Today’s puzzle, like the final fight of Enter the Dragon, takes place in a hall of mirrors. Using the lenses from yesterday, you shine a beam of light into the hall. Part 1 asks you to track how many tiles it traverses as it bounces off mirrors. Mirrors either redirect (/, \) or split (|,-) the beam.
Simulating the beam proved tricky but not truly difficult. Part 2 asks you to shine a beam from all 440 edge tiles of the input and find the greatest number of illuminated tiles. This took a long time; I should go back and optimize my solution. Nonetheless, the real difficulty spike has yet to come.
def simulate_beam(grid, start, initial):
# coord, direction
current = beam_dict()
current[start].add(initial)
traversed = set()
seen = set()
while current:
new = beam_dict()
for coord, directions in current.items():
for direction in directions:
new_coord = coord + direction
# Moving off grid
if new_coord not in grid:
continue
traversed.add(new_coord)
new_directions = grid[new_coord][direction]
# Add to count of beam in relevant direction on tile if it exists, otherwise create new beam
new[new_coord].update(new_directions)
hashed = frozenset(
(coord, frozenset(d))
for coord, d in new.items()
)
# print(hashed)
if hashed in seen:
break
seen.add(hashed)
current = new
return len(traversed)