection(
 349           [fm.start.x - x, fm.start.y - y]);
 350         const nbSteps =
 351           [V.PAWN, V.KING, V.KNIGHT].includes(piece)
 352             ? 1
 353             : null;
 354         return this.getMovesInDirection([x, y], dir, nbSteps);
 355       }
 356       return [];
 357     }
 358     const getPushMoves = () => {
 359       // Piece from subTurn 1 is still on board:
 360       const dirM = this.getNormalizedDirection(
 361         [fm.end.x - fm.start.x, fm.end.y - fm.start.y]);
 362       const dir = this.getNormalizedDirection(
 363         [fm.start.x - x, fm.start.y - y]);
 364       // Normalized directions should match
 365       if (dir[0] == dirM[0] && dir[1] == dirM[1]) {
 366         // We don't know if first move is a pushed piece or normal move,
 367         // so still must check if the push is valid.
 368         const deltaX = Math.abs(fm.start.x - x);
 369         const deltaY = Math.abs(fm.start.y - y);
 370         switch (piece) {
 371           case V.PAWN:
 372             if (x == pawnStartRank) {
 373               if (
 374                 (fm.start.x - x) * pawnShift < 0 ||
 375                 deltaX >= 3 ||
 376                 deltaY >= 2 ||
 377                 (fm.vanish[0].c == color && deltaY > 0) ||
 378                 (fm.vanish[0].c != color && deltaY == 0) ||
 379                 Math.abs(fm.end.x - fm.start.x) > deltaX ||
 380                 fm.end.y - fm.start.y != fm.start.y - y
 381               ) {
 382                 return [];
 383               }
 384             }
 385             else {
 386               if (
 387                 fm.start.x - x != pawnShift ||
 388                 deltaY >= 2 ||
 389                 (fm.vanish[0].c == color && deltaY == 1) ||
 390                 (fm.vanish[0].c != color && deltaY == 0) ||
 391                 fm.end.x - fm.start.x != pawnShift ||
 392                 fm.end.y - fm.start.y != fm.start.y - y
 393               ) {
 394                 return [];
 395               }
 396             }
 397             break;
 398           case V.KNIGHT:
 399             if (
 400               (deltaX + deltaY != 3 || (deltaX == 0 && deltaY == 0)) ||
 401               (fm.end.x - fm.start.x != fm.start.x - x) ||
 402               (fm.end.y - fm.start.y != fm.start.y - y)
 403             ) {
 404               return [];
 405             }
 406             break;
 407           case V.KING:
 408             if (
 409               (deltaX >= 2 || deltaY >= 2) ||
 410               (fm.end.x - fm.start.x != fm.start.x - x) ||
 411               (fm.end.y - fm.start.y != fm.start.y - y)
 412             ) {
 413               return [];
 414             }
 415             break;
 416           case V.BISHOP:
 417             if (deltaX != deltaY) return [];
 418             break;
 419           case V.ROOK:
 420             if (deltaX != 0 && deltaY != 0) return [];
 421             break;
 422           case V.QUEEN:
 423             if (deltaX != deltaY && deltaX != 0 && deltaY != 0) return [];
 424             break;
 425         }
 426         // Nothing should stand between [x, y] and the square fm.start
 427         let [i, j] = [x + dir[0], y + dir[1]];
 428         while (
 429           (i != fm.start.x || j != fm.start.y) &&
 430           this.board[i][j] == V.EMPTY
 431         ) {
 432           i += dir[0];
 433           j += dir[1];
 434         }
 435         if (i == fm.start.x && j == fm.start.y)
 436           return this.getMovesInDirection([x, y], dir);
 437       }
 438       return [];
 439     }
 440     const getPullExit = () => {
 441       // Piece at subTurn 1 exited: can I be pulled?
 442       // Note: kings cannot suicide, so fm.vanish[0].p is not KING.
 443       // Could be PAWN though, if a pawn was pushed out of board.
 444       if (
 445         fm.vanish[0].p != V.PAWN && //pawns cannot pull
 446         this.isAprioriValidExit(
 447           [x, y],
 448           [fm.start.x, fm.start.y],
 449           fm.vanish[0].c,
 450           fm.vanish[0].p
 451         )
 452       ) {
 453         // Seems so:
 454         const dir = this.getNormalizedDirection(
 455           [fm.start.x - x, fm.start.y - y]);
 456         const nbSteps = (fm.vanish[0].p == V.KNIGHT ? 1 : null);
 457         return this.getMovesInDirection([x, y], dir, nbSteps);
 458       }
 459       return [];
 460     };
 461     const getPullMoves = () => {
 462       if (fm.vanish[0].p == V.PAWN)
 463         // pawns cannot pull
 464         return [];
 465       const dirM = this.getNormalizedDirection(
 466         [fm.end.x - fm.start.x, fm.end.y - fm.start.y]);
 467       const dir = this.getNormalizedDirection(
 468         [fm.start.x - x, fm.start.y - y]);
 469       // Normalized directions should match
 470       if (dir[0] == dirM[0] && dir[1] == dirM[1]) {
 471         // Am I at the right distance?
 472         const deltaX = Math.abs(x - fm.start.x);
 473         const deltaY = Math.abs(y - fm.start.y);
 474         if (
 475           (fm.vanish[0].p == V.KING && (deltaX > 1 || deltaY > 1)) ||
 476           (fm.vanish[0].p == V.KNIGHT &&
 477             (deltaX + deltaY != 3 || deltaX == 0 || deltaY == 0))
 478         ) {
 479           return [];
 480         }
 481         // Nothing should stand between [x, y] and the square fm.start
 482         let [i, j] = [x + dir[0], y + dir[1]];
 483         while (
 484           (i != fm.start.x || j != fm.start.y) &&
 485           this.board[i][j] == V.EMPTY
 486         ) {
 487           i += dir[0];
 488           j += dir[1];
 489         }
 490         if (i == fm.start.x && j == fm.start.y)
 491           return this.getMovesInDirection([x, y], dir);
 492       }
 493       return [];
 494     };
 495     if (fm.vanish[0].c != color) {
 496       // Only possible action is a push:
 497       if (fm.appear.length == 0) return getPushExit();
 498       return getPushMoves();
 499     }
 500     else if (sqCol != color) {
 501       // Only possible action is a pull, considering moving piece abilities
 502       if (fm.appear.length == 0) return getPullExit();
 503       return getPullMoves();
 504     }
 505     else {
 506       // My color + my color: both actions possible
 507       // Structure to avoid adding moves twice (can be action & move)
 508       let movesHash = {};
 509       if (fm.appear.length == 0) {
 510         const pushes = getPushExit();
 511         pushes.forEach(m => { movesHash[getMoveHash(m)] = true; });
 512         return (
 513           pushes.concat(getPullExit().filter(m => !movesHash[getMoveHash(m)]))
 514         );
 515       }
 516       const pushes = getPushMoves();
 517       pushes.forEach(m => { movesHash[getMoveHash(m)] = true; });
 518       return (
 519         pushes.concat(getPullMoves().filter(m => !movesHash[getMoveHash(m)]))
 520       );
 521     }
 522     return [];
 523   }
 524
 525   getSlideNJumpMoves([x, y], steps, oneStep) {
 526     let moves = [];
 527     const c = this.getColor(x, y);
 528     const piece = this.getPiece(x, y);
 529     outerLoop: for (let step of steps) {
 530       let i = x + step[0];
 531       let j = y + step[1];
 532       while (V.OnBoard(i, j) && this.board[i][j] == V.EMPTY) {
 533         moves.push(this.getBasicMove([x, y], [i, j]));
 534         if (oneStep) continue outerLoop;
 535         i += step[0];
 536         j += step[1];
 537       }
 538       if (V.OnBoard(i, j)) {
 539         if (this.canTake([x, y], [i, j]))
 540           moves.push(this.getBasicMove([x, y], [i, j]));
 541       }
 542       else {
 543         // Add potential board exit (suicide), except for the king
 544         if (piece != V.KING) {
 545           moves.push({
 546             start: { x: x, y: y},
 547             end: { x: this.kingPos[c][0], y: this.kingPos[c][1] },
 548             appear: [],
 549             vanish: [
 550               new PiPo({
 551                 x: x,
 552                 y: y,
 553                 c: c,
 554                 p: piece
 555               })
 556             ]
 557           });
 558         }
 559       }
 560     }
 561     return moves;
 562   }
 563
 564   // Does m2 un-do m1 ? (to disallow undoing actions)
 565   oppositeMoves(m1, m2) {
 566     const isEqual = (av1, av2) => {
 567       for (let av of av1) {
 568         const avInAv2 = av2.find(elt => {
 569           return (
 570             elt.x == av.x &&
 571             elt.y == av.y &&
 572             elt.c == av.c &&
 573             elt.p == av.p
 574           );
 575         });
 576         if (!avInAv2) return false;
 577       }
 578       return true;
 579     };
 580     // All appear and vanish arrays must have the same length
 581     const mL = m1.appear.length;
 582     return (
 583       m2.appear.length == mL &&
 584       m1.vanish.length == mL &&
 585       m2.vanish.length == mL &&
 586       isEqual(m1.appear, m2.vanish) &&
 587       isEqual(m1.vanish, m2.appear)
 588     );
 589   }
 590
 591   getAmove(move1, move2) {
 592     // Just merge (one is action one is move, one may be empty)
 593     return {
 594       appear: move1.appear.concat(move2.appear),
 595       vanish: move1.vanish.concat(move2.vanish)
 596     }
 597   }
 598
 599   filterValid(moves) {
 600     const color = this.turn;
 601     const La = this.amoves.length;
 602     if (this.subTurn == 1) {
 603       return moves.filter(m => {
 604         // A move is valid either if it doesn't result in a check,
 605         // or if a second move is possible to counter the check
 606         // (not undoing a potential move + action of the opponent)
 607         this.play(m);
 608         let res = this.underCheck(color);
 609         let isOpposite = La > 0 && this.oppositeMoves(this.amoves[La-1], m);
 610         if (res || isOpposite) {
 611           const moves2 = this.getAllPotentialMoves();
 612           for (let m2 of moves2) {
 613             this.play(m2);
 614             const res2 = this.underCheck(color);
 615             const amove = this.getAmove(m, m2);
 616             isOpposite =
 617               La > 0 && this.oppositeMoves(this.amoves[La-1], amove);
 618             this.undo(m2);
 619             if (!res2 && !isOpposite) {
 620               res = false;
 621               break;
 622             }
 623           }
 624         }
 625         this.undo(m);
 626         return !res;
 627       });
 628     }
 629     if (La == 0) return super.filterValid(moves);
 630     const Lf = this.firstMove.length;
 631     return (
 632       super.filterValid(
 633         moves.filter(m => {
 634           // Move shouldn't undo another:
 635           const amove = this.getAmove(this.firstMove[Lf-1], m);
 636           return !this.oppositeMoves(this.amoves[La-1], amove);
 637         })
 638       )
 639     );
 640   }
 641
 642   isAttackedBySlideNJump([x, y], color, piece, steps, oneStep) {
 643     for (let step of steps) {
 644       let rx = x + step[0],
 645           ry = y + step[1];
 646       while (V.OnBoard(rx, ry) && this.board[rx][ry] == V.EMPTY && !oneStep) {
 647         rx += step[0];
 648         ry += step[1];
 649       }
 650       if (
 651         V.OnBoard(rx, ry) &&
 652         this.getPiece(rx, ry) == piece &&
 653         this.getColor(rx, ry) == color
 654       ) {
 655         // Continue some steps in the same direction (pull)
 656         rx += step[0];
 657         ry += step[1];
 658         while (
 659           V.OnBoard(rx, ry) &&
 660           this.board[rx][ry] == V.EMPTY &&
 661           !oneStep
 662         ) {
 663           rx += step[0];
 664           ry += step[1];
 665         }
 666         if (!V.OnBoard(rx, ry)) return true;
 667         // Step in the other direction (push)
 668         rx = x - step[0];
 669         ry = y - step[1];
 670         while (
 671           V.OnBoard(rx, ry) &&
 672           this.board[rx][ry] == V.EMPTY &&
 673           !oneStep
 674         ) {
 675           rx -= step[0];
 676           ry -= step[1];
 677         }
 678         if (!V.OnBoard(rx, ry)) return true;
 679       }
 680     }
 681     return false;
 682   }
 683
 684   isAttackedByPawn([x, y], color) {
 685     // The king can be pushed out by a pawn on last rank or near the edge
 686     const pawnShift = (color == "w" ? 1 : -1);
 687     for (let i of [-1, 1]) {
 688       if (
 689         V.OnBoard(x + pawnShift, y + i) &&
 690         this.board[x + pawnShift][y + i] != V.EMPTY &&
 691         this.getPiece(x + pawnShift, y + i) == V.PAWN &&
 692         this.getColor(x + pawnShift, y + i) == color
 693       ) {
 694         if (!V.OnBoard(x - pawnShift, y - i)) return true;
 695       }
 696     }
 697     return false;
 698   }
 699
 700   static OnTheEdge(x, y) {
 701     return (x == 0 || x == 7 || y == 0 || y == 7);
 702   }
 703
 704   isAttackedByKing([x, y], color) {
 705     // Attacked if I'm on the edge and the opponent king just next,
 706     // but not on the edge.
 707     if (V.OnTheEdge(x, y)) {
 708       for (let step of V.steps[V.ROOK].concat(V.steps[V.BISHOP])) {
 709         const [i, j] = [x + step[0], y + step[1]];
 710         if (
 711           V.OnBoard(i, j) &&
 712           !V.OnTheEdge(i, j) &&
 713           this.board[i][j] != V.EMPTY &&
 714           this.getPiece(i, j) == V.KING
 715           // NOTE: since only one king of each color, and (x, y) is occupied
 716           // by our king, no need to check other king's color.
 717         ) {
 718           return true;
 719         }
 720       }
 721     }
 722     return false;
 723   }
 724
 725   // No consideration of color: all pieces could be played
 726   getAllPotentialMoves() {
 727     let potentialMoves = [];
 728     for (let i = 0; i < V.size.x; i++) {
 729       for (let j = 0; j < V.size.y; j++) {
 730         if (this.board[i][j] != V.EMPTY) {
 731           Array.prototype.push.apply(
 732             potentialMoves,
 733             this.getPotentialMovesFrom([i, j])
 734           );
 735         }
 736       }
 737     }
 738     return potentialMoves;
 739   }
 740
 741   doClick(square) {
 742     // A click to promote a piece on subTurn 2 would trigger this.
 743     // For now it would then return [NaN, NaN] because surrounding squares
 744     // have no IDs in the promotion modal. TODO: improve this?
 745     if (isNaN(square[0])) return null;
 746     // If subTurn == 2 && square is empty && !underCheck && !isOpposite,
 747     // then return an empty move, allowing to "pass" subTurn2
 748     const La = this.amoves.length;
 749     const Lf = this.firstMove.length;
 750     if (
 751       this.subTurn == 2 &&
 752       this.board[square[0]][square[1]] == V.EMPTY &&
 753       !this.underCheck(this.turn) &&
 754       (La == 0 || !this.oppositeMoves(this.amoves[La-1], this.firstMove[Lf-1]))
 755     ) {
 756       return {
 757         start: { x: -1, y: -1 },
 758         end: { x: -1, y: -1 },
 759         appear: [],
 760         vanish: []
 761       };
 762     }
 763     return null;
 764   }
 765
 766   play(move) {
 767     move.flags = JSON.stringify(this.aggregateFlags());
 768     V.PlayOnBoard(this.board, move);
 769     if (this.subTurn == 2) {
 770       const L = this.firstMove.length;
 771       this.amoves.push(this.getAmove(this.firstMove[L-1], move));
 772       this.turn = V.GetOppCol(this.turn);
 773       this.movesCount++;
 774     }
 775     else this.firstMove.push(move);
 776     this.subTurn = 3 - this.subTurn;
 777     this.postPlay(move);
 778   }
 779
 780   postPlay(move) {
 781     if (move.start.x < 0) return;
 782     for (let a of move.appear)
 783       if (a.p == V.KING) this.kingPos[a.c] = [a.x, a.y];
 784     this.updateCastleFlags(move);
 785   }
 786
 787   updateCastleFlags(move) {
 788     const firstRank = { 'w': V.size.x - 1, 'b': 0 };
 789     for (let v of move.vanish) {
 790       if (v.p == V.KING) this.castleFlags[v.c] = [V.size.y, V.size.y];
 791       else if (v.x == firstRank[v.c] && this.castleFlags[v.c].includes(v.y)) {
 792         const flagIdx = (v.y == this.castleFlags[v.c][0] ? 0 : 1);
 793         this.castleFlags[v.c][flagIdx] = V.size.y;
 794       }
 795     }
 796   }
 797
 798   undo(move) {
 799     this.disaggregateFlags(JSON.parse(move.flags));
 800     V.UndoOnBoard(this.board, move);
 801     if (this.subTurn == 1) {
 802       this.amoves.pop();
 803       this.turn = V.GetOppCol(this.turn);
 804       this.movesCount--;
 805     }
 806     else this.firstMove.pop();
 807     this.subTurn = 3 - this.subTurn;
 808     this.postUndo(move);
 809   }
 810
 811   postUndo(move) {
 812     // (Potentially) Reset king position
 813     for (let v of move.vanish)
 814       if (v.p == V.KING) this.kingPos[v.c] = [v.x, v.y];
 815   }
 816
 817   getComputerMove() {
 818     let moves = this.getAllValidMoves();
 819     if (moves.length == 0) return null;
 820     // "Search" at depth 1 for now
 821     const maxeval = V.INFINITY;
 822     const color = this.turn;
 823     const emptyMove = {
 824       start: { x: -1, y: -1 },
 825       end: { x: -1, y: -1 },
 826       appear: [],
 827       vanish: []
 828     };
 829     moves.forEach(m => {
 830       this.play(m);
 831       m.eval = (color == "w" ? -1 : 1) * maxeval;
 832       const moves2 = this.getAllValidMoves().concat([emptyMove]);
 833       m.next = moves2[0];
 834       moves2.forEach(m2 => {
 835         this.play(m2);
 836         const score = this.getCurrentScore();
 837         let mvEval = 0;
 838         if (score != "1/2") {
 839           if (score != "*") mvEval = (score == "1-0" ? 1 : -1) * maxeval;
 840           else mvEval = this.evalPosition();
 841         }
 842         if (
 843           (color == 'w' && mvEval > m.eval) ||
 844           (color == 'b' && mvEval < m.eval)
 845         ) {
 846           m.eval = mvEval;
 847           m.next = m2;
 848         }
 849         this.undo(m2);
 850       });
 851       this.undo(m);
 852     });
 853     moves.sort((a, b) => {
 854       return (color == "w" ? 1 : -1) * (b.eval - a.eval);
 855     });
 856     let candidates = [0];
 857     for (let i = 1; i < moves.length && moves[i].eval == moves[0].eval; i++)
 858       candidates.push(i);
 859     const mIdx = candidates[randInt(candidates.length)];
 860     const move2 = moves[mIdx].next;
 861     delete moves[mIdx]["next"];
 862     return [moves[mIdx], move2];
 863   }
 864
 865   getNotation(move) {
 866     if (move.start.x < 0)
 867       // A second move is always required, but may be empty
 868       return "-";
 869     const initialSquare = V.CoordsToSquare(move.start);
 870     const finalSquare = V.CoordsToSquare(move.end);
 871     if (move.appear.length == 0)
 872       // Pushed or pulled out of the board
 873       return initialSquare + "R";
 874     return move.appear[0].p.toUpperCase() + initialSquare + finalSquare;
 875   }
 876 };