#wcag {
  @aa-contrast: 4.5;

  // Calculates the contrast between a dark colour and a background colour,
  // and returns the foreground colour that gives the best contrast (whether
  // that passes AA or not).
  .contrast(@background, @light, @dark) {
    @contrast-ratio-light: #wcag.contrast-ratio(@background, @light)[@result];
    @contrast-ratio-dark: #wcag.contrast-ratio(@background, @dark)[@result];

    @result: if(@contrast-ratio-dark >= @contrast-ratio-light,
      @dark,
      @light
    );
  }

  // Similar to .contrast, but it will strongly prefer the preferred colour
  // if it passes AA, and will only use the fallback colour if it has to.
  //
  // There is an assumption here that @background and @fallback pass AA.
  // If they don't, you may get a result that doesn't pass AA.
  .contrast-fallback(@background, @preferred, @fallback) {
    @contrast-ratio-preferred: #wcag.contrast-ratio(@background, @preferred)[@result];

    @result: if(@contrast-ratio-preferred >= #wcag[@aa-contrast],
      @preferred,
      @fallback
    );
  }



  // Contrast on steroids. This function may also modify the background colour
  // in order to make a colour pass the contrast threshold, and it heavily
  // favours the light colour over the dark colour because of situations such
  // as navigation where the background colour is not known.
  .ultra-contrast(@background, @light, @dark) {
    @target-contrast: #wcag[@aa-contrast];

    @lighter-background: tint(@background, 40%);
    @darker-background: screen(@background, darken(white, 30%));

    @light-aa: boolean(#wcag.contrast-ratio(@background, @light)[@result] > @target-contrast);
    @dark-aa: boolean(#wcag.contrast-ratio(@background, @dark)[@result] > @target-contrast);

    @enhanced-dark-aa: boolean(#wcag.contrast-ratio(@lighter-background, @dark)[@result] > @target-contrast);
    @enhanced-light-aa: boolean(#wcag.contrast-ratio(@darker-background, @light)[@result] > @target-contrast);

    @choice: if(@light-aa,
      {
        @msg: "Light colour passes AA";
        @fg: @light;
        @bg: @background;
      },
      if(@dark-aa,
        {
            @msg: "Dark colour passes AA";
            @fg: @dark;
            // @background would pass AA here but we've decided to go lighter for that extra boost,
            // as AA black text just doesn't appear as contrasting in practice.
            @bg: @lighter-background;
        },
        if(@enhanced-dark-aa,
          {
            @msg: "Dark colour passes AA with tweak";
            @fg: @dark;
            @bg: @lighter-background;
          },
          if(@enhanced-light-aa,
            {
              @msg: "Light colour passes AA with tweak";
              @fg: @light;
              @bg: @darker-background;
            },
            // final else: nothing worked, fall back to light text :|
            {
              @msg: "No colour passes AA, falling back to light text";
              @fg: @light;
              @bg: @background;
            }
          )
        )
      )
    );

    @msg: @choice[@msg];
    @fg: @choice[@fg];
    @bg: @choice[@bg];
  }

  // Calculates the WCAG contrast ratio between two colours.
  // Result properties:
  // - @result: the contrast ratio between the two colours.
  // - @c1-luma: the luma of the first colour.
  // - @c2-luma: the luma of the second colour.
  // - @light-luma: the luma of the lighter of the two colours.
  // - @dark-luma: the luma of the darker of the two colours.
  .contrast-ratio(@c1, @c2) {
    @c1-luma: luma(@c1);
    @c2-luma: luma(@c2);

    @light-luma: max(@c1-luma, @c2-luma);
    @dark-luma: min(@c1-luma, @c2-luma);

    @result: round(unit(((@light-luma+5)/(@dark-luma+5))), 2);
  }
}