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Merge pull request #1 from xerra-eo/master
Add support for NMEA 4.10 TAG Blocks in aisnmea convenience library
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,110 @@ | ||
| package aisnmea | ||
|
|
||
| import ( | ||
| "fmt" | ||
| "strings" | ||
|
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| nmea "github.com/adrianmo/go-nmea" | ||
| ) | ||
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| // BlankTagBlock is a pseudo-constant for convenient comparisons to a blank value | ||
| var BlankTagBlock nmea.TagBlock | ||
|
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| // addTagBlockChecksum performs the NMEA checksum from byte 0 (unlike addChecksum from nmea.go, which starts at 1) | ||
| func addTagBlockChecksum(sentence string) string { | ||
| checksum := byte(0) | ||
| for i := 0; i < len(sentence); i++ { | ||
| checksum ^= sentence[i] | ||
| } | ||
|
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| return fmt.Sprintf("%s*%02X", sentence, checksum) | ||
| } | ||
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| /* mergeTagBlocks tries to summarise NMEA 4.10 TAG Blocks relating to a multi-sentence | ||
| VDMVDO message (src) into one (dst) upon decoding. It is neccessarily lossy. | ||
| Here are the assumptions: | ||
| - Certain tags should be never have more than one value (time, source, etc), so we pick | ||
| the first value, and ignore subsequent ones. If for some weird reason the sentences are | ||
| timestamped instead of the message, we'd want to use the earliest timestamp. | ||
| - The grouping tag should be different for every sentence, so there is no sense in including it. | ||
| - Most examples in the wild seem to send the same tag block for every sentence of a message, | ||
| in those cases this function does no useful work, but no harm either. | ||
| - If each sentence contributes a different subset of tags, we'll get a complete set at the end. | ||
| */ | ||
| func mergeTagBlocks(dst *nmea.TagBlock, src *nmea.TagBlock) { | ||
| if dst == nil || src == nil { | ||
| return | ||
| } | ||
|
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| if src.Time != 0 && dst.Time == 0 { | ||
| dst.Time = src.Time | ||
| } | ||
|
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| if src.Text != "" && dst.Text == "" { | ||
| dst.Text = src.Text | ||
| } | ||
|
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| if src.Destination != "" && dst.Destination == "" { | ||
| dst.Destination = src.Destination | ||
| } | ||
|
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| if src.Source != "" && dst.Source == "" { | ||
| dst.Source = src.Source | ||
| } | ||
|
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| if src.RelativeTime != 0 && dst.RelativeTime == 0 { | ||
| dst.RelativeTime = src.RelativeTime | ||
| } | ||
|
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| if src.LineCount != 0 && dst.LineCount == 0 { | ||
| dst.LineCount = src.LineCount | ||
| } | ||
| } | ||
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| // encodeTagBlock encodes the fields of tagBlock into a NMEA 4.10 TAG Block string | ||
| func encodeTagBlock(tagBlock *nmea.TagBlock, msgIndex, msgNum, seqNo int, addLineCount bool) string { | ||
| if tagBlock == nil || *tagBlock == BlankTagBlock { | ||
| return "" | ||
| } | ||
|
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| tags := []string{} | ||
|
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| if msgNum > 1 { | ||
| // We add the group tag for all sentences in a multi-sentence message | ||
| tags = append(tags, fmt.Sprintf("g:%d-%d-%d", msgIndex, msgNum, seqNo)) | ||
|
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| // We don't duplicate the rest of the tags for subsequent sentences of multi-sentence message | ||
| if msgIndex > 1 { | ||
| return fmt.Sprintf("\\%s\\", addTagBlockChecksum(tags[0])) | ||
| } | ||
|
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| // If we got this far, it is a multi-sentence message AND this is the first sentence | ||
| if addLineCount { | ||
| tags = append(tags, fmt.Sprintf("n:%d", msgNum)) | ||
| } | ||
| } | ||
|
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| if tagBlock.Source != "" { | ||
| tags = append(tags, fmt.Sprintf("s:%s", tagBlock.Source)) | ||
| } | ||
|
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| if tagBlock.Time != 0 { | ||
| tags = append(tags, fmt.Sprintf("c:%d", tagBlock.Time)) | ||
| } | ||
|
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| if tagBlock.RelativeTime != 0 { | ||
| tags = append(tags, fmt.Sprintf("r:%d", tagBlock.RelativeTime)) | ||
| } | ||
|
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| if tagBlock.Destination != "" { | ||
| tags = append(tags, fmt.Sprintf("d:%s", tagBlock.Destination)) | ||
| } | ||
|
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| if tagBlock.Text != "" { | ||
| tags = append(tags, fmt.Sprintf("t:%s", tagBlock.Text)) | ||
| } | ||
|
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| return fmt.Sprintf("\\%s\\", addTagBlockChecksum(strings.Join(tags, ","))) | ||
| } |
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