* as a pivot. Returns the index which the slice is partitioned around, i.e. if it returns {@code
   * ret} then we know that the values with indexes in [{@code from}, {@code ret}) are less than or
   * equal to the value at {@code ret} and the values with indexes in ({@code ret}, {@code to}] are
   * greater than or equal to that.
   */
  private static int partition(double[] array, int from, int to) {