use std::fs::File;
use std::io::prelude::*;
use std::io::ErrorKind;
use std::io::Result;
use std::time::Instant;

fn main() {
    let now = Instant::now();
    copy_n_bytes(
        "/path/to/20_megbayte/file",
        "path/to/target/file",
        std::usize::MAX,
        1 << 5,
    )
    .unwrap();
    println!("Time elapsed: {}ms", now.elapsed().as_millis());
}

fn copy_n_bytes(source: &str, dest: &str, n: usize, cache: usize) -> Result<()> {
    let mut src = File::open(source)?;
    let mut dst = File::create(dest)?;

    let mut buf = vec![0; cache];
    let mut bytes_read: usize = 0;
    let mut k;

    while bytes_read < n {
        match src.read(&mut buf) {
            Ok(n) => {
                k = n;
            }
            Err(e) => match e.kind() {
                ErrorKind::Interrupted => {
                    continue;
                }
                _ => {
                    eprintln!("Error {:?}", e);
                    return Err(e);
                }
            },
        }
        if k == 0 {
            println!("EOF reached");
            break;
        }
        dst.write_all(&buf[..k])?;
        bytes_read += k;
    }

    Ok(())
}