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y2022/p23.py

+rows = open(0).read().splitlines()
+grid = {complex(x, y) for y, l in enumerate(rows)
+                      for x, c in enumerate(l) if c in '#'}
+dir8 = {dx + dy for dx in [-1, 0, 1]
+                for dy in [-1j, 0, 1j] if not dx + dy == 0}
+dir3 = [(k, {k, k + k * 1j, k - k * 1j}) for k in [-1j, 1j, -1, 1]]
+
+
+def render(grid):
+    xmin, *_, xmax = sorted(int(p.real) for p in grid)
+    ymin, *_, ymax = sorted(int(p.imag) for p in grid)
+    sab = ''
+    for y in range(ymin, ymax + 1):
+        for x in range(xmin, xmax + 1):
+            sab += grid.get(complex(x, y), ' ')
+        sab += '\n'
+    return sab
+
+
+def look(p):
+    for k, ds in dir3:
+        if not {p + d for d in ds} & grid:
+            return p + k
+
+
+for i in range(1, 100000):
+
+    moves = {}
+    for p in grid:
+        if not any(p + dp in grid for dp in dir8):
+            continue
+        moves.setdefault(look(p), []).append(p)
+    dir3 = dir3[1:] + dir3[:1]
+
+    if not moves:
+        print(i)
+        break
+
+    for k, vs in moves.items():
+        if k is not None:
+            if len(vs) == 1:
+                grid = grid - set(vs) | {k}
+
+    if i == 10:
+        print(render({k: '#' for k in grid}).count(' '))