slice: add Reverse function, deduplicate code a bit

pkg/core/native/std.go

@@ -16,6 +16,7 @@ import (
 	"github.com/nspcc-dev/neo-go/pkg/smartcontract"
 	"github.com/nspcc-dev/neo-go/pkg/smartcontract/callflag"
 	"github.com/nspcc-dev/neo-go/pkg/smartcontract/manifest"
+	"github.com/nspcc-dev/neo-go/pkg/util/slice"
 	"github.com/nspcc-dev/neo-go/pkg/vm/stackitem"
 )
 
@@ -232,7 +233,7 @@ func (s *Std) itoa(_ *interop.Context, args []stackitem.Item) stackitem.Item {
 			break
 		}
 		bs := bigint.ToBytes(num)
-		reverse(bs)
+		slice.Reverse(bs)
 		str = hex.EncodeToString(bs)
 		if pad := bs[0] & 0xF8; pad == 0 || pad == 0xF8 {
 			str = str[1:]
@@ -280,7 +281,7 @@ func (s *Std) atoi(_ *interop.Context, args []stackitem.Item) stackitem.Item {
 		if changed && bs[0]&0x8 != 0 {
 			bs[0] |= 0xF0
 		}
-		reverse(bs)
+		slice.Reverse(bs)
 		bi = bigint.FromBytes(bs)
 	default:
 		panic(ErrInvalidBase)
@@ -289,13 +290,6 @@ func (s *Std) atoi(_ *interop.Context, args []stackitem.Item) stackitem.Item {
 	return stackitem.NewBigInteger(bi)
 }
 
-func reverse(b []byte) {
-	l := len(b)
-	for i := 0; i < l/2; i++ {
-		b[i], b[l-i-1] = b[l-i-1], b[i]
-	}
-}
-
 func (s *Std) base64Encode(_ *interop.Context, args []stackitem.Item) stackitem.Item {
 	src := s.toLimitedBytes(args[0])
 	result := base64.StdEncoding.EncodeToString(src)

pkg/util/slice/array.go

@@ -7,8 +7,17 @@ package slice
 // original.
 func CopyReverse(b []byte) []byte {
 	dest := make([]byte, len(b))
-	for i, j := 0, len(b)-1; i <= j; i, j = i+1, j-1 {
-		dest[i], dest[j] = b[j], b[i]
-	}
+	reverse(dest, b)
 	return dest
 }
+
+// Reverse does in-place reversing of byte slice.
+func Reverse(b []byte) {
+	reverse(b, b)
+}
+
+func reverse(dst []byte, src []byte) {
+	for i, j := 0, len(src)-1; i <= j; i, j = i+1, j-1 {
+		dst[i], dst[j] = src[j], src[i]
+	}
+}

pkg/util/slice/array_test.go

@@ -41,5 +41,8 @@ func TestCopyReverse(t *testing.T) {
 			have[i] = ^have[i]
 		}
 		require.Equal(t, tc.arr, arg)
+
+		Reverse(arg)
+		require.Equal(t, tc.rev, arg)
 	}
 }

pkg/vm/vm.go

@@ -21,6 +21,7 @@ import (
 	"github.com/nspcc-dev/neo-go/pkg/smartcontract/nef"
 	"github.com/nspcc-dev/neo-go/pkg/smartcontract/trigger"
 	"github.com/nspcc-dev/neo-go/pkg/util"
+	"github.com/nspcc-dev/neo-go/pkg/util/slice"
 	"github.com/nspcc-dev/neo-go/pkg/vm/opcode"
 	"github.com/nspcc-dev/neo-go/pkg/vm/stackitem"
 )
@@ -1167,10 +1168,8 @@ func (v *VM) execute(ctx *Context, op opcode.Opcode, parameter []byte) (err erro
 				a[i], a[j] = a[j], a[i]
 			}
 		case *stackitem.Buffer:
-			slice := t.Value().([]byte)
-			for i, j := 0, t.Len()-1; i < j; i, j = i+1, j-1 {
-				slice[i], slice[j] = slice[j], slice[i]
-			}
+			b := t.Value().([]byte)
+			slice.Reverse(b)
 		default:
 			panic(fmt.Sprintf("invalid item type %s", t))
 		}