feat(tracker): add a rudimentary VU-meter to show master volume, peaks & clipping

Closes #16

tracker/vumeter.go

@@ -0,0 +1,78 @@
+package tracker
+
+import (
+	"image"
+	"math"
+
+	"gioui.org/f32"
+	"gioui.org/op"
+	"gioui.org/op/clip"
+	"gioui.org/op/paint"
+	"gioui.org/unit"
+)
+
+type VuMeter struct {
+	avg     [2]float32
+	max     [2]float32
+	speed   [2]float32
+	FallOff float32
+	Decay   float32
+	RangeDb float32
+}
+
+func (v *VuMeter) Update(buffer []float32) {
+	for j := 0; j < 2; j++ {
+		for i := 0; i < len(buffer); i += 2 {
+			sample2 := buffer[i+j] * buffer[i+j]
+			db := float32(10*math.Log10(float64(sample2))) + v.RangeDb
+			v.speed[j] += v.FallOff
+			v.max[j] -= v.speed[j]
+			if v.max[j] < 0 {
+				v.max[j] = 0
+			}
+			if v.max[j] < db {
+				v.max[j] = db
+				v.speed[j] = 0
+			}
+			v.avg[j] += (sample2 - v.avg[j]) * v.Decay
+			if math.IsNaN(float64(v.avg[j])) {
+				v.avg[j] = 0
+			}
+		}
+	}
+}
+
+func (v *VuMeter) Reset() {
+	v.avg = [2]float32{}
+	v.max = [2]float32{}
+}
+
+func (v *VuMeter) Layout(gtx C) D {
+	defer op.Save(gtx.Ops).Load()
+	gtx.Constraints.Max.Y = gtx.Px(unit.Dp(12))
+	height := gtx.Px(unit.Dp(6))
+	for j := 0; j < 2; j++ {
+		value := float32(10*math.Log10(float64(v.avg[j]))) + v.RangeDb
+		if value > 0 {
+			x := int(value/v.RangeDb*float32(gtx.Constraints.Max.X) + 0.5)
+			if x > gtx.Constraints.Max.X {
+				x = gtx.Constraints.Max.X
+			}
+			paint.FillShape(gtx.Ops, mediumEmphasisTextColor, clip.Rect(image.Rect(0, 0, x, height)).Op())
+		}
+		valueMax := v.max[j]
+		if valueMax > 0 {
+			color := white
+			if valueMax >= v.RangeDb {
+				color = errorColor
+			}
+			x := int(valueMax/v.RangeDb*float32(gtx.Constraints.Max.X) + 0.5)
+			if x > gtx.Constraints.Max.X {
+				x = gtx.Constraints.Max.X
+			}
+			paint.FillShape(gtx.Ops, color, clip.Rect(image.Rect(x-1, 0, x, height)).Op())
+		}
+		op.Offset(f32.Pt(0, float32(height))).Add(gtx.Ops)
+	}
+	return D{Size: gtx.Constraints.Max}
+}