// @flow
import { invariant } from '../../invariant';
import type { TypeId, DraggableId, DroppableId } from '../../types';
import type {
  Registry,
  DraggableAPI,
  DroppableAPI,
  DraggableEntry,
  DroppableEntry,
  RegistryEvent,
  Subscriber,
  Unsubscribe,
  DraggableEntryMap,
  DroppableEntryMap,
} from './registry-types';
import { values } from '../../native-with-fallback';

type EntryMap = {
  draggables: DraggableEntryMap,
  droppables: DroppableEntryMap,
};

export default function createRegistry(): Registry {
  const entries: EntryMap = {
    draggables: {},
    droppables: {},
  };

  const subscribers: Subscriber[] = [];

  function subscribe(cb: Subscriber): Unsubscribe {
    subscribers.push(cb);

    return function unsubscribe(): void {
      const index: number = subscribers.indexOf(cb);

      // might have been removed by a clean
      if (index === -1) {
        return;
      }

      subscribers.splice(index, 1);
    };
  }

  function notify(event: RegistryEvent) {
    if (subscribers.length) {
      subscribers.forEach(cb => cb(event));
    }
  }

  function findDraggableById(id: DraggableId): ?DraggableEntry {
    return entries.draggables[id] || null;
  }

  function getDraggableById(id: DraggableId): DraggableEntry {
    const entry: ?DraggableEntry = findDraggableById(id);
    invariant(entry, `Cannot find draggable entry with id [${id}]`);
    return entry;
  }

  const draggableAPI: DraggableAPI = {
    register: (entry: DraggableEntry) => {
      entries.draggables[entry.descriptor.id] = entry;
      notify({ type: 'ADDITION', value: entry });
    },
    update: (entry: DraggableEntry, last: DraggableEntry) => {
      const current: ?DraggableEntry = entries.draggables[last.descriptor.id];

      // item already removed
      if (!current) {
        return;
      }

      // id already used for another mount
      if (current.uniqueId !== entry.uniqueId) {
        return;
      }

      // We are safe to delete the old entry and add a new one
      delete entries.draggables[last.descriptor.id];
      entries.draggables[entry.descriptor.id] = entry;
    },
    unregister: (entry: DraggableEntry) => {
      const draggableId: DraggableId = entry.descriptor.id;
      const current: ?DraggableEntry = findDraggableById(draggableId);

      // can occur if cleaned before unregistration
      if (!current) {
        return;
      }

      // outdated uniqueId
      if (entry.uniqueId !== current.uniqueId) {
        return;
      }

      delete entries.draggables[draggableId];
      notify({ type: 'REMOVAL', value: entry });
    },
    getById: getDraggableById,
    findById: findDraggableById,
    exists: (id: DraggableId): boolean => Boolean(findDraggableById(id)),
    getAllByType: (type: TypeId): DraggableEntry[] =>
      values(entries.draggables).filter(
        (entry: DraggableEntry): boolean => entry.descriptor.type === type,
      ),
  };

  function findDroppableById(id: DroppableId): ?DroppableEntry {
    return entries.droppables[id] || null;
  }

  function getDroppableById(id: DroppableId): DroppableEntry {
    const entry: ?DroppableEntry = findDroppableById(id);
    invariant(entry, `Cannot find droppable entry with id [${id}]`);
    return entry;
  }

  const droppableAPI: DroppableAPI = {
    register: (entry: DroppableEntry) => {
      entries.droppables[entry.descriptor.id] = entry;
    },
    unregister: (entry: DroppableEntry) => {
      const current: ?DroppableEntry = findDroppableById(entry.descriptor.id);

      // can occur if cleaned before an unregistry
      if (!current) {
        return;
      }

      // already changed
      if (entry.uniqueId !== current.uniqueId) {
        return;
      }

      delete entries.droppables[entry.descriptor.id];
    },
    getById: getDroppableById,
    findById: findDroppableById,
    exists: (id: DroppableId): boolean => Boolean(findDroppableById(id)),
    getAllByType: (type: TypeId): DroppableEntry[] =>
      values(entries.droppables).filter(
        (entry: DroppableEntry): boolean => entry.descriptor.type === type,
      ),
  };

  function clean(): void {
    // kill entries
    entries.draggables = {};
    entries.droppables = {};
    // remove all subscribers
    subscribers.length = 0;
  }

  return {
    draggable: draggableAPI,
    droppable: droppableAPI,
    subscribe,
    clean,
  };
}
