Better play in tablebase drawn endgames

Author: Torom

lichess_game.py

@@ -635,6 +635,8 @@ def _probe_tablebase(self, moves: Iterable[chess.Move], tb_type: TablebaseType)
         best_wdl = -2
         best_metric = 1_000_000
         best_real_distance = 0
+        best_opponent_draw_ratio = 1.0
+
         board_copy = self.board.copy(stack=False)
 
         if tb_type == TablebaseType.GAVIOTA:
@@ -662,21 +664,46 @@ def _probe_tablebase(self, moves: Iterable[chess.Move], tb_type: TablebaseType)
                 elif wdl > 0:
                     metric -= 10_000
 
+            opponent_draw_ratio = 1.0
+            if best_wdl <= 0 and wdl == 0:
+                opponent_moves = list(board_copy.legal_moves)
+                if len(opponent_moves) > 0:
+                    opponent_draw_moves = 0
+                    for opponent_move in opponent_moves:
+                        board_copy.push(opponent_move)
+
+                        opponent_distance = -probe_distance(board_copy)
+                        opponent_wdl = self._value_to_wdl(opponent_distance, board_copy.halfmove_clock)
+
+                        if opponent_wdl == 0:
+                            opponent_draw_moves += 1
+
+                        board_copy.pop()
+
+                    opponent_draw_ratio = opponent_draw_moves / len(opponent_moves)
+
             if best_move:
                 if wdl > best_wdl:
                     best_move = move
                     best_wdl = wdl
                     best_metric = metric
                     best_real_distance = real_distance
-                elif wdl == best_wdl and metric < best_metric:
-                    best_move = move
-                    best_metric = metric
-                    best_real_distance = real_distance
+                    best_opponent_draw_ratio = opponent_draw_ratio
+                elif wdl == best_wdl:
+                    if wdl == 0:
+                        if opponent_draw_ratio < best_opponent_draw_ratio:
+                            best_move = move
+                            best_opponent_draw_ratio = opponent_draw_ratio
+                    elif metric < best_metric:
+                        best_move = move
+                        best_metric = metric
+                        best_real_distance = real_distance
             else:
                 best_move = move
                 best_wdl = wdl
                 best_metric = metric
                 best_real_distance = real_distance
+                best_opponent_draw_ratio = opponent_draw_ratio
 
             board_copy.pop()