/// Parse the prerelease of $version
/// @access private
/// @param {String} $version
/// @return {Map}
/// @example scss
///   $prerelease: _prerelease('1.2.3');
///    -> ( 'name': null, 'number': -1 )
///   $prerelease: _prerelease('1.2.3-alpha.0');
///    -> ( 'name': 'alpha', 'number': 0 )
@function _prerelease($version) {
  $name: null;
  $number: -1;
  $hyphen-index: str-index($version, '-');

  @if ($hyphen-index) {
    $prerelease: str-slice($version, $hyphen-index + 1);
    $dot-index: _str-last-index($prerelease, '.');
    $name: $prerelease;

    @if ($dot-index) {
      $number: _to-number(str-slice($prerelease, $dot-index + 1));
      $name: if($number,
        str-slice($prerelease, 1, $dot-index - 1),
        str-slice($prerelease, 1)
      );
      $number: $number or -1;
    } @else {
      $number: _to-number($prerelease);
      $name: if($number, null, $prerelease);
      $number: $number or -1;
    }
  }

  @return (
    'name': $name,
    'number': $number,
  );
}