crypto/rand: add tests for Int, Prime

LGTM=rsc, dave
R=golang-codereviews, dave, josharian, rsc
CC=golang-codereviews
https://golang.org/cl/46490043

src/pkg/crypto/rand/util_test.go

@@ -6,6 +6,7 @@ package rand_test
 
 import (
 	"crypto/rand"
+	"math/big"
 	"testing"
 )
 
@@ -24,3 +25,41 @@ func TestPrimeSmall(t *testing.T) {
 		}
 	}
 }
+
+// Test that passing bits < 2 causes Prime to return nil, error
+func TestPrimeBitsLt2(t *testing.T) {
+	if p, err := rand.Prime(rand.Reader, 1); p != nil || err == nil {
+		t.Errorf("Prime should return nil, error when called with bits < 2")
+	}
+}
+
+func TestInt(t *testing.T) {
+	// start at 128 so the case of (max.BitLen() % 8) == 0 is covered
+	for n := 128; n < 140; n++ {
+		b := new(big.Int).SetInt64(int64(n))
+		if i, err := rand.Int(rand.Reader, b); err != nil {
+			t.Fatalf("Can't generate random value: %v, %v", i, err)
+		}
+	}
+}
+
+func testIntPanics(t *testing.T, b *big.Int) {
+	defer func() {
+		if err := recover(); err == nil {
+			t.Errorf("Int should panic when called with max <= 0: %v", b)
+		}
+	}()
+	rand.Int(rand.Reader, b)
+}
+
+// Test that passing a new big.Int as max causes Int to panic
+func TestIntEmptyMaxPanics(t *testing.T) {
+	b := new(big.Int)
+	testIntPanics(t, b)
+}
+
+// Test that passing a negative value as max causes Int to panic
+func TestIntNegativeMaxPanics(t *testing.T) {
+	b := new(big.Int).SetInt64(int64(-1))
+	testIntPanics(t, b)
+}