(function() {
  'use strict';

  if (self.fetch) {
    return
  }

  function Headers(headers) {
    this.map = {}

    var self = this
    if (headers instanceof Headers) {
      headers.forEach(function(name, values) {
        values.forEach(function(value) {
          self.append(name, value)
        })
      })

    } else if (headers) {
      Object.getOwnPropertyNames(headers).forEach(function(name) {
        self.append(name, headers[name])
      })
    }
  }

  Headers.prototype.append = function(name, value) {
    name = name.toLowerCase()
    var list = this.map[name]
    if (!list) {
      list = []
      this.map[name] = list
    }
    list.push(value)
  }

  Headers.prototype['delete'] = function(name) {
    delete this.map[name.toLowerCase()]
  }

  Headers.prototype.get = function(name) {
    var values = this.map[name.toLowerCase()]
    return values ? values[0] : null
  }

  Headers.prototype.getAll = function(name) {
    return this.map[name.toLowerCase()] || []
  }

  Headers.prototype.has = function(name) {
    return this.map.hasOwnProperty(name.toLowerCase())
  }

  Headers.prototype.set = function(name, value) {
    this.map[name.toLowerCase()] = [value]
  }

  // Instead of iterable for now.
  Headers.prototype.forEach = function(callback) {
    var self = this
    Object.getOwnPropertyNames(this.map).forEach(function(name) {
      callback(name, self.map[name])
    })
  }

  function consumed(body) {
    if (body.bodyUsed) {
      return Promise.reject(new TypeError('Already read'))
    }
    body.bodyUsed = true
  }

  function fileReaderReady(reader) {
    return new Promise(function(resolve, reject) {
      reader.onload = function() {
        resolve(reader.result)
      }
      reader.onerror = function() {
        reject(reader.error)
      }
    })
  }

  function readBlobAsArrayBuffer(blob) {
    var reader = new FileReader()
    reader.readAsArrayBuffer(blob)
    return fileReaderReady(reader)
  }

  function readBlobAsText(blob) {
    var reader = new FileReader()
    reader.readAsText(blob)
    return fileReaderReady(reader)
  }

  var blobSupport = 'FileReader' in self && 'Blob' in self && (function() {
    try {
      new Blob();
      return true
    } catch(e) {
      return false
    }
  })();

  function Body() {
    this.bodyUsed = false

    if (blobSupport) {
      this._initBody = function(body) {
        this._bodyInit = body
        if (typeof body === 'string') {
          this._bodyText = body
        } else if ('Blob' in self && Blob.prototype.isPrototypeOf(body)) {
          this._bodyBlob = body
        } else if ('FormData' in self && FormData.prototype.isPrototypeOf(body)) {
          this._bodyFormData = body
        } else if (!body) {
          this._bodyText = ''
        } else {
          throw new Error('unsupported BodyInit type')
        }
      }

      this.blob = function() {
        var rejected = consumed(this)
        if (rejected) {
          return rejected
        }

        if (this._bodyBlob) {
          return Promise.resolve(this._bodyBlob)
        } else if (this._bodyFormData) {
          throw new Error('could not read FormData body as blob')
        } else {
          return Promise.resolve(new Blob([this._bodyText]))
        }
      }

      this.arrayBuffer = function() {
        return this.blob().then(readBlobAsArrayBuffer)
      }

      this.text = function() {
        var rejected = consumed(this)
        if (rejected) {
          return rejected
        }

        if (this._bodyBlob) {
          return readBlobAsText(this._bodyBlob)
        } else if (this._bodyFormData) {
          throw new Error('could not read FormData body as text')
        } else {
          return Promise.resolve(this._bodyText)
        }
      }
    } else {
      this._initBody = function(body) {
        this._bodyInit = body
        if (typeof body === 'string') {
          this._bodyText = body
        } else if ('FormData' in self && FormData.prototype.isPrototypeOf(body)) {
          this._bodyFormData = body
        } else if (!body) {
          this._bodyText = ''
        } else {
          throw new Error('unsupported BodyInit type')
        }
      }

      this.text = function() {
        var rejected = consumed(this)
        return rejected ? rejected : Promise.resolve(this._bodyText)
      }
    }

    if ('FormData' in self) {
      this.formData = function() {
        return this.text().then(decode)
      }
    }

    this.json = function() {
      return this.text().then(JSON.parse)
    }

    return this
  }

  // HTTP methods whose capitalization should be normalized
  var methods = ['DELETE', 'GET', 'HEAD', 'OPTIONS', 'POST', 'PUT']

  function normalizeMethod(method) {
    var upcased = method.toUpperCase()
    return (methods.indexOf(upcased) > -1) ? upcased : method
  }

  function Request(url, options) {
    options = options || {}
    this.url = url

    this.credentials = options.credentials || 'omit'
    this.headers = new Headers(options.headers)
    this.method = normalizeMethod(options.method || 'GET')
    this.mode = options.mode || null
    this.referrer = null

    if ((this.method === 'GET' || this.method === 'HEAD') && options.body) {
      throw new TypeError()
    }
    this._initBody(options.body)
  }

  function decode(body) {
    var form = new FormData()
    body.trim().split('&').forEach(function(bytes) {
      if (bytes) {
        var split = bytes.split('=')
        var name = split.shift().replace(/\+/g, ' ')
        var value = split.join('=').replace(/\+/g, ' ')
        form.append(decodeURIComponent(name), decodeURIComponent(value))
      }
    })
    return form
  }

  function headers(xhr) {
    var head = new Headers()
    var pairs = xhr.getAllResponseHeaders().trim().split('\n')
    pairs.forEach(function(header) {
      var split = header.trim().split(':')
      var key = split.shift().trim()
      var value = split.join(':').trim()
      head.append(key, value)
    })
    return head
  }

  Request.prototype.fetch = function() {
    var self = this

    return new Promise(function(resolve, reject) {
      var xhr = new XMLHttpRequest()

      function responseURL() {
        if ('responseURL' in xhr) {
          return xhr.responseURL
        }

        // Avoid security warnings on getResponseHeader when not allowed by CORS
        if (/^X-Request-URL:/m.test(xhr.getAllResponseHeaders())) {
          return xhr.getResponseHeader('X-Request-URL')
        }

        return;
      }

      xhr.onload = function() {
        var status = (xhr.status === 1223) ? 204 : xhr.status
        if (status < 100 || status > 599) {
          reject(new TypeError('Network request failed'))
          return
        }
        var options = {
          status: status,
          statusText: xhr.statusText,
          headers: headers(xhr),
          url: responseURL()
        }
        var body = 'response' in xhr ? xhr.response : xhr.responseText;
        resolve(new Response(body, options))
      }

      xhr.onerror = function() {
        reject(new TypeError('Network request failed'))
      }

      xhr.open(self.method, self.url)
      if ('responseType' in xhr && blobSupport) {
        xhr.responseType = 'blob'
      }

      self.headers.forEach(function(name, values) {
        values.forEach(function(value) {
          xhr.setRequestHeader(name, value)
        })
      })

      xhr.send(typeof self._bodyInit === 'undefined' ? null : self._bodyInit)
    })
  }

  Body.call(Request.prototype)

  function Response(bodyInit, options) {
    if (!options) {
      options = {}
    }

    this._initBody(bodyInit)
    this.type = 'default'
    this.url = null
    this.status = options.status
    this.statusText = options.statusText
    this.headers = options.headers
    this.url = options.url || ''
  }

  Body.call(Response.prototype)

  self.Headers = Headers;
  self.Request = Request;
  self.Response = Response;

  self.fetch = function (url, options) {
    return new Request(url, options).fetch()
  }
  self.fetch.polyfill = true
})();