ac-3

assignment2/csp.py

@@ -56,8 +56,47 @@ class CSP:
         bool
             False if a domain becomes empty, otherwise True
         """
-        # YOUR CODE HERE (and remove the assertion below)
+        queue = Queue()
-        assert False, "Not implemented"
+        for edge in self.binary_constraints.keys():
+            queue.put(edge)
+
+        while not queue.empty():
+            (xi, xj) = queue.get()
+
+            if self._revise(xi, xj):
+                if len(self.domains[xi]) == 0:
+                    return False
+                for neighboring_edge in [
+                    (a, b)
+                    for (a, b) in self.binary_constraints.keys()
+                    if a != xj and b == xi
+                ]:
+                    queue.put(neighboring_edge)
+        return True
+
+    def _revise(self, xi, xj) -> bool:
+        """Internal of ac_3
+        Makes an arc consistent by shrinking xi's domain.
+
+        Parameters
+        ----------
+        xi, xj
+            Nodes of the arc
+
+        Returns
+        -------
+        bool
+            True if a change was made to the domain of xi, False otherwise
+
+        """
+        revised = False
+        for x in set(self.domains[xi]):
+            if not any(
+                [(x, y) in self.binary_constraints[(xi, xj)] for y in self.domains[xj]]
+            ):
+                self.domains[xi].remove(x)
+                revised = True
+        return revised
 
     def _consistent(self, var, value, assignment) -> bool:
         for v in assignment: