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
}

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

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

  function post(msg) {
    if (!win) _msg = msg
    else win.postMessage(JSON.stringify(msg, fixJSON), 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
        var msg = JSON.parse(e.data)
        switch (msg[0]) {
          case 'read': return onRead(msg[1])
          case 'next': return onNext(msg[1], msg[2])
        }
      }
    }

    function onRead(end) {
      read(end, function (end, data) {
        post(['next', end, 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
      post(['read', end])
    }
  }
}