Fix checkmating ep capture in chess.gaviota.PythonTablebase

Author: niklasf

chess/gaviota.py

@@ -1493,8 +1493,8 @@ def probe_dtm(self, board: chess.Board) -> int:
         Probes for depth to mate information.
 
         The absolute value is the number of half-moves until forced mate
-        (or ``0`` in drawn positions). The value is positive if the
-        side to move is winning, otherwise it is negative.
+        (or ``0`` in drawn or checkmated positions). The value is positive if
+        the side to move is winning, otherwise it is negative or 0.
 
         In the example position, white to move will get mated in 10 half-moves:
 
@@ -1532,11 +1532,14 @@ def probe_dtm(self, board: chess.Board) -> int:
             try:
                 board.push(move)
 
-                child_dtm = -self._probe_dtm_no_ep(board)
-                if child_dtm > 0:
-                    child_dtm += 1
-                elif child_dtm < 0:
-                    child_dtm -= 1
+                if board.is_checkmate():
+                    child_dtm = 1
+                else:
+                    child_dtm = -self._probe_dtm_no_ep(board)
+                    if child_dtm > 0:
+                        child_dtm += 1
+                    elif child_dtm < 0:
+                        child_dtm -= 1
 
                 dtm = min(dtm, child_dtm) if dtm * child_dtm > 0 else max(dtm, child_dtm)
             finally:

test.py

@@ -4353,6 +4353,16 @@ def test_ep_is_best(self):
         board = chess.Board("8/8/7k/8/1pP5/7K/8/8 b - c3 0 1")
         self.assertEqual(self.tablebase.probe_dtm(board), 19)
 
+    @catchAndSkip(chess.gaviota.MissingTableError, "need KQPvKP.gtb.cp4")
+    def test_ep_is_mate(self):
+        # The resulting mate.
+        board = chess.Board("5Q2/7k/6P1/5K2/8/8/8/8 b - - 0 1")
+        self.assertEqual(self.tablebase.probe_dtm(board), 0)
+
+        # Ep leads to the previously tested mate position.
+        board = chess.Board("5Q2/7k/8/5KpP/8/8/8/8 w - g6 0 1")
+        self.assertEqual(self.tablebase.probe_dtm(board), 1)
+
 
 class SvgTestCase(unittest.TestCase):