encoding/bigint: allow to convert unsigned integers

Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>

pkg/encoding/bigint/bigint.go

@@ -4,6 +4,8 @@ import (
 	"encoding/binary"
 	"math/big"
 	"math/bits"
+
+	"github.com/nspcc-dev/neo-go/pkg/util"
 )
 
 const (
@@ -13,6 +15,12 @@ const (
 	wordSizeBytes = bits.UintSize / 8
 )
 
+// FromBytesUnsigned converts data in little-endian format to an unsigned integer.
+func FromBytesUnsigned(data []byte) *big.Int {
+	bs := util.ArrayReverse(data)
+	return new(big.Int).SetBytes(bs)
+}
+
 // FromBytes converts data in little-endian format to
 // an integer.
 func FromBytes(data []byte) *big.Int {

pkg/encoding/bigint/bigint_test.go

@@ -122,6 +122,31 @@ func TestBytesToInt(t *testing.T) {
 	})
 }
 
+var unsignedCases = []struct {
+	number int64
+	buf    []byte
+}{
+	{0xff00000000, []byte{0x00, 0x00, 0x00, 0x00, 0xff}},
+	{0xfd00000000, []byte{0x00, 0x00, 0x00, 0x00, 0xfd}},
+	{0x8000000000, []byte{0x00, 0x00, 0x00, 0x00, 0x80}},
+	{0xff0200000000, []byte{0x00, 0x00, 0x00, 0x00, 0x02, 0xff}},
+	{0xff0100000000, []byte{0x00, 0x00, 0x00, 0x00, 0x01, 0xff}},
+	{0xff0100000000, []byte{0x00, 0x00, 0x00, 0x00, 0x01, 0xff, 0x00}},
+}
+
+func TestBytesToUnsigned(t *testing.T) {
+	for _, tc := range testCases {
+		if tc.number > 0 {
+			num := FromBytesUnsigned(tc.buf)
+			assert.Equal(t, tc.number, num.Int64(), "expected %x, got %x", tc.number, num.Int64())
+		}
+	}
+	for _, tc := range unsignedCases {
+		num := FromBytesUnsigned(tc.buf)
+		assert.Equal(t, tc.number, num.Int64(), "expected %x, got %x", tc.number, num.Int64())
+	}
+}
+
 func TestEquivalentRepresentations(t *testing.T) {
 	for _, tc := range testCases {
 		buf := tc.buf