add function ExponentialRandom to generate random number following Ex…

…ponential Distribution

random/random.go

@@ -114,3 +114,23 @@ func RandomPickN(a []int, m int) []int {
 	}
 	return result
 }
+
+// ExponentialRandom generate a random number following Exponential Distribution, use rand.Seed in init() of this file
+// range is [lowerBound, upperBound] and the expectation is 1/lambda.
+// lambda is "rate parameter"
+func ExponentialRandom(lowerBound, upperBound float64, lambda float64) float64 {
+	if lowerBound > upperBound || float64(1)/lambda < lowerBound || float64(1)/lambda > upperBound {
+		return -1
+	}
+	if !(lambda > 0) {
+		return -1 // lambda must be larger than 0
+	}
+	var result float64
+	for {
+		result = rand.ExpFloat64() * 1 / lambda
+		if result <= upperBound && result >= lowerBound {
+			break
+		}
+	}
+	return result
+}

random/random_test.go

@@ -134,3 +134,14 @@ func TestRandomPickN(t *testing.T) {
 	testInt := []int{1, 2, 3, 4, 5, 3, 3, 3, 3, 3, 3, 3}
 	t.Log(RandomPickN(testInt, 7))
 }
+
+func TestExponentialRandom(t *testing.T) {
+	fmt.Println("Generate 40 Exponential random float64 in [10,20]:")
+	for i := 0; i < 40; i++ {
+		fmt.Println(ExponentialRandom(3, 35, float64(1)/float64(15)))
+	}
+	fmt.Println("Generate 40 Exponential random float64 in [0,max]:")
+	for i := 0; i < 40; i++ {
+		fmt.Println(ExponentialRandom(0, math.MaxFloat64, float64(1)/float64(15)))
+	}
+}