Solve 2021:12 "Passage Pathing"

I struggled a lot with the direction, but realised way too much later
that the mappings are bidirectional.

Part 2 was exceptionally hard, but I realised early that the order did
not matter and that a simple boolean lock was all that was needed. The
challenge was to know when to watch for duplicates.

The seen was initially a set, but since new sets must be sent instead of
references, a list is used instead to be able to use the seen + [] hack.

2021-python/solutions/day_12.py

@@ -0,0 +1,54 @@
+from solutions import BaseSolution
+from collections import defaultdict, deque, Counter
+
+
+class Solution(BaseSolution):
+    input_file = "12.txt"
+
+    def __str__(self):
+        return "Day 12: Passage Pathing"
+
+    def parse_input(self, data):
+        return [l.split("-") for l in data.split()]
+
+    def solve(self, puzzle_input):
+        return self._solve(puzzle_input, lambda seen: True)
+
+    def solve_again(self, puzzle_input):
+        return self._solve(
+            puzzle_input,
+            lambda seen: any(
+                [
+                    k.islower() and k != "start" and v > 1
+                    for k, v in Counter(seen).items()
+                ]
+            ),
+        )
+
+    def _solve(self, puzzle_input, isdup):
+        ft = defaultdict(lambda: [])
+        for f, t in puzzle_input:
+            ft[f].append(t)
+        for f, t in puzzle_input:
+            ft[t].append(f)
+        queue = deque([("start", list(), False)])
+        s2e = 0
+        while queue:
+            pos, seen, dup = queue.pop()
+            seen.append(pos)
+            dup = isdup(seen)
+            for t in ft[pos]:
+                if t == "end":
+                    s2e += 1
+                    continue
+                elif t == "start":
+                    continue
+                elif dup and t.islower() and t in seen:
+                    continue
+                queue.append((t, seen + [], dup))
+        return s2e
+
+
+if __name__ == "__main__":
+    solution = Solution()
+    solution.show_results()

2021-python/tests/test_day_12.py

@@ -0,0 +1,75 @@
+import unittest
+
+from solutions.day_12 import Solution
+
+
+class Day12TestCase(unittest.TestCase):
+    def setUp(self):
+        self.solution = Solution()
+        self.puzzle_input = self.solution.parse_input(
+            """
+        start-A
+start-b
+A-c
+A-b
+b-d
+A-end
+b-end
+        """
+        )
+
+    def test_parse_puzzle_input(self):
+        data = """
+        a-b
+        q1w2-e3r4
+        """
+        assert self.solution.parse_input(data) == [["a", "b"], ["q1w2", "e3r4"]]
+
+    def test_solve_first_part(self):
+        assert self.solution.solve(self.puzzle_input) == 10
+
+    def test_solve_second_part(self):
+        assert self.solution.solve_again(self.puzzle_input) == 36
+
+
+if __name__ == "__main__":
+    unittest.main()
+
+"""
+start,A,b,A,b,A,c,A,end
+start,A,b,A,b,A,end
+start,A,b,A,b,end
+start,A,b,A,c,A,b,A,end
+start,A,b,A,c,A,b,end
+start,A,b,A,c,A,c,A,end
+start,A,b,A,c,A,end
+start,A,b,A,end
+start,A,b,d,b,A,c,A,end
+start,A,b,d,b,A,end
+start,A,b,d,b,end
+start,A,b,end
+start,A,c,A,b,A,b,A,end
+start,A,c,A,b,A,b,end
+start,A,c,A,b,A,c,A,end
+start,A,c,A,b,A,end
+start,A,c,A,b,d,b,A,end
+start,A,c,A,b,d,b,end
+start,A,c,A,b,end
+start,A,c,A,c,A,b,A,end
+start,A,c,A,c,A,b,end
+start,A,c,A,c,A,end
+start,A,c,A,end
+start,A,end
+start,b,A,b,A,c,A,end
+start,b,A,b,A,end
+start,b,A,b,end
+start,b,A,c,A,b,A,end
+start,b,A,c,A,b,end
+start,b,A,c,A,c,A,end
+start,b,A,c,A,end
+start,b,A,end
+start,b,d,b,A,c,A,end
+start,b,d,b,A,end
+start,b,d,b,end
+start,b,end
+"""