1 files changed, 44 insertions, 0 deletions
diff --git a/danmaku/dmpkg/decode.go b/danmaku/dmpkg/decode.go
new file mode 100644
index 0000000..7d9f796
--- /dev/null
+++ b/danmaku/dmpkg/decode.go
@@ -0,0 +1,44 @@
+package dmpkg
+
+import (
+	"bytes"
+	"fmt"
+	"github.com/lunixbochs/struc"
+)
+
+func DecodeExchange(data []byte) (exc DanmakuExchange, err error) {
+	if ln := len(data); ln < kHeaderLength {
+		err = fmt.Errorf("incomplete datagram: length = %v < %v", ln, kHeaderLength)
+		return
+	}
+
+	// unpack header
+	var exchangeHeader DanmakuExchangeHeader
+	err = struc.Unpack(bytes.NewReader(data[:kHeaderLength]), &exchangeHeader)
+	if err != nil {
+		err = fmt.Errorf("cannot unpack exchange header: %w", err)
+		return
+	}
+	headerLength := exchangeHeader.HeaderLength
+
+	// validate header length, fail fast if not match
+	if headerLength != kHeaderLength {
+		err = fmt.Errorf("invalid header length, "+
+			"the protocol implementation might be obsolete: %v != %v", headerLength, kHeaderLength)
+		return
+	}
+
+	// special process
+	// TODO decouple this
+	// The server OpHeartbeatAck contains an extra 4-bytes header entry in the body, maybe a heat value
+	var body []byte
+	// copy body
+	body = make([]byte, exchangeHeader.Length-uint32(headerLength))
+	copy(body, data[headerLength:])
+
+	exc = DanmakuExchange{
+		DanmakuExchangeHeader: exchangeHeader,
+		Body:                  body,
+	}
+	return
+}