I forgot that this started yesterday, so I’m already behind. I quite like my solution for part one, but part two will have to wait.

Rust

use color_eyre::eyre;
use std::{fs, num, str::FromStr};

#[derive(Debug, PartialEq, Eq)]
struct Report(Vec<isize>);

impl FromStr for Report {
    type Err = num::ParseIntError;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let v: Result<Vec<isize>, _> = s
            .split_whitespace()
            .map(|num| num.parse::<isize>())
            .collect();
        Ok(Report(v?))
    }
}

impl Report {
    fn is_safe(&self) -> bool {
        let ascending = self.0[1] > self.0[0];
        let (low, high) = if ascending { (1, 3) } else { (-3, -1) };
        self.0.windows(2).all(|w| {
            let a = w[0];
            let b = w[1];
            b >= a + low && b <= a + high
        })
    }
}

fn main() -> eyre::Result<()> {
    color_eyre::install()?;

    let part1 = part1("d02/input.txt")?;
    println!("Part 1: {part1}");
    Ok(())
}

fn part1(filepath: &str) -> eyre::Result<isize> {
    let mut num_safe = 0;
    for l in fs::read_to_string(filepath)?.lines() {
        if Report::from_str(l)?.is_safe() {
            num_safe += 1;
        }
    }
    Ok(num_safe)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn sample_part1() {
        assert_eq!(part1("test.txt").unwrap(), 2);
    }
}