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@@ -0,0 +1,121 @@
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+from queue import PriorityQueue
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+
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+lines = [line.strip() for line in open("24.input")]
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+
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+blizzards = set()
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+
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+size = (len(lines[0]), len(lines))
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+
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+DIRS = {
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+ "<": (-1, 0),
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+ ">": (1, 0),
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+ "v": (0, 1),
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+ "^": (0, -1),
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+ }
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+
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+for y, line in enumerate(lines):
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+ for x, c in enumerate(line):
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+ if c in ("<", ">", "v", "^"):
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+ blizzards.add((x, y, c))
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+
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+def show(grid):
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+ for y in range(size[1]):
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+ for x in range(size[0]):
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+ if x == 0 or y == 0 or x == size[0] - 1 or y == size[1] - 1:
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+ print("#", end="")
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+ continue
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+ blizz = [c for (x_, y_, c) in grid if x == x_ and y == y_]
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+ if len(blizz) > 1:
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+ print(len(blizz), end="")
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+ elif len(blizz) == 1:
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+ print(blizz[0], end="")
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+ else:
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+ print(".", end="")
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+ print("")
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+ print("-" * 20)
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+
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+def in_bound(pos):
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+
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+ start = (1, 0)
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+ end = (size[0] - 2, size[1] - 1)
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+ if pos in [start, end]:
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+ return True
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+
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+ nx, ny = pos
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+ if nx <= 0:
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+ return False
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+ if nx >= size[0] - 1:
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+ return False
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+ if ny <= 0:
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+ return False
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+ if ny >= size[1] - 1:
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+ return False
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+ return True
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+
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+def next_map(current):
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+ new = set()
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+ for (x, y, d) in current:
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+ dx, dy = DIRS[d]
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+ nx, ny = x + dx, y + dy
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+ if nx == 0:
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+ nx = size[0] - 2
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+ if nx == size[0] - 1:
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+ nx = 1
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+ if ny == 0:
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+ ny = size[1] - 2
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+ if ny == size[1] - 1:
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+ ny = 1
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+ new.add((nx, ny, d))
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+ return new
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+
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+def search(grid_cache, runs):
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+
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+ start = (1, 0)
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+ end = (size[0] - 2, size[1] - 1)
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+
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+ q = PriorityQueue()
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+ q.put((0, start, 0))
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+
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+ dist_cache = {(0, start, 0): 0}
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+
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+ while not q.empty():
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+ tick, pos, run = q.get()
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+ #print(tick, pos, run)
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+
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+ nxt_map = grid_cache[(tick + 1) % len(grid_cache)]
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+
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+ if pos == start and run == 1:
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+ q.put((tick, pos, 2))
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+
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+ if pos == end:
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+ if run == runs:
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+ return tick
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+ elif run == 0:
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+ q.put((tick, pos, run + 1))
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+
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+ for dx, dy in [(1, 0), (0, 1), (-1, 0), (0, -1), (0, 0)]:
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+ dest = (pos[0] + dx, pos[1] + dy)
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+ if not in_bound(dest):
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+ continue
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+
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+ if dest not in nxt_map:
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+ new_state = ((tick + 1) % len(grid_cache), dest, run)
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+ if dist_cache.get(new_state, 100000000) > tick + 1:
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+ dist_cache[new_state] = tick + 1
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+ q.put((tick + 1, dest, run))
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+
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+
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+grid_cache = []
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+
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+current = blizzards
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+
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+while True:
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+ grid_cache.append(current)
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+ current = next_map(current)
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+ if current in grid_cache:
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+ break
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+
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+grid_cache = [set((x, y) for (x, y, c) in current) for current in grid_cache]
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+
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+print(search(grid_cache, 0))
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+print(search(grid_cache, 2))
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