class PaginationParametersHelper {
  constructor(request) {
    this.query = request.query;
  }

  /**
   * Handle boolean options
   * If the 'option'-Parameter is a string, check if it equals 'true'
   * If not, it should be a boolean, and can be returned as it is.
   *
   * @param {string|boolean} option
   * @return {boolean}
   * */
  booleanOpt(option) {
    return typeof option === 'string' ? option === 'true' : option;
  }

  /**
   * Handle options that are strings or objects (including arrays)
   *
   * @param {object|string} option
   * @return {object|string}
   * */
  optObjectOrString(option) {
    // Since the JSON in the query object will be strings,
    // we need to be able to detect this, in order to differentiate between JSON-objects and pure strings.
    // a pure string, e.g. 'field -test', might not be parsed as wished by JSON.parse
    const openingBrackets = ['{', '['];
    const closingBrackets = ['}', ']'];
    const firstCharIsBracket = option[0] && openingBrackets.includes(option[0]);
    const lastCharIsBracket =
      option[option.length - 1] &&
      closingBrackets.includes(option[option.length - 1]);
    const optionIsObject = firstCharIsBracket && lastCharIsBracket;

    try {
      return optionIsObject ? JSON.parse(option) : option;
    } catch (err) {
      // Fallback for parsing errors of objects
      return {};
    }
  }

  /**
   * Yields the "query" parameter for Model.paginate()
   * given any attributes of the Express req.query-Object,
   * */
  getQuery() {
    const filtersQueryParameter = this.query?.query;

    if (!filtersQueryParameter) return {};

    try {
      return JSON.parse(filtersQueryParameter);
    } catch (err) {
      return {};
    }
  }

  /**
   * Yields the "options" parameter for Model.paginate(),
   * given any attributes of the Express req.query-Object
   * */
  getOptions() {
    if (!this.query) return {};

    const options = {};

    // Instantiate variables with all the possible options for Model.paginate()
    const select = this.query.select,
      collation = this.query.collation,
      sort = this.query.sort,
      populate = this.query.populate,
      projection = this.query.projection,
      lean = this.query.lean,
      leanWithId = this.query.leanWithId,
      offset = this.query.offset,
      page = this.query.page,
      limit = this.query.limit,
      customLabels = this.query.customLabels,
      pagination = this.query.pagination,
      useEstimatedCount = this.query.useEstimatedCount,
      useCustomCountFn = this.query.useCustomCountFn,
      forceCountFn = this.query.forceCountFn,
      allowDiskUse = this.query.allowDiskUse,
      read = this.query.read,
      mongooseOptions = this.query.options;

    // For every option that is set, add it to the 'options' object-literal
    if (select) options['select'] = this.optObjectOrString(select);
    if (collation) options['collation'] = this.optObjectOrString(collation);
    if (sort) options['sort'] = this.optObjectOrString(sort);
    if (populate) options['populate'] = this.optObjectOrString(populate);
    if (projection !== undefined)
      options['projection'] = this.optObjectOrString(projection);
    if (lean !== undefined) options['lean'] = this.booleanOpt(lean);
    if (leanWithId !== undefined)
      options['leanWithId'] = this.booleanOpt(leanWithId);
    if (offset) options['offset'] = Number(offset);
    if (page) options['page'] = Number(page);
    if (limit) options['limit'] = Number(limit);
    if (customLabels)
      options['customLabels'] = this.optObjectOrString(customLabels);
    if (pagination !== undefined)
      options['pagination'] = this.booleanOpt(pagination);
    if (useEstimatedCount !== undefined)
      options['useEstimatedCount'] = this.booleanOpt(useEstimatedCount);
    if (useCustomCountFn !== undefined)
      options['useCustomCountFn'] = this.booleanOpt(useCustomCountFn);
    if (forceCountFn !== undefined)
      options['forceCountFn'] = this.booleanOpt(forceCountFn);
    if (allowDiskUse) options['allowDiskUse'] = this.booleanOpt(allowDiskUse);
    if (read) options['read'] = this.optObjectOrString(read);
    if (mongooseOptions) options['options'] = this.getOptions(mongooseOptions);

    return options;
  }

  /**
   * Yields an array with positions:
   * [0] "query" parameter, for Model.paginate()
   * [1] "options" parameter, for Model.paginate()
   * */
  get() {
    return [{ ...this.getQuery() }, { ...this.getOptions() }];
  }
}

module.exports = PaginationParametersHelper;