2022/src/bin/day14.rs

use std::{
    cmp::max,
    collections::{BTreeSet, VecDeque},
    str::FromStr,
};

use aoc_2022::*;

type Input = Vec<Line>;
type Output = usize;

type Coord = Vector2D<i32>;

#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Clone)]
struct Line {
    from: Coord,
    to: Coord,
}

#[derive(Debug)]
struct Multiline {
    lines: BTreeSet<Line>,
}

impl FromStr for Multiline {
    type Err = Report;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        fn parse_coord(s: &str) -> Result<Coord, Report> {
            let (x, y) = s.split(',').collect_tuple().unwrap();

            Ok(Coord::new(x.parse()?, y.parse()?))
        }

        let segs: Vec<Coord> = s
            .split(" -> ")
            .map(|c| parse_coord(c).unwrap())
            .collect_vec();
        let mut lines: BTreeSet<Line> = BTreeSet::new();

        for i in 0..segs.len() - 1 {
            lines.insert(Line {
                from: segs[i],
                to: segs[i + 1],
            });
        }

        Ok(Multiline { lines })
    }
}

fn can_move(grain: &Coord, set: &BTreeSet<Coord>) -> Option<Coord> {
    for direction in [Coord::new(0, 1), Coord::new(-1, 1), Coord::new(1, 1)] {
        let new_position = *grain + direction;

        if !set.contains(&new_position) {
            return Some(new_position);
        }
    }

    None
}

fn simulate_falling<F>(lines: &Input, terminate: F) -> usize
where
    F: Fn(&Coord, bool) -> bool,
{
    let mut grains: VecDeque<Coord> = VecDeque::new();
    let mut set_grains: BTreeSet<Coord> = BTreeSet::new();

    // convert lines to points
    for line in lines {
        let d = Coord::new(
            (line.to.x() - line.from.x()).signum(),
            (line.to.y() - line.from.y()).signum(),
        );

        let mut p = line.from;
        while p != line.to {
            set_grains.insert(p);
            p = p + d;
        }
        set_grains.insert(p);
    }
    let lines_count = set_grains.len();

    loop {
        for i in (0..grains.len()).rev() {
            let grain = grains[i];

            let mut is_set = false;
            if let Some(new_grain) = can_move(&grain, &set_grains) {
                grains[i] = new_grain;
            } else {
                let grain = grains.pop_back().unwrap();
                set_grains.insert(grain);
                is_set = true;
            }

            if terminate(&grain, is_set) {
                return set_grains.len() - lines_count;
            }
        }

        grains.push_front(Coord::new(500, 0));
    }
}

struct Day14;
impl Solution<Input, Output> for Day14 {
    fn parse_input<P: AsRef<Path>>(pathname: P) -> Input {
        file_to_structs::<P, Multiline>(pathname)
            .into_iter()
            .flat_map(|m| m.lines)
            .collect_vec()
    }

    fn part_1(input: &Input) -> Output {
        let max_y = input
            .iter()
            .map(|l| max(l.from.y(), l.to.y()))
            .max()
            .unwrap();

        let terminate = |grain: &Coord, _is_set: bool| grain.y() >= max_y;

        simulate_falling(input, terminate)
    }

    fn part_2(input: &Input) -> Output {
        fn terminate(grain: &Coord, is_set: bool) -> bool {
            is_set && grain == &Coord::new(500, 0)
        }

        let y = input
            .iter()
            .map(|l| max(l.from.y(), l.to.y()))
            .max()
            .unwrap();

        let mut lines = input.clone();
        lines.push(Line {
            from: Coord::new(500 - (y + 2), y + 2),
            to: Coord::new(500 + (y + 2), y + 2),
        });

        simulate_falling(&lines, terminate)
    }
}

fn main() -> Result<()> {
    // Day14::run("sample")
    Day14::main()
}

test_sample!(day_14, Day14, 24, 93);