import { IterableX } from '../iterablex.js';
import { bindCallback } from '../../util/bindcallback.js';
import { OperatorFunction } from '../../interfaces.js';

/** @ignore */
export class MapIterable<TSource, TResult> extends IterableX<TResult> {
  private _source: Iterable<TSource>;
  private _selector: (value: TSource, index: number) => TResult;

  constructor(source: Iterable<TSource>, selector: (value: TSource, index: number) => TResult) {
    super();
    this._source = source;
    this._selector = selector;
  }

  *[Symbol.iterator]() {
    let i = 0;
    for (const item of this._source) {
      yield this._selector(item, i++);
    }
  }
}

/**
 * Projects each element of an async-enumerable sequence into a new form.
 *
 * @template TSource The type of the elements in the source sequence.
 * @template TResult The type of the elements in the result sequence, obtained by running the selector
 * function for each element in the source sequence.
 * @param {((value: TSource, index: number) => TResult)} selector A transform function
 * to apply to each source element.
 * @param {*} [thisArg] Optional this for binding to the selector.
 * @returns {OperatorFunction<TSource, TResult>} An iterable sequence whose elements are the result of invoking the transform
 * function on each element of source.
 */
export function map<TSource, TResult>(
  selector: (value: TSource, index: number) => TResult,
  thisArg?: any
): OperatorFunction<TSource, TResult> {
  return function mapOperatorFunction(source: Iterable<TSource>): IterableX<TResult> {
    return new MapIterable<TSource, TResult>(source, bindCallback(selector, thisArg, 2));
  };
}