av/stream/mts/psi/op.go

155 lines
4.6 KiB
Go

/*
NAME
op.go
DESCRIPTION
op.go provides functionality for editing and reading bytes slices
directly in order to insert/read timestamp and location data in psi.
AUTHOR
Saxon Milton <saxon@ausocean.org>
LICENSE
op.go is Copyright (C) 2018 the Australian Ocean Lab (AusOcean)
It is free software: you can redistribute it and/or modify them
under the terms of the GNU General Public License as published by the
Free Software Foundation, either version 3 of the License, or (at your
option) any later version.
It is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
for more details.
You should have received a copy of the GNU General Public License
along with revid in gpl.txt. If not, see http://www.gnu.org/licenses.
*/
package psi
import (
"bytes"
"encoding/binary"
"errors"
"hash/crc32"
"math/bits"
)
// TimeBytes takes a timestamp as an uint64 and converts to an 8 byte slice -
// allows for updating of timestamp in pmt time descriptor.
func TimeBytes(t uint64) []byte {
var s [timeDataSize]byte
binary.BigEndian.PutUint64(s[:], t)
return s[:]
}
// HasTime takes a psi as a byte slice and checks to see if it has a time descriptor
// - if so return nil, otherwise return error
func HasTime(p []byte) error {
if p[timeTagIndx] != timeDescTag {
return errors.New("PSI does not contain a time descriptor, cannot update")
}
return nil
}
// HasLocation takes a psi as a byte slice and checks to see if it has a location descriptor
// - if so return nil, otherwise return error
func HasLocation(p []byte) error {
if p[locationTagIndx] != locationDescTag {
return errors.New("PSI does not contain a location descriptor, cannot update")
}
return nil
}
// UpdateTime takes the byte slice representation of a psi-pmt as well as a time
// as an integer and attempts to update the time descriptor in the pmt with the
// given time if the time descriptor exists, otherwise an error is returned
func UpdateTime(dst []byte, t uint64) error {
err := HasTime(dst)
if err != nil {
return err
}
ts := TimeBytes(uint64(t))
for i := range dst[timeDataIndx : timeDataIndx+timeDataSize] {
dst[i+timeDataIndx] = ts[i]
}
dst = updateCrc(dst)
return nil
}
// TimeFrom takes a byte slice representation of a psi-pmt and extracts it's
// timestamp, returning as a uint64 if it exists, otherwise returning 0 and nil
// if it does not exist
func TimeFrom(p []byte) (t uint64, err error) {
err = HasTime(p)
if err != nil {
return 0, err
}
t = binary.BigEndian.Uint64(p[timeDataIndx : timeDataIndx+timeDataSize])
return t, nil
}
// LocationFrom takes a byte slice representation of a psi-pmt and extracts it's
// timestamp, returning as a uint64 if it exists, otherwise returning 0 and nil
// if it does not exist
func LocationFrom(p []byte) (g string, err error) {
err = HasLocation(p)
if err != nil {
return "", err
}
gBytes := p[locationDataIndx : locationDataIndx+locationDataSize]
gBytes = bytes.Trim(gBytes, "\x00")
g = string(gBytes)
return g, nil
}
// LocationStrBytes take a string of location data and converts to a 32 byte slice -
// easy update of slice representation of a pmt with location descriptor
func LocationStrBytes(s string) []byte {
var b [locationDataSize]byte
copy(b[:], s)
return b[:]
}
// UpdateLocation takes a byte slice representation of a psi-pmt containing a location
// descriptor and attempts to update the location data value with the passed string.
// If the psi does not contain a location descriptor, and error is returned.
func UpdateLocation(d []byte, s string) error {
err := HasLocation(d)
if err != nil {
return err
}
gb := LocationStrBytes(s)
for i := range d[locationDataIndx : locationDataIndx+locationDataSize] {
d[i+locationDataIndx] = gb[i]
}
d = updateCrc(d)
return nil
}
// addCrc appends a crc table to a given psi table in bytes
func addCrc(out []byte) []byte {
out = append(out, make([]byte, 4)...)
out = updateCrc(out)
return out
}
// updateCrc updates the crc of psi bytes slice that may have been modified
func updateCrc(out []byte) []byte {
crc32 := crc32_Update(0xffffffff, crc32_MakeTable(bits.Reverse32(crc32.IEEE)), out[1:len(out)-4])
out[len(out)-4] = byte(crc32 >> 24)
out[len(out)-3] = byte(crc32 >> 16)
out[len(out)-2] = byte(crc32 >> 8)
out[len(out)-1] = byte(crc32)
return out
}
// addPadding adds an appropriate amount of padding to a pat or pmt table for
// addition to an mpegts packet
func addPadding(d []byte) []byte {
for len(d) < PktLen {
d = append(d, 0xff)
}
return d
}