scss gradient mixin

_gradient.scss

/// Mixin printing a linear-gradient
/// as well as a plain color fallback
/// and the `-webkit-` prefixed declaration
/// @access public
/// @param {Keyword | Angle} $direction - Linear gradient direction
/// @param {Arglist} $color-stops - List of color-stops composing the gradient
@mixin linear-gradient($direction, $color-stops...) {
  // Direction has been omitted and happens to be a color-stop
  @if is-direction($direction) == false {
    $color-stops: $direction, $color-stops;
    $direction: 180deg;
  }

  background: nth(nth($color-stops, 1), 1);
  background: -webkit-linear-gradient(legacy-direction($direction), $color-stops);
  background: linear-gradient($direction, $color-stops);
  filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#{nth(nth($color-stops, 1), 1)}', endColorstr='#{nth(nth($color-stops, length($color-stops)), 1)}', GradientType=0 ); /* IE6-9 */
}

/// Test if `$value` is a valid direction
/// @param {*} $value - Value to test
/// @return {Bool}
@function is-direction($value) {
  $is-keyword: index((to top, to top right, to right top, to right, to bottom right, to right bottom, to bottom, to bottom left, to left bottom, to left, to left top, to top left), $value);
  $is-angle: type-of($value) == 'number' and index('deg' 'grad' 'turn' 'rad', unit($value));

  @return $is-keyword or $is-angle;
}

/// Convert a direction to legacy syntax
/// @param {Keyword | Angle} $value - Value to convert
/// @require {function} is-direction
/// @require {function} convert-angle
/// @throw Cannot convert `#{$value}` to legacy syntax because it doesn't seem to be a direction.;
@function legacy-direction($value) {
  @if is-direction($value) == false {
    @error "Cannot convert `#{$value}` to legacy syntax because it doesn't seem to be a direction.";
  }

  $conversion-map: (
    to top          : bottom,
    to top right    : bottom left,
    to right top    : left bottom,
    to right        : left,
    to bottom right : top left,
    to right bottom : left top,
    to bottom       : top,
    to bottom left  : top right,
    to left bottom  : right top,
    to left         : right,
    to left top     : right bottom,
    to top left     : bottom right
  );

  @if map-has-key($conversion-map, $value) {
    @return map-get($conversion-map, $value);
  }

  @return 90deg - $value;
}