var nest = require('depnest')
var Value = require('mutant/value')
var computed = require('mutant/computed')
var Abortable = require('pull-abortable')
var resolve = require('mutant/resolve')
var pull = require('pull-stream')
var onceIdle = require('mutant/once-idle')

exports.needs = nest({
  'sbot.pull.backlinks': 'first'
})

exports.gives = nest('backlinks.obs.for', true)

exports.create = function (api) {
  var cache = {}

  // cycle remove sets for fast cleanup
  var newRemove = new Set()
  var oldRemove = new Set()

  // run cache cleanup every 5 seconds
  // an item will be removed from cache between 5 - 10 seconds after release
  // this ensures that the data is still available for a page reload
  var timer = setInterval(() => {
    oldRemove.forEach(id => {
      if (cache[id]) {
        cache[id].destroy()
        delete cache[id]
      }
    })
    oldRemove.clear()

    // cycle
    var hold = oldRemove
    oldRemove = newRemove
    newRemove = hold
  }, 5e3)

  if (timer.unref) timer.unref()

  return nest({
    'backlinks.obs.for': (id) => backlinks(id)
  })

  function backlinks (id) {
    if (!cache[id]) {
      var sync = Value(false)
      var aborter = Abortable()
      var collection = Value([])

      // try not to saturate the thread
      onceIdle(() => {
        pull(
          api.sbot.pull.backlinks({
            query: [ {$filter: { dest: id }} ],
            index: 'DTA', // use asserted timestamps
            live: true
          }),
          aborter,
          pull.drain((msg) => {
            if (msg.sync) {
              sync.set(true)
            } else {
              var value = resolve(collection)
              value.push(msg)
              collection.set(value)
            }
          })
        )
      })

      cache[id] = computed([collection], x => x, {
        onListen: () => use(id),
        onUnlisten: () => release(id)
      })

      cache[id].destroy = aborter.abort
      cache[id].sync = sync
    }
    return cache[id]
  }

  function use (id) {
    newRemove.delete(id)
    oldRemove.delete(id)
  }

  function release (id) {
    newRemove.add(id)
  }
}