require("isomorphic-fetch");
const fs = require("fs");
const path = require("path");
const tmpdir = require("os-tmpdir");
const gunzip = require("gunzip-maybe");
const mkdirp = require("mkdirp");
const tar = require("tar-fs");
const createMutex = require("./createMutex");

function createTempPath(name, version) {
  const normalName = name.replace(/\//g, "-");
  return path.join(tmpdir(), `unpkg-${normalName}-${version}`);
}

function stripNamePrefix(headers) {
  // Most packages have header names that look like "package/index.js"
  // so we shorten that to just "index.js" here. A few packages use a
  // prefix other than "package/". e.g. the firebase package uses the
  // "firebase_npm/" prefix. So we just strip the first dir name.
  headers.name = headers.name.replace(/^[^/]+\//, "");
  return headers;
}

function ignoreLinks(file, headers) {
  return headers.type === "link" || headers.type === "symlink";
}

function extractResponse(response, outputDir) {
  return new Promise((resolve, reject) => {
    const extract = tar.extract(outputDir, {
      readable: true, // All dirs/files should be readable.
      map: stripNamePrefix,
      ignore: ignoreLinks
    });

    response.body
      .pipe(gunzip())
      .pipe(extract)
      .on("finish", resolve)
      .on("error", reject);
  });
}

function fetchAndExtract(tarballURL, outputDir) {
  console.log(`info: Fetching ${tarballURL} and extracting to ${outputDir}`);

  return fetch(tarballURL).then(response => {
    return extractResponse(response, outputDir);
  });
}

const fetchMutex = createMutex((payload, callback) => {
  const { tarballURL, outputDir } = payload;

  fs.access(outputDir, function(error) {
    if (error) {
      if (error.code === "ENOENT" || error.code === "ENOTDIR") {
        // ENOENT or ENOTDIR are to be expected when we haven't yet
        // fetched a package for the first time. Carry on!
        mkdirp(outputDir, function(error) {
          if (error) {
            callback(error);
          } else {
            fetchAndExtract(tarballURL, outputDir).then(() => {
              callback();
            }, callback);
          }
        });
      } else {
        callback(error);
      }
    } else {
      // Best case: we already have this package cached on disk!
      callback();
    }
  });
});

function getPackage(packageConfig, callback) {
  const tarballURL = packageConfig.dist.tarball;
  const outputDir = createTempPath(packageConfig.name, packageConfig.version);

  fetchMutex(tarballURL, { tarballURL, outputDir }, function(error) {
    callback(error, outputDir);
  });
}

module.exports = getPackage;