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package nmea
import (
"fmt"
"strconv"
"strings"
)
// TagBlock struct
type TagBlock struct {
Time int64 // TypeUnixTime unix timestamp (unit is likely to be s, but might be ms, YMMV), parameter: -c
RelativeTime int64 // TypeRelativeTime relative time, parameter: -r
Destination string // TypeDestinationID destination identification 15 char max, parameter: -d
Grouping string // TypeGrouping sentence grouping, parameter: -g
LineCount int64 // TypeLineCount line count, parameter: -n
Source string // TypeSourceID source identification 15 char max, parameter: -s
Text string // TypeTextString valid character string, parameter -t
}
// ParseTagBlock parses tag blocks from a sentence string.
// The second return value is the length of the tag block prefix.
// See: https://gpsd.gitlab.io/gpsd/AIVDM.html#_nmea_tag_blocks
func ParseTagBlock(raw string) (TagBlock, int, error) {
startOfTagBlock := strings.IndexByte(raw, TagBlockSep)
if startOfTagBlock == -1 {
return TagBlock{}, 0, nil
}
// tag block is always at the start of line (unless IEC 61162-450). Starts with `\` and ends with `\` and has valid sentence
// following or <CR><LF>
//
// Note: tag block group can span multiple lines but we only parse ones that have sentence
endOfTagBlock := strings.LastIndexByte(raw, TagBlockSep)
if endOfTagBlock <= startOfTagBlock {
return TagBlock{}, 0, fmt.Errorf("nmea: sentence tag block is missing '\\' at the end")
}
tags := raw[startOfTagBlock+1 : endOfTagBlock]
sumSepIndex := strings.Index(tags, ChecksumSep)
if sumSepIndex == -1 {
return TagBlock{}, 0, fmt.Errorf("nmea: tagblock does not contain checksum separator")
}
var (
fieldsRaw = tags[0:sumSepIndex]
checksumRaw = strings.ToUpper(tags[sumSepIndex+1:])
checksum = Checksum(fieldsRaw)
tagBlock TagBlock
err error
)
// Validate the checksum
if checksum != checksumRaw {
return TagBlock{}, 0, fmt.Errorf("nmea: tagblock checksum mismatch [%s != %s]", checksum, checksumRaw)
}
items := strings.Split(tags[:sumSepIndex], ",")
for _, item := range items {
parts := strings.SplitN(item, ":", 2)
if len(parts) != 2 {
return TagBlock{}, 0, fmt.Errorf("nmea: tagblock field is malformed (should be <key>:<value>) [%s]", item)
}
key, value := parts[0], parts[1]
switch key {
case "c": // UNIX timestamp
tagBlock.Time, err = parseInt64(value)
if err != nil {
return TagBlock{}, 0, err
}
case "d": // Destination ID
tagBlock.Destination = value
case "g": // Grouping
tagBlock.Grouping = value
case "n": // Line count
tagBlock.LineCount, err = parseInt64(value)
if err != nil {
return TagBlock{}, 0, err
}
case "r": // Relative time
tagBlock.RelativeTime, err = parseInt64(value)
if err != nil {
return TagBlock{}, 0, err
}
case "s": // Source ID
tagBlock.Source = value
case "t": // Text string
tagBlock.Text = value
}
}
return tagBlock, endOfTagBlock + 1, nil
}
func parseInt64(raw string) (int64, error) {
i, err := strconv.ParseInt(raw, 10, 64)
if err != nil {
return 0, fmt.Errorf("nmea: tagblock unable to parse uint64 [%s]", raw)
}
return i, nil
}
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