# Before running Forward checking we must ensure

# that constraints are okay for this state.

basic = basic_constraint_checker(state, verbose)

if not basic:

return False

# Add your forward checking logic here.

# 1. Find X = x being the current assignment.

# 2. for each Y that is associated with X in a binary constraint; do

# for each y in Y's Domain; do

# if X=x and Y=y don't hold:

# remove y from Y's Domain

# if Y's Domain is empty:

# return False

# return True

# NOTE: This function is not a pure function. Its side-effect is

# to update the state

curr_var_name = state.get_current_variable_name()

if not curr_var_name:

# ROOT

return True

for constraint in state.get_constraints_by_name(curr_var_name):

var_j = state.get_variable_by_name( constraint.get_variable_j_name() )

for val in var_j.get_domain():

if not constraint.check(state, value_i = None, value_j = val):

var_j.reduce_domain(val)

if var_j.domain_size() == 0:

return False

# Run forward checking first.

fc_checker = forward_checking(state, verbose)

if not fc_checker:

return False

# Add your propagate singleton logic here.

singleton_queue = [ var for var in state.get_all_variables() if var.domain_size() == 1 ]

visited_singletons = set([])

while singleton_queue:

var = singleton_queue.pop(0)

visited_singletons.add(var)

for constraint in state.get_constraints_by_name(var.get_name()):

var_j = state.get_variable_by_name( constraint.get_variable_j_name() )

for val_j in var_j.get_domain():

if not constraint.check(state, var.get_domain()[0], val_j):

var_j.reduce_domain(val_j)

if var_j.domain_size() == 0:

return False

if var_j.domain_size() == 1 and not visited_singletons.issuperset(set([var_j])):

singleton_queue.append(var_j)