ex_merge_sort 0.4.0

use io::prelude::BufRead;
use std::fs;
use std::io;
use std::io::Seek;
use std::io::Write;

#[derive(Debug, PartialEq)]
pub(crate) enum RoughCount {
    Zero,
    One,
    Two,
    ThreeOrMore,
}

pub(crate) fn count_roughly(f: &fs::File) -> io::Result<RoughCount> {
    let mut reader = io::BufReader::new(f);
    let mut buf = String::new();
    let mut n = 0;

    while reader.read_line(&mut buf)? > 0 {
        buf.clear();
        n += 1;

        if n > 2 {
            break;
        }
    }

    let mut f = reader.into_inner();
    f.seek(io::SeekFrom::Start(0))?;

    let rc = match n {
        0 => RoughCount::Zero,
        1 => RoughCount::One,
        2 => RoughCount::Two,
        _ => RoughCount::ThreeOrMore,
    };

    Ok(rc)
}

pub(crate) fn rewind(mut f: &fs::File) -> io::Result<u64> {
    f.seek(io::SeekFrom::Start(0))
}

pub(crate) fn copy<T>(fin: &fs::File, fout: T) -> io::Result<()>
where
    T: io::Write,
{
    let mut reader = io::BufReader::new(fin);
    let mut writer = io::BufWriter::new(fout);
    let mut buf = String::new();

    while reader.read_line(&mut buf)? > 0 {
        writer.write(buf.as_bytes())?;
        buf.clear();
    }

    Ok(())
}