Bostocks_queue_simplified_untested.js

// Context: https://news.ycombinator.com/item?id=5474833
(function() {
  if (typeof module === "undefined") self.queue = queue;
  else module.exports = queue;
  queue.version = "1.0.3dev";

  var slice = [].slice;

  function queue(parallelism) {
    var queue = {},
        error = null,       // The first error result, if any.
        ncalled = 0,        // Number of deferrals called so far
        nresults = 0,       // Number of fulfilled deferrals
        array = [],         // The first ncalled are results (undefined
                            // if in flight); the rest are deferrals.
        awaitAll = false,   // true iff we're to pass await an array to await
        await = noop;       // Function to call on when all resolved.

    if (!parallelism) parallelism = Infinity;

    // Invariants:
    //   0 <= nresults <= ncalled <= array.length
    //   # in flight <= parallelism. That is,
    //   ncalled - nresults <= parallelism
    //   # queued === 0 unless # in flight is at max. That is,
    //   ncalled === min(array.length, nresults + parallelism)
    //   (N.B. This invariant implies the previous one.)

    queue.defer = function() {
      if (!error) {
        array.push(arguments);
        pop();
      }
      return queue;
    };

    queue.await = function(f) {
      awaitAll = false;
      await = f;
      if (nresults === array.length) notify();
      return queue;
    };

    queue.awaitAll = function(f) {
      awaitAll = true;
      await = f;
      if (nresults === array.length) notify();
      return queue;
    };

    function pop() {
      if (Math.min(array.length, nresults + parallelism) <= ncalled)
        return;
      var i = ncalled++;
      var fn = array[i][0], args = slice.call(array[i], 1);
      args.push(function(e, r) {
        if (error !== null) return;
        if (e !== null) {
          error = e;
          array = [], nresults = ncalled = 0;
          notify();
        } else {
          array[i] = r;
          if (++nresults === array.length) notify();
          else pop();
        }
      });
      fn.apply(null, args);
    }

    function notify() {
      if (awaitAll) await(error, array);
      else await.apply(null, [error].concat(array));
    }

    return queue;
  }

  function noop() {}
})();