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 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
}