path: root/day14b/src/main.rs

/// --- Part Two ---
///
/// You realize you misread the scan.  There isn't an endless void at the bottom of the scan -
/// there's floor, and you're standing on it!
///
/// You don't have time to scan the floor, so assume the floor is an infinite horizontal line with
/// a y coordinate equal to two plus the highest y coordinate of any point in your scan.
///
/// In the example above, the highest y coordinate of any point is 9, and so the floor is at y=11.
/// (This is as if your scan contained one extra rock path like -infinity,11 -> infinity,11.) With
/// the added floor, the example above now looks like this:
///
/// ```
///         ...........+........
///         ....................
///         ....................
///         ....................
///         .........#...##.....
///         .........#...#......
///         .......###...#......
///         .............#......
///         .............#......
///         .....#########......
///         ....................
/// <-- etc #################### etc -->
/// ```
///
/// To find somewhere safe to stand, you'll need to simulate falling sand until a unit of sand
/// comes to rest at 500,0, blocking the source entirely and stopping the flow of sand into the
/// cave.  In the example above, the situation finally looks like this after 93 units of sand come
/// to rest:
///
/// ```
/// ............o............
/// ...........ooo...........
/// ..........ooooo..........
/// .........ooooooo.........
/// ........oo#ooo##o........
/// .......ooo#ooo#ooo.......
/// ......oo###ooo#oooo......
/// .....oooo.oooo#ooooo.....
/// ....oooooooooo#oooooo....
/// ...ooo#########ooooooo...
/// ..ooooo.......ooooooooo..
/// #########################
/// ```
///
/// Using your scan, simulate the falling sand until the source of the sand becomes blocked.  How
/// many units of sand come to rest?
use clap::Parser;
use itertools::Itertools;
use nom::bytes::complete::tag;
use nom::character::complete::i32;
use nom::combinator::{map, opt};
use nom::error::{ContextError, ErrorKind as NomErrorKind, ParseError};
use nom::multi::many1;
use nom::sequence::{terminated, tuple};
use nom::IResult;

use std::fs::File;
use std::io::prelude::*;
use std::io::BufReader;
use std::path::PathBuf;

const FILEPATH: &'static str = "examples/input.txt";
const SAND_SOURCE: Coords = Coords { x: 500, y: 0 };
const FLOOR_OFFSET: usize = 2;

pub type Input<'a> = &'a str;
pub type Result<'a, T> = IResult<Input<'a>, T, Error<Input<'a>>>;

#[derive(Clone, Debug, PartialEq, Eq)]
pub enum ErrorKind {
    Nom(NomErrorKind),
    Context(&'static str),
    Custom(String),
}

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
enum Terrain {
    Air,
    Rock,
    Sand,
    SandSource,
}

#[derive(Parser, Debug)]
#[clap(author, version, about, long_about = None)]
struct Cli {
    #[clap(short, long, default_value = FILEPATH)]
    file: PathBuf,
}

#[derive(Clone, Debug, PartialEq, Eq)]
pub struct Error<I> {
    pub errors: Vec<(I, ErrorKind)>,
}

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
struct Coords {
    x: i32,
    y: i32,
}

impl Coords {
    fn path_between(&self, other: &Coords) -> Vec<Coords> {
        if self == other {
            panic!()
        }
        if self.x == other.x {
            let yrange = {
                if self.y > other.y {
                    other.y..=self.y
                } else if other.y > self.y {
                    self.y..=other.y
                } else {
                    panic!()
                }
            };
            yrange.map(|y| Coords { x: self.x, y }).collect_vec()
        } else {
            let xrange = {
                if self.x > other.x {
                    other.x..=self.x
                } else if other.x > self.x {
                    self.x..=other.x
                } else {
                    panic!()
                }
            };
            xrange.map(|x| Coords { x, y: self.y }).collect_vec()
        }
    }
}

#[derive(Clone, Debug)]
struct Grid {
    data: Vec<Vec<Terrain>>,
    bounds: GridBoundaries,
}

impl Grid {
    fn new(bounds: GridBoundaries) -> Self {
        let xlen = (bounds.maxx - bounds.minx) as usize + 1;
        let ylen = (bounds.maxy - bounds.miny) as usize + 1;
        let data = vec![vec![Terrain::Air; ylen]; xlen];
        Self { data, bounds }
    }

    fn get(&self, coord: &Coords) -> &Terrain {
        &self.data[(coord.x - self.bounds.minx) as usize][(coord.y - self.bounds.miny) as usize]
    }

    fn set(&mut self, coord: &Coords, terrain: Terrain) {
        self.data[(coord.x - self.bounds.minx) as usize][(coord.y - self.bounds.miny) as usize] =
            terrain;
    }

    fn populate(&mut self, geography: Vec<RockStructure>) {
        geography
            .into_iter()
            .map(|rstruct| {
                rstruct
                    .0
                    .as_slice()
                    .windows(2)
                    .map(|window| window[0].path_between(&window[1]))
                    .flatten()
                    .collect_vec()
            })
            .flatten()
            .scan(self, |state, coords| {
                state.data[(coords.x - state.bounds.minx) as usize]
                    [(coords.y - state.bounds.miny) as usize] = Terrain::Rock;
                Some(())
            })
            .last()
            .unwrap()
    }

    fn run(&mut self) -> usize {
        let mut count = 0;
        let mut pos = SAND_SOURCE;
        let mut history = vec![SAND_SOURCE];
        'block: loop {
            for (dx, dy) in [(0, 1), (-1, 1), (1, 1)] {
                let next_pos = Coords {
                    x: &pos.x + dx,
                    y: &pos.y + dy,
                };

                if (next_pos.x < self.bounds.minx)
                    || (next_pos.x > self.bounds.maxx)
                {
                    continue;
                }

                if self.get(&next_pos) == &Terrain::Air {
                    history.push(next_pos);
                    pos = next_pos;
                    continue 'block;
                }
            }

            count += 1;
            if pos == SAND_SOURCE {
                break 'block;
            }
            self.set(&pos, Terrain::Sand);
            pos = {
                loop {
                    let prev_pos = history[history.len() - 1];
                    match self.get(&prev_pos) {
                        Terrain::Air | Terrain::SandSource => break prev_pos,
                        _ => {
                            let _ = history.pop();
                            continue;
                        }
                    };
                }
            };
        }
        count
    }
}

#[derive(Clone, Debug)]
struct GridBoundaries {
    minx: i32,
    maxx: i32,
    miny: i32,
    maxy: i32,
}

impl GridBoundaries {
    fn new() -> Self {
        GridBoundaries {
            minx: i32::max_value(),
            maxx: i32::min_value(),
            miny: 0,
            maxy: i32::min_value(),
        }
    }

    fn combine(&mut self, other: &GridBoundaries) {
        self.minx = i32::min(self.minx, other.minx);
        self.maxx = i32::max(self.maxx, other.maxx);
        self.miny = i32::min(self.miny, other.miny);
        self.maxy = i32::max(self.maxy, other.maxy);
    }

    fn fold(&self, other: &Coords) -> Self {
        let mut res = GridBoundaries::new();
        res.minx = i32::min(self.minx, other.x);
        res.maxx = i32::max(self.maxx, other.x);
        res.miny = i32::min(self.miny, other.y);
        res.maxy = i32::max(self.maxy, other.y);
        res
    }
}

#[derive(Clone, Debug, PartialEq, Eq)]
struct RockStructure(Vec<Coords>);

impl<I> ParseError<I> for Error<I> {
    fn from_error_kind(input: I, kind: NomErrorKind) -> Self {
        let errors = vec![(input, ErrorKind::Nom(kind))];
        Self { errors }
    }

    fn append(input: I, kind: NomErrorKind, mut other: Self) -> Self {
        other.errors.push((input, ErrorKind::Nom(kind)));
        other
    }
}

impl<I> ContextError<I> for Error<I> {
    fn add_context(input: I, ctx: &'static str, mut other: Self) -> Self {
        other.errors.push((input, ErrorKind::Context(ctx)));
        other
    }
}

fn parse_coord(input: &str) -> Result<Coords> {
    map(tuple((terminated(i32, tag(",")), i32)), |(x, y)| Coords {
        x,
        y,
    })(input)
}

fn parse_line(input: &str) -> Result<RockStructure> {
    map(many1(terminated(parse_coord, opt(tag(" -> ")))), |v| {
        RockStructure(v)
    })(input)
}

fn main() {
    let args = Cli::parse();

    let file = File::open(&args.file).unwrap();
    let reader = BufReader::new(file);

    let mut bounds = GridBoundaries::new();
    let mut geography = reader
        .lines()
        .map(|l| parse_line(l.unwrap().as_str()).unwrap().1)
        .scan(&mut bounds, |state, rstruct| {
            state.combine(
                &rstruct
                    .0
                    .iter()
                    .fold(GridBoundaries::new(), |acc, x| acc.fold(x)),
            );
            Some(rstruct)
        })
        .collect_vec();

    bounds.maxy = bounds.maxy + FLOOR_OFFSET as i32;
    let max_x_len = 2 * bounds.maxy - 1;
    let minx = SAND_SOURCE.x - (max_x_len - 1) / 2;
    let maxx = SAND_SOURCE.x + (max_x_len - 1) / 2;
    assert!(minx < bounds.minx);
    assert!(maxx > bounds.maxx);
    bounds.minx = minx;
    bounds.maxx = maxx;

    geography.push(RockStructure(vec![
        Coords {
            x: minx,
            y: bounds.maxy,
        },
        Coords {
            x: maxx,
            y: bounds.maxy,
        },
    ]));

    let mut grid = Grid::new(bounds);
    grid.set(&SAND_SOURCE, Terrain::SandSource);
    grid.populate(geography);
    assert_eq!(grid.get(&SAND_SOURCE), &Terrain::SandSource);

    let res = grid.run();
    println!("{res:?}");
}