json-iterator/feature_reflect.go

package jsoniter

import (
	"fmt"
	"reflect"
	"sync/atomic"
	"unsafe"
)

/*
Reflection on type to create decoders, which is then cached
Reflection on value is avoided as we can, as the reflect.Value itself will allocate, with following exceptions
1. create instance of new value, for example *int will need a int to be allocated
2. append to slice, if the existing cap is not enough, allocate will be done using Reflect.New
3. assignment to map, both key and value will be reflect.Value
For a simple struct binding, it will be reflect.Value free and allocation free
*/

type Decoder interface {
	decode(ptr unsafe.Pointer, iter *Iterator)
}
type Encoder interface {
	encode(ptr unsafe.Pointer, stream *Stream)
}

type DecoderFunc func(ptr unsafe.Pointer, iter *Iterator)
type ExtensionFunc func(typ reflect.Type, field *reflect.StructField) ([]string, DecoderFunc)

type funcDecoder struct {
	fun DecoderFunc
}

func (decoder *funcDecoder) decode(ptr unsafe.Pointer, iter *Iterator) {
	decoder.fun(ptr, iter)
}

var DECODERS unsafe.Pointer
var ENCODERS unsafe.Pointer

var typeDecoders map[string]Decoder
var fieldDecoders map[string]Decoder
var extensions []ExtensionFunc

func init() {
	typeDecoders = map[string]Decoder{}
	fieldDecoders = map[string]Decoder{}
	extensions = []ExtensionFunc{}
	atomic.StorePointer(&DECODERS, unsafe.Pointer(&map[string]Decoder{}))
	atomic.StorePointer(&ENCODERS, unsafe.Pointer(&map[string]Encoder{}))
}

func addDecoderToCache(cacheKey reflect.Type, decoder Decoder) {
	done := false
	for !done {
		ptr := atomic.LoadPointer(&DECODERS)
		cache := *(*map[reflect.Type]Decoder)(ptr)
		copied := map[reflect.Type]Decoder{}
		for k, v := range cache {
			copied[k] = v
		}
		copied[cacheKey] = decoder
		done = atomic.CompareAndSwapPointer(&DECODERS, ptr, unsafe.Pointer(&copied))
	}
}

func addEncoderToCache(cacheKey reflect.Type, encoder Encoder) {
	done := false
	for !done {
		ptr := atomic.LoadPointer(&ENCODERS)
		cache := *(*map[reflect.Type]Encoder)(ptr)
		copied := map[reflect.Type]Encoder{}
		for k, v := range cache {
			copied[k] = v
		}
		copied[cacheKey] = encoder
		done = atomic.CompareAndSwapPointer(&ENCODERS, ptr, unsafe.Pointer(&copied))
	}
}

func getDecoderFromCache(cacheKey reflect.Type) Decoder {
	ptr := atomic.LoadPointer(&DECODERS)
	cache := *(*map[reflect.Type]Decoder)(ptr)
	return cache[cacheKey]
}

func getEncoderFromCache(cacheKey reflect.Type) Encoder {
	ptr := atomic.LoadPointer(&ENCODERS)
	cache := *(*map[reflect.Type]Encoder)(ptr)
	return cache[cacheKey]
}

// RegisterTypeDecoder can register a type for json object
func RegisterTypeDecoder(typ string, fun DecoderFunc) {
	typeDecoders[typ] = &funcDecoder{fun}
}

// RegisterFieldDecoder can register a type for json field
func RegisterFieldDecoder(typ string, field string, fun DecoderFunc) {
	fieldDecoders[fmt.Sprintf("%s/%s", typ, field)] = &funcDecoder{fun}
}

// RegisterExtension can register a custom extension
func RegisterExtension(extension ExtensionFunc) {
	extensions = append(extensions, extension)
}

// CleanDecoders cleans decoders registered
func CleanDecoders() {
	typeDecoders = map[string]Decoder{}
	fieldDecoders = map[string]Decoder{}
}

type optionalDecoder struct {
	valueType    reflect.Type
	valueDecoder Decoder
}

func (decoder *optionalDecoder) decode(ptr unsafe.Pointer, iter *Iterator) {
	if iter.ReadNil() {
		*((*unsafe.Pointer)(ptr)) = nil
	} else {
		if *((*unsafe.Pointer)(ptr)) == nil {
			// pointer to null, we have to allocate memory to hold the value
			value := reflect.New(decoder.valueType)
			decoder.valueDecoder.decode(unsafe.Pointer(value.Pointer()), iter)
			*((*uintptr)(ptr)) = value.Pointer()
		} else {
			// reuse existing instance
			decoder.valueDecoder.decode(*((*unsafe.Pointer)(ptr)), iter)
		}
	}
}

type optionalEncoder struct {
	valueType    reflect.Type
	valueEncoder Encoder
}

func (encoder *optionalEncoder) encode(ptr unsafe.Pointer, stream *Stream) {
	if *((*unsafe.Pointer)(ptr)) == nil {
		stream.WriteNil()
	} else {
		encoder.valueEncoder.encode(*((*unsafe.Pointer)(ptr)), stream)
	}
}

type mapDecoder struct {
	mapType      reflect.Type
	elemType     reflect.Type
	elemDecoder  Decoder
	mapInterface emptyInterface
}

func (decoder *mapDecoder) decode(ptr unsafe.Pointer, iter *Iterator) {
	// dark magic to cast unsafe.Pointer back to interface{} using reflect.Type
	mapInterface := decoder.mapInterface
	mapInterface.word = ptr
	realInterface := (*interface{})(unsafe.Pointer(&mapInterface))
	realVal := reflect.ValueOf(*realInterface).Elem()

	for field := iter.ReadObject(); field != ""; field = iter.ReadObject() {
		elem := reflect.New(decoder.elemType)
		decoder.elemDecoder.decode(unsafe.Pointer(elem.Pointer()), iter)
		// to put into map, we have to use reflection
		realVal.SetMapIndex(reflect.ValueOf(string([]byte(field))), elem.Elem())
	}
}

type mapEncoder struct {
	mapType      reflect.Type
	elemType     reflect.Type
	elemEncoder  Encoder
	mapInterface emptyInterface
}

func (encoder *mapEncoder) encode(ptr unsafe.Pointer, stream *Stream) {
	mapInterface := encoder.mapInterface
	mapInterface.word = ptr
	realInterface := (*interface{})(unsafe.Pointer(&mapInterface))
	realVal := reflect.ValueOf(*realInterface)

	stream.WriteObjectStart()
	for i, key := range realVal.MapKeys() {
		if i != 0 {
			stream.WriteMore()
		}
		stream.WriteObjectField(key.String())
		val := realVal.MapIndex(key).Interface()
		e := (*emptyInterface)(unsafe.Pointer(&val))
		encoder.elemEncoder.encode(e.word, stream)
	}
	stream.WriteObjectEnd()
}

// emptyInterface is the header for an interface{} value.
type emptyInterface struct {
	typ  *struct{}
	word unsafe.Pointer
}

// Read converts an Iterator instance into go interface, same as json.Unmarshal
func (iter *Iterator) ReadVal(obj interface{}) {
	typ := reflect.TypeOf(obj)
	cacheKey := typ.Elem()
	cachedDecoder := getDecoderFromCache(cacheKey)
	if cachedDecoder == nil {
		decoder, err := decoderOfType(cacheKey)
		if err != nil {
			iter.Error = err
			return
		}
		cachedDecoder = decoder
		addDecoderToCache(cacheKey, decoder)
	}
	e := (*emptyInterface)(unsafe.Pointer(&obj))
	cachedDecoder.decode(e.word, iter)
}


func (stream *Stream) WriteVal(val interface{}) {
	if nil == val {
		stream.WriteNil()
		return
	}
	typ := reflect.TypeOf(val)
	cacheKey := typ
	cachedEncoder := getEncoderFromCache(cacheKey)
	if cachedEncoder == nil {
		encoder, err := encoderOfType(cacheKey)
		if err != nil {
			stream.Error = err
			return
		}
		cachedEncoder = encoder
		addEncoderToCache(cacheKey, encoder)
	}

	e := (*emptyInterface)(unsafe.Pointer(&val))
	if typ.Kind() == reflect.Ptr {
		cachedEncoder.encode(unsafe.Pointer(&e.word), stream)
	} else {
		cachedEncoder.encode(e.word, stream)
	}
}

type prefix string

func (p prefix) addToDecoder(decoder Decoder, err error) (Decoder, error) {
	if err != nil {
		return nil, fmt.Errorf("%s: %s", p, err.Error())
	}
	return decoder, err
}

func (p prefix) addToEncoder(encoder Encoder, err error) (Encoder, error) {
	if err != nil {
		return nil, fmt.Errorf("%s: %s", p, err.Error())
	}
	return encoder, err
}

func decoderOfType(typ reflect.Type) (Decoder, error) {
	typeName := typ.String()
	typeDecoder := typeDecoders[typeName]
	if typeDecoder != nil {
		return typeDecoder, nil
	}
	switch typ.Kind() {
	case reflect.String:
		return &stringCodec{}, nil
	case reflect.Int:
		return &intCodec{}, nil
	case reflect.Int8:
		return &int8Codec{}, nil
	case reflect.Int16:
		return &int16Codec{}, nil
	case reflect.Int32:
		return &int32Codec{}, nil
	case reflect.Int64:
		return &int64Codec{}, nil
	case reflect.Uint:
		return &uintCodec{}, nil
	case reflect.Uint8:
		return &uint8Codec{}, nil
	case reflect.Uint16:
		return &uint16Codec{}, nil
	case reflect.Uint32:
		return &uint32Codec{}, nil
	case reflect.Uint64:
		return &uint64Codec{}, nil
	case reflect.Float32:
		return &float32Codec{}, nil
	case reflect.Float64:
		return &float64Codec{}, nil
	case reflect.Bool:
		return &boolCodec{}, nil
	case reflect.Interface:
		return &interfaceCodec{}, nil
	case reflect.Struct:
		return prefix(fmt.Sprintf("[%s]", typeName)).addToDecoder(decoderOfStruct(typ))
	case reflect.Slice:
		return prefix("[slice]").addToDecoder(decoderOfSlice(typ))
	case reflect.Map:
		return prefix("[map]").addToDecoder(decoderOfMap(typ))
	case reflect.Ptr:
		return prefix("[optional]").addToDecoder(decoderOfOptional(typ))
	default:
		return nil, fmt.Errorf("unsupported type: %v", typ)
	}
}

func encoderOfType(typ reflect.Type) (Encoder, error) {
	typeName := typ.String()
	switch typ.Kind() {
	case reflect.String:
		return &stringCodec{}, nil
	case reflect.Int:
		return &intCodec{}, nil
	case reflect.Int8:
		return &int8Codec{}, nil
	case reflect.Int16:
		return &int16Codec{}, nil
	case reflect.Int32:
		return &int32Codec{}, nil
	case reflect.Int64:
		return &int64Codec{}, nil
	case reflect.Uint:
		return &uintCodec{}, nil
	case reflect.Uint8:
		return &uint8Codec{}, nil
	case reflect.Uint16:
		return &uint16Codec{}, nil
	case reflect.Uint32:
		return &uint32Codec{}, nil
	case reflect.Uint64:
		return &uint64Codec{}, nil
	case reflect.Float32:
		return &float32Codec{}, nil
	case reflect.Float64:
		return &float64Codec{}, nil
	case reflect.Bool:
		return &boolCodec{}, nil
	case reflect.Interface:
		return &interfaceCodec{}, nil
	case reflect.Struct:
		return prefix(fmt.Sprintf("[%s]", typeName)).addToEncoder(encoderOfStruct(typ))
	case reflect.Slice:
		return prefix("[slice]").addToEncoder(encoderOfSlice(typ))
	case reflect.Map:
		return prefix("[map]").addToEncoder(encoderOfMap(typ))
	case reflect.Ptr:
		return prefix("[optional]").addToEncoder(encoderOfOptional(typ))
	default:
		return nil, fmt.Errorf("unsupported type: %v", typ)
	}
}

func decoderOfOptional(typ reflect.Type) (Decoder, error) {
	elemType := typ.Elem()
	decoder, err := decoderOfType(elemType)
	if err != nil {
		return nil, err
	}
	return &optionalDecoder{elemType, decoder}, nil
}

func encoderOfOptional(typ reflect.Type) (Encoder, error) {
	elemType := typ.Elem()
	decoder, err := encoderOfType(elemType)
	if err != nil {
		return nil, err
	}
	return &optionalEncoder{elemType, decoder}, nil
}

func decoderOfMap(typ reflect.Type) (Decoder, error) {
	decoder, err := decoderOfType(typ.Elem())
	if err != nil {
		return nil, err
	}
	mapInterface := reflect.New(typ).Interface()
	return &mapDecoder{typ, typ.Elem(), decoder, *((*emptyInterface)(unsafe.Pointer(&mapInterface)))}, nil
}

func encoderOfMap(typ reflect.Type) (Encoder, error) {
	encoder, err := encoderOfType(typ.Elem())
	if err != nil {
		return nil, err
	}
	mapInterface := reflect.New(typ).Elem().Interface()
	return &mapEncoder{typ, typ.Elem(), encoder, *((*emptyInterface)(unsafe.Pointer(&mapInterface)))}, nil
}