require("isomorphic-fetch")
const createCache = require("./createCache")
const createMutex = require("./createMutex")

const RegistryURL = process.env.NPM_REGISTRY_URL || "https://registry.npmjs.org"

const PackageInfoCache = createCache("packageInfo")

function fetchPackageInfo(packageName) {
  console.log(`info: Fetching package info for ${packageName}`)

  let encodedPackageName
  if (packageName.charAt(0) === "@") {
    encodedPackageName = `@${encodeURIComponent(packageName.substring(1))}`
  } else {
    encodedPackageName = encodeURIComponent(packageName)
  }

  const url = `${RegistryURL}/${encodedPackageName}`

  return fetch(url, {
    headers: {
      Accept: "application/json"
    }
  }).then(res => {
    return res.status === 404 ? null : res.json()
  })
}

const PackageNotFound = "PackageNotFound"

// This mutex prevents multiple concurrent requests to
// the registry for the same package info.
const fetchMutex = createMutex((packageName, callback) => {
  fetchPackageInfo(packageName).then(
    function(value) {
      if (value == null) {
        // Cache 404s for 5 minutes. This prevents us from making
        // unnecessary requests to the registry for bad package names.
        // In the worst case, a brand new package's info will be
        // available within 5 minutes.
        PackageInfoCache.set(packageName, PackageNotFound, 300, function() {
          callback(null, value)
        })
      } else {
        // Cache valid package info for 1 minute.
        PackageInfoCache.set(packageName, value, 60, function() {
          callback(null, value)
        })
      }
    },
    function(error) {
      // Do not cache errors.
      PackageInfoCache.del(packageName, function() {
        callback(error)
      })
    }
  )
})

function getPackageInfo(packageName, callback) {
  PackageInfoCache.get(packageName, function(error, value) {
    if (error || value != null) {
      callback(error, value === PackageNotFound ? null : value)
    } else {
      fetchMutex(packageName, packageName, callback)
    }
  })
}

module.exports = getPackageInfo