HORIZON PROJECT

var wrappy = require('wrappy')

var reqs = Object.create(null)

var once = require('once')

module.exports = wrappy(inflight)

function inflight (key, cb) {

  if (reqs[key]) {

    reqs[key].push(cb)

    return null

  } else {

    reqs[key] = [cb]

    return makeres(key)

  }

}

function makeres (key) {

  return once(function RES () {

    var cbs = reqs[key]

    var len = cbs.length

    var args = slice(arguments)

    // XXX It's somewhat ambiguous whether a new callback added in this

    // pass should be queued for later execution if something in the

    // list of callbacks throws, or if it should just be discarded.

    // However, it's such an edge case that it hardly matters, and either

    // choice is likely as surprising as the other.

    // As it happens, we do go ahead and schedule it for later execution.

    try {

      for (var i = 0; i < len; i++) {

        cbs[i].apply(null, args)

      }

    } finally {

      if (cbs.length > len) {

        // added more in the interim.

        // de-zalgo, just in case, but don't call again.

        cbs.splice(0, len)

        process.nextTick(function () {

          RES.apply(null, args)

        })

      } else {

        delete reqs[key]

      }

    }

  })

}

function slice (args) {

  var length = args.length

  var array = []

  for (var i = 0; i < length; i++) array[i] = args[i]

  return array

}