module.exports = runParallel

function runParallel (tasks, cb) {
  var results, pending, keys
  var isSync = true

  if (Array.isArray(tasks)) {
    results = []
    pending = tasks.length
  } else {
    keys = Object.keys(tasks)
    results = {}
    pending = keys.length
  }

  function done (err) {
    function end () {
      if (cb) cb(err, results)
      cb = null
    }
    if (isSync) process.nextTick(end)
    else end()
  }

  function each (i, err, result) {
    results[i] = result
    if (--pending === 0 || err) {
      done(err)
    }
  }

  if (!pending) {
    // empty
    done(null)
  } else if (keys) {
    // object
    keys.forEach(function (key) {
      tasks[key](function (err, result) { each(key, err, result) })
    })
  } else {
    // array
    tasks.forEach(function (task, i) {
      task(function (err, result) { each(i, err, result) })
    })
  }

  isSync = false
}