aoc2020_day_03.rs

use std::collections::HashSet;
use std::fs::read_to_string;

fn main() {
    let input = read_to_string("input.txt").unwrap();

    let mut x = 0;
    let mut max_x = 0;
    let mut y = 0;
    let mut trees: HashSet<(u16, u16)> = HashSet::new();
    for c in input.chars() {
        if c == '\n' {
            y = y + 1;
            max_x = x - 1;
            x = 0;
            continue;
        }

        if c == '#' {
            trees.insert((x, y));
        }

        x = x + 1;
    }

    let max_y = y - 1;

    let rt1_dn1 = count_trees(&trees, max_x, max_y, 1, 1);
    let rt3_dn1 = count_trees(&trees, max_x, max_y, 3, 1);
    let rt5_dn1 = count_trees(&trees, max_x, max_y, 5, 1);
    let rt7_dn1 = count_trees(&trees, max_x, max_y, 7, 1);
    let rt1_dn2 = count_trees(&trees, max_x, max_y, 1, 2);

    println!("Tree count for RT1, DN1: {}", rt1_dn1);
    println!("Tree count for RT3, DN1: {}", rt3_dn1);
    println!("Tree count for RT5, DN1: {}", rt5_dn1);
    println!("Tree count for RT7, DN1: {}", rt7_dn1);
    println!("Tree count for RT1, DN2: {}", rt1_dn2);

    let product: u64 = rt1_dn1 * rt3_dn1 * rt5_dn1 * rt7_dn1 * rt1_dn2;
    println!("Product: {}", product);
}

fn count_trees(
    trees: &HashSet<(u16, u16)>,
    max_x: u16,
    max_y: u16,
    right_count: u16,
    down_count: u16,
) -> u64 {
    let mut tob_x = 0;
    let mut tob_y = 0;
    let mut tree_count = 0;
    loop {
        if trees.contains(&(tob_x, tob_y)) {
            tree_count = tree_count + 1;
        }

        tob_x = (tob_x + right_count) % (max_x + 1);
        tob_y = tob_y + down_count;

        if tob_y > max_y {
            break;
        }
    }

    tree_count
}