Shared via Rust Playground

playground.rs

use std::io::Write;

struct LfIter<I> where I: Iterator {
    iter: I,
    lf_every: usize,
    items: usize
}

impl<I> LfIter<I> where I: Iterator {
    fn new(iter: I, lf_every: usize) -> LfIter<I> {
        LfIter {
            iter: iter,
            lf_every: lf_every,
            items: 0
        }
    }
}

impl<I> Iterator for LfIter<I> where I: Iterator<Item=u8> {
    type Item = u8;
    
    fn next(&mut self) -> Option<Self::Item> {
        if self.items >= self.lf_every {
            self.items = 0;
            Some(SEP)
        } else {
            self.items = self.items + 1;
            self.iter.next()
        }
    }
}

const SEP: u8 = 10u8; // LF
const BUF_SIZE: usize = 10; // 64 * 1024
const WRAP_LEN: usize = 4; // 60

fn main() {
    let d = "ABCDEFGHIJKLNMOPQRSTUVWXYZ".as_bytes();
    let sout = std::io::stdout();
    let mut out = sout.lock();

    let mut lf_iter =  LfIter::new(d.iter().cloned(), WRAP_LEN);
    
    let mut buf = [0 as u8; BUF_SIZE];
    loop {
        for i in 0..BUF_SIZE {
            if let Some(b) = lf_iter.next() {
                buf[i] = b;
            } else {
                let _ = out.write(&buf[0..i]);
                return;
            }
        }
        
        let _ = out.write(&buf);
    }
}