function isOriginCompatible(a, b) {
  return !a || a === '*' || a.slice(0, b.length) === b
}

module.exports = function (win, origin) {
  var cbs = [], queue = [], _read

  if(origin && origin[0] === '/') {
    origin = location.origin + origin
  }

  function gotWin(_win) {
    win = _win
    if(_read) doRead()
  }

  function onMessage(e) {
    if(isOriginCompatible(origin, e.origin)) {
      if(!win) gotWin(e.source)
      if(!origin) origin = e.origin
      if(cbs.length) cbs.shift()(null, e.data)
      else queue.push(e.data)
    }
  }

  var sinkEnded, sourceEnded

  window.addEventListener('message', onMessage, false)

  return {
    source: function (end, cb) {
      if(sourceEnded = end) {
        window.removeEventListener('message', onMessage, false)
        return cb(null)
      }
      if(queue.length) cb(null, queue.shift())
      else cbs.push(cb)
    },

    sink: function (read) {
      _read = read
      if(win) doRead()
    }
  }

  function doRead() {
    _read(null, function next(end, data) {
      if(sinkEnded = end) return
      win.postMessage(data, origin || '*')
      _read(null, next)
    })
  }
}