const fs = require('fs')
const path = require('path')
const semver = require('semver')
const createPackageURL = require('../utils/createPackageURL')
const createSearch = require('./utils/createSearch')
const getPackageInfo = require('./utils/getPackageInfo')
const getPackage = require('./utils/getPackage')

function getBasename(file) {
  return path.basename(file, path.extname(file))
}

/**
 * File extensions to look for when automatically resolving.
 */
const FindExtensions = [ '', '.js', '.json' ]

/**
 * Resolves a path like "lib/file" into "lib/file.js" or "lib/file.json"
 * depending on which one is available, similar to require('lib/file').
 */
function findFile(base, useIndex, callback) {
  FindExtensions.reduceRight(function (next, ext) {
    const file = base + ext

    return function () {
      fs.stat(file, function (error, stats) {
        if (error) {
          if (error.code === 'ENOENT' || error.code === 'ENOTDIR') {
            next()
          } else {
            callback(error)
          }
        } else if (useIndex && stats.isDirectory()) {
          findFile(path.join(file, 'index'), false, function (error, indexFile, indexStats) {
            if (error) {
              callback(error)
            } else if (indexFile) {
              callback(null, indexFile, indexStats)
            } else {
              next()
            }
          })
        } else {
          callback(null, file, stats)
        }
      })
    }
  }, callback)()
}

/**
 * Fetch the file from the registry and get its stats. Redirect if the URL
 * does not specify an exact version number or targets a directory with no
 * trailing slash.
 */
function fetchFile(req, res, next) {
  getPackageInfo(req.packageName, function (error, packageInfo) {
    if (error) {
      console.error(error)
      return res.status(500).type('text').send(`Cannot get info for package "${req.packageName}"`)
    }

    if (packageInfo == null || packageInfo.versions == null)
      return res.status(404).type('text').send(`Cannot find package "${req.packageName}"`)

    req.packageInfo = packageInfo

    if (req.packageVersion in req.packageInfo.versions) {
      // A valid request for a package we haven't downloaded yet.
      req.packageConfig = req.packageInfo.versions[req.packageVersion]

      getPackage(req.packageConfig, function (error, outputDir) {
        if (error) {
          console.error(error)
          res.status(500).type('text').send(`Cannot fetch package ${req.packageSpec}`)
        } else {
          req.packageDir = outputDir

          let filename = req.filename
          let useIndex = true

          if (req.query.module != null) {
            // They want an ES module. Try "module", "jsnext:main", and "/"
            // https://github.com/rollup/rollup/wiki/pkg.module
            if (!filename)
              filename = req.packageConfig.module || req.packageConfig['jsnext:main'] || '/'
          } else if (filename) {
            // They are requesting an explicit filename. Only try to find an
            // index file if they are NOT requesting an HTML directory listing.
            useIndex = filename[filename.length - 1] !== '/'
          } else if (req.query.main && typeof req.packageConfig[req.query.main] === 'string') {
            // They specified a custom ?main field.
            filename = req.packageConfig[req.query.main]
          } else if (typeof req.packageConfig.unpkg === 'string') {
            // The "unpkg" field allows packages to explicitly declare the
            // file to serve at the bare URL (see #59).
            filename = req.packageConfig.unpkg
          } else if (typeof req.packageConfig.browser === 'string') {
            // Fall back to the "browser" field if declared (only support strings).
            filename = req.packageConfig.browser
          } else {
            // Fall back to "main" or / (same as npm).
            filename = req.packageConfig.main || '/'
          }

          findFile(path.join(req.packageDir, filename), useIndex, function (error, file, stats) {
            if (error)
              console.error(error)

            if (file == null)
              return res.status(404).type('text').send(`Cannot find module "${filename}" in package ${req.packageSpec}`)

            filename = file.replace(req.packageDir, '')

            if (req.query.main != null || getBasename(req.filename) !== getBasename(filename)) {
              // Need to redirect to the module file so relative imports resolve
              // correctly. Cache module redirects for 1 minute.
              delete req.query.main
              res.set({
                'Cache-Control': 'public, max-age=60',
                'Cache-Tag': 'redirect,module-redirect'
              }).redirect(302, createPackageURL(req.packageName, req.packageVersion, filename, createSearch(req.query)))
            } else {
              req.filename = filename
              req.stats = stats
              next()
            }
          })
        }
      })
    } else if (req.packageVersion in req.packageInfo['dist-tags']) {
      // Cache tag redirects for 1 minute.
      res.set({
        'Cache-Control': 'public, max-age=60',
        'Cache-Tag': 'redirect,tag-redirect'
      }).redirect(302, createPackageURL(req.packageName, req.packageInfo['dist-tags'][req.packageVersion], req.filename, req.search))
    } else {
      const maxVersion = semver.maxSatisfying(Object.keys(req.packageInfo.versions), req.packageVersion)

      if (maxVersion) {
        // Cache semver redirects for 1 minute.
        res.set({
          'Cache-Control': 'public, max-age=60',
          'Cache-Tag': 'redirect,semver-redirect'
        }).redirect(302, createPackageURL(req.packageName, maxVersion, req.filename, req.search))
      } else {
        res.status(404).type('text').send(`Cannot find package ${req.packageSpec}`)
      }
    }
  })
}

module.exports = fetchFile