(function (global, factory) {
  typeof exports === 'object' && typeof module !== 'undefined' ? factory(exports) :
  typeof define === 'function' && define.amd ? define(['exports'], factory) :
  (factory((global.d3_queue = {})));
}(this, function (exports) { 'use strict';

  var slice = [].slice;

  function noop() {}

  var noabort = {};

  function newQueue(concurrency) {
    if (!(concurrency >= 1)) throw new Error;

    var q,
        tasks = [],
        results = [],
        waiting = 0,
        active = 0,
        ended = 0,
        starting, // inside a synchronous task callback?
        error = null,
        notify = noop;

    function poke() {
      if (!starting) try { start(); } // let the current task complete
      catch (e) { if (tasks[ended + active - 1]) abort(e); } // task errored synchronously
    }

    function start() {
      while (starting = waiting && active < concurrency) {
        var i = ended + active,
            t = tasks[i],
            j = t.length - 1,
            c = t[j];
        t[j] = end(i);
        --waiting, ++active;
        t = c.apply(null, t);
        if (!tasks[i]) continue; // task finished synchronously
        tasks[i] = t || noabort;
      }
    }

    function end(i) {
      return function(e, r) {
        if (!tasks[i]) return; // ignore multiple callbacks
        --active, ++ended;
        tasks[i] = null;
        if (error != null) return; // ignore secondary errors
        if (e != null) {
          abort(e);
        } else {
          results[i] = r;
          if (waiting) poke();
          else if (!active) notify(error, results);
        }
      };
    }

    function abort(e) {
      var i = tasks.length, t;
      error = e; // ignore active callbacks
      results = undefined; // allow gc
      waiting = NaN; // prevent starting

      while (--i >= 0) {
        if (t = tasks[i]) {
          tasks[i] = null;
          if (t.abort) try { t.abort(); }
          catch (e) { /* ignore */ }
        }
      }

      active = NaN; // allow notification
      notify(error, results);
    }

    return q = {
      defer: function(callback) {
        if (typeof callback !== "function" || notify !== noop) throw new Error;
        if (error != null) return q;
        var t = slice.call(arguments, 1);
        t.push(callback);
        ++waiting, tasks.push(t);
        poke();
        return q;
      },
      abort: function() {
        if (error == null) abort(new Error("abort"));
        return q;
      },
      await: function(callback) {
        if (typeof callback !== "function" || notify !== noop) throw new Error;
        notify = function(error, results) { callback.apply(null, [error].concat(results)); };
        if (!active) notify(error, results);
        return q;
      },
      awaitAll: function(callback) {
        if (typeof callback !== "function" || notify !== noop) throw new Error;
        notify = callback;
        if (!active) notify(error, results);
        return q;
      }
    };
  }

  function queue(concurrency) {
    return newQueue(arguments.length ? +concurrency : Infinity);
  }

  var version = "2.0.1";

  exports.version = version;
  exports.queue = queue;

}));