Day 12

day12/part2.py

@@ -0,0 +1,29 @@
+#!/usr/bin/env python3
+from collections import defaultdict
+
+lines = (x.strip() for x in open("input.txt"))
+paths = defaultdict(set)
+stored_sum = {}
+
+for line in lines:
+    a, b = line.split("-")
+    paths[a].add(b)
+    paths[b].add(a)
+
+
+def visit(node, visited):
+    store_key = hash((node, frozenset(hash((k, v)) for k, v in visited.items())))
+    if node == 'end':
+        return 1
+    elif store_key not in stored_sum:
+        has_twice = max(visited.values()) > 1
+        stored_sum[store_key] = sum(
+            visit(nxt, {**visited, nxt: visited.get(nxt, 0) + 1} if nxt != nxt.upper() else visited)
+            for nxt in paths[node].difference(
+                set(k for k, v in visited.items() if v >= (1 if has_twice else 2) or (v == 1 and k == 'start'))
+            )
+        )
+    return stored_sum[store_key]
+
+
+print(visit('start', {'start': 1}))