jackson

y2021/p25.py

+text = open(0).read()
+state = {}
+for y, row in enumerate(text.splitlines()):
+    for x, ch in enumerate(row):
+        state[x, y] = ch
+X, Y = x + 1, y + 1
+
+seen = set()
+for turn in range(1000):
+    for ch, dx, dy in [('>', 1, 0), ('v', 0, 1)]:
+        copy = state.copy()
+        for old in copy:
+            if copy[old] == ch:
+                x, y = old
+                new = (x + dx) % X, (y + dy) % Y
+                if copy[new] == '.':
+                    state[old] = '.'
+                    state[new] = ch
+
+    hashable = frozenset(state.items())
+    if hashable in seen:
+        print(turn)
+        break
+    seen.add(hashable)