def get_best_var_to_flip(self, instance, best_solution,
current_no_of_unsat_clauses):
best_no_of_unsat_clauses = current_no_of_unsat_clauses
best = best_solution.copy()
utils = SatUtils()
for i in range(1, len(best)):
# checking if given variable is part of tabu list. If it is then next var is selected for the flip
if self.is_var_tabu(i):
continue
tmp_solution = best_solution.copy()
# working copy of the proposed solution
var_to_flip = tmp_solution[i]
# flipping a selected variable to opposing value
tmp_solution[i] = SatUtils.flip_var(var_to_flip)
_, no_of_unsat_clauses = SatUtils.solution_status(
instance, tmp_solution)
# if solution hasn't been found check if proposed temp solution is better than previous best
if no_of_unsat_clauses < best_no_of_unsat_clauses:
best_no_of_unsat_clauses = no_of_unsat_clauses
# add current selection to tabu list
self.add_to_tabu(i)
# remember the best solution found so far for the next iteration
best = tmp_solution.copy()
return best
def execute_search(self, proposed_solution, currently_flipped_variable,
random_clause):
"""
Choose a variable to be flipped.
If a randomly chosen variable is present in the randomly selected unsatisfied clause then return that variable.
Otherwise find best and second best variable to be flipped. The choice between which variable to select, best
or second best is made based on the probability provided.
:param proposed_solution:
:return:
"""
if currently_flipped_variable not in random_clause:
return currently_flipped_variable
else:
# Chose best or second best variable to flip based on the probability provided
tmp_solution = proposed_solution.copy()
flip_scores = {}
utils = SatUtils()
# loop over the variables to see which flip generates the lowest number of unsatisfied clauses
for i in range(len(self.variables)):
tmp_solution[i] = SatUtils.flip_var(self.variables[i])
_, unsat_clauses = SatUtils.solution_status(
self.instance, tmp_solution)
# keep track of which variable flip generated what number of unsatisfied clauses
flip_scores[self.variables[i]] = unsat_clauses
# flip the selected variable
tmp_solution[i] = SatUtils.flip_var(self.variables[i])
# Find best and second best - extremely efficient according to stack overflow
two_smallest_keys = heapq.nsmallest(2,
flip_scores,
key=flip_scores.get)
best = two_smallest_keys[0]
second_best = two_smallest_keys[1]
# generate noise value
noise_value = random.uniform(0, 1)
# compare noise value generated to the probability of second best variable being selected
if noise_value < self.probability:
return second_best
else:
return best
def execute_walk(self, formula, proposed_solution):
solution_found, unsat_clause, unsat_clause_list = SatUtils.solution_status_with_unsat_clauses(formula,
proposed_solution)
random_unsat_clause = random.choice(unsat_clause_list)
random_variable_in_clause = random.choice(random_unsat_clause)
# as we're choosing a variable from the unsat clause we can come across negative numbers, where negative
# sign would indicate variable is in 0 state, however, negative index are not present in the proposed solution
# array, so it needs to be guaranteed that a positive number is passed as index, hence use of abs()
proposed_solution[abs(random_variable_in_clause)] = SatUtils.flip_var(abs(random_variable_in_clause))
return proposed_solution
def main(self, wp, p, max_iterations, instance_path):
cnf_contents = SatUtils.read_instance(instance_path)
# instantiate required search and util objects
novelty_search = NoveltySearch(cnf_contents, p, 1)
variables = cnf_contents[0]
walkSat = WalkSat()
# initialize first solution proposal
proposed_solution = SatUtils.initialize_variables(len(variables))
# start the timer
start = time.time()
end = None
for i in range(max_iterations):
# checking for timeout terminate condition
if time.time() > start + 60:
return
solution_found, unsat_clause, unsat_clause_list = SatUtils.solution_status_with_unsat_clauses(
cnf_contents, proposed_solution)
# if a solution has been identified break out of the search loop and record it
if solution_found is True:
end = time.time()
print("Iteration,{0},Duration,{1}".format(i, end - start))
return
# pick algorithm to run the solution search based on probability
if wp < random.uniform(0, 1):
proposed_solution = walkSat.execute_walk(
cnf_contents, proposed_solution)
else:
random_variable_to_flip = random.choice(variables)
random_unsat_clause = random.choice(unsat_clause_list)
best_flip = novelty_search.execute_search(
proposed_solution, random_variable_to_flip,
random_unsat_clause)
proposed_solution[best_flip] = SatUtils.flip_var(
proposed_solution[best_flip])
def main(self):
proposed_solution = SatUtils.initialize_variables(self.variables)
start = time.time()
end = None
for i in range(self.max_iterations):
solution_found, unsat_clause, unsat_clause_list = SatUtils.solution_status_with_unsat_clauses(
self.instance, proposed_solution)
if solution_found is True:
end = time.time()
print("Iteration,{0},Duration,{1}".format(i, end - start))
return
random_flip = random.choice(self.variables)
random_clause = random.choice(unsat_clause_list)
flip_this_index = self.execute_search(proposed_solution,
random_flip, random_clause)
proposed_solution[flip_this_index] = SatUtils.flip_var(
proposed_solution[flip_this_index])
def test_flip_var(self):
result = SatUtils.flip_var(0)
self.assertEqual(result, 1)