Add is_suicidal method

go.py

@@ -88,6 +88,25 @@ def is_eyeish(board, c):
     else:
         return color
 
+def is_suicidal(position, move):
+    potential_libs = set()
+    for n in NEIGHBORS[move]:
+        neighbor_group_id = position.lib_tracker.group_index[n]
+        if neighbor_group_id == MISSING_GROUP_ID:
+            # at least one liberty after playing here, so not a suicide
+            return False
+        neighbor_group = position.lib_tracker.groups[neighbor_group_id]
+        if neighbor_group.color == position.to_play:
+            potential_libs |= neighbor_group.liberties
+        elif len(neighbor_group.liberties) == 1:
+            # would capture an opponent group if they only had one lib.
+            return False
+    # it's possible to suicide by connecting several friendly groups
+    # each of which had one liberty.
+    potential_libs -= set([move])
+    return not potential_libs
+
+
 def is_reasonable(position, move):
     'Checks that a move is both legal and not self-eye filling'
     if is_eyeish(position.board, move) == position.to_play:

tests/test_go.py

@@ -88,7 +88,32 @@ def test_is_reasonable(self):
         for move in reasonable_moves:
             self.assertTrue(go.is_reasonable(position, move))
         for move in unreasonable_moves:
-            self.assertFalse(go.is_reasonable(position, move))
+            self.assertFalse(go.is_reasonable(position, move), str(move))
+
+    def test_is_suicidal(self):
+        board = load_board('''
+            ...O.O...
+            ....O....
+            XO.....O.
+            OXO...OXO
+            O.XO.OX.O
+            OXO...OOX
+            XO.......
+            ......XXO
+            .....XOO.
+        ''')
+        position = Position(
+            board=board,
+            to_play=BLACK,
+        )
+        suicidal_moves = pc_set('E9 H5')
+        nonsuicidal_moves = pc_set('B5 J1 A9')
+        for move in suicidal_moves:
+            assert(position.board[move] == go.EMPTY) #sanity check my coordinate input
+            self.assertTrue(go.is_suicidal(position, move), str(move))
+        for move in nonsuicidal_moves:
+            assert(position.board[move] == go.EMPTY) #sanity check my coordinate input
+            self.assertFalse(go.is_suicidal(position, move), str(move))
 
 
 class TestLibertyTracker(unittest.TestCase):