proto/message_set.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

/*
 * Support for message sets.
 */

import (
	"bytes"
	"os"
)

// ErrNoMessageTypeId occurs when a protocol buffer does not have a message type ID.
// A message type ID is required for storing a protocol buffer in a message set.
var ErrNoMessageTypeId = os.NewError("proto does not have a message type ID")

// The first two types (_MessageSet_Item and MessageSet)
// model what the protocol compiler produces for the following protocol message:
//   message MessageSet {
//     repeated group Item = 1 {
//       required int32 type_id = 2;
//       required string message = 3;
//     };
//   }
// That is the MessageSet wire format. We can't use a proto to generate these
// because that would introduce a circular dependency between it and this package.
//
// When a proto1 proto has a field that looks like:
//   optional message<MessageSet> info = 3;
// the protocol compiler produces a field in the generated struct that looks like:
//   Info *_proto_.MessageSet  "PB(bytes,3,opt,name=info)"
// The package is automatically inserted so there is no need for that proto file to
// import this package.

type _MessageSet_Item struct {
	TypeId  *int32 "PB(varint,2,req,name=type_id)"
	Message []byte "PB(bytes,3,req,name=message)"
}

type MessageSet struct {
	Item             []*_MessageSet_Item "PB(group,1,rep)"
	XXX_unrecognized *bytes.Buffer
	// TODO: caching?
}

// messageTypeIder is an interface satisfied by a protocol buffer type
// that may be stored in a MessageSet.
type messageTypeIder interface {
	MessageTypeId() int32
}

func (ms *MessageSet) find(pb interface{}) *_MessageSet_Item {
	mti, ok := pb.(messageTypeIder)
	if !ok {
		return nil
	}
	id := mti.MessageTypeId()
	for _, item := range ms.Item {
		if *item.TypeId == id {
			return item
		}
	}
	return nil
}

func (ms *MessageSet) Has(pb interface{}) bool {
	if ms.find(pb) != nil {
		return true
	}
	return false
}

func (ms *MessageSet) Unmarshal(pb interface{}) os.Error {
	if item := ms.find(pb); item != nil {
		return Unmarshal(item.Message, pb)
	}
	if _, ok := pb.(messageTypeIder); !ok {
		return ErrNoMessageTypeId
	}
	return nil // TODO: return error instead?
}

func (ms *MessageSet) Marshal(pb interface{}) os.Error {
	msg, err := Marshal(pb)
	if err != nil {
		return err
	}
	if item := ms.find(pb); item != nil {
		// reuse existing item
		item.Message = msg
		return nil
	}

	mti, ok := pb.(messageTypeIder)
	if !ok {
		return ErrWrongType // TODO: custom error?
	}

	mtid := mti.MessageTypeId()
	ms.Item = append(ms.Item, &_MessageSet_Item{
		TypeId:  &mtid,
		Message: msg,
	})
	return nil
}

// Support for the message_set_wire_format message option.

func skipVarint(buf []byte) []byte {
	i := 0
	for ; buf[i]&0x80 != 0; i++ {
	}
	return buf[i+1:]
}

// MarshalMessageSet encodes the extension map represented by m in the message set wire format.
// It is called by generated Marshal methods on protocol buffer messages with the message_set_wire_format option.
func MarshalMessageSet(m map[int32][]byte) ([]byte, os.Error) {
	ms := &MessageSet{Item: make([]*_MessageSet_Item, len(m))}
	i := 0
	for k, v := range m {
		// Remove the wire type and field number varint, as well as the length varint.
		v = skipVarint(skipVarint(v))

		ms.Item[i] = &_MessageSet_Item{
			TypeId:  Int32(k),
			Message: v,
		}
		i++
	}
	return Marshal(ms)
}

// UnmarshalMessageSet decodes the extension map encoded in buf in the message set wire format.
// It is called by generated Unmarshal methods on protocol buffer messages with the message_set_wire_format option.
func UnmarshalMessageSet(buf []byte, m map[int32][]byte) os.Error {
	ms := new(MessageSet)
	if err := Unmarshal(buf, ms); err != nil {
		return err
	}
	for _, item := range ms.Item {
		// restore wire type and field number varint, plus length varint.
		b := EncodeVarint(uint64(*item.TypeId)<<3 | WireBytes)
		b = append(b, EncodeVarint(uint64(len(item.Message)))...)
		b = append(b, item.Message...)

		m[*item.TypeId] = b
	}
	return nil
}