instrumental/app/javascript/mastodon/reducers/timelines.js

import {
  TIMELINE_UPDATE,
  TIMELINE_DELETE,
  TIMELINE_CLEAR,
  TIMELINE_EXPAND_SUCCESS,
  TIMELINE_EXPAND_REQUEST,
  TIMELINE_EXPAND_FAIL,
  TIMELINE_SCROLL_TOP,
  TIMELINE_CONNECT,
  TIMELINE_DISCONNECT,
  TIMELINE_LOAD_PENDING,
} from '../actions/timelines';
import {
  ACCOUNT_BLOCK_SUCCESS,
  ACCOUNT_MUTE_SUCCESS,
  ACCOUNT_UNFOLLOW_SUCCESS,
} from '../actions/accounts';
import { Map as ImmutableMap, List as ImmutableList, fromJS } from 'immutable';
import compareId from '../compare_id';

const initialState = ImmutableMap();

const initialTimeline = ImmutableMap({
  unread: 0,
  online: false,
  top: true,
  isLoading: false,
  hasMore: true,
  pendingItems: ImmutableList(),
  items: ImmutableList(),
});

const expandNormalizedTimeline = (state, timeline, statuses, next, isPartial, isLoadingRecent, usePendingItems) => {
  return state.update(timeline, initialTimeline, map => map.withMutations(mMap => {
    mMap.set('isLoading', false);
    mMap.set('isPartial', isPartial);

    if (!next && !isLoadingRecent) mMap.set('hasMore', false);

    if (timeline.endsWith(':pinned')) {
      mMap.set('items', statuses.map(status => status.get('id')));
    } else if (!statuses.isEmpty()) {
      usePendingItems = isLoadingRecent && (usePendingItems || !mMap.get('pendingItems').isEmpty());

      mMap.update(usePendingItems ? 'pendingItems' : 'items', ImmutableList(), oldIds => {
        const newIds = statuses.map(status => status.get('id'));

        const lastIndex = oldIds.findLastIndex(id => id !== null && compareId(id, newIds.last()) >= 0) + 1;
        const firstIndex = oldIds.take(lastIndex).findLastIndex(id => id !== null && compareId(id, newIds.first()) > 0);

        if (firstIndex < 0) {
          return (isPartial ? newIds.unshift(null) : newIds).concat(oldIds.skip(lastIndex));
        }

        return oldIds.take(firstIndex + 1).concat(
          isPartial && oldIds.get(firstIndex) !== null ? newIds.unshift(null) : newIds,
          oldIds.skip(lastIndex)
        );
      });
    }
  }));
};

const updateTimeline = (state, timeline, status, usePendingItems) => {
  const top = state.getIn([timeline, 'top']);

  if (usePendingItems || !state.getIn([timeline, 'pendingItems']).isEmpty()) {
    if (state.getIn([timeline, 'pendingItems'], ImmutableList()).includes(status.get('id')) || state.getIn([timeline, 'items'], ImmutableList()).includes(status.get('id'))) {
      return state;
    }

    return state.update(timeline, initialTimeline, map => map.update('pendingItems', list => list.unshift(status.get('id'))).update('unread', unread => unread + 1));
  }

  const ids        = state.getIn([timeline, 'items'], ImmutableList());
  const includesId = ids.includes(status.get('id'));
  const unread     = state.getIn([timeline, 'unread'], 0);

  if (includesId) {
    return state;
  }

  let newIds = ids;

  return state.update(timeline, initialTimeline, map => map.withMutations(mMap => {
    if (!top) mMap.set('unread', unread + 1);
    if (top && ids.size > 40) newIds = newIds.take(20);
    mMap.set('items', newIds.unshift(status.get('id')));
  }));
};

const deleteStatus = (state, id, accountId, references, exclude_account = null) => {
  state.keySeq().forEach(timeline => {
    if (exclude_account === null || (timeline !== `account:${exclude_account}` && !timeline.startsWith(`account:${exclude_account}:`))) {
      const helper = list => list.filterNot(item => item === id);
      state = state.updateIn([timeline, 'items'], helper).updateIn([timeline, 'pendingItems'], helper);
    }
  });

  // Remove reblogs of deleted status
  references.forEach(ref => {
    state = deleteStatus(state, ref[0], ref[1], [], exclude_account);
  });

  return state;
};

const clearTimeline = (state, timeline) => {
  return state.set(timeline, initialTimeline);
};

const filterTimelines = (state, relationship, statuses) => {
  let references;

  statuses.forEach(status => {
    if (status.get('account') !== relationship.id) {
      return;
    }

    references = statuses.filter(item => item.get('reblog') === status.get('id')).map(item => [item.get('id'), item.get('account')]);
    state      = deleteStatus(state, status.get('id'), status.get('account'), references, relationship.id);
  });

  return state;
};

const filterTimeline = (timeline, state, relationship, statuses) => {
  const helper = list => list.filterNot(statusId => statuses.getIn([statusId, 'account']) === relationship.id);
  return state.updateIn([timeline, 'items'], ImmutableList(), helper).updateIn([timeline, 'pendingItems'], ImmutableList(), helper);
};

const updateTop = (state, timeline, top) => {
  return state.update(timeline, initialTimeline, map => map.withMutations(mMap => {
    if (top) mMap.set('unread', mMap.get('pendingItems').size);
    mMap.set('top', top);
  }));
};

export default function timelines(state = initialState, action) {
  switch(action.type) {
  case TIMELINE_LOAD_PENDING:
    return state.update(action.timeline, initialTimeline, map =>
      map.update('items', list => map.get('pendingItems').concat(list.take(40))).set('pendingItems', ImmutableList()).set('unread', 0));
  case TIMELINE_EXPAND_REQUEST:
    return state.update(action.timeline, initialTimeline, map => map.set('isLoading', true));
  case TIMELINE_EXPAND_FAIL:
    return state.update(action.timeline, initialTimeline, map => map.set('isLoading', false));
  case TIMELINE_EXPAND_SUCCESS:
    return expandNormalizedTimeline(state, action.timeline, fromJS(action.statuses), action.next, action.partial, action.isLoadingRecent, action.usePendingItems);
  case TIMELINE_UPDATE:
    return updateTimeline(state, action.timeline, fromJS(action.status), action.usePendingItems);
  case TIMELINE_DELETE:
    return deleteStatus(state, action.id, action.accountId, action.references, action.reblogOf);
  case TIMELINE_CLEAR:
    return clearTimeline(state, action.timeline);
  case ACCOUNT_BLOCK_SUCCESS:
  case ACCOUNT_MUTE_SUCCESS:
    return filterTimelines(state, action.relationship, action.statuses);
  case ACCOUNT_UNFOLLOW_SUCCESS:
    return filterTimeline('home', state, action.relationship, action.statuses);
  case TIMELINE_SCROLL_TOP:
    return updateTop(state, action.timeline, action.top);
  case TIMELINE_CONNECT:
    return state.update(action.timeline, initialTimeline, map => map.set('online', true));
  case TIMELINE_DISCONNECT:
    return state.update(
      action.timeline,
      initialTimeline,
      map => map.set('online', false).update(action.usePendingItems ? 'pendingItems' : 'items', items => items.first() ? items.unshift(null) : items)
    );
  default:
    return state;
  }
};