use std::io::Read;

use crate::common::LineIter;

fn read_input(input: &mut dyn Read) -> (i32, i32) {
    let mut reader = LineIter::new(input);

    let a = reader
        .next()
        .unwrap()
        .split(' ')
        .last()
        .unwrap()
        .parse()
        .unwrap();

    let b = reader
        .next()
        .unwrap()
        .split(' ')
        .last()
        .unwrap()
        .parse()
        .unwrap();

    (a, b)
}

#[inline]
fn simulate(die: i32, pos: i32) -> i32 {
    (pos + 3 * die + 3 - 1) % 10 + 1
}

fn find_repetition(mut pos: i32, mut die: i32) -> i32 {
    let mut advance = 0;

    for _ in 0..10 {
        pos = simulate(die, pos);
        advance += pos;
        die = (die + 6) % 10;
    }

    advance
}

pub fn part1(input: &mut dyn Read) -> String {
    const TARGET_SCORE: i32 = 1000;

    let (a, b) = read_input(input);

    let a10 = find_repetition(a, 1);
    let b10 = find_repetition(b, 4);

    let a_win = TARGET_SCORE / a10;
    let b_win = TARGET_SCORE / b10;

    let mut rolls = 3 * 20 * a_win.min(b_win);
    let mut a_score = rolls / 3 / 20 * a10;
    let mut b_score = rolls / 3 / 20 * b10;
    let mut a_pos = a;
    let mut b_pos = b;
    let mut die = 1;

    let (loser_score, rolls) = if a_win < b_win {
        while a_score < TARGET_SCORE {
            a_pos = simulate(die, a_pos);
            a_score += a_pos;
            rolls += 3;

            if a_score < TARGET_SCORE {
                b_pos = simulate(die + 3, b_pos);
                b_score += b_pos;
                rolls += 3;
            }

            die = (die + 6) % 10;
        }

        (b_score, rolls)
    } else {
        while b_score < TARGET_SCORE {
            a_pos = simulate(die, a_pos);
            a_score += a_pos;

            b_pos = simulate(die + 3, b_pos);
            b_score += b_pos;

            rolls += 6;

            die = (die + 6) % 10;
        }

        (a_score, rolls)
    };

    (loser_score * rolls).to_string()
}

pub fn part2(_input: &mut dyn Read) -> String {
    todo!()
}

#[cfg(test)]
mod tests {
    use crate::test_implementation;

    use super::*;

    const SAMPLE: &[u8] = include_bytes!("samples/21.txt");

    #[test]
    fn sample_part1() {
        test_implementation(part1, SAMPLE, 739785);
    }
}