modify AC3 algorithm (aimacode#717)

* added submodule

* fixed ac3 in csp.py

* added a test to verify the modified ac3 algorithm in csp.py

* Update .gitmodules

Author: nvinayvarma189

csp.py

@@ -168,7 +168,7 @@ def AC3(csp, queue=None, removals=None):
             if not csp.curr_domains[Xi]:
                 return False
             for Xk in csp.neighbors[Xi]:
-                if Xk != Xi:
+                if Xk != Xj:
                     queue.append((Xk, Xi))
     return True
 

tests/test_csp.py

@@ -210,6 +210,13 @@ def test_AC3():
     assert AC3(csp, removals=removals) is True
     assert (removals == [('A', 1), ('A', 3), ('B', 1), ('B', 3)] or
             removals == [('B', 1), ('B', 3), ('A', 1), ('A', 3)])
+    
+    domains = {'A': [ 2, 4], 'B': [ 3, 5]}
+    constraints = lambda X, x, Y, y: int(x) > int (y)
+    removals=[]
+    csp = CSP(variables=None, domains=domains, neighbors=neighbors, constraints=constraints)
+
+    assert AC3(csp, removals=removals)
 
 
 def test_first_unassigned_variable():