const access = require('libnpmaccess')
const {add, sub, format} = require('date-fns')
const fetch = require('make-fetch-happen').defaults({
  cacheManager: './node_modules/.cache/make-fetch-happen',
})

const sortBy = (getProp, dir = 'asc') => (a, z) => {
  const aP = getProp(a)
  const zP = getProp(z)
  const aBigger = dir === 'asc' ? 1 : -1
  const zBigger = dir === 'asc' ? -1 : 1
  return aP > zP ? aBigger : aP < zP ? zBigger : 0
}

async function getUserDownloadStats(user) {
  const pkgs = Object.keys(await access.lsPackages(user))
  const pkgStats = []
  for (const pkg of pkgs) {
    try {
      pkgStats.push({pkg, stats: await getAllDownloadStats(pkg)})
    } catch (error) {
      console.error(`Error processing ${pkg}`)
      throw error
    }
  }
  const totalDownloads = pkgStats.reduce(
    (sum, d) => d.stats.totalDownloads + sum,
    0,
  )
  const firstDownload = pkgStats.reduce(
    (oldest, data) => {
      if (
        data.stats.firstDownload &&
        new Date(data.stats.firstDownload.day) < new Date(oldest.day)
      ) {
        return {pkg: data.pkg, ...data.stats.firstDownload}
      }
      return oldest
    },
    {pkg: '[Unknown]', downloads: 0, day: format(new Date(), 'yyyy-MM-dd')},
  )
  pkgStats.sort(sortBy(a => a.stats.averageDailyDownloads, 'desc'))
  return {user, totalDownloads, firstDownload, pkgStats}
}

async function getAllDownloadStats(pkg) {
  const npmLimitDays = 540
  const today = new Date()
  const npmDataStart = new Date('2015-01-10 00:00:00 UTC')
  const groups = []
  let currentStart
  let currentEnd = sub(npmDataStart, {days: 1})
  const requests = []
  do {
    currentStart = add(currentEnd, {days: 1})
    currentEnd = add(currentStart, {days: npmLimitDays})
    const start = format(currentStart, 'yyyy-MM-dd')
    const end = format(currentEnd, 'yyyy-MM-dd')
    requests.push(
      fetch(
        `https://api.npmjs.org/downloads/range/${start}:${end}/${pkg}`,
      ).then(response => {
        return response.json()
      }),
    )
    groups.push({start: currentStart, end: currentEnd})
  } while (currentEnd < today)

  const results = await Promise.all(requests)
  const allDownloads = results.flatMap(r => (r.error ? [] : r.downloads))
  const firstDownloadIndex = allDownloads.findIndex(d => d.downloads > 0)
  const firstDownload = allDownloads[firstDownloadIndex]
  const totalDownloads = allDownloads.reduce((sum, d) => sum + d.downloads, 0)
  const downloadDaysCount = allDownloads.length - (1 + firstDownloadIndex)
  const averageDailyDownloads = Number(
    (totalDownloads / (downloadDaysCount || 1)).toFixed(4),
  )
  const combinedResult = {
    start: results[0].start,
    end: results[results.length - 1].end,
    package: pkg,
    // downloads: allDownloads, (uncomment this if you want a silly amount of data...)
    firstDownload,
    downloadDaysCount,
    averageDailyDownloads,
    totalDownloads,
  }

  return combinedResult
}

module.exports = getUserDownloadStats