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y2019/p15.py

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+import sys
+
+from intcode import compute
+
+
+WALL, MOVE, GOAL = range(3)
+N, S, W, E = range(1, 5)
+ORIENTATIONS = {-1j: N, 1j: S, -1: W, 1: E}
+
+commands = []
+command_iter = iter(commands)
+text = sys.stdin.read()
+bot = compute(text, command_iter)
+
+# hug the wall until we get back home
+pos, ori, grid = 0, -1j, {}
+while 0 not in grid:
+    commands += [ORIENTATIONS[ori]]
+    result = next(bot)
+    if result == WALL:
+        grid[pos + ori] = 1
+        ori *= -1j
+    else:
+        if result == GOAL:
+            target = pos
+        grid[pos + ori] = 0
+        pos += ori
+        ori *= 1j
+
+# determine viable spaces
+valid = {p for p, v in grid.items() if v == 0}
+
+# from start to end
+hops = 0
+seen = set()
+edge = {0}
+while target not in seen:
+    hops += 1
+    seen |= edge
+    edge = {p + d for p in edge for d in ORIENTATIONS} & valid - seen
+print(hops)
+
+# from end to all
+hops = 0
+seen = set()
+edge = {target}
+while seen != valid:
+    hops += 1
+    seen |= edge
+    edge = {p + d for p in edge for d in ORIENTATIONS} & valid - seen
+print(hops)