$modules: () !default;

@mixin exports($name) {
    @if (not index($modules, $name)) {
        $modules: append($modules, $name) !global;

        @content;
    }
}

@function alpha($color, $amount) {
    @if type-of($color) == "color" {
        @return fade-out($color, (1 - $amount));
    } @else {
        @return unquote("alpha(#{$color},#{$amount})");
    }
}

@function shade($color, $amount) {
    @if type-of($color) == "color" {
        @if ($amount > 1) {
            @return lighten($color, ($amount - 1) * lightness($color))
        } @else {
            @return darken($color, (1 - $amount) * lightness($color))
        }
    } @else {
        @return unquote("shade(#{$color},#{$amount})");
    }
}

@function mix($color1, $color2, $amount) {
    @return unquote("mix(#{$color1},#{$color2},#{$amount})");
}

@function border_normal($color) {
    @return shade($color, $contrast);
}

@function border_focus($color) {
    @return shade($color, ($contrast - .05));
}

@function border_active($color) {
    @return shade($color, ($contrast - .1));
}

@function border_insensitive($color) {
    @return shade($color, ($contrast + .05));
}

@mixin linear-gradient($color, $direction: to bottom) {
    @if $gradient == 0 {
        background-color: $color;
        background-image: none;
    } @else {
        $amount: $gradient / 2;

        background-color: $color;
        background-image: linear-gradient($direction,
            shade($color, (1 + $amount)),
            shade($color, (1 - $amount))
        );
    }
}

@mixin border($color) {
    border-color: border_normal($color);

    &:focus, &:hover { border-color: border_focus($color); }

    &:active, &:active:hover,
    &:active:focus, &:active:hover:focus,
    &:checked, &:checked:hover,
    &:checked:focus, &:checked:hover:focus { border-color: border_active($color); }

    &:disabled { border-color: border_insensitive($color); }

    &:active:disabled, &:checked:disabled { border-color: border_normal($color); }
}

@function _text_shadow_color($tc: $fg_color, $bg: $bg_color) {
    //
    // calculate the color of text shadows
    //
    // $tc is the text color
    // $bg is the background color
    //
    $_lbg: lightness($bg) / 100%;

    @if lightness($tc) < 50% {
        @return transparentize(white, 1 - $_lbg / ($_lbg * 1.3));
    } @else {
        @return transparentize(black, $_lbg * .8);
    }
}
