(bug) The `mix` function has a fatally wrong algorithm

[debiru]

• polished version: v4.3.1
• JSS-in_CSS library and version: unknown
• Any relevant JS library and version: unknown

What You Are Seeing

I was looking into the implementation of the mix colors function.

Among them I found polished and dart-sass implementations.

However, upon closer inspection, I found a bug in the polished implementation.

• dart-sass: https://github.com/sass/dart-sass/blob/1.77.8/lib/src/functions/color.dart#L830-L835
• polished: https://github.com/styled-components/polished/blob/v4.3.1/src/color/mix.js#L48-L55

dart-sass:

  var normalizedWeight = weightScale * 2 - 1;
  var alphaDistance = color1.alpha - color2.alpha;

  var combinedWeight1 = normalizedWeight * alphaDistance == -1
      ? normalizedWeight
      : (normalizedWeight + alphaDistance) /
          (1 + normalizedWeight * alphaDistance);
  var weight1 = (combinedWeight1 + 1) / 2;
  var weight2 = 1 - weight1;

polished:

  // The formula is copied from the original Sass implementation:
  // http://sass-lang.com/documentation/Sass/Script/Functions.html#mix-instance_method
  const alphaDelta = color1.alpha - color2.alpha
  const x = parseFloat(weight) * 2 - 1
  const y = x * alphaDelta === -1 ? x : x + alphaDelta
  const z = 1 + x * alphaDelta
  const weight1 = (y / z + 1) / 2.0
  const weight2 = 1 - weight1

Converting the dart-sass implementation to polished variable names looks like this:

  var alphaDelta = color1.alpha - color2.alpha;
  var x = weightScale * 2 - 1;

  var z = 1 + x * alphaDelta;
  var y = x * alphaDelta == -1 ? x : (x + alphaDelta) / z;
  var weight1 = (y + 1) / 2;
  var weight2 = 1 - weight1;

What You Expected To See

The code of polished is wrong:

  const y = x * alphaDelta === -1 ? x : x + alphaDelta
  const z = 1 + x * alphaDelta
  const weight1 = (y / z + 1) / 2.0

Here's the correct code:

  const z = 1 + x * alphaDelta
  const y = x * alphaDelta === -1 ? x : (x + alphaDelta) / z
  const weight1 = (y + 1) / 2.0

Do not divide by z when x * alphaDelta === -1.

Notes

Furthermore, there is also the following problem. #613

dart-sass:

  return SassColor.rgb(
      fuzzyRound(color1.red * weight1 + color2.red * weight2),
      fuzzyRound(color1.green * weight1 + color2.green * weight2),
      fuzzyRound(color1.blue * weight1 + color2.blue * weight2),
      color1.alpha * weightScale + color2.alpha * (1 - weightScale));
}

polished:

  const mixedColor = {
    red: Math.floor(color1.red * weight1 + color2.red * weight2),
    green: Math.floor(color1.green * weight1 + color2.green * weight2),
    blue: Math.floor(color1.blue * weight1 + color2.blue * weight2),
    alpha:
      color1.alpha * parseFloat(weight) + color2.alpha * (1 - parseFloat(weight)),
  }

Math.round should be used instead of Math.floor.