/*
 * Copyright (c) 2014 The Polymer Project Authors. All rights reserved.
 * This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
 * The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
 * The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
 * Code distributed by Google as part of the polymer project is also
 * subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
 */

(function(scope) {
  var endOfMicrotask = Polymer.endOfMicrotask;

  // Generic url loader
  function Loader(regex) {
    this.cache = Object.create(null);
    this.map = Object.create(null);
    this.requests = 0;
    this.regex = regex;
  }
  Loader.prototype = {

    // TODO(dfreedm): there may be a better factoring here
    // extract absolute urls from the text (full of relative urls)
    extractUrls: function(text, base) {
      var matches = [];
      var matched, u;
      while ((matched = this.regex.exec(text))) {
        u = new URL(matched[1], base);
        matches.push({matched: matched[0], url: u.href});
      }
      return matches;
    },
    // take a text blob, a root url, and a callback and load all the urls found within the text
    // returns a map of absolute url to text
    process: function(text, root, callback) {
      var matches = this.extractUrls(text, root);

      // every call to process returns all the text this loader has ever received
      var done = callback.bind(null, this.map);
      this.fetch(matches, done);
    },
    // build a mapping of url -> text from matches
    fetch: function(matches, callback) {
      var inflight = matches.length;

      // return early if there is no fetching to be done
      if (!inflight) {
        return callback();
      }

      // wait for all subrequests to return
      var done = function() {
        if (--inflight === 0) {
          callback();
        }
      };

      // start fetching all subrequests
      var m, req, url;
      for (var i = 0; i < inflight; i++) {
        m = matches[i];
        url = m.url;
        req = this.cache[url];
        // if this url has already been requested, skip requesting it again
        if (!req) {
          req = this.xhr(url);
          req.match = m;
          this.cache[url] = req;
        }
        // wait for the request to process its subrequests
        req.wait(done);
      }
    },
    handleXhr: function(request) {
      var match = request.match;
      var url = match.url;

      // handle errors with an empty string
      var response = request.response || request.responseText || '';
      this.map[url] = response;
      this.fetch(this.extractUrls(response, url), request.resolve);
    },
    xhr: function(url) {
      this.requests++;
      var request = new XMLHttpRequest();
      request.open('GET', url, true);
      request.send();
      request.onerror = request.onload = this.handleXhr.bind(this, request);

      // queue of tasks to run after XHR returns
      request.pending = [];
      request.resolve = function() {
        var pending = request.pending;
        for(var i = 0; i < pending.length; i++) {
          pending[i]();
        }
        request.pending = null;
      };

      // if we have already resolved, pending is null, async call the callback
      request.wait = function(fn) {
        if (request.pending) {
          request.pending.push(fn);
        } else {
          endOfMicrotask(fn);
        }
      };

      return request;
    }
  };

  scope.Loader = Loader;
})(Polymer);