2017: d24: ex2: add solution

2017/d24/ex2/ex2.py

@@ -0,0 +1,74 @@
+#!/usr/bin/env python
+
+import itertools
+import sys
+from collections.abc import Iterator
+from typing import NamedTuple
+
+
+class Component(NamedTuple):
+    left: int
+    right: int
+
+    @classmethod
+    def from_str(cls, input: str) -> "Component":
+        return cls(*map(int, input.split("/")))
+
+    def remaining_port(self, connection: int) -> int:
+        if connection == self.left:
+            return self.right
+        elif connection == self.right:
+            return self.left
+        assert False  # Sanity check
+
+
+class Bridge(NamedTuple):
+    components: tuple[Component, ...]
+    head: int
+
+    @property
+    def length(self):
+        return len(self.components)
+
+    @property
+    def strength(self):
+        return sum(itertools.chain.from_iterable(self.components))
+
+    def add(self, c: Component) -> "Bridge":
+        assert self.head in c  # Sanity check
+        return Bridge(self.components + (c,), c.remaining_port(self.head))
+
+
+def solve(input: str) -> int:
+    def parse(input: str) -> list[Component]:
+        return [Component.from_str(line) for line in input.splitlines()]
+
+    def longest_bridge(components: set[Component]) -> Bridge:
+        def build_bridges(
+            bridge: Bridge, components: set[Component]
+        ) -> Iterator[Bridge]:
+            candidates = {c for c in components if bridge.head in c}
+            if not candidates:
+                yield bridge
+                return
+            for c in candidates:
+                yield from build_bridges(bridge.add(c), components - {c})
+
+        return max(
+            build_bridges(Bridge((), 0), components),
+            key=lambda b: (b.length, b.strength),
+        )
+
+    components = parse(input)
+    assert len(set(components)) == len(components)  # Sanity check
+    bridge = longest_bridge(set(components))
+    return bridge.strength
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()