range.ts

type IterableRange = IterableIterator<number> & {
  start: number;
  end: number;
  step: number;
};

function range(
  startOrEnd: number,
  maybeEnd?: number,
  maybeStep?: number
): IterableRange {
  const start = maybeEnd == null ? 0 : startOrEnd;
  const end = maybeEnd == null ? startOrEnd : maybeEnd;
  const step = maybeStep == null ? (end < 0 ? -1 : 1) : maybeStep;
  const predicate =
    step < 0
      ? (current: number) => current > end
      : (current: number) => current < end;

  return Object.assign(
    (function* () {
      let current = start;

      while (predicate(current)) {
        yield current;
        current += step;
      }
    })(),
    { start, end, step }
  );
}