///Computes the "brightness" of a color
@function brightness($color) {
  @if type-of($color) == color {
    @return (red($color) * 0.299 + green($color) * 0.587 + blue($color) * 0.114) / 255 * 100%;
  }
  @else {
    @return unquote("brightness(#{$color})");
  }
}


///Select the more readable foreground color for a given background color.
@function contrast-color($color, $dark: $contrasted-dark, $light: $contrasted-light) {
  @if $color == null {
    @return null;
  }
  @else {
    $color-brightness: brightness($color);
    $dark-text-brightness: brightness($dark);
    $light-text-brightness: brightness($light);
    @return if(abs($color-brightness - $light-text-brightness) > abs($color-brightness - $dark-text-brightness), $light, $dark);
  }
}

@function add-z-index($key, $value) {
  @return map-merge($z-indexes, ($key: $value));
}

@function z-index($key) {
  @if map-has-key($z-indexes, $key) {
    @return map-get($z-indexes, $key);
  }

  @warn "Unknown key `#{$key}` in $z-indexes";
  @return null;
}