vm: increase BigInt parsing precision

Follow the neo-project/neo#2883.

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

Author: AnnaShaleva

pkg/vm/stackitem/json.go

@@ -27,9 +27,9 @@ const MaxAllowedInteger = 2<<53 - 1
 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
+// It allows to properly parse integer numbers that our 256-bit VM is able to
+// handle.
+const MaxIntegerPrec = 1<<8 + 1
 
 // ErrInvalidValue is returned when an item value doesn't fit some constraints
 // during serialization or deserialization.

pkg/vm/stackitem/json_test.go

@@ -134,10 +134,10 @@ func TestFromToJSON(t *testing.T) {
 // 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",
+		`9.05e+28`:   "90500000000000000000000000000",
 		`1.871e+21`:  "1871000000000000000000",
-		`3.0366e+32`: "303660000000000004445016810323968",
-		`1e+30`:      "1000000000000000019884624838656",
+		`3.0366e+32`: "303660000000000000000000000000000",
+		`1e+30`:      "1000000000000000000000000000000",
 	}
 	for in, expected := range tcs {
 		t.Run(in, func(t *testing.T) {