ChatManager = class ChatManager {
  /**
   * Constructor.
   *
   * @param {Collection} chats
   *   A Chats collection instance.
   *
   * @returns {void}
   */
  constructor(chats) {
    this.chats = chats;
  }

  /**
   * Order two comparable objects (usually strings) as min, max.
   *
   * @param {Object} s1
   *   The first object.
   * @param {Object} s2
   *   The second object.
   * @returns {*[]}
   *   An array of the two objects, minimum first, maximum last.
   */
  static ordered(s1, s2) {
    let id1;
    let id2;
    // Ensure ordering as id1 <= id2. Equality should not happen, but handle it correctly nonetheless.
    if (s1 <= s2) {
      id1 = s1;
      id2 = s2;
    }
    else {
      id1 = s2;
      id2 = s1;
    }
    return [id1, id2];
  }

  /**
   * Find the id of a chat between two users.
   *
   * @param {String} myId
   *   The current user id.
   * @param {String} otherUserId
   *   The id of an other user.
   * @param {Session} session
   *   The session global.
   *
   * @returns {void}
   */
  assignChatId(myId, otherUserId, session) {
    let [user1Id, user2Id] = ChatManager.ordered(myId, otherUserId);

    // Since ids are ordered, we do not need a $or.
    const chat = this.chats.findOne({ user1Id, user2Id });
    Meteor._debug("filter", { user1Id, user2Id }, chat);

    // No chat matching the filter - need to insert a new one.
    if (!chat) {
      Meteor.call("chats.insert", { user1Id, user2Id }, function (err, chatId) {
        if (err) {
          throw new Meteor.Error("access-denied", "Could not insert new chat.");
        }
        session.set("chatId", chatId);
        console.log("-- No chat found between " + user1Id + " and " + user2Id + ". Created " + chatId);
      });
    }
    // There is a chat going already - use that.
    else {
      const chatId = chat._id;
      session.set("chatId", chatId);
      console.log("-- Chat " + chatId + " found between " + user1Id + " and " + user2Id + ".");
    }
  }

  /**
   * Meteor Method: insert a chat.
   *
   * @param {Object} chat
   *   A chat object, with user1Id, user2Id, and messages keys.
   *
   * @returns {String}
   *   The id of the inserted chat document.
   */
  static insertMethod(chat) {
    check(chat, {
      user1Id: String,
      user2Id: String
    });
    chat.messages = [];
    const chatId = Chats.insert(chat);
    // Meteor._debug("chats.insert", chat, chatId);
    return chatId;
  }

  /**
   * Meteor Method: push a message to a chat.
   *
   * @param {Object} chat
   *   A chat object, with user1Id, user2Id, and messages keys.
   * @param {String} chatValue
   *   The message to push.
   *
   * @returns {void}
   */
  static pushMessageMethod(chat, chatValue) {
    check(chat, {
      _id: String,
      user1Id: String,
      user2Id: String,
      messages: Array
    });

    // is a good idea to insert data straight from the form
    // (i.e. the user) into the database?? certainly not.
    // push adds the message to the end of the array
    const changes = {
      $push: {
        messages: {
          text: chatValue,
          sender: this.userId
        }
      }
    };
    // update the chat object in the database.
    Chats.update(chat._id, changes);
  }
};