mirror of https://bitbucket.org/ausocean/av.git
132 lines
3.9 KiB
Go
132 lines
3.9 KiB
Go
/*
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NAME
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op.go
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DESCRIPTION
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op.go provides functionality for editing and reading bytes slices
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directly in order to insert/read timestamp and gps data in psi.
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AUTHOR
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Saxon Milton <saxon@ausocean.org>
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LICENSE
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op.go is Copyright (C) 2018 the Australian Ocean Lab (AusOcean)
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It is free software: you can redistribute it and/or modify them
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under the terms of the GNU General Public License as published by the
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Free Software Foundation, either version 3 of the License, or (at your
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option) any later version.
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It is distributed in the hope that it will be useful, but WITHOUT
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ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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for more details.
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You should have received a copy of the GNU General Public License
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along with revid in gpl.txt. If not, see http://www.gnu.org/licenses.
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*/
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package psi
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import (
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"encoding/binary"
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"errors"
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"hash/crc32"
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"math/bits"
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)
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// timeBytes takes a timestamp as an uint64 and converts to an 8 byte slice -
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// allows for updating of timestamp in pmt time descriptor.
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func TimeBytes(t uint64) (s []byte) {
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s = make([]byte, timeDataSize)
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binary.BigEndian.PutUint64(s[:], t)
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return s
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}
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// ChkTime takes a psi as a byte slice and checks to see if it has a time descriptor
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// - if so return nil, otherwise return error
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func ChkTime(p []byte) error {
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if p[timeTagIndx] != timeDescTag {
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return errors.New("PSI does not contain a time descriptor, cannot update")
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}
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return nil
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}
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// ChkGps takes a psi as a byte slice and checks to see if it has a gps descriptor
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// - if so return nil, otherwise return error
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func ChkGps(p []byte) error {
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if p[gpsTagIndx] != gpsDescTag {
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return errors.New("PSI does not contain a gps descriptor, cannot update")
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}
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return nil
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}
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// UpdateTime takes the byte slice representation of a psi-pmt as well as a time
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// as an integer and attempts to update the time descriptor in the pmt with the
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// given time if the time descriptor exists, otherwise an error is returned
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func UpdateTime(d []byte, t uint64) error {
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err := ChkTime(d)
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if err != nil {
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return err
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}
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ts := TimeBytes(uint64(t))
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for i := range d[timeDataIndx : timeDataIndx+timeDataSize] {
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d[i+timeDataIndx] = ts[i]
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}
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d = updateCrc(d)
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return nil
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}
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// TimeOf takes a byte slice representation of a psi-pmt and extracts it's
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// timestamp, returning as a uint64 if it exists, otherwise returning 0 and nil
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// if it does not exist
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func TimeOf(p []byte) (t uint64, err error) {
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err = ChkTime(p)
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if err != nil {
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return 0, err
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}
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// TODO: could probably write a byte slice to int func, so that this would
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// be replaced by extraction of time data slice and then call to conversion func
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for i := range p[timeDataIndx : timeDataIndx+timeDataSize] {
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t |= uint64(p[i+timeDataIndx]) << uint(56-i*timeDataSize)
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}
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return t, nil
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}
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// GpsStrBytes take a string of gps data and converts to a 32 byte slice -
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// easy update of slice representation of a pmt with gps descriptor
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func GpsStrBytes(l string) (out []byte) {
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out = make([]byte, gpsDataSize)
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copy(out, []byte(l))
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return
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}
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// UpdateGps takes a byte slice representation of a psi-pmt containing a gps
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// descriptor and attempts to update the gps data value with the passed string.
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// If the psi does not contain a gps descriptor, and error is returned.
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func UpdateGps(d []byte, s string) error {
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err := ChkGps(d)
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if err != nil {
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return err
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}
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gb := GpsStrBytes(s)
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for i := range d[gpsDataIndx : gpsDataIndx+gpsDataSize] {
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d[i+gpsDataIndx] = gb[i]
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}
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d = updateCrc(d)
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return nil
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}
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func addCrc(out []byte) []byte {
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out = append(out, make([]byte, 4)...)
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out = updateCrc(out)
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return out
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}
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func updateCrc(out []byte) []byte {
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crc32 := crc32_Update(0xffffffff, crc32_MakeTable(bits.Reverse32(crc32.IEEE)), out[1:len(out)-4])
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out[len(out)-4] = byte(crc32 >> 24)
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out[len(out)-3] = byte(crc32 >> 16)
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out[len(out)-2] = byte(crc32 >> 8)
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out[len(out)-1] = byte(crc32)
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return out
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}
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