/// Compares two string to determine which comes first
/// @access private
/// @param {String} $a - first string
/// @param {String} $b - second string
/// @param {List} $order - order to deal with
/// @return {Bool}
@function _str-compare($a, $b, $order) {
  @if type-of($a) == "number" and type-of($b) == "number" {
    @return $a < $b;
  }

  $a: to-lower-case($a + unquote(""));
  $b: to-lower-case($b + unquote(""));

  @for $i from 1 through min(str-length($a), str-length($b)) {
    $char-a: str-slice($a, $i, $i);
    $char-b: str-slice($b, $i, $i);

    @if $char-a and $char-b and index($order, $char-a) != index($order, $char-b) {
      @return index($order, $char-a) < index($order, $char-b);
    }
  }

  @return str-length($a) < str-length($b);
}