/**
 * Returns a sorted string of numbers, grouping consecutive numbers
 * of at least [range] _as_ a range of numbers, e.g. "8-11"
 * @param {Iterable<number>} collection Numbers to sort and group into ranges
 * @param {number} [range=2] min number of consecutive ints that form a range
 * @return {string}
 */
function getSortedWithRanges(collection, range = 2) {
  // Roughly assert that we have an iterable
  // note: this doesn't validate the elements
  if (!(collection && typeof collection[Symbol.iterator] === 'function')) {
    throw new TypeError(
      `Expected [collection] to be an iterable,` +
        ` saw type "${Object.prototype.toString.call(collection)}"`
    );
  }

  // Assert that the range is a finite number greater than 1
  if (!(Number.isFinite(range) && range > 1)) {
    throw new TypeError(
      `Expected [range] to be a finite number` +
        ` greater than 1, saw "${range}" of type "${typeof range}"`
    );
  }

  const sorted = Array.from(collection).sort((a, b) => a - b);

  // Bail early if we don't have enough elements to create ranges
  if (sorted.length < range) {
    return sorted.join();
  }

  for (let i = 0; i < sorted.length; i++) {
    const n = sorted[i];

    // Count of consecutive numbers found
    // offset by 1 so we skip checking the
    // current number
    let cursor = 1;

    // Scan forward for said consecutive numbers
    while (sorted[i + cursor] === n + cursor) cursor++;

    // If we found consective numbers that satisfy the min range,
    // splice it into the result and remove the elements that make
    // up the range
    if (cursor >= range) {
      sorted.splice(
        // Insertion index
        i,
        // Number of elements to be replaced with the range
        cursor,
        // Range to insert
        `${n}-${n + cursor - 1}`
      );
    } else if (cursor !== 1) {
      // Advance the index by however many consecutive
      // elements were found. We know we can't create a
      // range from them so it's safe to step over
      i += cursor - 1;
    }
  }

  return sorted.join();
}

// doit
// const numbers = new Set([1, 2, 5, 9, 8, 7, 3, 4, 100]);
// console.log(getSortedWithRanges(numbers, 4));