av/stream/mts/psi/op.go

/*
NAME
	op.go
DESCRIPTION
  op.go provides functionality for editing and reading bytes slices
	directly in order to insert/read timestamp and gps 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 (
	"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) (s []byte) {
	s = make([]byte, timeDataSize)
	binary.BigEndian.PutUint64(s[:], t)
	return s
}

// ChkTime 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 ChkTime(p []byte) error {
	if p[timeTagIndx] != timeDescTag {
		return errors.New("PSI does not contain a time descriptor, cannot update")
	}
	return nil
}

// ChkGps takes a psi as a byte slice and checks to see if it has a gps descriptor
// - if so return nil, otherwise return error
func ChkGps(p []byte) error {
	if p[gpsTagIndx] != gpsDescTag {
		return errors.New("PSI does not contain a gps 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(d []byte, t uint64) error {
	err := ChkTime(d)
	if err != nil {
		return err
	}
	ts := TimeBytes(uint64(t))
	for i := range d[timeDataIndx : timeDataIndx+timeDataSize] {
		d[i+timeDataIndx] = ts[i]
	}
	d = updateCrc(d)
	return nil
}

// TimeOf 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 TimeOf(p []byte) (t uint64, err error) {
	err = ChkTime(p)
	if err != nil {
		return 0, err
	}
	// TODO: could probably write a byte slice to int func, so that this would
	// be replaced by extraction of time data slice and then call to conversion func
	for i := range p[timeDataIndx : timeDataIndx+timeDataSize] {
		t |= uint64(p[i+timeDataIndx]) << uint(56-i*timeDataSize)
	}
	return t, nil
}

// GpsStrBytes take a string of gps data and converts to a 32 byte slice -
// easy update of slice representation of a pmt with gps descriptor
func GpsStrBytes(l string) (out []byte) {
	out = make([]byte, gpsDataSize)
	copy(out, []byte(l))
	return
}

// UpdateGps takes a byte slice representation of a psi-pmt containing a gps
// descriptor and attempts to update the gps data value with the passed string.
// If the psi does not contain a gps descriptor, and error is returned.
func UpdateGps(d []byte, s string) error {
	err := ChkGps(d)
	if err != nil {
		return err
	}
	gb := GpsStrBytes(s)
	for i := range d[gpsDataIndx : gpsDataIndx+gpsDataSize] {
		d[i+gpsDataIndx] = gb[i]
	}
	d = updateCrc(d)
	return nil
}

func addCrc(out []byte) []byte {
	out = append(out, make([]byte, 4)...)
	out = updateCrc(out)
	return out
}
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
}