randtest controls the math/rand.Rand behavior from the Source for testing purpose.
type Behavior struct{}
func (b *Behavior) Intn(n int) int {
return n / 2
}
func main() {
source := randtest.NewSource(&Behavior{})
r := rand.New(source)
fmt.Println(r.Intn(10)) // Always 5.
}This is experimental package. It can be broken by updating Go because the logic is highly depends on undocumented behavior of math/rand package. Only tested on Go1.11.