2021 day 12.

src/holt59/aoc/2021/day12.py

@@ -1,7 +1,50 @@
-from typing import Any, Iterator
+import string
+from collections import defaultdict
+from functools import cache
+from typing import Any, Iterator, Mapping, Sequence
 
 from ..base import BaseSolver
 
 
+@cache
+def is_small(node: str):
+    return all(c in string.ascii_lowercase for c in node)
+
+
+def enumerate_paths(
+    neighbors: Mapping[str, Sequence[str]],
+    duplicate_smalls: int = 0,
+    start: str = "start",
+    current: tuple[str, ...] = ("start",),
+) -> Iterator[tuple[str, ...]]:
+    if start == "end":
+        yield current
+
+    for neighbor in neighbors[start]:
+        if not is_small(neighbor):
+            yield from enumerate_paths(
+                neighbors, duplicate_smalls, neighbor, current + (neighbor,)
+            )
+        elif neighbor not in current:
+            yield from enumerate_paths(
+                neighbors, duplicate_smalls, neighbor, current + (neighbor,)
+            )
+        elif duplicate_smalls > 0:
+            yield from enumerate_paths(
+                neighbors, duplicate_smalls - 1, neighbor, current + (neighbor,)
+            )
+
+
 class Solver(BaseSolver):
-    def solve(self, input: str) -> Iterator[Any]: ...
+    def solve(self, input: str) -> Iterator[Any]:
+        neighbors: dict[str, list[str]] = defaultdict(list)
+
+        for row in input.splitlines():
+            a, b = row.split("-")
+            if a != "end" and b != "start":
+                neighbors[a].append(b)
+            if b != "end" and a != "start":
+                neighbors[b].append(a)
+
+        yield len(list(enumerate_paths(neighbors)))
+        yield len(list(enumerate_paths(neighbors, 1)))