#! /usr/bin/env python3
from itertools import product


def main(filename):
    results = {}
    with open(filename) as inp:
        for line in inp:
            boarding_pass = line.rstrip()
            row, col = parse_boarding_pass(boarding_pass)
            seat_id = get_seat_id(row, col)
            results[boarding_pass] = (row, col, seat_id)
    part1(results)
    part2(results)


def part1(results):
    # part 1
    max_seat_id = max(x[2] for x in results.values())
    print("Max seat ID: ", max_seat_id)


def part2(results):
    seat_ids = sorted(x[2] for x in results.values())
    missing_seat_ids = set(range(max(seat_ids))) - set(seat_ids)
    print(missing_seat_ids)


def parse_boarding_pass(boarding_pass):
    row = _parse(boarding_pass[:7], "F", "B", 128)
    col = _parse(boarding_pass[7:], "L", "R", 7)
    return row, col


def _parse(inp, lower_option, upper_option, number):
    rows = slice(0, number - 1)
    for l in inp:
        if l == lower_option:
            rows = lower_half(rows)
        else:
            rows = upper_half(rows)
    return rows.start


def lower_half(sl: slice):
    length = sl.stop - sl.start
    return slice(sl.start, sl.start + length // 2)


def upper_half(sl: slice):
    length = sl.stop - sl.start
    return slice(1 + sl.start + length // 2, sl.stop)


def get_seat_id(row, col):
    return 8 * row + col


if __name__ == "__main__":
    main('input.txt')