Compass rails Sass mixin for resizing sprite

mixin.scss

@mixin scale-sprite($map, $sprite, $scale : 1) {
  $sprite-file: sprite-file($map, $sprite);

  $width-to-map-ratio: sprite-map-width($map)/image-width($sprite-file);
  $pos: sprite-position($map, $sprite, 0, 0);

  $height: image-height($sprite-file) * $scale;
  $width: image-width($sprite-file) * $scale;
  $x: nth($pos, 1) * $scale;
  $y: nth($pos, 2) * $scale;

  display: block;
  background-image: $map;
  background-position: $x $y;
  background-size: percentage($width-to-map-ratio);
  height: ceil($height) !important;
  width: ceil($width) !important;
}

sass.rb

module Sass::Script::Functions
  def sprite_map_width map
    binding.pry
    Sass::Script::Number.new(map.width,["px"])
  end
  declare :sprite_map_width, args: [:map]
end