2024: Day 5

Author: MiLk

2024/python/day4/__main__.py

@@ -1,4 +1,3 @@
-import re
 from itertools import product
 
 from utils.file import read_input

2024/python/day5/__init__.py

@@ -0,0 +1,68 @@
+from collections.abc import Iterable
+from math import floor
+
+from black.trans import defaultdict
+from more_itertools.recipes import partition
+
+from utils.file import read_input
+
+lines = read_input(__package__)
+
+rules: list[tuple[int, int]] = []
+updates: list[list[int]] = []
+for line in lines:
+    if line[2] == "|":
+        rules.append((int(line[:2]), int(line[3:])))
+    elif "," in line:
+        updates.append(list(map(int, line.split(","))))
+
+rules_after = defaultdict(set)
+rules_before = defaultdict(set)
+for rule in rules:
+    rules_after[rule[0]].add(rule[1])
+    rules_before[rule[1]].add(rule[0])
+
+
+def check_rule(page: int, before: list[int], after: list[int]) -> bool:
+    return all(p not in rules_after[page] for p in before) and all(p not in rules_before[page] for p in after)
+
+
+def is_ordered(pages: list[int]) -> bool:
+    return all(check_rule(pages[i], pages[:i], pages[i + 1 :]) for i in range(len(pages)))
+
+
+def middle_pages(updates_: Iterable[list[int]]) -> Iterable[int]:
+    return (update[floor((len(update) - 1) / 2)] for update in updates_)
+
+
+unordered, ordered = partition(is_ordered, updates)
+print("Part 1", sum(middle_pages(ordered)))
+
+
+def reorder(pages_: list[int]):
+    pages = pages_[:]  # Copy before mutations
+    while not is_ordered(pages):  # Pray for no infinite loop
+        for i in range(len(pages)):
+            page = pages[i]
+            before, after = pages[:i], pages[i + 1 :]
+            rule_before, rule_after = rules_before[page], rules_after[page]
+
+            move_before = next((p for p in before if p in rule_after), None)
+            if move_before:
+                pages = pages[:i] + pages[i + 1 :]
+                idx = pages.index(move_before)
+                pages.insert(idx, page)
+                break
+            move_after = next((p for p in after if p in rule_before), None)
+            if move_after:
+                pages = pages[:i] + pages[i + 1 :]
+                idx = -pages[::-1].index(move_after)
+                if idx == 0:
+                    pages.append(page)
+                else:
+                    pages.insert(idx, page)
+    return pages
+
+
+reordered = list(map(reorder, unordered))
+print("Part 2", sum(middle_pages(reordered)))