ix/src/iterable/operators/scan.ts

The Interactive Extensions for JavaScript

import { IterableX } from '../iterablex.js';
import { OperatorFunction } from '../../interfaces.js';
import { ScanOptions } from './scanoptions.js';

/** @ignore */
export class ScanIterable<T, R> extends IterableX<R> {
  private _source: Iterable<T>;
  private _fn: (acc: R, x: T, index: number) => R;
  private _seed?: T | R;
  private _hasSeed: boolean;

  constructor(source: Iterable<T>, options: ScanOptions<T, R>) {
    super();
    this._source = source;
    this._fn = options['callback'];
    this._hasSeed = options.hasOwnProperty('seed');
    this._seed = options['seed'];
  }

  *[Symbol.iterator]() {
    let i = 0;
    let hasValue = false;
    let acc = this._seed;
    for (const item of this._source) {
      if (hasValue || (hasValue = this._hasSeed)) {
        acc = this._fn(<R>acc, item, i++);
        yield acc;
      } else {
        acc = item;
        hasValue = true;
        i++;
      }
    }
    if (i === 1 && !this._hasSeed) {
      yield acc as R;
    }
  }
}

/**
 * Applies an accumulator function over an iterable sequence and returns each intermediate result.
 * The specified seed value, if given, is used as the initial accumulator value.
 *
 * @template T The type of the elements in the source sequence.
 * @template R The type of the result of the aggregation.
 * @param {ScanOptions<T, R>} options The options including the accumulator function and seed.
 * @returns {OperatorFunction<T, R>} An async-enumerable sequence containing the accumulated values.
 */
export function scan<T, R = T>(options: ScanOptions<T, R>): OperatorFunction<T, R>;
export function scan<T, R = T>(
  accumulator: (accumulator: R, current: T, index: number) => R,
  seed?: R
): OperatorFunction<T, R>;
export function scan<T, R = T>(
  optionsOrAccumulator: ScanOptions<T, R> | ((accumulator: R, current: T, index: number) => R),
  seed?: R
): OperatorFunction<T, R> {
  const options =
    // eslint-disable-next-line no-nested-ternary
    typeof optionsOrAccumulator === 'function'
      ? arguments.length > 1
        ? // prettier-ignore
          { 'callback': optionsOrAccumulator, 'seed': seed }
        : // prettier-ignore
          { 'callback': optionsOrAccumulator }
      : optionsOrAccumulator;
  return function scanOperatorFunction(source: Iterable<T>): IterableX<R> {
    return new ScanIterable(source, options);
  };
}