def day15_2():
    with open("input15.txt", "r") as file:
        data = file.read().split(",")

    last_seen = {int(n): i+1 for i, n in enumerate(data[:-1])}

    turn = len(data) + 1
    num = int(data[-1])

    while turn <= 30000000:
        if num not in last_seen:
            last_seen[num] = turn - 1
            next_num = 0
        else:
            next_num = turn - last_seen[num] - 1
            last_seen[num] = turn - 1

        num = next_num
        turn += 1

    return num