--- /dev/null
+@charset "UTF-8";
+
+/// Switches between two colors based on the contrast to another color. It’s
+/// like a [ternary operator] for color contrast and can be useful for building
+/// a button system.
+///
+/// The calculation of the contrast ratio is based on the [WCAG 2.0
+/// specification]. However, we cannot guarantee full compliance, though all of
+/// our manual testing passed.
+///
+/// [ternary operator]: https://goo.gl/ccfLqi
+/// [WCAG 2.0 specification]: https://goo.gl/zhQuYA
+///
+/// @argument {color} $base-color
+/// The color to evaluate lightness against.
+///
+/// @argument {color} $dark-color [#000]
+/// The color to be output when `$base-color` is light. Can also be set
+/// globally using the `contrast-switch-dark-color` key in the
+/// Bourbon settings.
+///
+/// @argument {color} $light-color [#fff]
+/// The color to be output when `$base-color` is dark. Can also be set
+/// globally using the `contrast-switch-light-color` key in the
+/// Bourbon settings.
+///
+/// @return {color}
+///
+/// @example scss
+/// .element {
+/// color: contrast-switch(#bae6e6);
+/// }
+///
+/// // CSS Output
+/// .element {
+/// color: #000;
+/// }
+///
+/// @example scss
+/// .element {
+/// $button-color: #2d72d9;
+/// background-color: $button-color;
+/// color: contrast-switch($button-color, #222, #eee);
+/// }
+///
+/// // CSS Output
+/// .element {
+/// background-color: #2d72d9;
+/// color: #eee;
+/// }
+///
+/// @require {function} _fetch-bourbon-setting
+///
+/// @require {function} _is-color
+///
+/// @require {function} _contrast-ratio
+///
+/// @since 5.0.0
+
+@function contrast-switch(
+ $base-color,
+ $dark-color: _fetch-bourbon-setting("contrast-switch-dark-color"),
+ $light-color: _fetch-bourbon-setting("contrast-switch-light-color")
+) {
+ @if not _is-color($base-color) {
+ @error "`#{$base-color}` is not a valid color for the `$base-color` " +
+ "argument in the `contrast-switch` function.";
+ } @else if not _is-color($dark-color) {
+ @error "`#{$dark-color}` is not a valid color for the `$dark-color` " +
+ "argument in the `contrast-switch` function.";
+ } @else if not _is-color($light-color) {
+ @error "`#{$light-color}` is not a valid color for the `$light-color` " +
+ "argument in the `contrast-switch` function.";
+ } @else {
+ $-contrast-to-dark: _contrast-ratio($base-color, $dark-color);
+ $-contrast-to-light: _contrast-ratio($base-color, $light-color);
+ $-prefer-dark: $-contrast-to-dark >= $-contrast-to-light;
+
+ @return if($-prefer-dark, $dark-color, $light-color);
+ }
+}