proto/extensions.go

// Go support for Protocol Buffers - Google's data interchange format
//
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// http://code.google.com/p/goprotobuf/
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//
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package proto


/*
 * Types and routines for supporting protocol buffer extensions.
 */

import (
	"os"
	"reflect"
	"unsafe"
)

// ExtensionRange represents a range of message extensions for a protocol buffer.
// Used in code generated by the protocol compiler.
type ExtensionRange struct {
	Start, End int32 // both inclusive
}

// extendableProto is an interface implemented by any protocol buffer that may be extended.
type extendableProto interface {
	ExtensionRangeArray() []ExtensionRange
	ExtensionMap() map[int32][]byte
}

// ExtensionDesc represents an extension specification.
// Used in generated code from the protocol compiler.
type ExtensionDesc struct {
	ExtendedType  interface{} // nil pointer to the type that is being extended
	ExtensionType interface{} // nil pointer to the extension type
	Field         int32       // field number
	Tag           string      // PB(...) tag style
}

// Return true iff the given field number is in an extension range.
func isExtensionField(extended extendableProto, field int32) bool {
	for _, er := range extended.ExtensionRangeArray() {
		if er.Start <= field && field <= er.End {
			return true
		}
	}
	return false
}

func checkExtensionTypes(extended extendableProto, extension *ExtensionDesc) os.Error {
	// Check the extended type.
	if a, b := reflect.Typeof(extended), reflect.Typeof(extension.ExtendedType); a != b {
		return os.NewError("bad extended type; " + b.String() + " does not extend " + a.String())
	}
	// Check the range.
	if !isExtensionField(extended, extension.Field) {
		return os.NewError("bad extension number; not in declared ranges")
	}
	return nil
}

func HasExtension(extended extendableProto, extension *ExtensionDesc) bool {
	// TODO: Check types, field numbers, etc.?
	_, ok := extended.ExtensionMap()[extension.Field]
	return ok
}

func ClearExtension(extended extendableProto, extension *ExtensionDesc) {
	// TODO: Check types, field numbers, etc.?
	extended.ExtensionMap()[extension.Field] = nil, false
}

func GetExtension(extended extendableProto, extension *ExtensionDesc) (interface{}, os.Error) {
	if err := checkExtensionTypes(extended, extension); err != nil {
		return nil, err
	}

	b, ok := extended.ExtensionMap()[extension.Field]
	if !ok {
		return nil, nil // not an error
	}

	// Discard wire type and field number varint. It isn't needed.
	_, n := DecodeVarint(b)
	o := NewBuffer(b[n:])

	t := reflect.Typeof(extension.ExtensionType).(*reflect.PtrType)
	props := &Properties{}
	props.Init(t, "irrelevant_name", extension.Tag, 0)

	base := unsafe.New(t)
	var sbase uintptr
	if _, ok := t.Elem().(*reflect.StructType); ok {
		// props.dec will be dec_struct_message, which does not refer to sbase.
		*(*unsafe.Pointer)(base) = unsafe.New(t.Elem())
	} else {
		sbase = uintptr(unsafe.New(t.Elem()))
	}
	if err := props.dec(o, props, uintptr(base), sbase); err != nil {
		return nil, err
	}
	return unsafe.Unreflect(t, base), nil
}

// TODO(: (needed for repeated extensions)
//   - ExtensionSize
//   - AddExtension

func SetExtension(extended extendableProto, extension *ExtensionDesc, value interface{}) os.Error {
	if err := checkExtensionTypes(extended, extension); err != nil {
		return err
	}
	if reflect.Typeof(extension.ExtensionType) != reflect.Typeof(value) {
		return os.NewError("bad extension value type")
	}

	props := new(Properties)
	props.Init(reflect.Typeof(extension.ExtensionType), "unknown_name", extension.Tag, 0)

	p := NewBuffer(nil)
	v := reflect.NewValue(value)
	if err := props.enc(p, props, v.Addr()); err != nil {
		return err
	}
	extended.ExtensionMap()[extension.Field] = p.buf
	return nil
}