More idiomatic rust

main.rs

use anyhow::{Context, Error, Result};
use itertools::Itertools;
use regex::Regex;
use std::{
    fs::File,
    io::{BufRead, BufReader},
};

fn main() -> Result<()> {
    let file_path = std::env::args()
        .nth(1)
        .expect("expected pathname argument");

    let file = File::open(&file_path)
        .context(format!("Unable to open {}", file_path))
        .map_err(Error::msg)?;

    println!("slowest 10 files:");
    for (file, speed) in slowest(10, BufReader::new(file))? {
        println!("{file} {speed}s");
    }
    Ok(())
}

fn slowest<R: BufRead>(n: usize, iterator: R) -> Result<Vec<(String, f64)>> {
    let re = Regex::new(r"took ([\d.]+) s")?;
    let mut times: Vec<(String, f64)> = BufReader::new(iterator)
        .lines()
        .map(|line| line.expect("error reading file"))
        .filter(|line| !line.is_empty())
        .chunks(2)
        .into_iter()
        .map(|mut chunk| {
            let filename = chunk
                .next()
                .expect("couldn't find the first line of a pair of lines");
            let statline = chunk
                .next()
                .expect("couldn't find the second line of a pair of lines");
            let caps = re
                .captures(&statline)
                .expect("couldn't take a regex of the stat line");
            (
                filename,
                caps.get(1)
                    .map_or("", |m| m.as_str())
                    .parse::<f64>()
                    .expect("couldn't parse a f64"),
            )
        })
        .collect();
    times.sort_by(|(_, speeda), (_, speedb)| speedb.partial_cmp(speeda).unwrap());
    Ok(times.into_iter().take(n).collect())
}

#[cfg(test)]
mod test {
    use std::io::Cursor;

    use super::slowest; // super slowest ROFL
    #[test]
    fn test_longest() {
        let lines = "file1\ntook 1 s\nfile2\ntook 2 s";
        assert_eq!(
            slowest(1, Cursor::new(lines)).unwrap(),
            vec![("file2".to_owned(), 2.0)]
        );
    }
}