function fixJSON(key, value) {
  if (value instanceof Error) return {
    message: value.message,
    stack: value.stack,
    name: value.name,
    code: value.code,
  }
  return value
}

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

var cmds = {
  read: 'r',
  readEnd: 'R',
  next: 'n',
  nextEnd: 'N',
}

module.exports = function (win, origin) {
  var _cb, _msg

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

  function post(cmd, data) {
    var msg = cmd + data
    if (!win) _msg = msg
    else win.postMessage(msg, origin || '*')
  }

  function gotWin(_win) {
    win = _win
    if (_msg) {
      var msg = _msg
      _msg = null
      post(msg)
    }
  }

  return function (read) {
    function onMessage(e) {
      if(isOriginCompatible(origin, e.origin)) {
        if(!win) gotWin(e.source)
        if(!origin) origin = e.origin
        switch (e.data[0]) {
          case cmds.read:     return onRead(null)
          case cmds.readEnd:  return onRead(JSON.parse(e.data.substr(1)))
          case cmds.next:     return onNext(null, e.data.substr(1))
          case cmds.nextEnd:  return onNext(JSON.parse(e.data.substr(1)))
        }
      }
    }

    function onRead(end) {
      read(end, function (end, data) {
        if (end) post(cmds.nextEnd, JSON.stringify(end, fixJSON))
        else post(cmds.next, data)
      })
    }

    function onNext(end, data) {
      if (end) window.removeEventListener('message', onMessage, false)
      if (!_cb) return console.error('unexpected data', end, data)
      var cb = _cb
      _cb = null
      return cb(end, data)
    }

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

    return function (end, cb) {
      if(_cb) return cb(new Error('read too soon'))
      _cb = cb
      if (end) post(cmds.readEnd, JSON.stringify(end, fixJSON))
      else post(cmds.read)
    }
  }
}