'use strict'

const MongoClient = require('mongodb').MongoClient
const express = require('express')
const LRU = require('lru-cache')
const fastq = require('fastq')
const app = express()

var url = 'mongodb://localhost:27017/test';

function sum (data) {
  var sum = 0
  const l = data.length
  for (var i = 0; i < l; i++) {
    sum += data[i].value
  }
  return sum
}

MongoClient.connect(url, function(err, db) {
  if (err) { throw err }
  const collection = db.collection('data')
  const queue = fastq(work)
  const cache = LRU({
    maxAge: 1000 * 5 // 5 seconds
  })

  function work (req, done) {
    const elem = cache.get('average')
    if (elem) {
      done(null, elem)
      return
    }
    collection.find({}).toArray(function (err, data) {
      if (err) {
        done(err)
        return
      }
      const result = sum(data) / data.length
      cache.set('average', result)
      done(null, result)
    })
  }

  app.get('/hello', (req, res) => {
    queue.push(req, function (err, result) {
      if (err) {
        res.send(err.message)
        return
      }
      res.send('' + result)
    })
  })

  app.listen(3000)
})