/**
 * QueryParams Class
 */
export class QueryParams {
  /**
   * NOTE:
   * This class uses a reducer in order to flatten arrays, this approach is inefficient.
   * The better solution is to use flatMap when it becomes available.
   * flatMap is an es2019 proposal in stage 4 (Finished) and should be part of newer TS versions
   * Angular 8 require a TypeScript update that might include es2019 already.
   * see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flatMap#Alternative
   */

  private readonly _params: any;

  constructor(params: any) {
    if (!(params instanceof Object) || !Object.keys(params)) {
      throw new Error(
        'QueryParams constructor expected an object with at least 1 key'
      );
    }
    this._params = params;
  }

  /**
   * Converts any value into an array of strings
   * @param key
   * @param value
   */
  static serialize(key: string, value: any): string[] {
    if (typeof value === 'object' && value !== null) {
      const isArray = value instanceof Array;

      return (isArray ? value : Object.keys(value)).reduce((acc, cur, idx) => {
        // TODO: Replace reduce with flatMap (see note above);
        const { _key, _value } = isArray
          ? { _key: idx, _value: cur }
          : { _key: cur, _value: value[cur] };
        return acc.concat(QueryParams.serialize(`${key}[${_key}]`, _value));
      }, []);
    }
    return [`${key}=${encodeURI(value)}`];
  }

  /**
   * Converts a QueryParams object into an url params string
   * eg: {a : 1, b : ['a', 'b']} => 'a=1&b[0]=a&b[1]=b'
   */
  toString(): string {
    let urlParams: string[] = [];

    for (const key of Object.keys(this._params)) {
      urlParams = urlParams.concat(
        QueryParams.serialize(key, this._params[key])
      );
    }

    return urlParams.join('&');
  }

  add(key: string, value: any): void{
    this._params[key] = value;
  }
}