From 84758bbacfaaceb5e328ff01fe2c003bc58e939c Mon Sep 17 00:00:00 2001
From: "Steinar H. Gunderson" <sgunderson@bigfoot.com>
Date: Sat, 24 Dec 2022 21:03:36 +0100
Subject: [PATCH] chess.js: Much faster disambiguation.

---
 www/js/chess.js | 48 ++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 46 insertions(+), 2 deletions(-)

diff --git a/www/js/chess.js b/www/js/chess.js
index f274628..9a2d383 100644
--- a/www/js/chess.js
+++ b/www/js/chess.js
@@ -708,6 +708,10 @@ var Chess = function(fen) {
       }
     }
 
+    return possibly_filter_moves(moves, us, legal);
+  }
+
+  function possibly_filter_moves(moves, us, legal) {
     /* return all pseudo-legal moves (this includes moves that allow the king
      * to be captured)
      */
@@ -1114,12 +1118,17 @@ var Chess = function(fen) {
 
   /* this function is used to uniquely identify ambiguous moves */
   function get_disambiguator(move, sloppy) {
-    var moves = generate_moves({legal: !sloppy});
-
     var from = move.from;
     var to = move.to;
     var piece = move.piece;
 
+    if (piece === 'p' || piece === 'k') {
+       // Pawn or king moves are never ambiguous.
+       return '';
+    }
+
+    let moves = find_attacking_moves(move.to, piece, move.color, sloppy);
+
     var ambiguities = 0;
     var same_rank = 0;
     var same_file = 0;
@@ -1167,6 +1176,41 @@ var Chess = function(fen) {
     return '';
   }
 
+  // Find all moves featuring the given piece attacking 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 on the destination square.
+  function find_attacking_moves(to, piece, us, sloppy) {
+    let moves = [];
+
+    function add_move(board, moves, from, to, flags, rook_sq) {
+      moves.push(build_move(board, from, to, flags, undefined, rook_sq));
+    }
+    for (let offset of PIECE_OFFSETS[piece]) {
+      var square = to;
+
+      while (true) {
+        square += offset;
+        if (square & 0x88) break;
+
+        if (board[square] != null) {
+          if (board[square].color !== us) break;
+          if (board[to] == null) {
+            add_move(board, moves, square, to, BITS.NORMAL);
+          } else {
+            add_move(board, moves, square, to, BITS.CAPTURE);
+          }
+          break;
+        }
+
+        /* break if knight */
+        if (piece === 'n') break;
+      }
+    }
+
+    return possibly_filter_moves(moves, us, !sloppy);
+  }
+
   function ascii() {
     var s = '   +------------------------+\n';
     for (var i = SQUARES.a8; i <= SQUARES.h1; i++) {
-- 
2.39.2