def find_pure_symbol(symbols, unknown_clauses):
"""
Find a symbol and its value if it appears only as a positive literal
(or only as a negative) in clauses.
>>> from diofant import symbols
>>> from diofant.abc import A, B, D
>>> from diofant.logic.algorithms.dpll import find_pure_symbol
>>> find_pure_symbol([A, B, D], [A|~B,~B|~D,D|A])
(A, True)
"""
for sym in symbols:
found_pos, found_neg = False, False
for c in unknown_clauses:
if not found_pos and sym in disjuncts(c):
found_pos = True
if not found_neg and Not(sym) in disjuncts(c):
found_neg = True
if found_pos != found_neg:
return sym, found_pos
return None, None
def find_unit_clause(clauses, model):
"""
A unit clause has only 1 variable that is not bound in the model.
>>> from diofant import symbols
>>> from diofant.abc import A, B, D
>>> from diofant.logic.algorithms.dpll import find_unit_clause
>>> find_unit_clause([A | B | D, B | ~D, A | ~B], {A:True})
(B, False)
"""
for clause in clauses:
num_not_in_model = 0
for literal in disjuncts(clause):
sym = literal_symbol(literal)
if sym not in model:
num_not_in_model += 1
P, value = sym, not (literal.func is Not)
if num_not_in_model == 1:
return P, value
return None, None
def test_disjuncts():
assert disjuncts(A | B | C) == {A, B, C}
assert disjuncts((A | B) & C) == {(A | B) & C}
assert disjuncts(A) == {A}
assert disjuncts(True) == {True}
assert disjuncts(False) == {False}