darkerThan

main.go

@@ -126,20 +126,20 @@ func (c *RGB) to_hsv() *HSV {
 	h = 0.0
 	if min != v {
 		if v == r {
-			h = math.Mod((g - b) / C, 6.0)
+			h = math.Mod((g-b)/C, 6.0)
 		}
 		if v == g {
-			h = (b - r) / C + 2.0
+			h = (b-r)/C + 2.0
 		}
 		if v == b {
-			h = (r - g) / C + 4.0
+			h = (r-g)/C + 4.0
 		}
 		h *= 60.0
 		if h < 0.0 {
 			h += 360.0
 		}
 	}
-	return f2hsv(h, s, v) 
+	return f2hsv(h, s, v)
 }
 
 func (c *HSV) to_rgb() *RGB {
@@ -189,10 +189,18 @@ func (c *RGB) brighterOrDarkerThan(ref RGB, delta uint8) string {
 }
 
 func (c *RGB) darkerThan(ref RGB, delta uint8) string {
-	/* XXX FIXME: THIS IS NOT CORRECT. The original implementation does darkening in HSV space.
-	 * cf. https://github.com/DiceBear/avatars/blob/master/packages/avatars/src/color.ts
-	 */
-	return fmt.Sprintf("#%02x%02x%02x", sub255(c.r, delta), sub255(c.g, delta), sub255(c.b, delta))
+	primary := c.to_hsv()
+	secondary := ref.to_hsv()
+	delta_f := float64(delta) * 1.0 / 255.0
+	if primary.v <= secondary.v-delta_f {
+		return fmt.Sprintf("#%02x%02x%02x", c.r, c.g, c.b)
+	}
+	primary.v = secondary.v - delta_f
+	if primary.v < 0 {
+		primary.v = 0
+	}
+	c = primary.to_rgb()
+	return fmt.Sprintf("#%02x%02x%02x", c.r, c.g, c.b)
 }
 
 func (c RGB) withAlpha(alpha float64) string {