DustJS Helper: CDN URL Rewriting

cdn-helper.js

/// Copyright (c) 2014 Christopher A. Watford
///
/// Permission is hereby granted, free of charge, to any person obtaining a copy
/// of this software and associated documentation files (the "Software"), to deal
/// in the Software without restriction, including without limitation the rights
/// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
/// copies of the Software, and to permit persons to whom the Software is
/// furnished to do so, subject to the following conditions:
/// 
/// The above copyright notice and this permission notice shall be included in
/// all copies or substantial portions of the Software.
/// 
/// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
/// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
/// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
/// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
/// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
/// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
/// THE SOFTWARE.

var url = require('url'),
    path = require('path'),
    crypto = require('crypto'),
    ReadStream = require('fs').ReadStream,
    _ = require('lodash');

var kPublicDirectory = '/public/';
var kProductionDirectory = './.build';
var kCdnHost = 'cdn.yourdomain.com';

var cache = {},
    md5 = {};

/** Adds the CDN related helpers to DustJS.
 *
 * @param {Object} [dust] A reference to the DustJS engine being configured.
 */
module.exports = function AddHelpers(dust, options) {
  
  // See krakenjs/adaro/pull/22
  options = _.extend({
    "env": process.env.NODE_ENV,
    "public": kPublicDirectory,
    "build": kProductionDirectory,
    "cdn": kCdnHost
  }, options || {});
  
  var IS_DEV = options.env === 'development';

  /** Rewires a resource to use a CDN host. In production it applies cache-busting as well (?v={MD5SUM}).
   *
   * @param {Object} [chunk] The current DustJS template chunk.
   * @param {Object} [ctx] The current DustJS rendering context.
   * @param {Array} [bodies] Any body text contained within the template (UNUSED).
   * @param {Object} [params] The parameters passed to the helper.
   * @param {String} [params.src] The source URL to rewire to use a CDN host.
   * @param {String} [params.type] The extension of the file if not given (useful for RequireJS setups).
   */
  function Helper_LoadFromCdn(chunk, ctx, bodies, params) {
    var src = dust.helpers.tap(params.src, chunk, ctx);
    var type = dust.helpers.tap(params.type, chunk, ctx);

    if (IS_DEV) {
      return chunk.write(src);
    }
    else {
      if (!_.has(cache, src)) {
        console.log('uncached CDN reference');
        cache[src] = src; // fallthrough for non-cached URL's

        var parsed = url.parse(src);
        if (parsed.pathname.indexOf(kPublicDirectory) === 0) {
          parsed.host = kCdnHost;
          if (!IS_DEV) {
            if (!_.has(md5, src)) {
              return chunk.map(function (chunk) {
                var md5sum = crypto.createHash('md5');

                var fn = parsed.pathname.substr(kPublicDirectory.length);
                if (!path.extname(fn) && type) {
                  fn += '.' + type;
                }

                var stream = ReadStream(path.join(kProductionDirectory, fn));
                stream.on('data', function (data) {
                  md5sum.update(data);
                });

                stream.on('error', function (error) {
                  console.error(error);
                  chunk.setError(error);
                  return chunk.end();
                });

                stream.on('end', function () {
                  parsed.search = '?v=' + md5sum.digest('hex');
                  cache[src] = url.format(parsed);

                  chunk.write(cache[src]);
                  return chunk.end();
                });
              });
            }
            else {
              parsed.search = '?v=' + md5[src];
              cache[src] = url.format(parsed);
            }
          }
        }
      }

      return chunk.write(cache[src]);
    }
  }

  dust.helpers.cdn = Helper_LoadFromCdn;
};