@use "sass:color";
@use "sass:math";

///
/// Private function - returns the relative luminance of a single color
/// channel where $n goes from 0 to 255.
///
@function _relative-lum($n) {
  @if $n < 0.03928 {
    @return math.div($n, 12.92);
  } @else {
    @return math.pow(math.div($n+0.055, 1.055), 2.4);
  }
}

///
/// Private function - returns the relative luminance of a given color.
///
@function _relative-luminance($c) {
  $r: _relative-lum(math.div(color.red($c), 255));
  $g: _relative-lum(math.div(color.green($c), 255));
  $b: _relative-lum(math.div(color.blue($c), 255));

  @return $r*0.2126+$g*0.7152+$b*0.0722;
}

///
/// Private function - returns a value for the contrast between the color $c
/// and white. If $on-black is truthy, will calculate contrast of $c and black
/// instead.
///
@function _contrast($c, $on-black: false) {
  @if $on-black {
    @return math.div(_relative-luminance($c) + 0.05, 0.05);
  } @else {
    @return math.div(1.05, _relative-luminance($c) + 0.05);
  }
}

///
/// returns either $black or $white, whichever has a the higher contrast value
/// with $c.
///
@function blackwhite($c, $black: #000, $white: #fff) {
    @if _contrast($c) < _contrast($c, true) {
        @return $black;
    } @else {
        @return $white;
    }
}