# `ccsds_packets`

The `ccsds_packets` library implements predicates for parsing and generating CCSDS (Consultative Committee for Space Data Systems) Space Packets following the CCSDS 133.0-B-2 standard (Space Packet Protocol):

- https://public.ccsds.org/Pubs/133x0b2e1.pdf

- https://jastoolbox.sandia.gov/topic/communication-specification/jas-packets/ccsds-telecommand-and-telemetry-format-packet-standard/

## API documentation

Open the ../../apis/library_index.html#ccsds_packets link in a web browser.

## Loading

To load all entities in this library, load the `loader.lgt` file:

    | ?- logtalk_load(ccsds_packets(loader)).

## Testing

To test this library predicates, load the `tester.lgt` file:

    | ?- logtalk_load(ccsds_packets(tester)).

To test the performance of the library parsing predicates, load the `tester_performance.lgt` file:

    | ?- logtalk_load(ccsds_packets(tester_performance)).

## Packet Structure

A CCSDS Space Packet consists of:

1. Primary Header (6 bytes):

   - Version Number (3 bits) - Always 000 for Space Packets

   - Packet Type (1 bit) - 0=Telemetry, 1=Telecommand

   - Secondary Header Flag (1 bit) - 0=absent, 1=present

   - Application Process ID (APID) (11 bits) - 0-2047

   - Sequence Flags (2 bits) - 00=continuation, 01=first, 10=last, 11=standalone

   - Packet Sequence Count (14 bits) - 0-16383

   - Packet Data Length (16 bits) - Number of octets in data field minus 1

2. User Data Field - Variable length payload

## Representation

Packets are represented using the compound term:

    ccsds_packet(Version, Type, SecHeaderFlag, APID, SeqFlags, SeqCount, SecHeader, UserData)

Where:

- `Version` is an integer (0-7, typically 0)

- `Type` is an integer (0=telemetry, 1=telecommand)

- `SecHeaderFlag` is an integer (0 or 1)

- `APID` is an integer (0-2047)

- `SeqFlags` is an integer (0-3)

- `SeqCount` is an integer (0-16383)

- `SecHeader` is either `none` or `secondary_header(Bytes)` where `Bytes` is a list of bytes

- `UserData` is a list of bytes

Note that the `DataLength` field from the wire format is not stored in the term representation as it can be computed from `SecHeader` and `UserData`. The `data_length/2` accessor predicate computes and returns this value when needed.

## Parsing

The `parse/2` predicate accepts a source term as its first argument. The source can be `file(File)`, `stream(Stream)`, or `bytes(Bytes)`. All source types return a list of packets for uniformity.

To parse packets from a list of bytes:

    | ?- ccsds_packets::parse(bytes([0x08, 0x01, 0xC0, 0x00, 0x00, 0x03, 0xDE, 0xAD, 0xBE, 0xEF]), Packets).
    Packets = [ccsds_packet(0, 0, 1, 1, 3, 0, none, [222, 173, 190, 239])]
    yes

To parse packets from a binary file:

    | ?- ccsds_packets::parse(file('telemetry.bin'), Packets).

To parse packets from a binary stream:

    | ?- ccsds_packets::parse(stream(Stream), Packets).

When the packets include a secondary header, the secondary header length must be known. In this case, use the `ccsds_packets(SecondaryHeaderLength)` object instead of the `ccsds_packets` object. For example, to parse packets with a secondary header of 6 bytes:

    | ?- ccsds_packets(6)::parse(bytes([0x08, 0x01, 0xC0, 0x00, 0x00, 0x07, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0xAA, 0xBB]), Packets).

## Generating

The `generate/2` predicate accepts a sink term as its first argument and a list of packet terms as the second argument. The sink can be `file(File)`, `stream(Stream)`, or `bytes(Bytes)`.

To generate bytes from a list of packet terms:

    | ?- ccsds_packets::generate(bytes(Bytes), [ccsds_packet(0, 0, 1, 1, 3, 0, none, [0xDE, 0xAD, 0xBE, 0xEF])]).
    Bytes = [8, 1, 192, 0, 0, 3, 222, 173, 190, 239]
    yes

To write the bytes generated from a list of packet terms to a binary file:

    | ?- ccsds_packets::generate(file('output.bin'), Packets).

To write the bytes generated from a list of packet terms to a binary stream:

    | ?- ccsds_packets::generate(stream(Stream), Packets).

## Accessor Predicates

The library provides convenient accessor predicates for extracting packet fields:

    | ?- ccsds_packets::apid(Packet, APID).

    % Returns telemetry or telecommand
    | ?- ccsds_packets::type(Packet, Type).

    % Returns continuation, first, last, or standalone
    | ?- ccsds_packets::sequence_flags(Packet, Flags).

    % Returns the secondary header as a list of bytes or none
    | ?- ccsds_packets::secondary_header(Packet, SecHeader).

    % Parses a self-describing secondary-header time field when Descriptor is a variable.
    | ?- ccsds_packets(7)::secondary_header_time(Packet, Descriptor, Time).

    % Parses raw secondary-header T-field bytes using an explicit descriptor term.
    | ?- ccsds_packets(10)::secondary_header_time(Packet, cuc_descriptor(5, 5, ccsds_epoch), Time).

    | ?- ccsds_packets::user_data(Packet, Data).

    % Returns the packet data length (computed from secondary header and user data)
    | ?- ccsds_packets::data_length(Packet, Length).

## Types and arbitrary generators

The library includes a `ccsds_packets_types` category that provides `ccsds_packet` and `ccsds_packet(SecondaryHeaderLength)` types and arbitrary generators for CCSDS packets. For example:

    | ?- type::check(ccsds_packet, Bytes).

    | ?- type::arbitrary(ccsds_packet(42), Bytes).

It also provides a `ccsds_packets(N)` and `ccsds_packets(SecondaryHeaderLength, N)` types for generating a list with `N` packets. For example:

    | ?- type::arbitrary(ccsds_packets(10), Bytes).

    | ?- type::check(ccsds_packets(42, 10), Bytes).

For a term representation of a packet, use the `ccsds_packet_term` and `ccsds_packet_term(SecondaryHeaderLength)` types and arbitrary generators. For example:

    | ?- type::check(ccsds_packet_term, Packet).

    | ?- type::arbitrary(ccsds_packet_term(42), Packet).