add fix and UT (#7)

* add fix and UT

* changed func signature

int256.go

@@ -112,6 +112,8 @@ func (z *Int) SetString(s string) (*Int, error) {
 // }
 
 func (z *Int) Add(x, y *Int) *Int {
+	z.initiateAbs()
+
 	neg := x.neg
 
 	if x.neg == y.neg {
@@ -134,6 +136,8 @@ func (z *Int) Add(x, y *Int) *Int {
 
 // Sub sets z to the difference x-y and returns z.
 func (z *Int) Sub(x, y *Int) *Int {
+	z.initiateAbs()
+
 	neg := x.neg
 	if x.neg != y.neg {
 		// x - (-y) == x + y
@@ -155,6 +159,8 @@ func (z *Int) Sub(x, y *Int) *Int {
 
 // Mul sets z to the product x*y and returns z.
 func (z *Int) Mul(x, y *Int) *Int {
+	z.initiateAbs()
+
 	z.abs = z.abs.Mul(x.abs, y.abs)
 	z.neg = x.neg != y.neg // 0 has no sign
 	return z
@@ -163,6 +169,8 @@ func (z *Int) Mul(x, y *Int) *Int {
 // Sqrt sets z to ⌊√x⌋, the largest integer such that z² ≤ x, and returns z.
 // It panics if x is negative.
 func (z *Int) Sqrt(x *Int) *Int {
+	z.initiateAbs()
+
 	if x.neg {
 		panic("square root of negative number")
 	}
@@ -173,6 +181,8 @@ func (z *Int) Sqrt(x *Int) *Int {
 
 // Rsh sets z = x >> n and returns z.
 func (z *Int) Rsh(x *Int, n uint) *Int {
+	z.initiateAbs()
+
 	if !x.neg {
 		z.abs.Rsh(x.abs, n)
 		z.neg = x.neg
@@ -187,6 +197,8 @@ func (z *Int) Rsh(x *Int, n uint) *Int {
 // If y == 0, a division-by-zero run-time panic occurs.
 // Quo implements truncated division (like Go); see QuoRem for more details.
 func (z *Int) Quo(x, y *Int) *Int {
+	z.initiateAbs()
+
 	z.abs = z.abs.Div(x.abs, y.abs)
 	z.neg = len(z.abs) > 0 && x.neg != y.neg // 0 has no sign
 	return z
@@ -196,6 +208,8 @@ func (z *Int) Quo(x, y *Int) *Int {
 // If y == 0, a division-by-zero run-time panic occurs.
 // Rem implements truncated modulus (like Go); see QuoRem for more details.
 func (z *Int) Rem(x, y *Int) *Int {
+	z.initiateAbs()
+
 	z.abs.Mod(x.abs, y.abs)
 	z.neg = len(z.abs) > 0 && x.neg // 0 has no sign
 	return z
@@ -207,6 +221,8 @@ func (z *Int) Rem(x, y *Int) *Int {
 //	 0 if x == y
 //	+1 if x >  y
 func (z *Int) Cmp(x *Int) (r int) {
+	z.initiateAbs()
+
 	// x cmp y == x cmp y
 	// x cmp (-y) == x
 	// (-x) cmp y == y
@@ -234,6 +250,8 @@ func (z *Int) Cmp(x *Int) (r int) {
 // Modular exponentiation of inputs of a particular size is not a
 // cryptographically constant-time operation.
 func (z *Int) Exp(x, y, m *Int) *Int {
+	z.initiateAbs()
+
 	if x == nil {
 		panic("x is nil")
 	}
@@ -253,6 +271,8 @@ func (z *Int) Exp(x, y, m *Int) *Int {
 }
 
 func (z *Int) Div(x, y *Int) *Int {
+	z.initiateAbs()
+
 	z.abs.Div(x.abs, y.abs)
 	if x.neg == y.neg {
 		z.neg = false
@@ -264,6 +284,7 @@ func (z *Int) Div(x, y *Int) *Int {
 
 // Lsh sets z = x << n and returns z.
 func (z *Int) Lsh(x *Int, n uint) *Int {
+	z.initiateAbs()
 	b := new(big.Int).Lsh(x.abs.ToBig(), n)
 	z.abs = uint256.MustFromBig(b)
 	z.neg = x.neg
@@ -272,6 +293,8 @@ func (z *Int) Lsh(x *Int, n uint) *Int {
 
 // Or sets z = x | y and returns z.
 func (z *Int) Or(x, y *Int) *Int {
+	z.initiateAbs()
+
 	if x.neg == y.neg {
 		if x.neg {
 			// (-x) | (-y) == ^(x-1) | ^(y-1) == ^((x-1) & (y-1)) == -(((x-1) & (y-1)) + 1)
@@ -306,6 +329,8 @@ func (z *Int) Or(x, y *Int) *Int {
 
 // And sets z = x & y and returns z.
 func (z *Int) And(x, y *Int) *Int {
+	z.initiateAbs()
+
 	if x.neg == y.neg {
 		if x.neg {
 			// (-x) & (-y) == ^(x-1) & ^(y-1) == ^((x-1) | (y-1)) == -(((x-1) | (y-1)) + 1)
@@ -338,3 +363,11 @@ func (z *Int) And(x, y *Int) *Int {
 	z = MustFromBig(big)
 	return z
 }
+
+// initiateAbs sets default value for `z.abs` value if is nil
+func (z *Int) initiateAbs() {
+	if z.abs == nil {
+		z.abs = new(uint256.Int)
+	}
+
+}

int256_test.go

@@ -362,6 +362,21 @@ func TestInt_Mul(t *testing.T) {
 	}
 }
 
+func TestInt_MulPanic(t *testing.T) {
+	i256 := new(Int)
+
+	defer func() {
+		if r := recover(); r != nil {
+			t.Error("should not have paniced", r)
+		}
+	}()
+
+	res := i256.Mul(NewInt(1), NewInt(2))
+	if res.Cmp(NewInt(2)) != 0 {
+		t.Errorf("want: 2, got: %v", res)
+	}
+}
+
 func TestInt_Sqrt(t *testing.T) {
 	type fields struct {
 		abs *uint256.Int

int_64.go

@@ -9,6 +9,8 @@ func (z *Int) Int64() int64 {
 }
 
 func (z *Int) String() string {
+	z.initiateAbs()
+
 	s := z.abs.ToBig().String()
 	if !z.neg {
 		return s