From bfe59b931653a1068a1bc677c05f36d55b02e032 Mon Sep 17 00:00:00 2001
From: "Steinar H. Gunderson" <sgunderson@bigfoot.com>
Date: Sun, 25 Dec 2022 21:02:13 +0100
Subject: [PATCH] Chess.js: In move_from_san(), only bother generating relevant
 candidate moves.

---
 www/js/chess.js | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/www/js/chess.js b/www/js/chess.js
index 531a08f..3c97b64 100644
--- a/www/js/chess.js
+++ b/www/js/chess.js
@@ -1176,10 +1176,10 @@ var Chess = function(fen) {
     return '';
   }
 
-  // Find all pseudolegal moves featuring the given piece attacking
+  // Find all pseudolegal moves featuring the given piece moving to
   // the given square (using symmetry of all non-pawn-or-castle moves,
-  // we simply generate moves backwards). Does not support kings
-  // or pawns. Assumes there's not already a piece of our own color
+  // we simply generate moves backwards). Does not support pawns.
+  // Assumes there's not already a piece of our own color
   // on the destination square.
   function find_attacking_moves(to, piece, us) {
     let moves = [];
@@ -1204,8 +1204,8 @@ var Chess = function(fen) {
           break;
         }
 
-        /* break if knight */
-        if (piece === 'n') break;
+        /* break if knight or king */
+        if (piece === 'n' || piece === 'k') break;
       }
     }
 
@@ -1259,7 +1259,23 @@ var Chess = function(fen) {
       }
     }
 
-    var moves = generate_moves();
+    let moves;
+    let piece_matches = clean_move.match(/^([NBRQK])x?([a-h][1-8])$/);
+    if (piece_matches) {
+      // Only look for moves by the given piece to the given square.
+      let to = SQUARES[piece_matches[2]];
+      if (board[to] != null && board[to].color === turn) {
+        // Cannot capture our own piece.
+        return null;
+      }
+      moves = find_attacking_moves(to, piece_matches[1].toLowerCase(), turn);
+      // Legal moves only.
+      moves = possibly_filter_moves(moves, turn, true);
+    } else {
+      // Fallback (also used for pawns): Any (legal) moves.
+      moves = generate_moves();
+    }
+
     for (var i = 0, len = moves.length; i < len; i++) {
       // try the strict parser first, then the sloppy parser if requested
       // by the user
-- 
2.39.2