text: function MessageListData(opts) {
      this.muting_enabled = opts.muting_enabled;
      if (this.muting_enabled) {
          this._all_items = [];
      }
      this._items = [];
      this._hash = {};
      this._local_only = {};
      this._selected_id = -1;
  
      var filter = opts.filter;
      if (filter === undefined) {
          filter = new Filter();
      }
  
      this.filter = filter;
  }
  
  MessageListData.prototype = {
      all_messages: function () {
          return this._items;
      },
  
      num_items: function () {
          return this._items.length;
      },
  
      empty: function () {
          return this._items.length === 0;
      },
  
      first: function () {
          return this._items[0];
      },
  
      last: function () {
          return this._items[this._items.length - 1];
      },
  
      select_idx: function () {
          if (this._selected_id === -1) {
              return;
          }
          var ids = _.pluck(this._items, 'id');
  
          var i = ids.indexOf(this._selected_id);
          if (i === -1) {
              return;
          }
          return i;
      },
  
      prev: function () {
          var i = this.select_idx();
  
          if (i === undefined) {
              return;
          }
  
          if (i === 0) {
              return;
          }
  
          return this._items[i - 1].id;
      },
  
      next: function () {
          var i = this.select_idx();
  
          if (i === undefined) {
              return;
          }
  
          if (i + 1 >= this._items.length) {
              return;
          }
  
          return this._items[i + 1].id;
      },
  
      is_at_end: function () {
          if (this._selected_id === -1) {
              return false;
          }
  
          var n = this._items.length;
  
          if (n === 0) {
              return false;
          }
  
          var last_msg = this._items[n - 1];
  
          return last_msg.id === this._selected_id;
      },
  
      nth_most_recent_id: function (n) {
          var i = this._items.length - n;
          if (i < 0) {
              return -1;
          }
          return this._items[i].id;
      },
  
      clear: function () {
          if (this.muting_enabled) {
              this._all_items = [];
          }
  
          this._items = [];
          this._hash = {};
      },
  
      get: function (id) {
          id = parseFloat(id);
          if (isNaN(id)) {
              return;
          }
          return this._hash[id];
      },
  
      clear_selected_id: function () {
          this._selected_id = -1;
      },
  
      selected_id: function () {
          return this._selected_id;
      },
  
      set_selected_id: function (id) {
          this._selected_id = id;
      },
  
      selected_idx: function () {
          return this._lower_bound(this._selected_id);
      },
  
      reset_select_to_closest: function () {
          this._selected_id = this.closest_id(this._selected_id);
      },
  
      is_search: function () {
          return this.filter.is_search();
      },
  
      _get_predicate: function () {
          // We cache this.
          if (!this.predicate) {
              this.predicate = this.filter.predicate();
          }
          return this.predicate;
      },
  
      valid_non_duplicated_messages: function (messages) {
          var predicate = this._get_predicate();
          var self = this;
          return _.filter(messages, function (msg) {
              return self.get(msg.id) === undefined && predicate(msg);
          });
      },
  
      filter_incoming: function (messages) {
          var predicate = this._get_predicate();
          return _.filter(messages, predicate);
      },
  
      unmuted_messages: function (messages) {
          return _.reject(messages, function (message) {
              return muting.is_topic_muted(message.stream_id, util.get_message_topic(message)) &&
                     !message.mentioned;
          });
      },
  
      update_items_for_muting: function () {
          if (!this.muting_enabled) {
              return;
          }
          this._items = this.unmuted_messages(this._all_items);
      },
  
      first_unread_message_id: function () {
          var first_unread = _.find(this._items, function (message) {
              return unread.message_unread(message);
          });
  
          if (first_unread) {
              return first_unread.id;
          }
  
          // if no unread, return the bottom message
          return this.last().id;
      },
  
      update_user_full_name: function (user_id, full_name) {
          _.each(this._items, function (item) {
              if (item.sender_id && item.sender_id === user_id) {
                  item.sender_full_name = full_name;
              }
          });
      },
  
      update_user_avatar: function (user_id, avatar_url) {
          // TODO:
          // We may want to de-dup some logic with update_user_full_name,
          // especially if we want to optimize this with some kind of
          // hash that maps sender_id -> messages.
          _.each(this._items, function (item) {
              if (item.sender_id && item.sender_id === user_id) {
                  item.small_avatar_url = avatar_url;
              }
          });
      },
  
      update_stream_name: function (stream_id, new_stream_name) {
          _.each(this._items, function (item) {
              if (item.stream_id && item.stream_id === stream_id) {
                  item.display_recipient = new_stream_name;
                  item.stream = new_stream_name;
              }
          });
      },
  
      add_messages: function (messages) {
          var self = this;
          var top_messages = [];
          var bottom_messages = [];
          var interior_messages = [];
  
          // If we're initially populating the list, save the messages in
          // bottom_messages regardless
          if (self.selected_id() === -1 && self.empty()) {
              var narrow_messages = self.filter_incoming(messages);
              bottom_messages = _.reject(narrow_messages, function (msg) {
                  return self.get(msg.id);
              });
          } else {
              // Filter out duplicates that are already in self, and all messages
              // that fail our filter predicate
              messages = self.valid_non_duplicated_messages(messages);
  
              _.each(messages, function (msg) {
                  // Put messages in correct order on either side of the
                  // message list.  This code path assumes that messages
                  // is a (1) sorted, and (2) consecutive block of
                  // messages that belong in this message list; those
                  // facts should be ensured by the caller.
                  if (self.empty() || msg.id > self.last().id) {
                      bottom_messages.push(msg);
                  } else if (msg.id < self.first().id) {
                      top_messages.push(msg);
                  } else {
                      interior_messages.push(msg);
                  }
              });
          }
  
          if (interior_messages.length > 0) {
              interior_messages = self.add_anywhere(interior_messages);
          }
  
          if (top_messages.length > 0) {
              top_messages = self.prepend(top_messages);
          }
  
          if (bottom_messages.length > 0) {
              bottom_messages = self.append(bottom_messages);
          }
  
          var info = {
              top_messages: top_messages,
              bottom_messages: bottom_messages,
              interior_messages: interior_messages,
          };
  
          return info;
      },
  
      add_anywhere: function (messages) {
          // Caller should have already filtered messages.
          // This should be used internally when we have
          // "interior" messages to add and can't optimize
          // things by only doing prepend or only doing append.
          var viewable_messages;
          if (this.muting_enabled) {
              this._all_items = messages.concat(this._all_items);
              this._all_items.sort(function (a, b) {return a.id - b.id;});
  
              viewable_messages = this.unmuted_messages(messages);
              this._items = viewable_messages.concat(this._items);
  
          } else {
              viewable_messages = messages;
              this._items = messages.concat(this._items);
          }
  
          this._items.sort(function (a, b) {return a.id - b.id;});
          this._add_to_hash(messages);
          return viewable_messages;
      },
  
      append: function (messages) {
          // Caller should have already filtered
          var viewable_messages;
          if (this.muting_enabled) {
              this._all_items = this._all_items.concat(messages);
              viewable_messages = this.unmuted_messages(messages);
          } else {
              viewable_messages = messages;
          }
          this._items = this._items.concat(viewable_messages);
          this._add_to_hash(messages);
          return viewable_messages;
      },
  
      prepend: function (messages) {
          // Caller should have already filtered
          var viewable_messages;
          if (this.muting_enabled) {
              this._all_items = messages.concat(this._all_items);
              viewable_messages = this.unmuted_messages(messages);
          } else {
              viewable_messages = messages;
          }
          this._items = viewable_messages.concat(this._items);
          this._add_to_hash(messages);
          return viewable_messages;
      },
  
      remove: function (messages) {
          var self = this;
          _.each(messages, function (message) {
              var stored_message = self._hash[message.id];
              if (stored_message !== undefined) {
                  delete self._hash[stored_message];
              }
              delete self._local_only[message.id];
          });
  
          var msg_ids_to_remove = {};
          _.each(messages, function (message) {
              msg_ids_to_remove[message.id] = true;
          });
          this._items = _.filter(this._items, function (message) {
              return !msg_ids_to_remove.hasOwnProperty(message.id);
          });
          if (this.muting_enabled) {
              this._all_items = _.filter(this._all_items, function (message) {
                  return !msg_ids_to_remove.hasOwnProperty(message.id);
              });
          }
      },
  
      // Returns messages from the given message list in the specified range, inclusive
      message_range: function (start, end) {
          if (start === -1) {
              blueslip.error("message_range given a start of -1");
          }
  
          var start_idx = this._lower_bound(start);
          var end_idx   = this._lower_bound(end);
          return this._items.slice(start_idx, end_idx + 1);
      },
  
      // Returns the index where you could insert the desired ID
      // into the message list, without disrupting the sort order
      // This takes into account the potentially-unsorted
      // nature of local message IDs in the message list
      _lower_bound: function (id) {
          var self = this;
          function less_func(msg, ref_id, a_idx) {
              if (self._is_localonly_id(msg.id)) {
                  // First non-local message before this one
                  var effective = self._next_nonlocal_message(self._items, a_idx,
                                                              function (idx) { return idx - 1; });
                  if (effective) {
                      // Turn the 10.02 in [11, 10.02, 12] into 11.02
                      var decimal = parseFloat((msg.id % 1).toFixed(0.02));
                      var effective_id = effective.id + decimal;
                      return effective_id < ref_id;
                  }
              }
              return msg.id < ref_id;
          }
  
          return util.lower_bound(self._items, id, less_func);
      },
  
      closest_id: function (id) {
          // We directly keep track of local-only messages,
          // so if we're asked for one that we know we have,
          // just return it directly
          if (this._local_only.hasOwnProperty(id)) {
              return id;
          }
  
          var items = this._items;
  
          if (items.length === 0) {
              return -1;
          }
  
          var closest = this._lower_bound(id);
  
          if (closest < items.length && id === items[closest].id) {
              return items[closest].id;
          }
  
          var potential_closest_matches = [];
          if (closest > 0 && this._is_localonly_id(items[closest - 1].id)) {
              // Since we treated all blocks of local ids as their left-most-non-local message
              // for lower_bound purposes, find the real leftmost index (first non-local id)
              do {
                  potential_closest_matches.push(closest);
                  closest -= 1;
              } while (closest > 0 && this._is_localonly_id(items[closest - 1].id));
          }
          potential_closest_matches.push(closest);
  
          if (closest === items.length) {
              closest = closest - 1;
          } else {
              // Any of the ids that we skipped over (due to them being local-only) might be the
              // closest ID to the desired one, in case there is no exact match.
              potential_closest_matches.unshift(_.last(potential_closest_matches) - 1);
              var best_match = items[closest].id;
  
              _.each(potential_closest_matches, function (potential_idx) {
                  if (potential_idx < 0) {
                      return;
                  }
                  var item = items[potential_idx];
  
                  if (item === undefined) {
                      blueslip.warn('Invalid potential_idx: ' + potential_idx);
                      return;
                  }
  
                  var potential_match = item.id;
                  // If the potential id is the closest to the requested, save that one
                  if (Math.abs(id - potential_match) < Math.abs(best_match - id)) {
                      best_match = potential_match;
                      closest = potential_idx;
                  }
              });
          }
          return items[closest].id;
      },
  
      advance_past_messages: function (msg_ids) {
          // Start with the current pointer, but then keep advancing the
          // pointer while the next message's id is in msg_ids.  See trac #1555
          // for more context, but basically we are skipping over contiguous
          // messages that we have recently visited.
          var next_msg_id = 0;
  
          var id_set = {};
  
          _.each(msg_ids, function (msg_id) {
              id_set[msg_id] = true;
          });
  
          var idx = this.selected_idx() + 1;
          while (idx < this._items.length) {
              var msg_id = this._items[idx].id;
              if (!id_set[msg_id]) {
                  break;
              }
              next_msg_id = msg_id;
              idx += 1;
          }
  
          if (next_msg_id > 0) {
              this.set_selected_id(next_msg_id);
          }
      },
  
      _add_to_hash: function (messages) {
          var self = this;
          messages.forEach(function (elem) {
              var id = parseFloat(elem.id);
              if (isNaN(id)) {
                  blueslip.fatal("Bad message id");
              }
              if (self._is_localonly_id(id)) {
                  self._local_only[id] = elem;
              }
              if (self._hash[id] !== undefined) {
                  blueslip.error("Duplicate message added to MessageListData");
                  return;
              }
              self._hash[id] = elem;
          });
      },
  
      _is_localonly_id: function (id) {
          return id % 1 !== 0;
      },
  
      _next_nonlocal_message: function (item_list, start_index, op) {
          var cur_idx = start_index;
          do {
              cur_idx = op(cur_idx);
          } while (item_list[cur_idx] !== undefined && this._is_localonly_id(item_list[cur_idx].id));
          return item_list[cur_idx];
      },
  
      change_message_id: function (old_id, new_id, opts) {
          // Update our local cache that uses the old id to the new id
          function message_sort_func(a, b) {return a.id - b.id;}
  
          if (this._hash.hasOwnProperty(old_id)) {
              var msg = this._hash[old_id];
              delete this._hash[old_id];
              this._hash[new_id] = msg;
          } else {
              return;
          }
  
          if (this._local_only.hasOwnProperty(old_id)) {
              if (this._is_localonly_id(new_id)) {
                  this._local_only[new_id] = this._local_only[old_id];
              }
              delete this._local_only[old_id];
          }
  
          if (this._selected_id === old_id) {
              this._selected_id = new_id;
          }
  
          // If this message is now out of order, re-order and re-render
          var self = this;
          setTimeout(function () {
              var current_message = self._hash[new_id];
              var index = self._items.indexOf(current_message);
  
              if (index === -1) {
                  if (!self.muting_enabled && opts.is_current_list()) {
                      blueslip.error("Trying to re-order message but can't find message with new_id in _items!");
                  }
                  return;
              }
  
              var next = self._next_nonlocal_message(self._items, index,
                                                     function (idx) { return idx + 1; });
              var prev = self._next_nonlocal_message(self._items, index,
                                                     function (idx) { return idx - 1; });
  
              if (next !== undefined && current_message.id > next.id ||
                  prev !== undefined && current_message.id < prev.id) {
                  blueslip.debug("Changed message ID from server caused out-of-order list, reordering");
                  self._items.sort(message_sort_func);
                  if (self.muting_enabled) {
                      self._all_items.sort(message_sort_func);
                  }
  
                  opts.re_render();
              }
          }, 0);
      },
  
      get_last_message_sent_by_me: function () {
          var msg_index = _.findLastIndex(this._items, {sender_id: page_params.user_id});
          if (msg_index === -1) {
              return;
          }
          var msg = this._items[msg_index];
          return msg;
      },
  };
  
  if (typeof module !== 'undefined') {
      module.exports = MessageListData;
  }
  
  window.MessageListData = MessageListData;
  