psi: HasTime and HasLocation now return bool instead of error

stream/mts/psi/crc.go

@@ -0,0 +1,72 @@
+/*
+NAME
+  crc.go
+DESCRIPTION
+  See Readme.md
+
+AUTHOR
+  Saxon Milton <saxon@ausocean.org>
+
+LICENSE
+  crc.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 (
+	"hash/crc32"
+	"math/bits"
+)
+
+// 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
+}
+
+func crc32_MakeTable(poly uint32) *crc32.Table {
+	var t crc32.Table
+	for i := range t {
+		crc := uint32(i) << 24
+		for j := 0; j < 8; j++ {
+			if crc&0x80000000 != 0 {
+				crc = (crc << 1) ^ poly
+			} else {
+				crc <<= 1
+			}
+		}
+		t[i] = crc
+	}
+	return &t
+}
+
+func crc32_Update(crc uint32, tab *crc32.Table, p []byte) uint32 {
+	for _, v := range p {
+		crc = tab[byte(crc>>24)^v] ^ (crc << 8)
+	}
+	return crc
+}

stream/mts/psi/helpers.go

@@ -0,0 +1,128 @@
+/*
+NAME
+	helpers.go
+DESCRIPTION
+  helpers.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
+  helpers.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"
+)
+
+// error message
+var (
+	ErrNoLocation = errors.New("psi does not have location descriptor")
+	ErrNoTime     = errors.New("psi does not have time descriptor")
+)
+
+// 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) bool {
+	if p[timeTagIndx] == timeDescTag {
+		return true
+	}
+	return false
+}
+
+// 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) bool {
+	if p[locationTagIndx] == locationDescTag {
+		return true
+	}
+	return false
+}
+
+// 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 {
+	if !HasTime(dst) {
+		return ErrNoTime
+	}
+	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) {
+	if !HasTime(p) {
+		return 0, ErrNoTime
+	}
+	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) {
+	if !HasLocation(p) {
+		return "", ErrNoLocation
+	}
+	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 {
+	if !HasLocation(d) {
+		return ErrNoLocation
+	}
+	gb := LocationStrBytes(s)
+	for i := range d[locationDataIndx : locationDataIndx+locationDataSize] {
+		d[i+locationDataIndx] = gb[i]
+	}
+	d = updateCrc(d)
+	return nil
+}