From d69091c6e07a04b212b593b7365e379a26db3dd1 Mon Sep 17 00:00:00 2001
From: Bertrand Madet <trambi@users.noreply.github.com>
Date: Thu, 28 Dec 2023 14:19:42 +0100
Subject: [PATCH] Fix HSL to RGB conversion

Adapts the implementation given in https://drafts.csswg.org/css-color/#hsl-to-rgb.

Fixes #142.
---
 lib/parsers.test.js     |  9 ++++++++-
 lib/utils/colorSpace.js | 28 +++++++++++++---------------
 2 files changed, 21 insertions(+), 16 deletions(-)

diff --git a/lib/parsers.test.js b/lib/parsers.test.js
index 926f7e74..1d70f3d4 100644
--- a/lib/parsers.test.js
+++ b/lib/parsers.test.js
@@ -107,7 +107,14 @@ describe('parseColor', () => {
 
     expect(output).toEqual('rgba(5, 5, 5, 0.5)');
   });
-
+  it.each([[120, 'rgb(0, 255, 0)'], [240, 'rgb(0, 0, 255)']])(
+    'should convert not zero hsl with non zero hue %s to %s',
+    (hue, rgbValue) => {
+      let input = 'hsl(' + hue + ', 100%, 50%)';
+      let output = parsers.parseColor(input);
+      expect(output).toEqual(rgbValue);
+    }
+  );
   it.todo('Add more tests');
 });
 describe('parseAngle', () => {
diff --git a/lib/utils/colorSpace.js b/lib/utils/colorSpace.js
index 92ca7bd0..419620bf 100644
--- a/lib/utils/colorSpace.js
+++ b/lib/utils/colorSpace.js
@@ -1,21 +1,19 @@
 'use strict';
 
-const hueToRgb = (t1, t2, hue) => {
-  if (hue < 0) hue += 6;
-  if (hue >= 6) hue -= 6;
-
-  if (hue < 1) return (t2 - t1) * hue + t1;
-  else if (hue < 3) return t2;
-  else if (hue < 4) return (t2 - t1) * (4 - hue) + t1;
-  else return t1;
-};
+const MAX_HUE = 360;
+const COLOR_NB = 12;
+const MAX_RGB_VALUE = 255;
 
 // https://www.w3.org/TR/css-color-4/#hsl-to-rgb
 exports.hslToRgb = (hue, sat, light) => {
-  const t2 = light <= 0.5 ? light * (sat + 1) : light + sat - light * sat;
-  const t1 = light * 2 - t2;
-  const r = hueToRgb(t1, t2, hue + 2);
-  const g = hueToRgb(t1, t2, hue);
-  const b = hueToRgb(t1, t2, hue - 2);
-  return [Math.round(r * 255), Math.round(g * 255), Math.round(b * 255)];
+  hue = hue % MAX_HUE;
+  if (hue < 0) {
+    hue += MAX_HUE;
+  }
+  function f(n) {
+    const k = (n + hue / (MAX_HUE / COLOR_NB)) % COLOR_NB;
+    const a = sat * Math.min(light, 1 - light);
+    return light - a * Math.max(-1, Math.min(k - 3, 9 - k, 1));
+  }
+  return [f(0), f(8), f(4)].map(value => Math.round(value * MAX_RGB_VALUE));
 };