////
/// @group contrast
///
/// Functions to help with contrast as defined by
/// [WCAG21](https://www.w3.org/TR/WCAG21/#dfn-contrast-ratio)
////

@use "sass:math";

/// @type color
$planifolia-contrast-dark-default: black !default;
/// @type color
$planifolia-contrast-light-default: white !default;

@function _threshold($threshold) {
  @if ($threshold == 'AA' or $threshold == 'AAALG') {
    @return 4.5;
  } @else if ($threshold == 'AALG') {
    @return 3;
  } @else if ($threshold == 'AAA') {
    @return 7;
  } @else {
    @return $threshold;
  }
}

@function _srgb($channel) {
  $x: math.div($channel, 255);
  @if $x <= 0.04045 {
    @return math.div($x, 12.92);
  } @else {
    $c: math.div($x + 0.055, 1.055);
    @return math.pow($c, 2.4);
  }
}

@function alpha-blend($fg, $bg: white) {
  $a1: alpha($bg);
  $a2: alpha($fg);

  @if ($a1 == 0) {
    @if ($a2 == 0) {
      @return $fg;
    }
  }

  $a: $a2 + (1 - $a2) * $a1;
  $r: math.div($a2 * red($fg) + (1 - $a2) * $a1 * red($bg), $a);
  $g: math.div($a2 * green($fg) + (1 - $a2) * $a1 * green($bg), $a);
  $b: math.div($a2 * blue($fg) + (1 - $a2) * $a1 * blue($bg), $a);

  @return rgba($r, $g, $b, $a);
}

@function luma($color) {
  $r: _srgb(red($color));
  $g: _srgb(green($color));
  $b: _srgb(blue($color));
  @return 0.2126 * $r + 0.7152 * $g + 0.0722 * $b;
}

@function _contrast($fg, $bg) {
  $lbg: luma($bg);
  $lfg: luma(alpha-blend($fg, $bg));
  @return math.div(max($lbg, $lfg) + 0.05, min($lbg, $lfg) + 0.05);
}

/// Calculate the minimum possible contrast between two colors.
///
/// Note that the "minimum" part of this is only relevant if `$bg` is
/// transparent. In that case, a backdrop color is chosen so that the resulting
/// contrast is minimal.
///
/// @param {color} $fg foreground color
/// @param {color} $bg background color
/// @return {number} between 1 and 21
@function contrast-min($fg, $bg) {
  // optimize for the common case
  @if alpha($bg) == 1 {
    @return _contrast($fg, $bg);
  } @else {
    $bg-black: alpha-blend($bg, black);
    $bg-white: alpha-blend($bg, white);
    $lfg: luma($fg);

    @if luma($bg-white) < $lfg {
      @return _contrast($fg, $bg-white);
    } @else if luma($bg-black) > $lfg {
      @return _contrast($fg, $bg-black);
    } @else {
      @return 1;
    }
  }
}

/// Calculate the contrast between two colors.
///
/// This function is different from `contrast-min` by not caring about the
/// order of inputs. This is achieved by calculating the average of both
/// possible results of `contrast-min`.
///
/// @param {color} $color1
/// @param {color} $color2
/// @return {number} between 1 and 21
/// @see contrast-min
@function contrast($color1, $color2) {
  // NOTE: optimized for the common case
  @if alpha($color1) + alpha($color2) == 2 {
    @return _contrast($color1, $color2);
  } @else {
    $c1: contrast-min($color1, $color2);
    $c2: contrast-min($color2, $color1);
    @return ($c1 + $c2) * 0.5;
  }
}

/// Pick the higher contrast option for a given base color.
///
/// @param {color} $base the base color to compare to
/// @param {color} $color1 [$planifolia-contrast-dark-default] first option
/// @param {color} $color2 [$planifolia-contrast-light-default] second option
/// @return {color} either `$color1` or `$color2`
@function color(
  $base,
  $color1: $planifolia-contrast-dark-default,
  $color2: $planifolia-contrast-light-default
) {
  @if contrast($color1, $base) >= contrast($color2, $base) {
    @return $color1;
  } @else {
    @return $color2;
  }
}

/// Mix color with black or white to increase contrast for a given base color.
///
/// @param {color} $base
/// @param {color} $color
/// @param {number} $threshold [4.5]
///    (can also be 'AA', 'AALG', 'AAA', or 'AAALG')
/// @return {color}
@function stretch($base, $color, $threshold: 4.5) {
  $threshold: _threshold($threshold);
  $lower: $color;
  $upper: if(luma($base) < 0.18, white, black);

  @if contrast($base, $lower) >= $threshold {
    @return $lower;
  }
  @if contrast($base, $upper) <= $threshold {
    @return $upper;
  }

  // NOTE: This is not a usual binary search. It is possible that the contrast
  // first decreases for a while when going from $lower to $upper.  However, we
  // checked that it starts below $contrast, so the algorithm still works.
  @for $i from 0 to 10 {
    $tmp: mix($lower, $upper);

    @if contrast($base, $tmp) < $threshold {
      $lower: $tmp;
    } @else {
      $upper: $tmp;
    }
  }

  @return $upper;
}

/// Warn if the contrast is below a threshold.
///
/// @param {color} $base
/// @param {color} $color
/// @param {number} $threshold [4.5]
///    (can also be 'AA', 'AALG', 'AAA', or 'AAALG')
/// @return {color} unchanged $color
@function check($base, $color, $threshold: 4.5) {
  $threshold: _threshold($threshold);
  $contrast: contrast($base, $color);
  @if $contrast < $threshold {
    @warn 'contrast #{$contrast} between #{$base} and #{$color} too low!';
  }
  @return $color;
}