text: var Dict = require('./dict').Dict;
  
  var user_groups = (function () {
  
  var exports = {};
  
  var user_group_name_dict;
  var user_group_by_id_dict;
  
  // We have an init() function so that our automated tests
  // can easily clear data.
  exports.init = function () {
      user_group_name_dict = new Dict({fold_case: true});
      user_group_by_id_dict = new Dict();
  };
  
  // WE INITIALIZE DATA STRUCTURES HERE!
  exports.init();
  
  exports.add = function (user_group) {
      // Reformat the user group members structure to be a dict.
      user_group.members = Dict.from_array(user_group.members);
      user_group_name_dict.set(user_group.name, user_group);
      user_group_by_id_dict.set(user_group.id, user_group);
  };
  
  exports.remove = function (user_group) {
      user_group_name_dict.del(user_group.name);
      user_group_by_id_dict.del(user_group.id);
  };
  
  exports.get_user_group_from_id = function (group_id, suppress_errors) {
      if (!user_group_by_id_dict.has(group_id)) {
          if (suppress_errors === undefined) {
              blueslip.error('Unknown group_id in get_user_group_from_id: ' + group_id);
          }
          return;
      }
      return user_group_by_id_dict.get(group_id);
  };
  
  exports.update = function (event) {
      var group = exports.get_user_group_from_id(event.group_id);
      if (event.data.name !== undefined) {
          group.name = event.data.name;
          user_group_name_dict.del(group.name);
          user_group_name_dict.set(group.name, group);
      }
      if (event.data.description !== undefined) {
          group.description = event.data.description;
          user_group_name_dict.del(group.name);
          user_group_name_dict.set(group.name, group);
      }
  };
  
  exports.get_user_group_from_name = function (name) {
      return user_group_name_dict.get(name);
  };
  
  exports.get_realm_user_groups = function () {
      return user_group_by_id_dict.values().sort(function (a, b) {
          return a.id - b.id;
      });
  };
  
  exports.is_member_of = function (user_group_id, user_id) {
      var user_group = user_group_by_id_dict.get(user_group_id);
      if (user_group === undefined) {
          blueslip.error("Could not find user group with ID " + user_group_id);
          return false;
      }
      return user_group.members.has(user_id);
  };
  
  exports.add_members = function (user_group_id, user_ids) {
      var user_group = user_group_by_id_dict.get(user_group_id);
      _.each(user_ids, function (user_id) {
          user_group.members.set(user_id, true);
      });
  };
  
  exports.remove_members = function (user_group_id, user_ids) {
      var user_group = user_group_by_id_dict.get(user_group_id);
      _.each(user_ids, function (user_id) {
          user_group.members.del(user_id);
      });
  };
  
  exports.initialize = function () {
      _.each(page_params.realm_user_groups, function (user_group) {
          exports.add(user_group);
      });
  
      delete page_params.realm_user_groups; // We are the only consumer of this.
  };
  
  exports.is_user_group = function (item) {
      return item.hasOwnProperty('members');
  };
  
  return exports;
  
  }());
  if (typeof module !== 'undefined') {
      module.exports = user_groups;
  }
  window.user_groups = user_groups;
  