use aoc_runner_derive::{aoc, aoc_generator};

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

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

    fn push(&mut self, button: DirButton) {
        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());
        }
    }

    fn reversed(&self) -> Self {
        let mut path = [None; 6];
        (0..self.steps).for_each(|i| {
            path[i] = match self.path[self.steps - i - 1] {
                Some(DirButton::Up) => Some(DirButton::Down),
                Some(DirButton::Down) => Some(DirButton::Up),
                Some(DirButton::Left) => Some(DirButton::Right),
                Some(DirButton::Right) => Some(DirButton::Left),
                Some(DirButton::A) => Some(DirButton::A),
                None => None,
            };
        });

        Self {
            steps: self.steps,
            path,
        }
    }
}

impl IntoIterator for Path {
    type Item = DirButton;
    type IntoIter = std::iter::Flatten<std::array::IntoIter<Option<DirButton>, 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>; 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),
        }
    }

    #[inline(always)]
    fn to(&self, other: &NumButton) -> [Option<Path>; 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 precompute_steps(steps: &mut [DistanceMatrix]) {
    if steps.len() == 1 {
        return;
    }

    precompute_steps(&mut steps[1..]);

    let buttons = [
        DirButton::Up,
        DirButton::Down,
        DirButton::Left,
        DirButton::Right,
        DirButton::A,
    ];

    buttons.iter().enumerate().for_each(|(i, start)| {
        buttons[i + 1..].iter().for_each(|end| {
            let (forward, reversed) = start
                .to(end)
                .into_iter()
                .flatten()
                .map(|mut path| {
                    path.steps -= 1;
                    let mut reversed = path.reversed();
                    let mut forward = path;
                    reversed.push(DirButton::A);
                    forward.push(DirButton::A);

                    (
                        forward
                            .path
                            .into_iter()
                            .flatten()
                            .fold((0, DirButton::A), |acc, next| {
                                let extension = steps[1][acc.1.index()][next.index()];

                                (acc.0 + extension, next)
                            })
                            .0,
                        reversed
                            .path
                            .into_iter()
                            .flatten()
                            .fold((0, DirButton::A), |acc, next| {
                                let extension = steps[1][acc.1.index()][next.index()];

                                (acc.0 + extension, next)
                            })
                            .0,
                    )
                })
                .fold((usize::MAX, usize::MAX), |acc, next| {
                    (acc.0.min(next.0), acc.1.min(next.1))
                });
            steps[0][start.index()][end.index()] = forward;
            steps[0][end.index()][start.index()] = reversed;
        });
    });
}

fn shortest_dir_path(path: Path, 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 = steps[0][pos.index()][next.index()];

            (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![[[1; 5]; 5]; 3];

    precompute_steps(&mut steps);

    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![[[1; 5]; 5]; 26];

    precompute_steps(&mut steps);

    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);
    }
}