Fixes recursion exception in davisb_putnamb_logemann_loveland

other/davisb_putnamb_logemannb_loveland.py

@@ -77,15 +77,21 @@ def evaluate(self, model: dict[str, bool]) -> bool:
         3. Return None(unable to complete evaluation) if a literal has no assignment.
         4. Compute disjunction of all values assigned in clause.
         """
+
+        # 1. literal and complement in clause
         for literal in self.literals:
             symbol = literal.rstrip("'") if literal.endswith("'") else literal + "'"
             if symbol in self.literals:
                 return True
 
+        # 2. Any literal is True and
+        # 3. Any literal is False
         self.assign(model)
         for value in self.literals.values():
             if value in (True, None):
                 return value
+
+        # 4. disjunction
         return any(self.literals.values())
 
 
@@ -282,6 +288,7 @@ def dpll_algorithm(
     2. If some clause in clauses is False, return False.
     3. Find pure symbols.
     4. Find unit symbols.
+    5. recurse on symbol = True or symbol = False
 
     >>> formula = Formula([Clause(["A4", "A3", "A5'", "A1", "A3'"]), Clause(["A4"])])
     >>> clauses, symbols = generate_parameters(formula)
@@ -292,6 +299,9 @@ def dpll_algorithm(
     >>> model
     {'A4': True}
     """
+
+    # 2. Check any False
+    # 1. Check All True
     check_clause_all_true = True
     for clause in clauses:
         clause_check = clause.evaluate(model)
@@ -304,54 +314,77 @@ def dpll_algorithm(
     if check_clause_all_true:
         return True, model
 
-    try:
-        pure_symbols, assignment = find_pure_symbols(clauses, symbols, model)
-    except RecursionError:
-        print("raises a RecursionError and is")
-        return None, {}
-    P = None
-    if len(pure_symbols) > 0:
+    # 3. Pure symbols
+    pure_symbols, assignment = find_pure_symbols(clauses, symbols, model)
+    if pure_symbols:
         P, value = pure_symbols[0], assignment[pure_symbols[0]]
+        model_true = model.copy()
+        model_true[P] = value
+        symbols_without_p = [i for i in symbols if i != P]
+        return dpll_algorithm(clauses, symbols_without_p, model_true)
 
-    if P:
-        tmp_model = model
-        tmp_model[P] = value
-        tmp_symbols = [i for i in symbols]
-        if P in tmp_symbols:
-            tmp_symbols.remove(P)
-        return dpll_algorithm(clauses, tmp_symbols, tmp_model)
-
+    # 4. Unit symbols
     unit_symbols, assignment = find_unit_clauses(clauses, model)
-    P = None
-    if len(unit_symbols) > 0:
+    if unit_symbols:
         P, value = unit_symbols[0], assignment[unit_symbols[0]]
-    if P:
-        tmp_model = model
-        tmp_model[P] = value
-        tmp_symbols = [i for i in symbols]
-        if P in tmp_symbols:
-            tmp_symbols.remove(P)
-        return dpll_algorithm(clauses, tmp_symbols, tmp_model)
+        if P in symbols:
+            model_with_p = model.copy()
+            model_with_p[P] = value
+            symbols_without_p = [i for i in symbols if i != P]
+            return dpll_algorithm(clauses, symbols_without_p, model_with_p)
+
+    # 5. recurse through next symbol True and False:
     P = symbols[0]
     rest = symbols[1:]
-    tmp1, tmp2 = model, model
-    tmp1[P], tmp2[P] = True, False
 
-    return dpll_algorithm(clauses, rest, tmp1) or dpll_algorithm(clauses, rest, tmp2)
+    # duplicate model for update, python passes objects by reference
+    model_true = model.copy()
+    model_true[P] = True
+    model_false = model.copy()
+    model_false[P] = False
+
+    return dpll_algorithm(clauses, rest, model_true) or dpll_algorithm(
+        clauses, rest, model_false
+    )
 
 
 if __name__ == "__main__":
     import doctest
 
     doctest.testmod()
 
-    formula = generate_formula()
-    print(f"The formula {formula} is", end=" ")
-
-    clauses, symbols = generate_parameters(formula)
-    solution, model = dpll_algorithm(clauses, symbols, {})
-
-    if solution:
-        print(f"satisfiable with the assignment {model}.")
-    else:
-        print("not satisfiable.")
+    # recursion example:
+    # {A4} , {A3} , {A2' , A1' , A3 , A4} , {A2'} , {A1' , A1 , A5'} , {A3' , A1}
+
+    # no recursion problem:
+    clauses_ok = [
+        Clause(x)
+        for x in [
+            ["A3'", "A1"],
+            ["A3"],
+            ["A4"],
+        ]
+    ]
+    clauses_recursion = [
+        Clause(x)
+        for x in [
+            ["A3'", "A1"],
+            ["A4"],
+            ["A3"],
+        ]
+    ]
+
+    for formula in [
+        Formula(clauses_ok),
+        Formula(clauses_recursion),
+        generate_formula(),
+    ]:
+        print(f"The formula {formula} is", end=" ")
+        clauses, symbols = generate_parameters(formula)
+        solution, model = dpll_algorithm(clauses, symbols, {})
+
+        if solution:
+            print(f"satisfiable with the assignment {model}.")
+        else:
+            print("not satisfiable.")
+        print()