vm: allow parsing scientific JSON numbers

52-bit precision is not enough for our 256-bit VM, but this value
matches the reference implementation, see the
https://github.com/neo-project/neo/issues/2879.

MaxIntegerPrec will be increased (or even removed) as soon as the
ref. issue is resolved.

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

pkg/vm/stackitem/json.go

@@ -26,6 +26,11 @@ const MaxAllowedInteger = 2<<53 - 1
 // MaxJSONDepth is the maximum allowed nesting level of an encoded/decoded JSON.
 const MaxJSONDepth = 10
 
+// MaxIntegerPrec is the maximum precision allowed for big.Integer parsing.
+// It equals to the reference value and doesn't allow to precisely parse big
+// numbers, see the https://github.com/neo-project/neo/issues/2879.
+const MaxIntegerPrec = 53
+
 // ErrInvalidValue is returned when an item value doesn't fit some constraints
 // during serialization or deserialization.
 var ErrInvalidValue = errors.New("invalid value")
@@ -213,6 +218,22 @@ func (d *decoder) decode() (Item, error) {
 		return NewByteArray([]byte(t)), nil
 	case json.Number:
 		ts := t.String()
+		var (
+			num *big.Int
+			ok  bool
+		)
+		isScientific := strings.Contains(ts, "e+") || strings.Contains(ts, "E+")
+		if isScientific {
+			// As a special case numbers like 2.8e+22 are allowed (SetString rejects them).
+			// That's the way how C# code works.
+			f, _, err := big.ParseFloat(ts, 10, MaxIntegerPrec, big.ToNearestEven)
+			if err != nil {
+				return nil, fmt.Errorf("%w (malformed exp value for int)", ErrInvalidValue)
+			}
+			num = new(big.Int)
+			_, acc := f.Int(num)
+			ok = acc == big.Exact
+		} else {
 			dot := strings.IndexByte(ts, '.')
 			if dot != -1 {
 				// As a special case numbers like 123.000 are allowed (SetString rejects them).
@@ -224,7 +245,8 @@ func (d *decoder) decode() (Item, error) {
 				}
 				ts = ts[:dot]
 			}
-		num, ok := new(big.Int).SetString(ts, 10)
+			num, ok = new(big.Int).SetString(ts, 10)
+		}
 		if !ok {
 			return nil, fmt.Errorf("%w (integer)", ErrInvalidValue)
 		}

pkg/vm/stackitem/json_test.go

@@ -10,6 +10,10 @@ import (
 )
 
 func getTestDecodeFunc(js string, expected ...interface{}) func(t *testing.T) {
+	return getTestDecodeEncodeFunc(js, true, expected...)
+}
+
+func getTestDecodeEncodeFunc(js string, needEncode bool, expected ...interface{}) func(t *testing.T) {
 	return func(t *testing.T) {
 		actual, err := FromJSON([]byte(js), 20)
 		if expected[0] == nil {
@@ -19,7 +23,7 @@ func getTestDecodeFunc(js string, expected ...interface{}) func(t *testing.T) {
 		require.NoError(t, err)
 		require.Equal(t, Make(expected[0]), actual)
 
-		if len(expected) == 1 {
+		if needEncode && len(expected) == 1 {
 			encoded, err := ToJSON(actual)
 			require.NoError(t, err)
 			require.Equal(t, js, string(encoded))
@@ -28,6 +32,8 @@ func getTestDecodeFunc(js string, expected ...interface{}) func(t *testing.T) {
 }
 
 func TestFromToJSON(t *testing.T) {
+	bigInt, ok := new(big.Int).SetString("28000000000000000000000", 10)
+	require.True(t, ok)
 	t.Run("ByteString", func(t *testing.T) {
 		t.Run("Empty", getTestDecodeFunc(`""`, []byte{}))
 		t.Run("Base64", getTestDecodeFunc(`"test"`, "test"))
@@ -36,6 +42,8 @@ func TestFromToJSON(t *testing.T) {
 	t.Run("BigInteger", func(t *testing.T) {
 		t.Run("ZeroFloat", getTestDecodeFunc(`12.000`, 12, nil))
 		t.Run("NonZeroFloat", getTestDecodeFunc(`12.01`, nil))
+		t.Run("ExpInteger", getTestDecodeEncodeFunc(`2.8e+22`, false, bigInt))
+		t.Run("ExpFloat", getTestDecodeEncodeFunc(`1.2345e+3`, false, nil)) // float value, parsing should fail for it.
 		t.Run("Negative", getTestDecodeFunc(`-4`, -4))
 		t.Run("Positive", getTestDecodeFunc(`123`, 123))
 	})
@@ -123,6 +131,24 @@ func TestFromToJSON(t *testing.T) {
 	})
 }
 
+// TestFromJSON_CompatBigInt ensures that maximum BigInt parsing precision matches
+// the C# one, ref. https://github.com/neo-project/neo/issues/2879.
+func TestFromJSON_CompatBigInt(t *testing.T) {
+	tcs := map[string]string{
+		`9.05e+28`:   "90499999999999993918259200000",
+		`1.871e+21`:  "1871000000000000000000",
+		`3.0366e+32`: "303660000000000004445016810323968",
+		`1e+30`:      "1000000000000000019884624838656",
+	}
+	for in, expected := range tcs {
+		t.Run(in, func(t *testing.T) {
+			actual, err := FromJSON([]byte(in), 5)
+			require.NoError(t, err)
+			require.Equal(t, expected, actual.Value().(*big.Int).String())
+		})
+	}
+}
+
 func testToJSON(t *testing.T, expectedErr error, item Item) {
 	data, err := ToJSON(item)
 	if expectedErr != nil {

pkg/vm/testdata/neo-vm

@@ -1 +1 @@
-Subproject commit 02f2c68e7ba2694aff88c143631e7acf158d378a
+Subproject commit 7e5996844a90b514739f879bc9f873f9a34c9a67