From fcb14d489c4b4b5bfad0de6d5b13b84e1f9cb562 Mon Sep 17 00:00:00 2001
From: Jesse McDonald <nybble41@gmail.com>
Date: Sun, 5 Apr 2020 04:42:10 -0500
Subject: [PATCH] add ability to detect obvious check (chains of 6 moves or
 less)

---
 js/iterator.js    | 215 +++++++++++++++++++----------
 js/pacosako.js    | 345 +++++++++++++++++++++++++++++-----------------
 js/pacosako_ui.js |  16 ++-
 3 files changed, 374 insertions(+), 202 deletions(-)

diff --git a/js/iterator.js b/js/iterator.js
index db0c086..b932189 100644
--- a/js/iterator.js
+++ b/js/iterator.js
@@ -1,63 +1,120 @@
 'use strict';
 
-export const IteratorMixin = {
-   map: function* map(f) {
-      let y;
-      while (!(y = this.next()).done) {
-         yield f(y.value);
-      }
-      return y.value;
-   },
+function identity(x) {
+   return x;
+}
 
-   filter: function* filter(f) {
-      let y;
-      while (!(y = this.next()).done) {
-         if (f(y.value)) {
-            yield y.value;
-         }
-      }
-      return y.value;
-   },
+export class Iterator {
+   constructor(from) {
+      let next;
 
-   take: function* take(limit) {
-      let remaining = Number(limit) & -1;
-      let y;
-      while (remaining > 0 && !(y = this.next()).done) {
-         yield y.value;
-         remaining -= 1;
+      if (Symbol.iterator in from.__proto__) {
+         const iter = from.__proto__[Symbol.iterator].call(from);
+         next = iter.next.bind(iter);
+      } else if (typeof from === 'function') {
+         next = from;
+      } else {
+         throw new TypeError('Iterator class needs iterable input');
       }
-   },
 
-   drop: function* drop(limit) {
-      let remaining = Number(limit) & -1;
-      let y;
-      while (!(y = this.next()).done) {
-         if (remaining <= 0) {
-            yield y.value;
+      Object.defineProperty(this, 'next', {
+         value: next,
+         writable: false,
+         enumerable: true,
+         configurable: false,
+      });
+   }
+
+   [Symbol.iterator]() {
+      return this;
+   }
+
+   map(f) {
+      const next = this.next;
+      return new Iterator(function() {
+         const y = next();
+         if (y.done) {
+            return y;
          } else {
+            return { value: f(y.value), done: false };
+         }
+      });
+   }
+
+   filter(f) {
+      const next = this.next;
+      return new Iterator(function() {
+         let y;
+         while (!(y = next()).done && !f(y.value)) {
+            continue;
+         }
+         return y;
+      });
+   }
+
+   take(limit) {
+      let remaining = Number(limit) & -1;
+      const next = this.next;
+      return new Iterator(function() {
+         if (remaining > 0) {
+            remaining -= 1;
+            return next();
+         } else {
+            return { value: undefined, done: true };
+         }
+      });
+   }
+
+   drop(limit) {
+      let remaining = Number(limit) & -1;
+      const next = this.next;
+      return new Iterator(function() {
+         while (remaining > 0 && !next().done) {
             remaining -= 1;
          }
-      }
-   },
+         return next();
+      });
+   }
 
-   asIndexedPairs: function* asIndexedPairs() {
+   asIndexedPairs() {
+      const next = this.next;
       let index = 0;
-      let y;
-      while (!(y = this.next()).done) {
-         yield [index++, x];
-      }
-      return y.value;
-   },
+      return new Iterator(function() {
+         const y = next();
+         if (y.done) {
+            return y;
+         } else {
+            return { value: [index++, y.value], done: false };
+         }
+      });
+   }
 
-   flatMap: function* flatMap(f) {
-      let y;
-      while (!(y = this.next()).done) {
-         yield* f(y.value);
-      }
-      return y.value;
-   },
+   flatMap(f) {
+      const next = this.next;
+      let innerNext;
 
-   reduce: function reduce(f, state) {
+      return new Iterator(function() {
+         for (;;) {
+            if (innerNext) {
+               let y = innerNext();
+               if (!y.done) {
+                  return y;
+               }
+            }
+
+            let z = next();
+            if (z.done) {
+               innerNext = undefined;
+               return z;
+            }
+
+            const iter = y.value.__proto__[Symbol.iterator].call(y.value);
+            innerNext = iter.next.bind(iter);
+         }
+      });
+   }
+
+   reduce(f, state) {
       if (typeof state === 'undefined') {
          const first = this.next();
          if (first.done) {
@@ -65,52 +122,60 @@ export const IteratorMixin = {
          }
          state = first.value;
       }
+
       let y;
       while (!(y = this.next()).done) {
          state = f(state, y.value);
       }
+
       return state;
-   },
+   }
 
-   toArray: function toArray() {
+   toArray() {
       return [...this];
-   },
+   }
 
-   forEach: function* forEach(f) {
+   forEach(f) {
       let y;
       while (!(y = this.next()).done) {
          f(y.value);
       }
-   },
+      /* extension: return final value from underlying iterator */
+      return y.value;
+   }
 
-   some: function some(f) {
+   some(f) {
       /* extension: if f is undefined, assume identity function */
-      let iter = (typeof f === 'undefined') ? this : IteratorMixin.map.call(this, f);
+      if (typeof f === 'undefined') {
+         f = identity;
+      }
       let y;
-      while (!(y = iter.next()).done) {
-         if (y.value) {
+      while (!(y = this.next()).done) {
+         if (f(y.value)) {
             return true;
          }
       }
       return false;
-   },
+   }
 
-   every: function every(f) {
+   every(f) {
       /* extension: if f is undefined, assume identity function */
-      let iter = (typeof f === 'undefined') ? this : IteratorMixin.map.call(this, f);
+      if (typeof f === 'undefined') {
+         f = identity;
+      }
       let y;
-      while (!(y = iter.next()).done) {
-         if (!y.value) {
+      while (!(y = this.next()).done) {
+         if (!f(y.value)) {
             return false;
          }
       }
       return true;
-   },
+   }
 
-   find: function find(f) {
+   find(f) {
+      /* extension: if f is undefined, return the first 'truthy' value */
       if (typeof f === undefined) {
-         /* extension */
-         f = function identity(x) { return x; };
+         f = identity;
       }
       let y;
       while (!(y = this.next()).done) {
@@ -118,19 +183,17 @@ export const IteratorMixin = {
             return y.value;
          }
       }
-   },
+   }
 
    /* extension */
-   includes: function includes(x) {
-      return IteratorMixin.some.call(this, function equalsX(y) { return y === x; });
-   },
+   includes(x) {
+      return this.some(function matches(y) { return y == x; });
+   }
+
+   /* extension */
+   strictlyIncludes(x) {
+      return this.some(function matches(y) { return y === x; });
+   }
 };
 
-export const Iterator = {};
-for (const fn in IteratorMixin) {
-   Iterator[fn] = function() {
-      return IteratorMixin[fn].call(...arguments);
-   };
-}
-
 export default Iterator;
diff --git a/js/pacosako.js b/js/pacosako.js
index 633d1f9..1cf091f 100644
--- a/js/pacosako.js
+++ b/js/pacosako.js
@@ -128,7 +128,14 @@ class Board {
    }
 
    toString() {
-      return new Buffer(this._board).toString('hex')
+      const bufferStr = new Buffer(this._board).toString('hex');
+      if (this._phantom) {
+         const phantom = this._phantom;
+         const phantomStr = ':' + phantom.side[0] + phantom.type + phantom.from;
+         return bufferStr + phantomStr;
+      } else {
+         return bufferStr;
+      }
    }
 
    get phantom() {
@@ -302,6 +309,114 @@ const NBSP = '\u00a0';  /* non-breaking space */
 const SHY = '\u00ad'  /* soft hyphen */
 const ZWSP = '\u200b';  /* zero-width space */
 
+function addHistory(game) {
+   const prior = game._undo;
+   const move = game.lastMove;
+
+   if (game._history === undefined) {
+      if (!move.phantom && !move.resign && move.side === LIGHT) {
+         game._turn += 1;
+      }
+
+      return;
+   }
+
+   let result = '';
+   if (move.phantom) {
+      result += SHY + '*';
+   } else if (!move.resign) {
+      if (game._turn > 0 || move.side === DARK) {
+         result += ' ';
+      }
+
+      if (move.side === LIGHT) {
+         game._turn += 1;
+         result += String(game._turn) + '.' + NBSP;
+      }
+   }
+
+   if (move.castle) {
+      result += 'O-O';
+   } else if (move.queen_castle) {
+      result += 'O-O-O';
+   } else if (move.side && move.type && move.from && move.to) {
+      let piece = '';
+
+      if (move.alongside) {
+         if (move.side === LIGHT) {
+            piece = move.type.toUpperCase() + move.alongside.toUpperCase();
+         } else {
+            piece = move.alongside.toUpperCase() + move.type.toUpperCase();
+         }
+      } else {
+         piece = move.type === PAWN ? '' : move.type.toUpperCase();
+      }
+
+      /* the second condition below is for en passant of a joined piece */
+      if (!move.phantom || move.from !== prior.lastMove.to) {
+         const sameKind = prior.board.findPieces(move.side, move.type, move.alongside || EMPTY);
+         const legalFrom = [];
+         let sameFile = 0;  /* column / letter */
+         let sameRank = 0;  /* row / number */
+
+         for (const where of sameKind) {
+            if (prior.isLegalMove(move.side, where, move.to, true)) {
+               legalFrom.push(where);
+
+               if (where[0] === move.from[0]) {
+                  sameFile += 1;
+               }
+
+               if (where[1] === move.from[1]) {
+                  sameRank += 1;
+               }
+            }
+         }
+
+         /* always disambiguate captures by pawns (standard convention) */
+         if (legalFrom.length !== 1 || (move.type === PAWN && move.took)) {
+            /* append file, rank, or both to disambiguate */
+            if (sameFile === 1) {
+               piece += move.from[0];
+            } else if (sameRank === 1) {
+               piece += move.from[1];
+            } else {
+               piece += move.from;
+            }
+         }
+      }
+
+      const took = move.took ? 'x' : '';
+      result += piece + took + move.to;
+   }
+
+   if (move.en_passant) {
+      result += 'e.p.';
+   }
+
+   if (move.promotion) {
+      result += '(' + move.promotion.toUpperCase() + ')';
+   }
+
+   if (move.took === KING) {
+      result += '#';
+   } else if (game.isInCheck()) {
+      result += '+';
+   }
+
+   let winner = game.winner;
+
+   if (winner === LIGHT) {
+      result += ' 1-0';
+   } else if (winner === DARK) {
+      result += ' 0-1';
+   } else if (game.status !== PLAYING) {
+      result += ' \u00bd-\u00bd'; /* 1/2-1/2 */
+   }
+
+   game._history += result;
+}
+
 class Game {
    constructor(original) {
       if (original !== undefined) {
@@ -314,13 +429,19 @@ class Game {
          this._status = original._status;
          this._moves = JSON.parse(JSON.stringify(original._moves));
          this._redo = JSON.parse(JSON.stringify(original._redo));
+         this._history = original._history;
+         this._turn = original._turn;
          this._castling = JSON.parse(JSON.stringify(original._castling));
+         this._undo = original._undo;
       } else {
          this._board = new Board();
          this._player = LIGHT;
          this._status = PLAYING;
          this._moves = [];
          this._redo = [];
+         this._history = '';
+         this._turn = 0;
+         this._undo = null;
 
          /* set to false when the king or rook moves */
          this._castling = {};
@@ -460,19 +581,88 @@ class Game {
    }
 
    isLegalMove(side, from, to, canCapture) {
-      const legals = this.legalMoves(side, from, canCapture);
-      return Iterator.some(legals, (where) => where === to);
+      const legals = new Iterator(this.legalMoves(side, from, canCapture));
+      return legals.strictlyIncludes(to);
    }
 
    isInCheck(_side) {
+      if (this._status !== PLAYING || this._board.phantom) {
+         /* can't be in check mid-move, or if the game is already over */
+         return false;
+      }
+
       const side = _side || this._player;
+      const other = otherSide(side);
       const kings = [...this._board.findPieces(side, KING)];
 
       if (kings.length !== 1) {
-         throw { message: "there should be exactly one king" };
+         throw { message: 'there should be exactly one king per side' };
       }
 
       const king = kings[0];
+
+      /* check is evaluated as if the current player passed without moving */
+      const sim = new Game(this);
+      sim.dropHistory();
+      sim._player = other;
+
+      /* start with the opponent's unpaired pieces, the ones that can capture */
+      let queue = [];
+      for (const from of sim._board.findPieces(other, true, false)) {
+         if (sim.isLegalMove(other, from, king, true)) {
+            /* this piece can directly capture the king */
+            return true;
+         }
+
+         queue.push({ game: sim, from });
+      }
+
+      /* arbitrary limit, but a human player would probably miss a 7-move chain too */
+      const moveLimit = this._moves.length + 6;
+      const seen = new Set();
+
+      while (queue.length > 0) {
+         const game = queue[0].game;
+         const from = queue[0].from;
+         queue = queue.slice(1);
+
+         seen.add(game.toString());
+
+         /* look for another piece that can reach the king or continue the chain */
+         const pairs = [...game.board.findPieces(other, true, true)];
+         for (const pair of pairs) {
+            if (!game.isLegalMove(other, from, pair, true)) {
+               /* can't reach it */
+               continue;
+            }
+
+            const game2 = new Game(game);
+            try {
+               game2.move(from, pair);
+            } catch (err) {
+               /* internal error, but keep looking at the rest of the queue */
+               console.log('isInCheck:', err);
+               continue;
+            }
+
+            if (seen.has(game2._board.toString())) {
+               /* we've already been here via another path */
+               continue;
+            }
+
+            if (game2.isLegalMove(other, PHANTOM, king, true)) {
+               /* we have our answer */
+               return true;
+            }
+
+            if (game2._moves.length < moveLimit) {
+               queue.push({ game: game2, from: PHANTOM });
+            }
+         }
+      }
+
+      /* didn't find anything */
+      return false;
    }
 
    move(from, to, meta) {
@@ -525,6 +715,7 @@ class Game {
          move.meta = JSON.parse(JSON.stringify(meta));
       }
 
+      this._undo = new Game(this);
       board.move(from, to);
 
       if (type === KING) {
@@ -582,12 +773,14 @@ class Game {
          this._player = otherSide(this._player);
       }
 
+      this._moves.push(move);
+      this._redo = [];
+
       if (took === KING) {
          this._status = ENDED;
       }
 
-      this._moves.push(move);
-      this._redo = [];
+      addHistory(this);
    }
 
    resign(meta) {
@@ -604,9 +797,12 @@ class Game {
          move.meta = meta;
       }
 
-      this._status = ENDED;
+      this._undo = new Game(this);
       this._moves.push(move);
       this._redo = [];
+      this._status = ENDED;
+
+      addHistory(this);
    }
 
    get lastMove() {
@@ -656,15 +852,10 @@ class Game {
          const lastMove = this._moves[this._moves.length - 1];
          const savedRedo = this._redo;
 
-         /* replay all moves except the last in a new game object */
-         const replay = new this.constructor();
-         for (let i = 0; i < this._moves.length - 1; ++i) {
-            replay.replayMove(this._moves[i]);
-         }
-
-         /* copy all the properties from the replayed game to this one */
-         for (const prop in replay) {
-            this[prop] = replay[prop];
+         /* copy all the properties from the saved prior game to this one */
+         const savedUndo = this._undo;
+         for (const prop in savedUndo) {
+            this[prop] = savedUndo[prop];
          }
 
          /* restore the original redo history and add the undone move */
@@ -685,122 +876,28 @@ class Game {
       this._redo = [];
    }
 
-   countTurns() {
-      let n = 0;
-      let player = null;
+   get turns() {
+      return this._turn;
+   }
 
-      for (const move of this._moves) {
-         /* multiple consecutive moves by the same player are a single turn */
-         if (move.side !== player) {
-            ++n;
-         }
+   get history() {
+      return this._history;
+   }
 
-         player = move.side;
-      }
-
-      return n;
+   dropHistory() {
+      this._history = undefined;
    }
 
    renderHistory() {
-      let replay = new Game();
-      let result = '';
-      let n = 0;
-
-      for (const move of this._moves) {
-         if (move.phantom) {
-            result += SHY + '*';
-         } else if (!move.resign) {
-            if (n > 0 || move.side === DARK) {
-               result += ' ';
-            }
-
-            if (move.side === LIGHT) {
-               ++n;
-               result += String(n) + '.' + NBSP;
-            }
+      if (this._history === undefined) {
+         const replay = new Game();
+         for (const move of this._moves) {
+            replay.replayMove(move);
          }
-
-         if (move.castle) {
-            result += 'O-O';
-         } else if (move.queen_castle) {
-            result += 'O-O-O';
-         } else if (move.side && move.type && move.from && move.to) {
-            let piece = '';
-
-            if (move.alongside) {
-               if (move.side === LIGHT) {
-                  piece = move.type.toUpperCase() + move.alongside.toUpperCase();
-               } else {
-                  piece = move.alongside.toUpperCase() + move.type.toUpperCase();
-               }
-            } else {
-               piece = move.type === PAWN ? '' : move.type.toUpperCase();
-            }
-
-            /* the second condition below is for en passant of a joined piece */
-            if (!move.phantom || move.from !== replay.lastMove.to) {
-               const sameKind = replay.board.findPieces(move.side, move.type, move.alongside || EMPTY);
-               const legalFrom = [];
-               let sameFile = 0;  /* column / letter */
-               let sameRank = 0;  /* row / number */
-
-               for (const where of sameKind) {
-                  if (replay.isLegalMove(move.side, where, move.to, true)) {
-                     legalFrom.push(where);
-
-                     if (where[0] === move.from[0]) {
-                        sameFile += 1;
-                     }
-
-                     if (where[1] === move.from[1]) {
-                        sameRank += 1;
-                     }
-                  }
-               }
-
-               /* always disambiguate captures by pawns (standard convention) */
-               if (legalFrom.length !== 1 || (move.type === PAWN && move.took)) {
-                  /* append file, rank, or both to disambiguate */
-                  if (sameFile === 1) {
-                     piece += move.from[0];
-                  } else if (sameRank === 1) {
-                     piece += move.from[1];
-                  } else {
-                     piece += move.from;
-                  }
-               }
-            }
-
-            const took = move.took ? 'x' : '';
-            result += piece + took + move.to;
-         }
-
-         if (move.en_passant) {
-            result += 'e.p.';
-         }
-
-         if (move.promotion) {
-            result += '(' + move.promotion.toUpperCase() + ')';
-         }
-
-         if (move.took === KING) {
-            result += '#';
-         }
-
-         replay.replayMove(move);
+         this._history = replay._history;
       }
 
-      let winner = replay.winner;
-
-      if (winner === LIGHT) {
-         result += ' 1-0';
-      } else if (winner === DARK) {
-         result += ' 0-1';
-      } else if (replay.status !== PLAYING) {
-         result += ' \u00bd-\u00bd'; /* 1/2-1/2 */
-      }
-
-      return result;
+      return this._history;
    }
 }
 
diff --git a/js/pacosako_ui.js b/js/pacosako_ui.js
index b291bfe..02e014e 100644
--- a/js/pacosako_ui.js
+++ b/js/pacosako_ui.js
@@ -144,6 +144,15 @@ $(function (){
       const from = piece.data('location');
       const legals = currentGame.legalMoves(side, from);
       for (const there of legals) {
+         try {
+            const preview = new PS.Game(currentGame);
+            preview.dropHistory();
+            preview.move(from, there);
+            if (preview.isInCheck(side)) {
+               continue;
+            }
+         } catch (err) {}
+
          const square = cbSquare(there);
          square.addClass('cb-legal')
          if (event === 'drag') {
@@ -336,13 +345,16 @@ $(function (){
          }
          msg += (winner === PS.LIGHT ? 'Light' : 'Dark') + ' player won!';
       } else if (playing) {
+         if (game.isInCheck()) {
+            msg += 'Check! ';
+         }
          msg += (game.player === PS.LIGHT ? 'Light' : 'Dark') + ' player\'s turn.';
       } else {
          msg += 'Game ended in a draw.';
       }
       $('#cb_message').text(msg);
 
-      $('#cb_history').text(game.renderHistory());
+      $('#cb_history').text(game.history || '');
 
       $('#cb_nav_first').attr('disabled', !game.canUndo);
       $('#cb_nav_prev_turn').attr('disabled', !game.canUndo);
@@ -438,7 +450,7 @@ $(function (){
       const gameId = $('#cb_board').data('gameId');
       const lightName = $('#cb_light_name').val();
       const darkName = $('#cb_dark_name').val();
-      const turns = currentGame.countTurns();
+      const turns = currentGame.turns;
       const winner = currentGame.winner;
       const lastMove = currentGame.lastMove || {};
       const lastMeta = lastMove.meta || {};