2022: d13: ex2: add solution

2022/d13/ex2/ex2.py

@@ -0,0 +1,52 @@
+#!/usr/bin/env python
+
+import functools
+import sys
+from typing import Union
+
+Packet = Union[int, list["Packet"]]
+
+
+def cmp_packet(lhs: Packet, rhs: Packet) -> int:
+    if isinstance(lhs, int) and isinstance(rhs, int):
+        return lhs - rhs
+    if isinstance(lhs, int):
+        lhs = [lhs]
+    if isinstance(rhs, int):
+        rhs = [rhs]
+    non_equal_cmp = (res for res in map(cmp_packet, lhs, rhs) if res != 0)
+    len_cmp = cmp_packet(len(lhs), len(rhs))
+    return next(non_equal_cmp, len_cmp)
+
+
+def solve(input: list[str]) -> int:
+    def to_packet(input: str) -> Packet:
+        # Sanity checks
+        assert "\n" not in input
+        assert input
+
+        return eval(input)  # Secret best way to parse JSON
+
+    packets = [to_packet(line) for line in input if line]
+    packets.extend(
+        (
+            [[2]],
+            [[6]],
+        )
+    )
+    packets.sort(key=functools.cmp_to_key(cmp_packet))
+
+    # Packets are 1-indexed
+    first = packets.index([[2]]) + 1
+    second = packets.index([[6]]) + 1
+
+    return first * second
+
+
+def main() -> None:
+    input = sys.stdin.read().splitlines()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()