import { mark, stop } from '../../_utils/marks'
import { RealmStore } from '../../_utils/RealmStore'
import { reselect } from '../../_utils/reselect'

const RENDER_BUFFER_FACTOR = 2.5

class VirtualListStore extends RealmStore {
  constructor (state) {
    super(state, /* maxSize */ 10)
  }
}

const virtualListStore = new VirtualListStore()

virtualListStore.computeForRealm('items', null)
virtualListStore.computeForRealm('showFooter', false)
virtualListStore.computeForRealm('footerHeight', 0)
virtualListStore.computeForRealm('showHeader', false)
virtualListStore.computeForRealm('headerHeight', 0)
virtualListStore.computeForRealm('scrollTop', 0)
virtualListStore.computeForRealm('scrollHeight', 0)
virtualListStore.computeForRealm('offsetHeight', 0)
virtualListStore.computeForRealm('listOffset', 0)
virtualListStore.computeForRealm('itemHeights', {})

virtualListStore.compute('rawVisibleItems',
  ['items', 'scrollTop', 'itemHeights', 'offsetHeight', 'showHeader', 'headerHeight', 'listOffset'],
  (items, scrollTop, itemHeights, offsetHeight, showHeader, headerHeight, listOffset) => {
    window.rawVisibleItemsComputed = (window.rawVisibleItemsComputed || 0) + 1
    mark('compute visibleItems')
    if (!items) {
      return null
    }
    let effectiveScrollTop = scrollTop - listOffset
    let renderBuffer = RENDER_BUFFER_FACTOR * offsetHeight
    let visibleItems = []
    let totalOffset = showHeader ? headerHeight : 0
    let len = items.length
    let i = -1
    while (++i < len) {
      let key = items[i]
      let height = itemHeights[key] || 0
      let currentOffset = totalOffset
      totalOffset += height
      let isAboveViewport = (currentOffset < effectiveScrollTop)
      if (isAboveViewport) {
        if ((effectiveScrollTop - height - renderBuffer) > currentOffset) {
          continue // above the area we want to render
        }
      } else {
        if (currentOffset > (effectiveScrollTop + offsetHeight + renderBuffer)) {
          break // below the area we want to render
        }
      }
      visibleItems.push({
        offset: currentOffset,
        key: key,
        index: i
      })
    }
    stop('compute visibleItems')
    return visibleItems
  })

reselect(virtualListStore, 'visibleItems', 'rawVisibleItems')

virtualListStore.compute('heightWithoutFooter',
  ['items', 'itemHeights', 'showHeader', 'headerHeight'],
  (items, itemHeights, showHeader, headerHeight) => {
    if (!items) {
      return 0
    }
    let sum = showHeader ? headerHeight : 0
    let i = -1
    let len = items.length
    while (++i < len) {
      sum += itemHeights[items[i]] || 0
    }
    return sum
  })

virtualListStore.compute('height',
  ['heightWithoutFooter', 'showFooter', 'footerHeight'],
  (heightWithoutFooter, showFooter, footerHeight) => {
    return showFooter ? (heightWithoutFooter + footerHeight) : heightWithoutFooter
  })

virtualListStore.compute('length', ['items'], (items) => items ? items.length : 0)

virtualListStore.compute('allVisibleItemsHaveHeight',
  ['visibleItems', 'itemHeights'],
  (visibleItems, itemHeights) => {
    if (!visibleItems) {
      return false
    }
    for (let visibleItem of visibleItems) {
      if (!itemHeights[visibleItem.key]) {
        return false
      }
    }
    return true
  })

if (process.browser && process.env.NODE_ENV !== 'production') {
  window.virtualListStore = virtualListStore

  virtualListStore.observe('visibleItems', () => {
    window.visibleItemsChangedCount = (window.visibleItemsChangedCount || 0) + 1
  })

  virtualListStore.observe('rawVisibleItems', () => {
    window.rawVisibleItemsChangedCount = (window.rawVisibleItemsChangedCount || 0) + 1
  })
}

export {
  virtualListStore
}