block chain moves which are known dead ends

js/pacosako.js

@@ -757,6 +757,72 @@ class Game {
       return recordCheck(this, false);
    }
 
+   isDeadEnd(from, to) {
+      const side = this._player;
+      const sim = new Game(this);
+      sim.dropHistory();
+
+      try {
+         sim.move(from, to);
+      } catch (err) {
+         /* illegal moves are automatic dead ends */
+         return true;
+      }
+
+      if (!sim._board.phantom) {
+         /* moves that leave the player in check are also dead ends */
+         /* otherwise this is a legal non-chain move, so not a dead end */
+         return sim.isInCheck(side);
+      }
+
+      const seen = new Set();
+      let queue = [sim];
+      let counter = 0;
+
+      while (queue.length > 0) {
+         const game = queue[0];
+         queue = queue.slice(1);
+
+         seen.add(game._board.toString());
+
+         /* look for another piece that can reach the king or continue the chain */
+         for (const toNext of game.legalMoves(side, PHANTOM, true)) {
+            if (counter >= 300) {
+               /* this is taking too long */
+               const newMoves = game._moves.length - this._moves.length;
+               console.log(`stopped looking for dead end after ${counter} moves (max length was ${newMoves})`);
+               return undefined;
+            }
+
+            ++counter;
+
+            const game2 = new Game(game);
+            try {
+               game2.move(PHANTOM, toNext);
+            } catch (err) {
+               /* internal error */
+               console.log('isDeadEnd:', err);
+               continue;
+            }
+
+            if (!game2._board.phantom) {
+               /* skip moves that would leave the player in check */
+               if (!game2.isInCheck(side)) {
+                  /* it's a legal non-chain move, so not a dead end */
+                  return false;
+               }
+            }
+            else if (!seen.has(game2._board.toString())) {
+               /* we haven't seen this exact board state before */
+               queue.push(game2);
+            }
+         }
+      }
+
+      /* an exhaustive search didn't uncover any legal ways to end the chain */
+      return true;
+   }
+
    move(from, to, meta) {
       if (this._status !== PLAYING) {
          throw { message: "can't move, game is already over" };

js/pacosako_ui.js

@@ -147,14 +147,9 @@ $(function (){
       const from = piece.data('location');
       const legals = currentGame.legalMoves(side, from);
       for (const there of legals) {
-         try {
+         if (currentGame.isDeadEnd(from, there)) {
-            const preview = new PS.Game(currentGame);
+            continue;
-            preview.dropHistory();
+         }
-            preview.move(from, there);
-            if (preview.isInCheck(side)) {
-               continue;
-            }
-         } catch (err) {}
 
          const square = cbSquare(there);
          square.addClass('cb-legal')