Added variable latency delay, normal and uniform based

- Allow providing new delays with your own rng / use shared rng

License: MIT
Signed-off-by: Or Rikon <rikonor@gmail.com>

delay.go

 package delay
 
 import (
+	"math/rand"
 	"sync"
 	"time"
 )
 
+var sharedRNG = rand.New(rand.NewSource(time.Now().UnixNano()))
+
 // Delay makes it easy to add (threadsafe) configurable delays to other
 // objects.
 type D interface {
@@ -23,8 +26,6 @@ type delay struct {
 	t time.Duration
 }
 
-// TODO func Variable(time.Duration) D returns a delay with probablistic latency
-
 func (d *delay) Set(t time.Duration) time.Duration {
 	d.l.Lock()
 	defer d.l.Unlock()
@@ -44,3 +45,61 @@ func (d *delay) Get() time.Duration {
 	defer d.l.Unlock()
 	return d.t
 }
+
+// VariableNormal is a delay following a normal distribution
+// Notice that to implement the D interface Set can only change the mean delay
+// the standard deviation is set only at initialization
+func VariableNormal(t, std time.Duration, rng *rand.Rand) D {
+	if rng == nil {
+		rng = sharedRNG
+	}
+
+	v := &variableNormal{
+		std: std,
+		rng: rng,
+	}
+	v.t = t
+	return v
+}
+
+type variableNormal struct {
+	delay
+	std time.Duration
+	rng *rand.Rand
+}
+
+func (d *variableNormal) Wait() {
+	d.l.RLock()
+	defer d.l.RUnlock()
+	randomDelay := time.Duration(d.rng.NormFloat64() * float64(d.std))
+	time.Sleep(randomDelay + d.t)
+}
+
+// VariableUniform is a delay following a uniform distribution
+// Notice that to implement the D interface Set can only change the minimum delay
+// the delta is set only at initialization
+func VariableUniform(t, d time.Duration, rng *rand.Rand) D {
+	if rng == nil {
+		rng = sharedRNG
+	}
+
+	v := &variableUniform{
+		d:   d,
+		rng: rng,
+	}
+	v.t = t
+	return v
+}
+
+type variableUniform struct {
+	delay
+	d   time.Duration // max delta
+	rng *rand.Rand
+}
+
+func (d *variableUniform) Wait() {
+	d.l.RLock()
+	defer d.l.RUnlock()
+	randomDelay := time.Duration(d.rng.Float64() * float64(d.d))
+	time.Sleep(randomDelay + d.t)
+}