Python

Also took advantage of cmp_to_key.

from functools import cmp_to_key
from pathlib import Path


def parse_input(input: str) -> tuple[dict[int, list[int]], list[list[int]]]:
    rules, updates = tuple(input.strip().split("\n\n")[:2])
    order = {}
    for entry in rules.splitlines():
        values = entry.split("|")
        order.setdefault(int(values[0]), []).append(int(values[1]))
    updates = [[int(v) for v in u.split(",")] for u in updates.splitlines()]
    return (order, updates)


def is_ordered(update: list[int], order: dict[int, list[int]]) -> bool:
    return update == sorted(
        update, key=cmp_to_key(lambda a, b: 1 if a in order.setdefault(b, []) else -1)
    )


def part_one(input: str) -> int:
    order, updates = parse_input(input)
    return sum([u[len(u) // 2] for u in (u for u in updates if is_ordered(u, order))])


def part_two(input: str) -> int:
    order, updates = parse_input(input)
    return sum(
        [
            sorted(u, key=cmp_to_key(lambda a, b: 1 if a in order[b] else -1))[
                len(u) // 2
            ]
            for u in (u for u in updates if not is_ordered(u, order))
        ]
    )


if __name__ == "__main__":
    input = Path("input").read_text("utf-8")
    print(part_one(input))
    print(part_two(input))