MulOverflow (#75)

uint256.go

@@ -327,6 +327,20 @@ func (z *Int) Mul(x, y *Int) *Int {
 	return z.Set(&res)
 }
 
+// MulOverflow sets z to the product x*y, and returns whether overflow occurred
+func (z *Int) MulOverflow(x, y *Int) bool {
+	p := umul(x, y)
+	var (
+		pl Int
+		ph Int
+	)
+	copy(pl[:], p[:4])
+	copy(ph[:], p[4:])
+
+	z.Set(&pl)
+	return !ph.IsZero()
+}
+
 func (z *Int) squared() {
 	var (
 		res                    Int

uint256_test.go

@@ -237,6 +237,27 @@ func TestRandomMul(t *testing.T) {
 		},
 	)
 }
+func TestRandomMulOverflow(t *testing.T) {
+	for i := 0; i < 10000; i++ {
+		b, f1, err := randNums()
+		if err != nil {
+			t.Fatal(err)
+		}
+		b2, f2, err := randNums()
+		if err != nil {
+			t.Fatal(err)
+		}
+		f1a, f2a := f1.Clone(), f2.Clone()
+		overflow := f1.MulOverflow(f1, f2)
+		b.Mul(b, b2)
+		if err := checkOverflow(b, f1, overflow); err != nil {
+			t.Fatal(err)
+		}
+		if eq := checkEq(b, f1); !eq {
+			t.Fatalf("Expected equality:\nf1= %x\nf2= %x\n[ - ]==\nf= %x\nb= %x\n", f1a, f2a, f1, b)
+		}
+	}
+}
 func TestRandomSquare(t *testing.T) {
 	testRandomOp(t,
 		func(f1, f2, f3 *Int) {