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@@ -0,0 +1,60 @@
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+#!/usr/bin/env python3
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+import sys
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+from heapq import heappop, heappush
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+from math import lcm
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+
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+def parse(inp):
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+ blizzards = {}
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+ DELTA = (1, 0), (-1, 0), (0, 1), (0, -1)
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+ for y, line in enumerate(inp):
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+ for x, cell in enumerate(line.rstrip()):
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+ if cell not in '#.':
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+ d = DELTA['><v^'.index(cell)]
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+ blizzards.setdefault((x - 1, y - 1), []).append(d)
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+ return blizzards, x - 1, y - 1
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+
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+def move(blizzards, w, h):
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+ for _ in range(lcm(w, h)):
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+ yield set(blizzards)
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+ newbliz = {}
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+ for (x, y), deltas in blizzards.items():
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+ for dx, dy in deltas:
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+ moved = (x + dx + w) % w, (y + dy + h) % h
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+ newbliz.setdefault(moved, []).append((dx, dy))
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+ blizzards = newbliz
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+
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+def walk(src, dst, step, blizzard_movement, w, h):
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+ start = src, step
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+ visited = {start}
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+ dist = {start: 0}
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+ work = [(0, start)]
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+ endx, endy = dst
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+
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+ while work:
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+ pos, step = current = heappop(work)[1]
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+ if pos == dst:
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+ return step
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+ x, y = pos
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+ bliz = blizzard_movement[(step + 1) % len(blizzard_movement)]
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+ for dx, dy in ((-1, 0), (1, 0), (0, 0), (0, -1), (0, 1)):
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+ nx = x + dx
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+ ny = y + dy
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+ if (0 <= nx < w and 0 <= ny < h and (nx, ny) not in bliz) or \
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+ (nx == w - 1 and ny == h) or (nx == 0 and ny == -1):
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+ nb = (nx, ny), step + 1
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+ if nb not in visited:
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+ visited.add(nb)
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+ alt = dist[current] + 1
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+ if alt < dist.get(nb, 1000000):
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+ dist[nb] = alt
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+ estimate = abs(endx - nx) + abs(endy - ny)
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+ heappush(work, (alt + estimate, nb))
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+
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+blizzards, w, h = parse(sys.stdin)
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+blizzard_movement = list(move(blizzards, w, h))
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+start = 0, -1
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+end = w - 1, h
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+to_end = walk(start, end, 0, blizzard_movement, w, h)
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+print(to_end)
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+to_start = walk(end, start, to_end, blizzard_movement, w, h)
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+print(walk(start, end, to_start, blizzard_movement, w, h))
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Taddeus Kroes