day 24, not mine

d24/run.py

@@ -0,0 +1,54 @@
+import sys
+from collections import deque, namedtuple
+
+MOVES = [(0, 1), (0, -1), (1, 0), (-1, 0), (0, 0)]
+
+Blizzards = namedtuple('Blizzards', ['rows', 'cols', 'left', 'right', 'up', 'down'])
+
+
+def is_free(blizz, r, c, t):
+    return not any((
+        (r, (c-t) % blizz.cols) in blizz.right,
+        (r, (c+t) % blizz.cols) in blizz.left,
+        ((r-t) % blizz.rows, c) in blizz.down,
+        ((r+t) % blizz.rows, c) in blizz.up
+    ))
+
+def parse(data):
+    lines = [l[1:-1] for l in data.splitlines()[1:-1]] # Cut off walls
+    rows, cols = len(lines), len(lines[0])
+    left = frozenset((r, c) for r in range(rows) for c in range(cols) if lines[r][c] == '<')
+    right = frozenset((r, c) for r in range(rows) for c in range(cols) if lines[r][c] == '>')
+    up = frozenset((r, c) for r in range(rows) for c in range(cols) if lines[r][c] == '^')
+    down = frozenset((r, c) for r in range(rows) for c in range(cols) if lines[r][c] == 'v')
+    return Blizzards(rows, cols, left, right, up, down)
+
+def shortest_path(blizz, start, end, start_time):
+    visited = set()
+    q = deque()
+    while True:
+        # When do we enter the board? We can enter at any time after the start_time
+        while not q:
+            start_time += 1 # Increment first, because it takes one timestep to enter the board
+            if is_free(blizz, start[0], start[1], start_time):
+                q.append((start[0], start[1], start_time))
+        r, c, t = q.popleft()
+        if (r, c, t) in visited:
+            continue
+        visited.add((r, c, t))
+        if (r, c) == end:
+            return t + 1
+        for dr, dc in MOVES:
+            nr, nc = r + dr, c + dc
+            if 0 <= nr < blizz.rows and 0 <= nc < blizz.cols and is_free(blizz, nr, nc, t+1):
+                q.append((nr, nc, t+1))
+
+if __name__ == '__main__':
+    b = parse(sys.stdin.read())
+    start = (0, 0)
+    end = (b.rows-1, b.cols-1)
+    t1 = shortest_path(b, start, end, 0)
+    t2 = shortest_path(b, end, start, t1)
+    t3 = shortest_path(b, start, end, t2)
+    print(t1)
+    print(t3)