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// Copyright 2019 Huan Du. All rights reserved.
// Licensed under the MIT license that can be found in the LICENSE file.
package clone
import (
"encoding/binary"
"hash/crc64"
"reflect"
"sync"
"unsafe"
)
var (
sizeOfChecksum = unsafe.Sizeof(uint64(0))
crc64Table = crc64.MakeTable(crc64.ECMA)
cachedWrapperTypes sync.Map
)
// Wrap creates a wrapper of v, which must be a pointer.
// If v is not a pointer, Wrap simply returns v and do nothing.
//
// The wrapper is a deep clone of v's value. It holds a shadow copy to v internally.
//
// t := &T{Foo: 123}
// v := Wrap(t).(*T) // v is a clone of t.
// reflect.DeepEqual(t, v) == true // v equals t.
// v.Foo = 456 // v.Foo is changed, but t.Foo doesn't change.
// orig := Unwrap(v) // Use `Unwrap` to discard wrapper and return original value, which is t.
// orig.(*T) == t // orig and t is exactly the same.
// Undo(v) // Use `Undo` to discard any change on v.
// v.Foo == t.Foo // Now, the value of v and t are the same again.
func Wrap(v interface{}) interface{} {
if v == nil {
return v
}
val := reflect.ValueOf(v)
pt := val.Type()
if val.Kind() != reflect.Ptr {
return v
}
t := pt.Elem()
elem := val.Elem()
ptr := unsafe.Pointer(val.Pointer())
cache, ok := cachedWrapperTypes.Load(t)
if !ok {
cache = reflect.StructOf([]reflect.StructField{
{
Name: "T",
Type: t,
Anonymous: true,
},
{
Name: "Checksum",
Type: reflect.TypeOf(uint64(0)),
},
{
Name: "Origin",
Type: pt,
},
})
cachedWrapperTypes.Store(t, cache)
}
wrapperType := cache.(reflect.Type)
pw := defaultAllocator.New(wrapperType)
wrapperPtr := unsafe.Pointer(pw.Pointer())
wrapper := pw.Elem()
// Equivalent code: wrapper.T = Clone(v)
field := wrapper.Field(0)
field.Set(heapCloneState.clone(elem))
// Equivalent code: wrapper.Checksum = makeChecksum(v)
checksumPtr := unsafe.Pointer((uintptr(wrapperPtr) + t.Size()))
*(*uint64)(checksumPtr) = makeChecksum(t, uintptr(wrapperPtr), uintptr(ptr))
// Equivalent code: wrapper.Origin = v
originPtr := unsafe.Pointer((uintptr(wrapperPtr) + t.Size() + sizeOfChecksum))
*(*uintptr)(originPtr) = uintptr(ptr)
return field.Addr().Interface()
}
func validateChecksum(t reflect.Type, ptr unsafe.Pointer) bool {
pw := uintptr(ptr)
orig := uintptr(getOrigin(t, ptr))
checksum := *(*uint64)(unsafe.Pointer(uintptr(ptr) + t.Size()))
expected := makeChecksum(t, pw, orig)
return checksum == expected
}
func makeChecksum(t reflect.Type, pw uintptr, orig uintptr) uint64 {
var data [binary.MaxVarintLen64 * 2]byte
binary.PutUvarint(data[:binary.MaxVarintLen64], uint64(pw))
binary.PutUvarint(data[binary.MaxVarintLen64:], uint64(orig))
return crc64.Checksum(data[:], crc64Table)
}
func getOrigin(t reflect.Type, ptr unsafe.Pointer) unsafe.Pointer {
return *(*unsafe.Pointer)(unsafe.Pointer(uintptr(ptr) + t.Size() + sizeOfChecksum))
}
// Unwrap returns v's original value if v is a wrapped value.
// Otherwise, simply returns v itself.
func Unwrap(v interface{}) interface{} {
if v == nil {
return v
}
val := reflect.ValueOf(v)
if !isWrapped(val) {
return v
}
origVal := origin(val)
return origVal.Interface()
}
func origin(val reflect.Value) reflect.Value {
pt := val.Type()
t := pt.Elem()
ptr := unsafe.Pointer(val.Pointer())
orig := getOrigin(t, ptr)
origVal := reflect.NewAt(t, orig)
return origVal
}
// Undo discards any change made in wrapped value.
// If v is not a wrapped value, nothing happens.
func Undo(v interface{}) {
if v == nil {
return
}
val := reflect.ValueOf(v)
if !isWrapped(val) {
return
}
origVal := origin(val)
elem := val.Elem()
elem.Set(heapCloneState.clone(origVal.Elem()))
}
func isWrapped(val reflect.Value) bool {
pt := val.Type()
if pt.Kind() != reflect.Ptr {
return false
}
t := pt.Elem()
ptr := unsafe.Pointer(val.Pointer())
return validateChecksum(t, ptr)
}
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