move the random number generator to the utils package

internal/utils/rand.go

@@ -0,0 +1,29 @@
+package utils
+
+import (
+	"crypto/rand"
+	"encoding/binary"
+)
+
+// Rand is a wrapper around crypto/rand that adds some convenience functions known from math/rand.
+type Rand struct {
+	buf [4]byte
+}
+
+func (r *Rand) Int31() int32 {
+	rand.Read(r.buf[:])
+	return int32(binary.BigEndian.Uint32(r.buf[:]) & ^uint32(1<<31))
+}
+
+// copied from the standard library math/rand implementation of Int63n
+func (r *Rand) Int31n(n int32) int32 {
+	if n&(n-1) == 0 { // n is power of two, can mask
+		return r.Int31() & (n - 1)
+	}
+	max := int32((1 << 31) - 1 - (1<<31)%uint32(n))
+	v := r.Int31()
+	for v > max {
+		v = r.Int31()
+	}
+	return v % n
+}

internal/utils/rand_test.go

@@ -0,0 +1,32 @@
+package utils
+
+import (
+	. "github.com/onsi/ginkgo"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("Rand", func() {
+	It("generates random numbers", func() {
+		const (
+			num = 1000
+			max = 123456
+		)
+
+		var values [num]int32
+		var r Rand
+		for i := 0; i < num; i++ {
+			v := r.Int31n(max)
+			Expect(v).To(And(
+				BeNumerically(">=", 0),
+				BeNumerically("<", max),
+			))
+			values[i] = v
+		}
+
+		var sum uint64
+		for _, n := range values {
+			sum += uint64(n)
+		}
+		Expect(float64(sum) / num).To(BeNumerically("~", max/2, max/25))
+	})
+})