day2.rs

use std::cmp::Ordering;

fn parse(data: &str) -> impl Iterator<Item = Vec<i32>> + use<'_> {
    data.lines().flat_map(|line| {
        line.split_ascii_whitespace()
            .map(|level| level.parse::<i32>().ok())
            .collect::<Option<Vec<_>>>()
    })
}

fn is_safe_1(report: &[i32]) -> bool {
    report
        .iter()
        .zip(report.iter().skip(1))
        .try_fold(Ordering::Equal, |cmp, (x, y)| {
            if x != y && (x - y).abs() <= 3 {
                Some(x.cmp(y)).filter(|cmp2| cmp != cmp2.reverse())
            } else {
                None
            }
        })
        .is_some()
}

fn is_safe_2(report: &[i32]) -> bool {
    if report.is_empty() {
        return true;
    }
    let mut report2 = report[1..].to_vec();
    is_safe_1(&report2)
        || (0..report2.len()).any(|i| {
            report2[i] = report[i];
            is_safe_1(&report2)
        })
}

pub fn part1(data: &str) -> usize {
    parse(data).filter(|report| is_safe_1(report)).count()
}

pub fn part2(data: &str) -> usize {
    parse(data).filter(|report| is_safe_2(report)).count()
}

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

    static EXAMPLE: &str = indoc! {"
        7 6 4 2 1
        1 2 7 8 9
        9 7 6 2 1
        1 3 2 4 5
        8 6 4 4 1
        1 3 6 7 9
    "};

    #[test]
    fn part1_examples() {
        assert_eq!(2, part1(EXAMPLE));
    }

    #[test]
    fn part2_examples() {
        assert_eq!(4, part2(EXAMPLE));
    }
}