const _ = require('lodash');

/**
 * SingletonResolver is a cached loader for a single result.
 */
class SingletonResolver {
  constructor(resolver) {
    this._cache = null;
    this._resolver = resolver;
  }

  load() {
    if (this._cache) {
      return this._cache;
    }

    let promise = this._resolver(arguments).then((result) => {
      return result;
    });

    // Set the promise on the cache.
    this._cache = promise;

    return promise;
  }
}

/**
 * This joins a set of results with a specific keys and sets an empty array in
 * place if it was not found.
 * @param  {Array}  ids ids to locate
 * @param  {String} key key to group by
 * @return {Array}      array of results
 */
const arrayJoinBy = (ids, key) => (items) => {
  const itemsByKey = _.groupBy(items, key);
  return ids.map((id) => {
    if (id in itemsByKey) {
      return itemsByKey[id];
    }

    return [];
  });
};

/**
 * This joins a set of results with a specific keys and sets null in place if it
 * was not found.
 * @param  {Array}  ids ids to locate
 * @param  {String} key key to group by
 * @return {Array}      array of results
 */
const singleJoinBy = (ids, key) => (items) => {
  const itemsByKey = _.groupBy(items, key);
  return ids.map((id) => {
    if (id in itemsByKey) {
      return itemsByKey[id][0];
    }

    return null;
  });
};

module.exports = {
  singleJoinBy,
  arrayJoinBy,
  SingletonResolver
};