/// Parses a JSON encoded string to see if it's a CSS length
/// @access private
/// @param {String} $string - JSON string
/// @return {Number | String} - string or number, depending on the match
/// @require {function} _json-decode--number
@function _length($string) {
  @if type-of($string) == "number" {
    @return $string;
  }

  $strings: 'px' 'cm' 'mm' '%' 'ch' 'pica' 'in' 'em' 'rem' 'pt' 'pc' 'ex' 'vw' 'vh' 'vmin' 'vmax';
  $units:   1px  1cm  1mm  1%  1ch  1pica  1in  1em  1rem  1pt  1pc  1ex  1vw  1vh  1vmin  1vmax;
  $number: "";
  $unit: "";

  @for $i from 1 through str-length($string) {
    $c: str-slice($string, $i, $i);
    @if $c == ' ' or $c == "	" {
      @if $number != "" {
        @return $string;
      }
    } @else if index('0' '1' '2' '3' '4' '5' '6' '7' '8' '9' '-' '.', $c) {
      $number: $number + $c;
    } @else {
      @if $number == "" {
        @return $string;
      }
      $unit: $unit + $c;
    }
  }

  $number: nth(_json-decode--number($number, 2), 2);
  $index: index($strings, to-lower-case($unit));

  @if $index and $index > 0 {
    @return $number * nth($units, $index);
  }

  @return $string;
}