better random color hue (not backward compatible)

Author: kpym

pattern.go

@@ -239,9 +239,45 @@ func (g *generator) setOpacity(opacity float64) {
 	g.opacity = opacity
 }
 
+type basecolor struct {
+	middle float64
+	delta  float64
+}
+
+var colors = [...]basecolor{
+	{0, 15},   // red
+	{30, 15},  // orange
+	{60, 15},  // yellow
+	{120, 45}, // green
+	{180, 15}, // cyan
+	{225, 30}, // blue
+	{270, 15}, // purple
+	{315, 30}, // magenta
+}
+
+// randomHue generate a random intéger in [0,360]
+// First it peaks one of the base colors,
+// then it adds a random deviation in [-delta, delta].
+// This is necessary to avoid 'green' to be 3 times more probable
+// than 'red', for example.
+
+func (g *generator) randomHue() float64 {
+	n := len(colors)
+	if n <= 0 {
+		return 0
+	}
+	i := g.rand.Intn(n)
+	c := colors[i]
+	h := c.middle + g.rd(c.delta)
+	if h < 0 {
+		h += 360
+	}
+	return h
+}
+
 // randomColor generate a random background color.
 func (g *generator) randomColor() {
-	randCol := colorful.Hsl(360*g.rand.Float64(), 0.3+0.1*g.rand.Float64(), 0.3+0.1*g.rand.Float64())
+	randCol := colorful.Hsl(g.randomHue(), 0.3+0.1*g.rand.Float64(), 0.3+0.1*g.rand.Float64())
 	g.setColor(randCol)
 }