From 66d1b272b39822a06fc5793eacd7f75a90957c18 Mon Sep 17 00:00:00 2001
From: xzb <223@qq.com>
Date: Thu, 2 Nov 2023 14:37:50 +0800
Subject: [PATCH] rssp lfsr

---
 third_party/message/rssp.go                   |  4 +-
 .../message/{rssp_crc.go => rssp_code.go}     | 50 ++++++++++++++++++-
 2 files changed, 52 insertions(+), 2 deletions(-)
 rename third_party/message/{rssp_crc.go => rssp_code.go} (80%)

diff --git a/third_party/message/rssp.go b/third_party/message/rssp.go
index 40bb183..973d23a 100644
--- a/third_party/message/rssp.go
+++ b/third_party/message/rssp.go
@@ -72,7 +72,7 @@ func RsspPackCrc16Check(pack []byte) bool {
 // RsspRsd 实时安全数据包
 type RsspRsd struct {
 	RsspHead
-	//安全校验域-序列号(4Byte)
+	//安全校验域-序列号(4Byte),可以是一个随时间周期变化的计数器
 	Sn uint32
 	//安全校验域-安全数据长度(2Byte)
 	Sdl uint16
@@ -325,3 +325,5 @@ func PackToString(pack []byte) string {
 	}
 	return b.String()
 }
+
+////////////////////////////////////////////////////////////////////
diff --git a/third_party/message/rssp_crc.go b/third_party/message/rssp_code.go
similarity index 80%
rename from third_party/message/rssp_crc.go
rename to third_party/message/rssp_code.go
index 1548a50..13adc52 100644
--- a/third_party/message/rssp_crc.go
+++ b/third_party/message/rssp_code.go
@@ -7,11 +7,15 @@ var crc16Table []uint32 = nil
 var crc32C1Table []uint32 = nil
 var crc32C2Table []uint32 = nil
 
-const (
+const ( //CRC生成多项式
 	RsspCrc16GX uint32 = 0b1_0000_1000_0001_0001 //生成多项式 G(X)=X16+X11+X4+1
 	RsspCrc32C1 uint32 = 0x100d4e63              //安全通道1 CRC32生成多项式
 	RsspCrc32C2 uint32 = 0x8ce56011              //安全通道1 CRC32生成多项式
 )
+const ( //时间戳生成多项式
+	RsspTsC1 uint32 = 0x0fc22f87
+	RsspTsC2 uint32 = 0xc3e887e1
+)
 
 // InitRsspCrcTable 初始化RSSP协议中需要的CRC表
 func InitRsspCrcTable() {
@@ -193,3 +197,47 @@ func Crc(data []byte,
 	}
 	return (register1 ^ final_xor_value) & significant_mask
 }
+
+////////////////////////////////////////////////////////
+
+// 线性反馈移位寄存器
+type RsspLFSR struct {
+	polynomial uint32 //生成多项式
+	width      int    //寄存器宽度
+	register   uint32 //寄存器
+}
+
+func NewRsspLFSR(polynomial uint32, width int, init_value uint32) *RsspLFSR {
+	var wd_msb_mask uint32 = 1 << (width - 1)
+	var wd_mask uint32 = 0xffffffff >> (32 - width)
+	polynomial &= wd_mask
+	polynomial = (polynomial >> 1) | wd_msb_mask
+	//
+	return &RsspLFSR{polynomial: polynomial, width: width, register: init_value}
+}
+func (r *RsspLFSR) move() {
+	var significant_mask uint32 = 0xffffffff >> (32 - r.width)
+	//
+	r.register &= significant_mask
+	cb := r.register & r.polynomial & significant_mask
+	out := bitXor(cb, r.width)
+	r.register >>= 1
+	r.register = r.register | (out << (r.width - 1))
+	r.register &= significant_mask
+}
+
+func (r *RsspLFSR) GetAndMove() uint32 {
+	rt := r.register
+	r.move()
+	return rt
+}
+
+// return 0 或 1
+func bitXor(data uint32, width int) uint32 {
+	var v uint32 = 0
+	var lsb_mask uint32 = 1
+	for i := 0; i < width; i++ {
+		v ^= data >> i
+	}
+	return v & lsb_mask
+}