advent_of_code_2023_in_rust/crates/solutions/src/day6.rs

mod parsing;
mod race_record;

type NumericValue = u64;

pub fn solve_task_1(input: &str) -> String {
    let parsed = parsing::parsing(input);
    let mut how_many_times_won = 1;

    for next_record in parsed {
        how_many_times_won *= all_rounds_of_durations(next_record.time())
            .filter(|&distance| distance > next_record.reached_distance())
            .count() as NumericValue;
    }
    how_many_times_won.to_string()
}

pub fn solve_task_2(input: &str) -> String {
    let parsed = parsing::parsing_part_2(input);

    let time_to_beat = parsed.time();
    assert!(time_to_beat > 1, "Race time must be at least 2");
    const FIRST_AND_LAST_CYCLE_DO_NOT_MOVE: NumericValue = 2;
    const HALF_AT_OR_OVER_PEAK: NumericValue = 2;
    let range: NumericValue =
        (parsed.time() - FIRST_AND_LAST_CYCLE_DO_NOT_MOVE) / HALF_AT_OR_OVER_PEAK;

    const FIRST_CYCLE_NO_DISTANCE: usize = 1;
    for (index, next_distance) in all_rounds_of_durations(parsed.time())
        .enumerate()
        .skip(FIRST_CYCLE_NO_DISTANCE)
    {
        if next_distance > parsed.reached_distance() {
            const EVEN_TIME_TO_BEAT_LEADS_UNEVEN_PICK: NumericValue = 1;
            let one_offset = if time_to_beat % 2 == 0 {
                EVEN_TIME_TO_BEAT_LEADS_UNEVEN_PICK
            } else {
                0
            };
            let lower_than_distance_over_pick = (index as NumericValue) - 1;
            let both_halves = (range - lower_than_distance_over_pick) * 2;
            let how_many_times_won = both_halves + one_offset;
            return how_many_times_won.to_string();
        }
    }
    unreachable!();
}

fn all_rounds_of_durations(duration: NumericValue) -> impl Iterator<Item = NumericValue> {
    let limit = duration as usize;
    std::iter::repeat(duration)
        .take(limit)
        .enumerate()
        .map(|(speed_duration, duration)| {
            simuluate_reached_distance_at(duration, speed_duration as NumericValue)
        })
}

fn simuluate_reached_distance_at(
    duration: NumericValue,
    speed_duration: NumericValue,
) -> NumericValue {
    assert!(
        duration >= speed_duration,
        "Duration {:?}. Speed Duration: {:?}",
        duration,
        speed_duration
    );
    let rest_time = duration - speed_duration;
    rest_time * speed_duration
}

#[cfg(test)]
mod testing {
    use crate::day6::{simuluate_reached_distance_at, solve_task_2, NumericValue};

    use super::solve_task_1;

    #[test]
    fn should_simuluate_reached_distance_at() {
        fn assert_case(
            duration: NumericValue,
            speed_duration: NumericValue,
            expected: NumericValue,
        ) {
            let actual = simuluate_reached_distance_at(duration, speed_duration);
            assert_eq!(
                expected, actual,
                "Duration: {}\n Speed Duration: {}",
                duration, speed_duration
            );
        }
        const DURATION: NumericValue = 7;

        for (speed_duration, expected) in [
            (0, 0),
            (1, 6),
            (2, 10),
            (3, 12),
            (4, 12),
            (5, 10),
            (6, 6),
            (7, 0),
        ] {
            assert_case(DURATION, speed_duration, expected)
        }
    }

    #[test]
    fn should_solve_task_1() {
        let input = include_str!("day6/day_example_input.txt");
        let actual = solve_task_1(input);
        let expected = "288";
        assert_eq!(expected, actual);
    }

    #[test]
    fn should_solve_task_2() {
        let input = include_str!("day6/day_example_input.txt");
        let actual = solve_task_2(input);
        let expected = "71503";
        assert_eq!(expected, actual);
    }
}