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')
const incrementCounter = require('./utils/incrementCounter')

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((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

            // Count which packages + versions are actually using this fallback
            // so we can warn them when we deprecate this functionality.
            // See https://github.com/unpkg/unpkg/issues/63
            incrementCounter(
              'package-json-browser-fallback',
              req.packageSpec,
              1
            )
          } 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