Correct least constraints value implementation

csp.py

@@ -83,6 +83,14 @@ def conflict(var2):
 
         return count(conflict(v) for v in self.neighbors[var])
 
+    def count_lost_values(self, var, val, assignment):
+        """Return how many values would be ruled out in the domains of the
+        unassigned neighbours of var if var were assigned val (the count used
+        by the least-constraining-value heuristic)."""
+        return count(not self.constraints(var, val, neighbor, dval)
+                     for neighbor in self.neighbors[var] if neighbor not in assignment
+                     for dval in self.domains[neighbor])
+
     def display(self, assignment):
         """Show a human-readable representation of the CSP."""
         # Subclasses can print in a prettier way, or display with a GUI
@@ -371,7 +379,7 @@ def unordered_domain_values(var, assignment, csp):
 
 def lcv(var, assignment, csp):
     """Least-constraining-values heuristic."""
-    return sorted(csp.choices(var), key=lambda val: csp.nconflicts(var, val, assignment))
+    return sorted(csp.choices(var), key=lambda val: csp.count_lost_values(var, val, assignment))
 
 
 # Inference

tests/test_csp.py

@@ -35,6 +35,12 @@ def test_csp_nconflicts():
     val = 'B'
     assert map_coloring_test.nconflicts(var, val, assignment) == 0
 
+def test_csp_count_lost_values():
+    map_coloring_test = MapColoringCSP(list('RGB'), 'A: B C; B: C; C: ')
+    assignment = {'A': 'G'}
+    var = 'C'
+    val = 'R'
+    assert map_coloring_test.count_lost_values(var, val, assignment) == 1
 
 def test_csp_actions():
     map_coloring_test = MapColoringCSP(list('123'), 'A: B C; B: C; C: ')
@@ -332,13 +338,13 @@ def test_lcv():
 
     var = 'B'
 
-    assert lcv(var, assignment, csp) == [4, 0, 1, 2, 3, 5]
-    assignment = {'A': 1, 'C': 3}
+    assert lcv(var, assignment, csp) == [0, 2, 4, 1, 3, 5]
+    assignment = {'A': 1}
 
     constraints = lambda X, x, Y, y: (x + y) % 2 == 0 and (x + y) < 5
     csp = CSP(variables=None, domains=domains, neighbors=neighbors, constraints=constraints)
 
-    assert lcv(var, assignment, csp) == [1, 3, 0, 2, 4, 5]
+    assert lcv(var, assignment, csp) == [0, 1, 2, 3, 4, 5]
 
 
 def test_forward_checking():