import { toChildArray, Component } from 'preact'
import { mapKeys } from 'lowline'

const CLASSNAME = '__preact_generated__'

const DOMAttributeNames = {
  acceptCharset: 'accept-charset',
  className: 'class',
  htmlFor: 'for',
  httpEquiv: 'http-equiv',
}

const isBrowser = typeof window !== 'undefined'

let mounted = []

function reducer(components) {
  return components
    .map((c) => toChildArray(c.props.children))
    .reduce((result, c) => result.concat(c), [])
    .reverse()
    .filter(unique())
    .reverse()
    .reduce(
      (result, c) => {
        if (c.type === 'title') {
          result.title += toChildArray(c.props.children).join('')
        } else {
          result.tags.push({
            type: c.type,
            attributes: mapKeys(c.props, (_value, key) => DOMAttributeNames[key] || key),
          })
        }

        return result
      },
      { title: '', tags: [] },
    )
}

function updateClient({ title, tags }) {
  const head = document.head

  const prevElements = Array.from(head.getElementsByClassName(CLASSNAME))

  for (const tag of tags) {
    const el = createDOMElement(tag)

    const prevIndex = prevElements.findIndex((prevEl) => prevEl.isEqualNode(el))

    if (~prevIndex) {
      prevElements.splice(prevIndex, 1)
    } else {
      head.appendChild(el)
    }
  }

  prevElements.forEach((prevEl) => prevEl.remove())

  document.title = title
}

function createDOMElement(tag) {
  const el = document.createElement(tag.type)
  const attributes = tag.attributes || {}

  el.setAttribute('class', CLASSNAME)

  for (const p in attributes || {}) {
    const attribute = DOMAttributeNames[p] || p.toLowerCase()

    el.setAttribute(attribute, attributes[p])
  }

  return el
}

const METATYPES = ['name', 'httpEquiv', 'charSet', 'itemProp']

// returns a function for filtering head child elements
// which shouldn't be duplicated, like <title/>.
function unique() {
  const tags = []
  const metaTypes = []
  const metaCategories = {}

  return (h) => {
    switch (h.type) {
      case 'base':
        if (~tags.indexOf(h.type)) {
          return false
        }

        tags.push(h.type)

        break
      case 'meta':
        for (let i = 0, len = METATYPES.length; i < len; i++) {
          const metatype = METATYPES[i]

          if (!(metatype in h)) {
            continue
          }

          if (metatype === 'charSet') {
            if (~metaTypes.indexOf(metatype)) {
              return false
            }

            metaTypes.push(metatype)
          } else {
            const category = h.props[metatype]
            const categories = metaCategories[metatype] || []

            if (~categories.indexOf(category)) {
              return false
            }

            categories.push(category)

            metaCategories[metatype] = categories
          }
        }
        break
    }

    return true
  }
}

function update() {
  if (isBrowser) updateClient(reducer(mounted))
}

export default class Head extends Component {
  static rewind() {
    const state = reducer(mounted)

    mounted = []

    return state
  }

  static clear() {
    mounted = []
  }

  componentDidUpdate() {
    update()
  }

  // eslint-disable-next-line react/no-deprecated
  componentWillMount() {
    mounted.push(this)

    update()
  }

  componentWillUnmount() {
    const i = mounted.indexOf(this)

    if (~i) {
      mounted.splice(i, 1)
    }

    update()
  }

  render() {
    return null
  }
}