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package eventstream
import (
"bytes"
"encoding/binary"
"hash"
"hash/crc32"
"io"
)
// Encoder provides EventStream message encoding.
type Encoder struct {
w io.Writer
headersBuf *bytes.Buffer
}
// NewEncoder initializes and returns an Encoder to encode Event Stream
// messages to an io.Writer.
func NewEncoder(w io.Writer) *Encoder {
return &Encoder{
w: w,
headersBuf: bytes.NewBuffer(nil),
}
}
// Encode encodes a single EventStream message to the io.Writer the Encoder
// was created with. An error is returned if writing the message fails.
func (e *Encoder) Encode(msg Message) error {
e.headersBuf.Reset()
err := encodeHeaders(e.headersBuf, msg.Headers)
if err != nil {
return err
}
crc := crc32.New(crc32IEEETable)
hashWriter := io.MultiWriter(e.w, crc)
headersLen := uint32(e.headersBuf.Len())
payloadLen := uint32(len(msg.Payload))
if err := encodePrelude(hashWriter, crc, headersLen, payloadLen); err != nil {
return err
}
if headersLen > 0 {
if _, err := io.Copy(hashWriter, e.headersBuf); err != nil {
return err
}
}
if payloadLen > 0 {
if _, err := hashWriter.Write(msg.Payload); err != nil {
return err
}
}
msgCRC := crc.Sum32()
return binary.Write(e.w, binary.BigEndian, msgCRC)
}
func encodePrelude(w io.Writer, crc hash.Hash32, headersLen, payloadLen uint32) error {
p := messagePrelude{
Length: minMsgLen + headersLen + payloadLen,
HeadersLen: headersLen,
}
if err := p.ValidateLens(); err != nil {
return err
}
err := binaryWriteFields(w, binary.BigEndian,
p.Length,
p.HeadersLen,
)
if err != nil {
return err
}
p.PreludeCRC = crc.Sum32()
err = binary.Write(w, binary.BigEndian, p.PreludeCRC)
if err != nil {
return err
}
return nil
}
func encodeHeaders(w io.Writer, headers Headers) error {
for _, h := range headers {
hn := headerName{
Len: uint8(len(h.Name)),
}
copy(hn.Name[:hn.Len], h.Name)
if err := hn.encode(w); err != nil {
return err
}
if err := h.Value.encode(w); err != nil {
return err
}
}
return nil
}
func binaryWriteFields(w io.Writer, order binary.ByteOrder, vs ...interface{}) error {
for _, v := range vs {
if err := binary.Write(w, order, v); err != nil {
return err
}
}
return nil
}
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