Refactor 2024 day 5 without networkx.

src/holt59/aoc/2024/day5.py

@@ -1,27 +1,42 @@
 import sys
 from collections import defaultdict
 
-import networkx as nx
-
 
 def in_correct_order(update: list[int], requirements: dict[int, set[int]]) -> bool:
-    for i_value, value in enumerate(update):
-        if any(value_2 in requirements[value] for value_2 in update[i_value:]):
-            return False
-    return True
-
-
-def fix_order(update: list[int], requirements: dict[int, set[int]]) -> list[int]:
-    graph = nx.DiGraph()
-    graph.add_nodes_from(update)
-    graph.add_edges_from(
-        (v, k)
-        for k, vs in requirements.items()
-        for v in vs
-        if k in update and v in update
+    return all(
+        not any(value_2 in requirements[value] for value_2 in update[i_value:])
+        for i_value, value in enumerate(update)
     )
 
-    return list(nx.topological_sort(graph))
+
+def fix_order(
+    update: list[int],
+    requirements: dict[int, set[int]],
+    max_update_length: int | None = None,
+) -> list[int]:
+    # copy requirements to update
+    requirements = {
+        value: {predecessor for predecessor in predecessors if predecessor in update}
+        for value, predecessors in requirements.items()
+        if value in update
+    }
+
+    max_update_length = max_update_length or len(update)
+
+    update = []
+    while requirements and len(update) < max_update_length:
+        value = next(
+            value for value, predecessors in requirements.items() if not predecessors
+        )
+
+        update.append(value)
+        del requirements[value]
+
+        for predecessors in requirements.values():
+            if value in predecessors:
+                predecessors.remove(value)
+
+    return update
 
 
 part1, part2 = sys.stdin.read().split("\n\n")
@@ -40,7 +55,7 @@ answer_1 = sum(
 )
 
 answer_2 = sum(
-    fix_order(update, requirements)[len(update) // 2]
+    fix_order(update, requirements, len(update) // 2 + 1)[-1]
     for update in updates
     if not in_correct_order(update, requirements)
 )