/// Shell sort
/// @author Hugo Giraudel
/// @param {List} $list - list to sort
/// @param {List} $order ($default-order) - order to use for sorting
/// @return {List}
/// @require {function} _str-compare
@function shell-sort($list, $order: $default-order) {
  $gaps: 701, 301, 132, 57, 23, 10, 4, 1;

  @for $k from 1 through length($gaps) {
    $gap: nth($gaps, $k);
    $i: $gap;

    @while $i <= length($list) {
      $current: nth($list, $i);
      $j: $i;

      @while $j >= $gap and $j - $gap > 0 and not _str-compare(nth($list, $j - $gap), $current, $order) {
        $list: set-nth($list, $j, nth($list, $j - $gap));
        $j: $j - $gap;
      }

      $list: set-nth($list, $j, $current);
      $i: $i + $gap;
    }
  }

  @return $list;
}