factor out the code to diff game history into a utility function

js/pacosako.js

@@ -397,6 +397,69 @@ function addHistory(game) {
    game._history += result;
 }
 
+function diffHistory(game1, game2) {
+   let prefixLength = 0;
+
+   while (prefixLength < game1._moves.length && prefixLength < game2._moves.length) {
+      const move1 = game1._moves[prefixLength];
+      const move2 = game2._moves[prefixLength];
+
+      if (move2.resign !== move1.resign ||
+            move2.from !== move1.from ||
+            move2.to !== move1.to) {
+         break;
+      }
+
+      prefixLength += 1;
+   }
+
+   const hist = new Game(game1._history === undefined ? game2 : game1);
+   game1 = undefined;
+
+   hist.clearRedo();
+   while (hist._moves.length > prefixLength) {
+      hist.undo();
+      hist.clearRedo();
+   }
+
+   const turn_before = hist._turn;
+   const history_before = hist.renderHistory();
+   hist.ignoreCheck = game2.ignoreCheck;
+
+   for (const move of game2._moves.slice(prefixLength)) {
+      if (hist._player !== move.side) {
+         if (hist._player === LIGHT) {
+            hist._turn += 1;
+            hist._history += `${hist._turn === 1 ? '' : ' '}${hist._turn}.${NBSP}…`;
+         } else {
+            hist._history += ` …`;
+         }
+
+         hist._player = move.side;
+      }
+
+      try {
+         hist._status = PLAYING;
+         hist.replayMove(move);
+      } catch(err) {
+         try {
+            hist.ignoreCheck = true;
+            hist.replayMove(move);
+            hist.ignoreCheck = game2.ignoreCheck;
+         } catch(err) {
+            return undefined;
+         }
+      }
+   }
+
+   let suffix = hist.history.slice(history_before.length).trimStart();
+   if (!suffix.match(/^\d+\./)) {
+      suffix = `${turn_before}.${NBSP}… ${suffix}`;
+   }
+
+   return suffix;
+}
+
 class Game {
    constructor(original) {
       if (original !== undefined) {
@@ -689,28 +752,12 @@ class Game {
             if (sim.isLegalMove(other, from, king, true)) {
                /* this piece can directly capture the king */
                let check = true;
-               try {
                if (this._history !== undefined) {
-                     const hist = new Game(this);
+                  try {
-                     hist.ignoreCheck = true;
+                     sim.move(from, king);
-                     if (hist._player !== other) {
+                     check = diffHistory(this, sim) || true;
-                        if (hist._player === LIGHT) {
-                           hist._turn += 1;
-                           hist._history += ` ${hist._turn}.${NBSP}…`;
-                        } else {
-                           hist._history += ` …`;
-                        }
-                        hist._player = other;
-                     }
-                     const turn_before = hist._turn;
-                     const history_before = hist._history;
-                     hist.move(from, king);
-                     check = hist.history.slice(history_before.length).trimStart();
-                     if (!check.match(/^\d+\./)) {
-                        check = `${turn_before}.${NBSP}… ${check}`;
-                     }
-                  }
                   } catch(err) {}
+               }
                return recordCheck(this, check);
             }
 
@@ -761,31 +808,12 @@ class Game {
             if (game2.isLegalMove(other, PHANTOM, king, true)) {
                /* we have our answer */
                let check = true;
-               try {
                if (this._history !== undefined) {
-                     const hist = new Game(this);
+                  try {
-                     hist.ignoreCheck = true;
+                     game2.move(PHANTOM, king);
-                     if (hist._player !== other) {
+                     check = diffHistory(this, game2) || true;
-                        if (hist._player === LIGHT) {
-                           hist._turn += 1;
-                           hist._history += ` ${hist._turn}.${NBSP}…`;
-                        } else {
-                           hist._history += ` …`;
-                        }
-                        hist._player = other;
-                     }
-                     const turn_before = hist._turn;
-                     const history_before = hist._history;
-                     for (const move of game2._moves.slice(hist._moves.length)) {
-                        hist.replayMove(move);
-                     }
-                     hist.move(PHANTOM, king);
-                     check = hist.history.slice(history_before.length).trimStart();
-                     if (!check.match(/^\d+\./)) {
-                        check = `${turn_before}.${NBSP}… ${check}`;
-                     }
-                  }
                   } catch(err) {}
+               }
                return recordCheck(this, check);
             }
 
@@ -1163,7 +1191,7 @@ export default {
    ROWS, COLUMNS, PHANTOM,
 
    /* Miscellaneous */
-   Util: { otherSide, offsetSquare },
+   Util: { otherSide, offsetSquare, diffHistory },
 };
 
 /* vim:set expandtab sw=3 ts=8: */