2024: d05: ex2: add solution

2024/d05/ex2/ex2.py

@@ -0,0 +1,60 @@
+#!/usr/bin/env python
+
+import collections
+import copy
+import functools
+import sys
+
+Graph = dict[int, set[int]]
+
+
+def solve(input: str) -> int:
+    def parse_ordering(input: str) -> Graph:
+        graph: Graph = collections.defaultdict(set)
+        for l in input.splitlines():
+            lhs, rhs = map(int, l.split("|"))
+            graph[lhs].add(rhs)
+        return graph
+
+    def parse_updates(input: str) -> list[list[int]]:
+        return [[int(n) for n in line.split(",")] for line in input.splitlines()]
+
+    def parse(input: str) -> tuple[Graph, list[list[int]]]:
+        ordering, updates = input.split("\n\n")
+        return parse_ordering(ordering), parse_updates(updates)
+
+    def validate_update(ordering: Graph, update: list[int]) -> tuple[int, int] | None:
+        for i in range(len(update)):
+            for j in range(i + 1, len(update)):
+                lhs, rhs = update[i], update[j]
+                if lhs in ordering[rhs]:
+                    return i, j
+        return None
+
+    def get_middle(update: list[int]) -> int:
+        assert len(update) % 2 == 1  # Sanity check
+        return update[len(update) // 2]
+
+    def fix_update(ordering: Graph, update: list[int]) -> list[int]:
+        # The graph has cycles, so using a topological sort is out
+        # Instead just swap the wrong pages until we fixed the update
+        update = copy.copy(update)
+        while (indices := validate_update(ordering, update)) is not None:
+            lhs, rhs = indices
+            update[lhs], update[rhs] = update[rhs], update[lhs]
+        return update
+
+    ordering, updates = parse(input)
+    invalid_updates = filter(functools.partial(validate_update, ordering), updates)
+    fixed_updates = map(functools.partial(fix_update, ordering), invalid_updates)
+
+    return sum(get_middle(update) for update in fixed_updates)
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()