commit b113263c7e5e7cb104c5fff7a9c61d0c10db31f4
parent 2c4c5d7ef79d88f67d5fc142395436952ee1a747
Author: Oliver Lowe <o@olowe.co>
Date: Tue, 28 May 2024 12:17:45 +1000
scte35: document some of the magic values for crc32
While here remove unused validation functions; we have tests for this.
Diffstat:
2 files changed, 15 insertions(+), 40 deletions(-)
diff --git a/scte35/crc_32.go b/scte35/crc_32.go
@@ -16,24 +16,18 @@
package scte35
-import (
- "encoding/binary"
- "errors"
-)
+import "hash/crc32"
-// ErrCRC32Invalid indicates that a splice_info_sections CRC_32 is
-// invalid with respect to the binary payload.
-var ErrCRC32Invalid = errors.New("CRC_32 not valid")
+var crctab = makeCRC32Table(crc32PolyNormal)
-// crc32Table provides values for calculating an SCTE35 object's CRC32 value.
-var crc32Table = makeCRC32Table()
+// The reverse of crc32.IEEE, from
+// https://en.wikipedia.org/wiki/Cyclic_redundancy_check#Polynomial_representations
+const crc32PolyNormal = 0x04C11DB7
-// makeCRC32Table generates CRC32/BZIP2 table
-func makeCRC32Table() [256]uint32 {
- const poly = 0x04C11DB7
- crctable := [256]uint32{}
-
- for i := range crctable {
+// makeCRC32Table generates CRC32/BZIP2 table using poly.
+func makeCRC32Table(poly uint32) crc32.Table {
+ var tab crc32.Table
+ for i := range tab {
crc := uint32(i) << 24
for j := 0; j < 8; j++ {
if crc&0x80000000 != 0 {
@@ -42,35 +36,16 @@ func makeCRC32Table() [256]uint32 {
crc = crc << 1
}
}
- crctable[i] = crc
+ tab[i] = crc
}
- return crctable
+ return tab
}
-// calculateCRC32 takes an byte array Writer containing all of the bytes of a
-// Splice Info Section object up to the CRC32 value, calculates a CRC32 value
-// matching the bits for those existing bytes, and returns it.
-func calculateCRC32(b []byte) uint32 {
+// checksum calculates the checksum for the given bytes using tab.
+func checksum(b []byte, tab *crc32.Table) uint32 {
crc := int32(-1)
for i := range b {
- crc = (crc << 8) ^ int32(crc32Table[((crc>>24)^int32(b[i]))&0xFF])
+ crc = (crc << 8) ^ int32(tab[((crc>>24)^int32(b[i]))&0xFF])
}
return uint32(crc)
}
-
-// verifyCRC32 verifies the base-64 encoded binary string is valid by confirming
-// the CRC_32 matches
-func verifyCRC32(b []byte) error {
- if len(b) < 4 {
- return ErrCRC32Invalid
- }
-
- // crc32 is the last 4 bytes.
- payload := b[:len(b)-4]
- crc := binary.BigEndian.Uint32(b[len(b)-4:])
- if calculateCRC32(payload) != crc {
- return ErrCRC32Invalid
- }
-
- return nil
-}
diff --git a/scte35/splice_info.go b/scte35/splice_info.go
@@ -119,7 +119,7 @@ func EncodeSpliceInfo(info *SpliceInfo) ([]byte, error) {
buf[1] |= byte(buflen >> 8)
buf[2] = byte(buflen)
- crc := calculateCRC32(buf)
+ crc := checksum(buf, &crctab)
return binary.BigEndian.AppendUint32(buf, crc), nil
}
