From b036517329a093908bec8a29272600095f3389ed Mon Sep 17 00:00:00 2001
From: Dan Kortschak <dan@kortschak.io>
Date: Sun, 19 Aug 2018 09:59:33 +0930
Subject: [PATCH] generator: add CRC32 calculation

This uses a big-endian CRC32 algorithm rather than the hash/crc32
package's little-endian implementation.
---
 generator/mpegts_generator.go | 59 ++++++++++++++++++++++++++++++-----
 1 file changed, 51 insertions(+), 8 deletions(-)

diff --git a/generator/mpegts_generator.go b/generator/mpegts_generator.go
index 42dceaa1..a3c6712c 100644
--- a/generator/mpegts_generator.go
+++ b/generator/mpegts_generator.go
@@ -29,6 +29,9 @@ LICENSE
 package generator
 
 import (
+	"encoding/binary"
+	"hash/crc32"
+	"math/bits"
 	"time"
 
 	"bitbucket.org/ausocean/av/mpegts"
@@ -42,6 +45,7 @@ var (
 	patTable = []byte{
 		0x00, // pointer
 
+		// ---- section included in data sent to CRC32 during check
 		// table header
 		0x00, // table id
 		0xb0, // section syntax indicator:1|private bit:1|reserved:2|section length:2|more bytes...:2
@@ -56,13 +60,15 @@ var (
 		0x00, 0x01, // Program number
 		0xf0, 0x00, // reserved:3|program map PID:13
 
-		0x2a, 0xb1, 0x04, 0xb2, // CRC
+		// 0x2a, 0xb1, 0x04, 0xb2, // CRC
+		// ----
 	}
 	pmtTable = []byte{
 		0x00, // pointer
 
+		// ---- section included in data sent to CRC32 during check
 		// table header
-		0x02, // table id.
+		0x02, // table id
 		0xb0, // section syntax indicator:1|private bit:1|reserved:2|section length:2|more bytes...:2
 		0x12, // more bytes...
 
@@ -79,19 +85,56 @@ var (
 		0x1b,       // stream type
 		0xe1, 0x00, // reserved:3|elementary PID:13
 		0xf0, 0x00, // reserved:4|unused:2|ES info length:10
-		// No elementary stream descriptos since ES info length is 0.
+		// No elementary stream descriptors since ES info length is 0.
 
-		0x15, 0xbd, 0x4d, 0x56, // CRC
+		// 0x15, 0xbd, 0x4d, 0x56, // CRC
+		// ----
 	}
 )
 
 func init() {
-	for len(patTable) < psiPacketSize {
-		patTable = append(patTable, 0xff)
+	// Generate IEEE polynomial table
+	// for the big-endian algorithm.
+	crcTable := crc32_MakeTable(bits.Reverse32(crc32.IEEE))
+
+	patTable = completePSI(patTable, crcTable)
+	pmtTable = completePSI(pmtTable, crcTable)
+}
+
+func completePSI(psi []byte, tab *crc32.Table) []byte {
+	var buf [4]byte
+	crc := crc32_Update(0xffffffff, tab, psi[1:])
+	binary.BigEndian.PutUint32(buf[:], crc)
+	dst := make([]byte, len(psi), psiPacketSize)
+	copy(dst, psi)
+	dst = append(dst, buf[:]...)
+	for len(dst) < cap(dst) {
+		dst = append(dst, 0xff)
 	}
-	for len(pmtTable) < psiPacketSize {
-		pmtTable = append(pmtTable, 0xff)
+	return dst
+}
+
+func crc32_MakeTable(poly uint32) *crc32.Table {
+	var t crc32.Table
+	for i := range t {
+		crc := uint32(i) << 24
+		for j := 0; j < 8; j++ {
+			if crc&0x80000000 != 0 {
+				crc = (crc << 1) ^ poly
+			} else {
+				crc <<= 1
+			}
+		}
+		t[i] = crc
 	}
+	return &t
+}
+
+func crc32_Update(crc uint32, tab *crc32.Table, p []byte) uint32 {
+	for _, v := range p {
+		crc = tab[byte(crc>>24)^v] ^ (crc << 8)
+	}
+	return crc
 }
 
 const (