use super::util;

pub fn solve() {
    let lines = util::read_file("input/day15.txt");
    let starting_numbers = lines[0].split(",").map(|s| s.parse::<usize>().unwrap()).collect::<Vec<_>>();

    let part1 = play_until(2020, &starting_numbers);
    println!("Day X Part 1: {}", part1);


    let part2 = play_until(30_000_000, &starting_numbers);
    println!("Day X Part 2: {}", part2);
}

fn play_until(max_turn: usize, starting_numbers: &Vec<usize>) -> usize {

    // Because numbers can never be bigger than the number of turns, this works:
    let mut last_occurrence = vec!{0usize; max_turn};

    // Note: we ignore the last number here; it will be the "previous_number" later.
    for i in 0..(starting_numbers.len() - 1) {
        let n = starting_numbers[i];
        last_occurrence[n] = i + 1;
    }

    let mut previous_number = starting_numbers[starting_numbers.len() - 1];
    for turn in starting_numbers.len()..max_turn {

        let prev_occurrence = last_occurrence[previous_number];
        last_occurrence[previous_number] = turn;
        if prev_occurrence == 0 {
            previous_number = 0;
        } else {
            previous_number = turn - prev_occurrence;
        }
        // println!("Turn {}: {}", turn + 1, previous_number);
    }

    return previous_number;

}