From 8e15d802865ed57db0018c15ea5559c8bd44c01f Mon Sep 17 00:00:00 2001
From: Keuin <keuinx@gmail.com>
Date: Wed, 7 Sep 2022 02:48:46 +0800
Subject: First working version. Just a POC.

---
 danmaku/dmpkg/package.go | 111 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 111 insertions(+)
 create mode 100644 danmaku/dmpkg/package.go

(limited to 'danmaku/dmpkg/package.go')

diff --git a/danmaku/dmpkg/package.go b/danmaku/dmpkg/package.go
new file mode 100644
index 0000000..3175a18
--- /dev/null
+++ b/danmaku/dmpkg/package.go
@@ -0,0 +1,111 @@
+package dmpkg
+
+import (
+	"bytes"
+	"compress/zlib"
+	"fmt"
+	"github.com/andybalholm/brotli"
+	"github.com/lunixbochs/struc"
+	"io"
+)
+
+type DanmakuExchangeHeader struct {
+	// Length total remaining bytes of this exchange, excluding `Length` itself
+	Length uint32
+	// HeaderLength = Length - len(Body) + 4, always equals to 16
+	HeaderLength uint16
+	ProtocolVer  ProtocolVer
+	Operation    Operation
+	// SequenceId is always 1
+	SequenceId uint32
+}
+
+// DanmakuExchange represents an actual message sent from client or server. This is an atomic unit.
+type DanmakuExchange struct {
+	DanmakuExchangeHeader
+	Body []byte
+}
+
+func (e *DanmakuExchange) String() string {
+	return fmt.Sprintf("DanmakuExchange{length=%v, protocol=%v, operation=%v, body=%v}",
+		e.Length, e.ProtocolVer, e.Operation, e.Body)
+}
+
+const kHeaderLength = 16
+const kSequenceId = 1
+
+type ProtocolVer uint16
+
+const (
+	// ProtoPlainJson the body is plain JSON text
+	ProtoPlainJson ProtocolVer = 0
+	// ProtoMinimal the body is uint32 watcher count (big endian)
+	ProtoMinimal ProtocolVer = 1
+	// ProtoZlib the body is a zlib compressed package
+	ProtoZlib ProtocolVer = 2
+	// ProtoBrotli the body is a brotli compressed package
+	ProtoBrotli ProtocolVer = 3
+)
+
+const kUidGuest = 0
+const kPlatformWeb = "web"
+const kAuthTypeDefault = 2 // magic number, not sure what does it mean
+
+func (e *DanmakuExchange) Marshal() (data []byte, err error) {
+	var buffer bytes.Buffer
+	// only unpack header with struc, since it does not support indirect variable field length calculation
+	err = struc.Pack(&buffer, &e.DanmakuExchangeHeader)
+	if err != nil {
+		err = fmt.Errorf("cannot pack an exchange into binary form: %w", err)
+		return
+	}
+	data = buffer.Bytes()
+	data = append(data, e.Body...)
+	return
+}
+
+// Inflate decompresses the body if it is compressed
+func (e *DanmakuExchange) Inflate() (ret DanmakuExchange, err error) {
+	switch e.ProtocolVer {
+	case ProtoMinimal:
+		fallthrough
+	case ProtoPlainJson:
+		ret = *e
+	case ProtoBrotli:
+		var data []byte
+		rd := brotli.NewReader(bytes.NewReader(e.Body))
+		data, err = io.ReadAll(rd)
+		if err != nil {
+			err = fmt.Errorf("cannot decompress exchange body: %w", err)
+			return
+		}
+		var nestedExchange DanmakuExchange
+		nestedExchange, err = DecodeExchange(data)
+		if err != nil {
+			err = fmt.Errorf("cannot decode nested exchange: %w", err)
+			return
+		}
+		return nestedExchange.Inflate()
+	case ProtoZlib:
+		var data []byte
+		var rd io.ReadCloser
+		rd, err = zlib.NewReader(bytes.NewReader(e.Body))
+		if err != nil {
+			err = fmt.Errorf("cannot create zlib reader: %w", err)
+			return
+		}
+		data, err = io.ReadAll(rd)
+		if err != nil {
+			err = fmt.Errorf("cannot decompress exchange body: %w", err)
+			return
+		}
+		var nestedExchange DanmakuExchange
+		nestedExchange, err = DecodeExchange(data)
+		if err != nil {
+			err = fmt.Errorf("cannot decode nested exchange: %w", err)
+			return
+		}
+		return nestedExchange.Inflate()
+	}
+	return
+}
-- 
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