libcotshrink takes CotEvent instances from ATAK and turns them into minimally sized byte arrays.
It uses a combination of EXI (Efficient XML Interchange), Protobufs, and GZip.
libcotshrink is now available on Maven Central
Edit your app/build.gradle and add the library to your app/plugin's dependencies:
dependencies {
...
implementation 'com.paulmandal.atak:libcotshrink:1.0.4'
}
Run Gradle Sync and you will have access to libcotshrink's classes.
The CotShrinker class contains the libcotshrink API. It has lossless and lossy variants:
CotShrinker.toByteArray(CotEvent) - Losslessly converts a CotEvent to a byte array, currently this uses EXI (Efficient XML Interchange)
CotShrinker.toByteArrayLossy(CotEvent) - Lossy conversion for a CotEvent to a byte array, uses custom protobufs. This will fall back to toByteArray(CotEvent) if the custom protobuf conversion fails.
CotShrinker.toCotEvent(byte[]) - Will attempt to convert back to a CotEvent, first using the lossy conversion and then using the lossless if that one fails.
Example usage:
CotEvent cotEvent = ...;
CotShrinkerFactory cotShrinkerFactory = new CotShrinkerFactory();
CotShrinker cotShrinker = cotShrinkerFactory.createCotShrinker();
// Convert to bytes
byte[] cotAsBytes = cotShrinker.toByteArray(cotEvent);
byte[] cotAsLossyBytes = cotShrinker.toByteArrayLossy(cotEvent);
// Convert back
CotEvent cotFromBytes = cotShrinker.toCotEvent(cotAsBytes);
CotEvent cotFromLossyBytes = cotShrinker.toCotEvent(cotAsLossyBytes);Using the CotShrinker.toByteArrayLossy(CotEvent) method will cause loss of precision in the following ways:
| Field | Description |
|---|---|
time |
to whole seconds since start of year |
stale |
to whole seconds since time |
start |
reuses valuefrom time |
point.lat |
to 7 decimal places |
point.lon |
to 7 decimal places |
point.hae |
to 16 bit, -900 to 20945 range, 0.3m |
point.ce |
to int from double |
point.le |
to int from double |
link.lat |
to 7 decimal places |
link.lon |
to 7 decimal places |
link.hae |
to 2 decimal places |
track.course |
to 3 decimal places |
track.speed |
to 1 decimal place |
height.value |
to 3 decimal places |
shape.major |
to 3 decimal places |
shape.minor |
to 3 decimal places |
__geofence.boundingSphere |
to 1 decimal place |
__geofence.minElevation |
to 1 decimal place |
__geofence.maxElevation |
to 1 decimal place |
To test your changes, make sure your app is in the same parent directory as your libcotshrink directory. In your app's settings.gradle:
include ':libcotshrink'
project(':libcotshrink').projectDir = file('../libcotshrink')and in your app's build.gradle include the dependency like this:
dependencies {
...
implementation project(':libcotshrink')
}You will need to comment out all the build.gradle lines in libcotshrink related to publishing to Maven, you can just click Try Again in Android Studio repeatedly and comment out each line that causes an issue.
- Figure out how to get unit tests working, they rely
CotEvent.parse()but this does not seem to work during unit tests - Use
sint32instead ofint32for fields that can be negative - Support more message types (please identify them and how they are generated in Issues)
- Create detail subobjects for each set of detail fields used by a specific message type, e.g. the detail field should have objs like GeoChatDetail, PliDetail, etc. that are composed of the existing detail fields. This MAY save some space, need to test with PLI or something like that first.
- Shrink messages further
- Use Dagger 2
| Message Type | Original Size | Lossless Size | Lossy Size |
|---|---|---|---|
| PLI | 731 | 545 (-25.44%) | 171 (-76.61%) |
| HA PLI | 732 | 546 (-25.41%) | 171 (-76.64%) |
| -HAE PLI | 720 | 543 (-24.58%) | 171 (-76.25%) |
| PLI w/o IP | 725 | 539 (-25.66%) | 166 (-77.10%) |
| Complex Shape | 800 | 575 (-28.13%) | 222 (-72.25%) |
| Drawn Shape | 963 | 687 (-28.66%) | 212 (-77.99%) |
| Freehand | 1867 | 958 (-48.69%) | 600 (-67.86%) |
| Group Chat | 1181 | 723 (-38.78%) | 346 (-70.70%) |
| Chat w/ MessageId | 932 | 647 (-30.58%) | 274 (-70.60%) |
| OP | 735 | 546 (-25.71%) | 194 (-73.61%) |
| P2P Chat | 951 | 642 (-32.49%) | 303 (-68.14%) |
| Route | 2826 | 1692 (-40.13%) | 544 (-80.75%) |
| Sensor | 1106 | 853 (-22.88%) | 285 (-74.23%) |
| Simple Shape | 881 | 656 (-25.54%) | 240 (-72.76%) |
| User Icon | 770 | 582 (-24.42%) | 252 (-67.27%) |
| Waypoint | 756 | 552 (-26.98%) | 198 (-73.81%) |
| Casevac w/ All Fields | 1810 | 1414 (-21.88%) | 601 (-66.80%) |
| Casevac w/ Empty Fields | 876 | 669 (-23.63%) | 236 (-73.06%) |
| Circle | 912 | 621 (-31.91%) | 209 (-77.08%) |
| GeoFence w/ Altitude | 1040 | 792 (-23.85%) | 222 (-78.65%) |
| GeoFence w/o Altitude | 1009 | 761 (-24.58%) | 217 (-78.49%) |