55 lines
2.0 KiB
Python
55 lines
2.0 KiB
Python
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import sys
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from collections import deque, namedtuple
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MOVES = [(0, 1), (0, -1), (1, 0), (-1, 0), (0, 0)]
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Blizzards = namedtuple('Blizzards', ['rows', 'cols', 'left', 'right', 'up', 'down'])
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def is_free(blizz, r, c, t):
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return not any((
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(r, (c-t) % blizz.cols) in blizz.right,
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(r, (c+t) % blizz.cols) in blizz.left,
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((r-t) % blizz.rows, c) in blizz.down,
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((r+t) % blizz.rows, c) in blizz.up
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))
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def parse(data):
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lines = [l[1:-1] for l in data.splitlines()[1:-1]] # Cut off walls
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rows, cols = len(lines), len(lines[0])
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left = frozenset((r, c) for r in range(rows) for c in range(cols) if lines[r][c] == '<')
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right = frozenset((r, c) for r in range(rows) for c in range(cols) if lines[r][c] == '>')
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up = frozenset((r, c) for r in range(rows) for c in range(cols) if lines[r][c] == '^')
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down = frozenset((r, c) for r in range(rows) for c in range(cols) if lines[r][c] == 'v')
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return Blizzards(rows, cols, left, right, up, down)
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def shortest_path(blizz, start, end, start_time):
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visited = set()
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q = deque()
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while True:
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# When do we enter the board? We can enter at any time after the start_time
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while not q:
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start_time += 1 # Increment first, because it takes one timestep to enter the board
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if is_free(blizz, start[0], start[1], start_time):
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q.append((start[0], start[1], start_time))
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r, c, t = q.popleft()
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if (r, c, t) in visited:
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continue
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visited.add((r, c, t))
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if (r, c) == end:
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return t + 1
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for dr, dc in MOVES:
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nr, nc = r + dr, c + dc
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if 0 <= nr < blizz.rows and 0 <= nc < blizz.cols and is_free(blizz, nr, nc, t+1):
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q.append((nr, nc, t+1))
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if __name__ == '__main__':
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b = parse(sys.stdin.read())
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start = (0, 0)
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end = (b.rows-1, b.cols-1)
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t1 = shortest_path(b, start, end, 0)
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t2 = shortest_path(b, end, start, t1)
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t3 = shortest_path(b, start, end, t2)
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print(t1)
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print(t3)
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