/**
 * Copyright 2013 Google Inc. All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

var querystring = require('querystring');
var Auth2Client = require('./oauth2client.js');
var util = require('util');

/**
 * @private
 * Google Compute Engine metadata server token endpoint.
 */
Compute.GOOGLE_OAUTH2_TOKEN_URL_ =
  'http://metadata/computeMetadata/v1beta1/instance/service-accounts/default/token';

/**
 * @constructor
 * Google Compute Engine service account credentials.
 *
 * Retrieve access token from the metadata server.
 * See: https://developers.google.com/compute/docs/authentication
 *
 */
function Compute() {
  Compute.super_.call(this);
}

/**
 * Inherit from Auth2Client.
 */
util.inherits(Compute, Auth2Client);

/**
 * Get the initial access token from compute metadata server.
 * @param {function=} opt_callback Optional callback.
 */
Compute.prototype.authorize = function(opt_callback) {
  var that = this;
  that.refreshToken_(null, function(err, result) {
    if (!err) {
      that.credentials = result;
      that.credentials.refresh_token = 'compute-placeholder';
    }
    opt_callback && opt_callback(err, result);
  });
};

/**
 * @private
 * Refreshes the access token.
 * @param {object=} ignored_
 * @param {function=} opt_callback Optional callback.
 */
Compute.prototype.refreshToken_ = function(ignored_, opt_callback) {
  var uri = this.opts.tokenUrl || Compute.GOOGLE_OAUTH2_TOKEN_URL_;
  // request for new token
  this.transporter.request({
    method: 'GET',
    uri: uri,
    json: true
  }, opt_callback);
};

/**
 * Export Compute.
 */
module.exports = Compute;