Codibre/fluent-iterable

import fluent from './fluent';
import { FluentIterable } from './types';

function internalInterval(
  fromInclusive: number | undefined,
  count: number | undefined,
  step: number,
): Iterable<number> {
  const from = fromInclusive || 0;
  let offset = 0;
  return {
    [Symbol.iterator]: () => ({
      next() {
        const done = count !== undefined && offset >= count;
        const value = from + offset;
        offset += step;
        return {
          value,
          done,
        };
      },
    }),
  };
}

/**
 * Generates a `count` long sequential integer interval starting from `fromInclusive`.
 *
 * * The interval starts at zero if `fromInclusive` is not specified.
 * * The interval ends in infinity if `count` is not specified.
 *
 * Examples:
 *
 * 1. `interval(5, 3)` generates an interval of `[5, 6, 7]`
 * 2. `interval(5)` generates an interval of `[5, 6, 7, ...]`
 * 3. `interval()` generates an interval of `[0, 1, 2, ...]`
 * 4. `fluent(interval(1)).take(10).forEach(console.log)` prints all numbers between 1 and 10 using [[fluent]].
 * @param fromInclusive Specifies the start of the interval. Defaults to zero.
 * @param count Specifies the length of the interval. The interval provides numbers indefinitely if omitted.
 * @returns The iterable of numbers in the interval.
 */
export function interval(
  fromInclusive?: number,
  count?: number,
): FluentIterable<number> {
  return fluent(internalInterval(fromInclusive, count, 1));
}

/**
 * Generates a `count` long sequential integer interval starting from `fromInclusive`.
 *
 * * The interval ends in negative infinity if `count` is not specified.
 *
 * Examples:
 *
 * 1. `decrement(5, 3)` generates an interval of `[5, 4, 3]`
 * 2. `decrement(5)` generates an interval of `[5, 4, 3, ...]`
 * 3. `decrement()` generates an interval of `[0, -1, -2, ...]`
 * 4. `decrement(1).take(10).forEach(console.log)` prints all numbers between 1 and -9.
 * @param fromInclusive Specifies the start of the interval. Defaults to zero.
 * @param count Specifies the length of the interval. The interval provides numbers indefinitely if omitted.
 * @returns The iterable of numbers in the interval.
 */
export function decrement(
  fromInclusive?: number,
  count?: number,
): FluentIterable<number> {
  return fluent(internalInterval(fromInclusive, count, -1));
}