cg.js

import Promise from 'bluebird';
import map from 'lodash/fp/map';

/**
 * @module utils/concurrentGenerator
 */

export default function ConcurrentGenerator ({
  concurrency = 5,
  items = [],
  operation = () => {}
}) {
  function* generator () {
    let ret = [];

    for (let i = 0; i < items.length; i++) {
      ret[i] = yield operation(items[i]);
    }

    return ret;
  }

  return runGenerator.bind(null, generator, concurrency);
}

function runGenerator (generator, concurrency) {
  return new Promise(resolve => {
    const it = generator();
    const iteratees = [];

    // asynchronously iterate over generator
    function iterate (){
      const iterator = it.next();
      if (!iterator.done) {
        iteratees.push(iterator);
        return iterator.value.then(() => iterate());
      } else {
        return Promise.all(map(n => n.value, iteratees))
          .then(result => resolve(result));
      }
    }

    for (let i = 0; i < concurrency; i++) {
      iterate();
    }
  });
}