Count constraint checks in CSP via self.nchecks (#1112)

The CSP class tracked nassigns but not the number of consistency checks, so
the constraint-check benchmarks from Figure 6.1 (BT, BT+MRV, FC, ...) could
not be reproduced. Expose the constraint relation through a constraints()
method that tallies every call in self.nchecks (the raw relation is kept as
self.constraint_relation). All callers - nconflicts, count_lost_values,
forward_checking, AC3/AC3b/AC4 - go through it, so nchecks measures the
total checks across backtracking and propagation. Add a test.

Author: dmeoli

csp.py

@@ -58,9 +58,20 @@ def __init__(self, variables, domains, neighbors, constraints):
         self.variables = variables
         self.domains = domains
         self.neighbors = neighbors
-        self.constraints = constraints
+        # store the constraint relation privately and expose it through the
+        # constraints() method below, which counts every consistency check
+        self.constraint_relation = constraints
         self.curr_domains = None
         self.nassigns = 0
+        self.nchecks = 0
+
+    def constraints(self, A, a, B, b):
+        """Return True if neighbors A, B satisfy the constraint when A=a, B=b.
+        Every call is tallied in self.nchecks, so callers can measure the number
+        of consistency checks performed (e.g. to reproduce the benchmarks in
+        Figure 6.1), just as self.nassigns tallies the assignments made."""
+        self.nchecks += 1
+        return self.constraint_relation(A, a, B, b)
 
     def assign(self, var, val, assignment):
         """Add {var: val} to assignment; Discard the old value if any."""

tests/test_csp.py

@@ -701,5 +701,20 @@ def test_zebra():
                    'Winston': 3, 'LuckyStrike': 4, 'Parliaments': 5}
 
 
+def test_constraint_check_count():
+    csp = MapColoringCSP(list('RGB'), 'SA: WA NT Q NSW V; NT: WA Q; NSW: Q V; T: ')
+    # before solving, no assignments or checks have been made
+    assert csp.nassigns == 0 and csp.nchecks == 0
+    solution = backtracking_search(csp)
+    assert solution is not None
+    # solving performs both variable assignments and constraint (consistency) checks
+    assert csp.nassigns > 0
+    assert csp.nchecks > 0
+    # every constraints() call is tallied
+    before = csp.nchecks
+    csp.constraints('WA', 'R', 'NT', 'G')
+    assert csp.nchecks == before + 1
+
+
 if __name__ == "__main__":
     pytest.main()