def __call__(self, source):
"""
Interesting moves include pushing the boulder and moving to a different side of the boulder to push it.
"""
boulder_location, player_location = source
# Keep a set of the next states to return at the end of the function.
next_states = set()
# Make sure that the state is valid.
assert self.rect.is_inbounds(boulder_location)
assert self.rect.is_inbounds(player_location)
assert self.level[boulder_location] not in '0-|^', (boulder_location, self.level[boulder_location])
assert self.level[player_location] not in '0-|^', (player_location, self.leve[player_location])
# Put the boulder on the floor.
self.level[boulder_location] = '0'
# Keep a set of the locations we want to visit to push the boulder in a different direction.
target_locations = set()
# This set includes all target locations and may include the current location.
all_desirable_sides = set()
for back, front in self.location_to_back_front_pairs[boulder_location]:
# You cannot push a boulder while standing on a trap or solid object.
if self.level[back] in '0^':
continue
# You cannot push a boulder into a solid object
if self.level[front] == '0':
continue
# At this point the boulder is pushable if we can get behind it.
all_desirable_sides.add(back)
if player_location == back:
# If we are already behind it then add a boulder push to the list of next states.
next_states.add((front, boulder_location))
else:
# Otherwise add the location behind the boulder to a list of places we want to reach.
target_locations.add(back)
# See which of the target locations we can reach while the boulder is on the floor.
if target_locations:
reachable_desirable_sides = self.side_cache.get(source, None)
if not reachable_desirable_sides:
reachable_desirable_sides = AscDP.flood_all_targets([player_location], all_desirable_sides, self.manhattan_transition)
for back in reachable_desirable_sides:
adjacent_state = (boulder_location, back)
self.side_cache[adjacent_state] = reachable_desirable_sides
if adjacent_state != source:
next_states.add(adjacent_state)
# Take the boulder back off of the floor.
self.level[boulder_location] = '.'
return next_states
