ju-tcs-tbop-24-lib-dcfk 1.0.1

use std::{
    fs::File,
    io::{self, BufRead},
    path::Path,
};

/// Returns n first lines from given file.
///
/// If the file does not contain n lines, then whole content is returned.
///
/// # Errors
///
/// This function panics if there is any problem when opening
/// or reading from the file.
///
pub fn head(path: &Path, n: usize) -> Vec<String> {
    let file = File::open(path).unwrap();
    let lines = io::BufReader::new(file).lines();
    lines.take(n).map(|f| f.unwrap()).collect()
}

fn get_line_count(path: &Path) -> usize {
    let file = File::open(path).unwrap();
    let lines = io::BufReader::new(file).lines();
    lines.count()
}

/// Returns n last lines from given file.
///
/// If the file does not contain n lines, then whole content is returned.
///
/// # Errors
///
/// This function panics if there is any problem when opening
/// or reading from the file.
///
pub fn tail(path: &Path, n: usize) -> Vec<String> {
    let total = get_line_count(path);
    let file = File::open(path).unwrap();
    let lines = io::BufReader::new(file).lines();
    lines
        .skip(total.saturating_sub(n))
        .map(|f| f.unwrap())
        .collect()
}

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

    #[test]
    #[should_panic]
    fn test_head_panic() {
        let _res = head(Path::new("abc"), 1usize);
    }

    #[test]
    #[should_panic]
    fn test_tail_panic() {
        let _res = tail(Path::new("abc"), 1usize);
    }

    #[test]
    fn test_head() {
        let res = head(Path::new("./test_files/test_file.txt"), 2usize);
        assert_eq!(res, vec!["Hello".to_string(), "Hello, World!".to_string()]);
    }

    #[test]
    fn test_tail() {
        let res = tail(Path::new("./test_files/test_file.txt"), 3usize);
        assert_eq!(
            res,
            vec!["abc".to_string(), "def".to_string(), "xyz".to_string()]
        );
    }
}