AoC/aoc_2024/src/day21.rs

use aoc_runner_derive::{aoc, aoc_generator};

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
struct Path<T> {
    steps: usize,
    path: [Option<T>; 6],
}

impl<T: Copy> Path<T> {
    fn new() -> Self {
        Self {
            steps: 0,
            path: [None, None, None, None, None, None],
        }
    }

    fn push(&mut self, button: T) {
        self.path[self.steps] = Some(button);
        self.steps += 1;
    }

    fn append(&mut self, other: &Self) {
        for i in 0..other.steps {
            self.push(other.path[i].unwrap());
        }
    }
}

impl<T> IntoIterator for Path<T> {
    type Item = T;
    type IntoIter = std::iter::Flatten<std::array::IntoIter<Option<T>, 6>>;

    fn into_iter(self) -> Self::IntoIter {
        self.path.into_iter().flatten()
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum DirButton {
    Up,
    Down,
    Left,
    Right,
    A,
}

impl DirButton {
    fn pos(&self) -> (usize, usize) {
        match self {
            DirButton::Up => (1, 0),
            DirButton::Down => (1, 1),
            DirButton::Left => (0, 1),
            DirButton::Right => (2, 1),
            DirButton::A => (2, 0),
        }
    }

    fn index(&self) -> usize {
        match self {
            DirButton::Up => 0,
            DirButton::Down => 1,
            DirButton::Left => 2,
            DirButton::Right => 3,
            DirButton::A => 4,
        }
    }

    fn to(&self, other: &DirButton) -> [Option<Path<DirButton>>; 2] {
        let start = self.pos();
        let end = other.pos();

        let mut x = Path::new();
        let mut y = Path::new();

        if start.0 < end.0 {
            (start.0..end.0).for_each(|_| x.push(DirButton::Right));
        } else {
            (end.0..start.0).for_each(|_| x.push(DirButton::Left));
        };
        if start.1 < end.1 {
            (start.1..end.1).for_each(|_| y.push(DirButton::Down));
        } else {
            (end.1..start.1).for_each(|_| y.push(DirButton::Up));
        };

        if start.0 == 0 && end.1 == 0 {
            x.append(&y);
            x.push(DirButton::A);
            [Some(x), None]
        } else if end.0 == 0 && start.1 == 0 {
            y.append(&x);
            y.push(DirButton::A);
            [Some(y), None]
        } else {
            let x_copy = x;
            x.append(&y);
            x.push(DirButton::A);
            y.append(&x_copy);
            y.push(DirButton::A);
            [Some(x), Some(y)]
        }
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum NumButton {
    One,
    Two,
    Three,
    Four,
    Five,
    Six,
    Seven,
    Eight,
    Nine,
    Zero,
    A,
}

impl NumButton {
    fn pos(&self) -> (usize, usize) {
        match self {
            NumButton::One => (0, 2),
            NumButton::Two => (1, 2),
            NumButton::Three => (2, 2),
            NumButton::Four => (0, 1),
            NumButton::Five => (1, 1),
            NumButton::Six => (2, 1),
            NumButton::Seven => (0, 0),
            NumButton::Eight => (1, 0),
            NumButton::Nine => (2, 0),
            NumButton::Zero => (1, 3),
            NumButton::A => (2, 3),
        }
    }

    fn to(&self, other: &NumButton) -> [Option<Path<DirButton>>; 2] {
        let start = self.pos();
        let end = other.pos();

        let mut x = Path::new();
        let mut y = Path::new();

        if start.0 < end.0 {
            (start.0..end.0).for_each(|_| x.push(DirButton::Right));
        } else {
            (end.0..start.0).for_each(|_| x.push(DirButton::Left));
        };
        if start.1 < end.1 {
            (start.1..end.1).for_each(|_| y.push(DirButton::Down));
        } else {
            (end.1..start.1).for_each(|_| y.push(DirButton::Up));
        };

        if start.0 == 0 && end.1 == 3 {
            x.append(&y);
            x.push(DirButton::A);
            [Some(x), None]
        } else if end.0 == 0 && start.1 == 3 {
            y.append(&x);
            y.push(DirButton::A);
            [Some(y), None]
        } else {
            let x_copy = x;
            x.append(&y);
            x.push(DirButton::A);
            y.append(&x_copy);
            y.push(DirButton::A);
            [Some(x), Some(y)]
        }
    }
}

type Input = Vec<(usize, Vec<NumButton>)>;

#[aoc_generator(day21)]
fn parse(input: &str) -> Input {
    input
        .lines()
        .map(|line| {
            (
                line[0..3].parse().unwrap(),
                line.chars()
                    .map(|c| match c {
                        '1' => NumButton::One,
                        '2' => NumButton::Two,
                        '3' => NumButton::Three,
                        '4' => NumButton::Four,
                        '5' => NumButton::Five,
                        '6' => NumButton::Six,
                        '7' => NumButton::Seven,
                        '8' => NumButton::Eight,
                        '9' => NumButton::Nine,
                        '0' => NumButton::Zero,
                        'A' => NumButton::A,
                        _ => unreachable!(),
                    })
                    .collect(),
            )
        })
        .collect()
}

type DistanceMatrix = [[usize; 5]; 5];

fn calculate_steps(start: DirButton, end: DirButton, steps: &mut [DistanceMatrix]) -> usize {
    if steps.is_empty() {
        return 1;
    }

    if steps[0][start.index()][end.index()] == 0 {
        let extension = start
            .to(&end)
            .into_iter()
            .flatten()
            .map(|path| shortest_dir_path(path, &mut steps[1..]))
            .min()
            .unwrap();
        steps[0][start.index()][end.index()] = extension;
    }

    steps[0][start.index()][end.index()]
}

fn shortest_dir_path(path: Path<DirButton>, steps: &mut [DistanceMatrix]) -> usize {
    path.path
        .into_iter()
        .flatten()
        .fold((0, DirButton::A), |acc, next| {
            let sum = acc.0;
            let pos = acc.1;

            let extension = calculate_steps(pos, next, steps);

            (sum + extension, next)
        })
        .0
}

fn shortest_path(path: Vec<NumButton>, steps: &mut [DistanceMatrix]) -> usize {
    path.into_iter()
        .fold((0, NumButton::A), |acc, end| {
            let sum = acc.0;
            let start = acc.1;

            let extension = start
                .to(&end)
                .into_iter()
                .flatten()
                .map(|path| shortest_dir_path(path, steps))
                .min()
                .unwrap();
            (sum + extension, end)
        })
        .0
}

#[aoc(day21, part1)]
fn part1(input: &Input) -> usize {
    let mut steps = vec![[[0; 5]; 5]; 3 - 1];

    input
        .iter()
        .cloned()
        .map(|(num, path)| num * shortest_path(path, &mut steps))
        .sum()
}

#[aoc(day21, part2)]
fn part2(input: &Input) -> usize {
    let mut steps = vec![[[0; 5]; 5]; 26 - 1];

    input
        .iter()
        .cloned()
        .map(|(num, path)| num * shortest_path(path, &mut steps))
        .sum()
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn part1_example() {
        assert_eq!(part1(&parse("179A")), 68 * 179);
        assert_eq!(part1(&parse("379A")), 64 * 379);
        assert_eq!(part1(&parse("456A")), 64 * 456);
        assert_eq!(part1(&parse("980A")), 60 * 980);
        assert_eq!(part1(&parse("029A")), 68 * 29);
    }
}