import { parse, format } from 'url'
import evalScript from '../eval-script'
import shallowEquals from '../shallow-equals'
import { EventEmitter } from 'events'

export default class Router extends EventEmitter {
  constructor (pathname, query, { Component, ErrorComponent, ctx } = {}) {
    super()
    // represents the current component key
    this.route = toRoute(pathname)

    // set up the component cache (by route keys)
    this.components = { [this.route]: { Component, ctx } }

    this.ErrorComponent = ErrorComponent
    this.pathname = pathname
    this.query = query
    this.subscriptions = new Set()

    this.componentLoadCancel = null
    this.onPopState = this.onPopState.bind(this)

    if (typeof window !== 'undefined') {
      // in order for `e.state` to work on the `onpopstate` event
      // we have to register the initial route upon initialization
      this.replace(format({ pathname, query }), getURL())

      window.addEventListener('popstate', this.onPopState)
    }
  }

  async onPopState (e) {
    this.abortComponentLoad()

    const { url, as } = e.state
    const { pathname, query } = parse(url, true)

    if (!this.urlIsNew(pathname, query)) {
      this.emit('routeChangeStart', as)
      this.emit('routeChangeComplete', as)
      return
    }

    const route = toRoute(pathname)

    this.emit('routeChangeStart', as)
    const {
      data,
      props,
      error
    } = await this.getRouteInfo(route, pathname, query)

    if (error) {
      this.emit('routeChangeError', error, as)
      // We don't need to throw here since the error is already logged by
      // this.getRouteInfo
      return
    }

    this.route = route
    this.set(pathname, query, { ...data, props })
    this.emit('routeChangeComplete', as)
  }

  update (route, Component) {
    const data = this.components[route] || {}
    const newData = { ...data, Component }
    this.components[route] = newData

    if (route === this.route) {
      this.notify(newData)
    }
  }

  async reload (route) {
    delete this.components[route]

    if (route !== this.route) return

    const url = window.location.href
    const { pathname, query } = parse(url, true)

    this.emit('routeChangeStart', url)
    const {
      data,
      props,
      error
    } = await this.getRouteInfo(route, pathname, query)

    if (error) {
      this.emit('routeChangeError', error, url)
      throw error
    }

    this.notify({ ...data, props })

    this.emit('routeChangeComplete', url)
  }

  back () {
    window.history.back()
  }

  push (url, as = url) {
    return this.change('pushState', url, as)
  }

  replace (url, as = url) {
    return this.change('replaceState', url, as)
  }

  async change (method, url, as) {
    this.abortComponentLoad()
    const { pathname, query } = parse(url, true)

    if (!this.urlIsNew(pathname, query)) {
      this.emit('routeChangeStart', as)
      changeState()
      this.emit('routeChangeComplete', as)
      return true
    }

    const route = toRoute(pathname)

    this.emit('routeChangeStart', as)
    const {
      data, props, error
    } = await this.getRouteInfo(route, pathname, query)

    if (error) {
      this.emit('routeChangeError', error, as)
      throw error
    }

    changeState()

    this.route = route
    this.set(pathname, query, { ...data, props })

    this.emit('routeChangeComplete', as)
    return true

    function changeState () {
      if (method !== 'pushState' || getURL() !== as) {
        window.history[method]({ url, as }, null, as)
      }
    }
  }

  async getRouteInfo (route, pathname, query) {
    const routeInfo = {}

    try {
      const data = routeInfo.data = await this.fetchComponent(route)
      const ctx = { ...data.ctx, pathname, query }
      routeInfo.props = await this.getInitialProps(data.Component, ctx)
    } catch (err) {
      if (err.cancelled) {
        return { error: err }
      }

      const data = routeInfo.data = { Component: this.ErrorComponent, ctx: { err } }
      const ctx = { ...data.ctx, pathname, query }
      routeInfo.props = await this.getInitialProps(data.Component, ctx)

      routeInfo.error = err
      console.error(err)
    }

    return routeInfo
  }

  set (pathname, query, data) {
    this.pathname = pathname
    this.query = query
    this.notify(data)
  }

  urlIsNew (pathname, query) {
    return this.pathname !== pathname || !shallowEquals(query, this.query)
  }

  async fetchComponent (route) {
    let data = this.components[route]
    if (!data) {
      let cancel

      data = await new Promise((resolve, reject) => {
        this.componentLoadCancel = cancel = () => {
          if (xhr.abort) {
            xhr.abort()
            const error = new Error('Fetching componenet cancelled')
            error.cancelled = true
            reject(error)
          }
        }

        const url = `/_next/pages${route}`
        const xhr = loadComponent(url, (err, data) => {
          if (err) return reject(err)
          resolve({
            Component: data.Component,
            ctx: { xhr, err: data.err }
          })
        })
      })

      if (cancel === this.componentLoadCancel) {
        this.componentLoadCancel = null
      }

      this.components[route] = data
    }
    return data
  }

  async getInitialProps (Component, ctx) {
    let cancelled = false
    const cancel = () => { cancelled = true }
    this.componentLoadCancel = cancel

    const props = await (Component.getInitialProps ? Component.getInitialProps(ctx) : {})

    if (cancel === this.componentLoadCancel) {
      this.componentLoadCancel = null
    }

    if (cancelled) {
      const err = new Error('Loading initial props cancelled')
      err.cancelled = true
      throw err
    }

    return props
  }

  abortComponentLoad () {
    if (this.componentLoadCancel) {
      this.componentLoadCancel()
      this.componentLoadCancel = null
    }
  }

  notify (data) {
    this.subscriptions.forEach((fn) => fn(data))
  }

  subscribe (fn) {
    this.subscriptions.add(fn)
    return () => this.subscriptions.delete(fn)
  }
}

function getURL () {
  return window.location.pathname + (window.location.search || '') + (window.location.hash || '')
}

function toRoute (path) {
  return path.replace(/\/$/, '') || '/'
}

function loadComponent (url, fn) {
  return loadJSON(url, (err, data) => {
    if (err) return fn(err)

    let module
    try {
      module = evalScript(data.component)
    } catch (err) {
      return fn(err)
    }

    const Component = module.default || module
    fn(null, { Component, err: data.err })
  })
}

function loadJSON (url, fn) {
  const xhr = new window.XMLHttpRequest()
  xhr.onload = () => {
    let data

    try {
      data = JSON.parse(xhr.responseText)
    } catch (err) {
      fn(new Error('Failed to load JSON for ' + url))
      return
    }

    fn(null, data)
  }
  xhr.onerror = () => {
    fn(new Error('XHR failed. Status: ' + xhr.status))
  }
  xhr.onabort = () => {
    const err = new Error('XHR aborted')
    err.cancelled = true
    fn(err)
  }
  xhr.open('GET', url)
  xhr.setRequestHeader('Accept', 'application/json')
  xhr.send()

  return xhr
}