aoc_2024/day12/src/main.rs

use std::collections::HashSet;
use std::collections::HashMap;
use std::fs;
use std::env;
use crate::Dir::{North, South, East, West };

// nodes are (coord, direction) pairs.
// neighbors are step1+turnleft, step2+turnleft, step3+turnleft, step1+turnright, etc
#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
enum Dir { North, South, East, West }

fn step(start: &Coord, d: Dir, steps: i32) -> Coord {
    match d {
        North => (start.0, start.1 - steps),
        South => (start.0, start.1 + steps),
        East => (start.0 + steps, start.1),
        West => (start.0 - steps, start.1),
    }
}

type Coord = (i32, i32);

fn main() {
    println!("Hello, AoC day 04!");

    let args: Vec<String> = env::args().collect();
    if args.len() != 2 {
        println!("wrong number of arguments!");
        std::process::exit(1);
    }

    let file_path = &args[1];

    let contents = fs::read_to_string(file_path).expect("Should have been able to read the file");
    let mut grid: HashMap<Coord, char> = HashMap::new();

    let mut x;
    // build our grid!
    let mut y = 0;
    for line in contents.lines() {
        x = 0;
        for c in line.chars() {
            let coords = (x,y);
            grid.insert(coords, c);
            x += 1;
        }
        y += 1;
    }


    // part a
    let mut sum = 0;

    let mut cache: Vec<HashSet<Coord>>= Vec::new();
    for (location, _color) in &grid {
        cache = fill_region_and_cache(location, &grid, cache);
    }

    println!("we found {} regions", cache.len());
    for region in &cache {
        println!("a region with color {} was found to have {} squares", 
            grid.get(region.iter().nth(0).unwrap()).unwrap(),
            region.len());
    }

    for region in &cache {
        let mut total_perimeter = 0;
        let mut area = 0;
        for location in region {
            total_perimeter += perimeter(&location, &grid);
            area += 1;
        }
        sum += area * total_perimeter;
    }

    println!("the total cost is {sum}");


    // part b
    // a list of sides (a side is a set of sidesegments)
    let mut sides_cache: Vec<HashSet<SideSegment>> = Vec::new();

    // for every location in the grid, 
    for (location, _color) in &grid {
        // for every side ajdacent to that location, update the side cache with its side. 
        for sideseg in sides(location, &grid) {
            sides_cache = spread_over_side_and_cache(
                &sideseg, 
                &grid, 
                sides_cache);
        }
    }

    sum = 0;

    for region in &cache {
        let mut sides = 0;
        let mut area = 0;
        for side in &sides_cache {
            let first_location = side.iter().nth(0).unwrap().1;
            if region.contains(&first_location) {
                sides += 1;
            }
        }

        area += region.len() as i32;

        let color = grid.get(region.iter().nth(0).unwrap()).unwrap();

        println!("a region with label {color} has area {area} and {sides} sides");

        sum += area * sides;
    }

    println!("the total cost is {sum}");



}

fn neighbors(head: &Coord) -> Vec<Coord> {
    return vec![North, South, East, West]
        .into_iter()
        .map(|x| step(head, x, 1))
        .collect();
}

fn same_neighbors(location: &Coord, grid: &HashMap<Coord, char>) -> Vec<Coord> {
    if let Some(me) = grid.get(location) {
        return vec![North, South, East, West]
            .into_iter()
            .map(|x| step(location, x, 1))
            .filter(|x| grid.contains_key(x))
            .filter(|x| me == grid.get(x).expect("tried to look for a missing grid square"))
            .collect();
    }
    return vec![];
}

fn fill_region_and_cache(
    location: &Coord, 
    grid: &HashMap<Coord, char>, 
    mut cache: Vec<HashSet<Coord>>
) -> Vec<HashSet<Coord>> {
    let mut new_region: HashSet<Coord> = HashSet::from([*location]);
    let mut accumulator: HashSet<Coord> = HashSet::new();
    while !new_region.is_empty() {
        let mut blah: HashSet<Coord> = HashSet::new();
        for x in &new_region {
            for existing_region in &cache {
                if existing_region.contains(x) {
                    return cache;
                }
            }
        }

        for x in &new_region {
            for n in same_neighbors(&x, grid) {
                if !accumulator.contains(&n) {
                    blah.insert(n);
                }
            }
        }

        for x in new_region.drain() {
            accumulator.insert(x);
        }

        new_region = blah;
    }

    cache.push(accumulator);

    return cache;
}

fn spread_over_side_and_cache(
    side: &SideSegment,
    grid: &HashMap<Coord, char>, 
    mut cache: Vec<HashSet<SideSegment>>
) -> Vec<HashSet<SideSegment>> {
    let mut wavefront: HashSet<SideSegment> = HashSet::from([*side]);
    let mut accumulator: HashSet<SideSegment> = HashSet::new();
    while !wavefront.is_empty() {
        // we're iterating over the wavefront of squares to look at for whether they're 
        // part of the side we're building.

        let mut new_wavefront: HashSet<SideSegment> = HashSet::new();
        
        // look at each side in the wavefront
        for x in &wavefront {
            // if it's in a side that we already have cached, then we're done.
            // this loop is what makes this slow i bet
            for existing_side in &cache {
                if existing_side.contains(x) {
                    // return the cache unchanged.
                    return cache;
                }
            }
        }

        // look at each side in the wavefrnot, to build up a new wavefront.
        for SideSegment(dir, coord) in &wavefront {

            for n in same_neighbors(&coord, grid) 
                // look at neighbors in this region.
                .into_iter()
                .flat_map(|next_coord| sides(&next_coord, grid)) 
                // look at sides of those neighbors.
                .filter(|SideSegment(dir2, _coord2)| dir2 == dir) { 
                // filter down to just sides in the same direction.
                // n is a neighboring side with the same direction as us.
                if !accumulator.contains(&n) {
                    // if n isn't already in the accumulator, add n to the new wavefront.
                    new_wavefront.insert(n);
                }
            }
        }

        // add all the siddes in the wavefront to the accumulator.
        for x in &wavefront {
            accumulator.insert(*x);
        }

        wavefront = new_wavefront;
    }

    cache.push(accumulator);

    return cache;
}

fn perimeter(location: &Coord, grid: &HashMap<Coord, char>) -> i32 {
    let mut perimeter = 0;
    for neighbor in neighbors(location) {
        let neighboring_region = grid.get(&neighbor);
        if neighboring_region != grid.get(location) {
            perimeter += 1;
        }
    }
    return perimeter;
}

#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
struct SideSegment (Dir, Coord);

fn sides(location: &Coord, grid: &HashMap<Coord, char>) -> Vec<SideSegment> {
    return vec![North, South, East, West]
        .into_iter()
        .map(|x| SideSegment(x, *location) )
        .filter(|SideSegment(dir, loc)| grid.get(loc) != grid.get(&step(loc, *dir, 1)) )
        .collect();
}