goboard_slow.py method does_move_violate_ko() Incorrect

code/dlgo/goboard_slow.py

@@ -213,10 +213,29 @@ def does_move_violate_ko(self, player, move):
         next_board.place_stone(player, move.point)
         next_situation = (player.other, next_board)
         past_state = self.previous_state
+
         while past_state is not None:
-            if past_state.situation == next_situation:
+            # Assume ko is violated.
+            ko_violated = True
+
+            past_board = past_state.situation[1]
+            next_board = next_situation[1]
+
+            for row in range(self.board.num_rows, 0, -1):
+                for col in range(1, self.board.num_cols + 1):
+                    past_stone = past_board.get(Point(row=row, col=col))
+                    next_stone = next_board.get(Point(row=row, col=col))
+
+                    # If a point/player between the boards does not match, ko is
+                    # not violated.
+                    if past_stone != next_stone:
+                        ko_violated = False
+
+            if ko_violated:
                 return True
+
             past_state = past_state.previous_state
+
         return False
 # end::is_ko[]
 

code/dlgo/goboard_test.py

@@ -76,6 +76,67 @@ def test_empty_triangle(self):
             [Point(3, 2), Point(2, 3), Point(1, 3)],
             black_string.liberties)
 
+    def test_GameState_does_move_violate_ko(self):
+        # Testing Figures 3.5-3.7.
+        game = GameState.new_game(6)
+        board = game.board
+        board.place_stone(Player.black, Point(1, 2))
+        board.place_stone(Player.black, Point(1, 3))
+        board.place_stone(Player.white, Point(1, 4))
+        board.place_stone(Player.white, Point(1, 6))
+        board.place_stone(Player.black, Point(2, 2))
+        board.place_stone(Player.white, Point(2, 3))
+        board.place_stone(Player.black, Point(2, 4))
+        board.place_stone(Player.black, Point(2, 5))
+        board.place_stone(Player.white, Point(2, 6))
+        board.place_stone(Player.black, Point(3, 2))
+        board.place_stone(Player.white, Point(3, 3))
+        board.place_stone(Player.white, Point(3, 4))
+        board.place_stone(Player.white, Point(3, 5))
+
+        next_player, _ = game.situation
+        self.assertTrue(next_player, Player.black)
+
+        move = Move(Point(1, 5), is_pass=False, is_resign=False)
+        self.assertFalse(game.does_move_violate_ko(Player.black, move))
+        move = move.play(Point(1, 5))
+        game = game.apply_move(move)
+
+        next_player, _ = game.situation
+        self.assertTrue(next_player, Player.white)
+
+        # Verify white can play at 1,4.
+        move = Move(Point(1, 4), is_pass=False, is_resign=False)
+        self.assertFalse(game.does_move_violate_ko(Player.white, move))
+        move = move.play(Point(1, 4))
+        game = game.apply_move(move)
+
+        # Testing a real Ko.  Figure 2.5
+        game = GameState.new_game(5)
+        board = game.board
+        board.place_stone(Player.black, Point(2, 2))
+        board.place_stone(Player.black, Point(3, 1))
+        board.place_stone(Player.white, Point(3, 2))
+        board.place_stone(Player.black, Point(3, 3))
+        board.place_stone(Player.white, Point(4, 1))
+        board.place_stone(Player.white, Point(4, 3))
+        board.place_stone(Player.white, Point(5, 2))
+
+        # Have black capture 4,2.
+        next_player, _ = game.situation
+        self.assertTrue(next_player, Player.black)
+
+        move = Move(Point(4, 2), is_pass=False, is_resign=False)
+        self.assertFalse(game.does_move_violate_ko(Player.black, move))
+        move = move.play(Point(4, 2))
+        game = game.apply_move(move)
+
+        # Have white break Ko rule.
+        next_player, _ = game.situation
+        self.assertTrue(next_player, Player.white)
+        move = Move(Point(3, 2), is_pass=False, is_resign=False)
+        move = move.play(Point(3, 2))
+        self.assertTrue(game.does_move_violate_ko(Player.white, move))
 
 class GameTest(unittest.TestCase):
     def test_new_game(self):