lazy-read.js

'use strict';

var fs = require('fs');
var path = require('path');

function lazyRead(path, chunkFn, done) {
  var input = fs.createReadStream(path)
    .on('readable', handleReadable)
    .on('end', handleEnd);

  var busy = 0;
  var readable = false;
  var ended = false;

  function handleReadable() {
    readable = true;
    if (!busy) readChunk();
  }

  function handleEnd() {
    ended = true;
    if (!busy) done();
  }

  function chunkDone() {
    if (--busy === 0) {
      if (readable) readChunk();
      else if (ended) done();
    }
  }

  function readChunk() {
    var chunk;
    while (null !== (chunk = input.read())) {
      busy++;
      chunkFn(chunk, chunkDone);
    }
    readable = false;
  }
}