MNT: Move CCSDS Header information into the CCSDSPacket class

docs/source/changelog.md

@@ -8,6 +8,9 @@ Release notes for the `space_packet_parser` library
 ### v5.1.0 (unreleased)
 - BUGFIX: Fix kbps calculation in packet generator for showing progress.
 - Add support for string and float encoded enumerated lookup parameters.
+- The global CCSDS_HEADER constants in packets.py are moved onto the CCSDSPacket
+  class itself accessible as `CCSDSPacket.HEADER_LENGTH` and `CCSDSPacket.HEADER_DEFINITION`
+  to more closely align with where these constants are useful.
 
 ### v5.0.1 (released)
 - BUGFIX: Allow raw_value representation for enums with falsy raw values. Previously these defaulted to the enum label.

space_packet_parser/packets.py

@@ -5,7 +5,6 @@
 import datetime as dt
 import io
 import logging
-from collections import namedtuple
 import socket
 import time
 from typing import BinaryIO, Iterator, List, Optional, Protocol, Union
@@ -130,21 +129,6 @@ def read_as_int(self, nbits: int) -> int:
         return int_data
 
 
-CcsdsPacketHeaderElement = namedtuple('CcsdsPacketHeaderElement', ['name', 'nbits'])
-
-CCSDS_HEADER_DEFINITION = [
-    CcsdsPacketHeaderElement('VERSION', 3),
-    CcsdsPacketHeaderElement('TYPE', 1),
-    CcsdsPacketHeaderElement('SEC_HDR_FLG', 1),
-    CcsdsPacketHeaderElement('PKT_APID', 11),
-    CcsdsPacketHeaderElement('SEQ_FLGS', 2),
-    CcsdsPacketHeaderElement('SRC_SEQ_CTR', 14),
-    CcsdsPacketHeaderElement('PKT_LEN', 16)
-]
-
-CCSDS_HEADER_LENGTH_BYTES = 6
-
-
 class CCSDSPacket(dict):
     """CCSDS Packet
 
@@ -164,6 +148,21 @@ class CCSDSPacket(dict):
     **kwargs : dict
         Additional packet items to store, passed to the dict() constructor.
     """
+    HEADER_LENGTH = 6
+    """CCSDS Packet Header Length in bytes."""
+
+    HEADER_DEFINITION = {"VERSION": 3,
+                         "TYPE": 1,
+                         "SEC_HDR_FLG": 1,
+                         "PKT_APID": 11,
+                         "SEQ_FLGS": 2,
+                         "SRC_SEQ_CTR": 14,
+                         "PKT_LEN": 16}
+    """CCSDS Packet Header Definition
+
+    Mapping of the field names to the number of bits they occupy in the header.
+    """
+
     def __init__(self, *args, raw_data: bytes = b"", **kwargs):
         self.raw_data = RawPacketData(raw_data)
         super().__init__(*args, **kwargs)
@@ -194,9 +193,9 @@ def _parse_header(packet_data: bytes) -> dict:
         """
         header = {}
         current_bit = 0
-        for item in CCSDS_HEADER_DEFINITION:
-            header[item.name] = _extract_bits(packet_data, current_bit, item.nbits)
-            current_bit += item.nbits
+        for name, nbits in CCSDSPacket.HEADER_DEFINITION.items():
+            header[name] = _extract_bits(packet_data, current_bit, nbits)
+            current_bit += nbits
         return header
 
 
@@ -332,21 +331,21 @@ def packet_generator(  # pylint: disable=too-many-branches,too-many-statements
             current_pos = 0
 
         # Fill buffer enough to parse a header
-        while len(read_buffer) - current_pos < skip_header_bytes + CCSDS_HEADER_LENGTH_BYTES:
+        while len(read_buffer) - current_pos < skip_header_bytes + CCSDSPacket.HEADER_LENGTH:
             result = read_bytes_from_source(buffer_read_size_bytes)
             if not result:  # If there is verifiably no more data to add, break
                 break
             read_buffer += result
         # Skip the header bytes
         current_pos += skip_header_bytes
-        header_bytes = read_buffer[current_pos:current_pos + CCSDS_HEADER_LENGTH_BYTES]
+        header_bytes = read_buffer[current_pos:current_pos + CCSDSPacket.HEADER_LENGTH]
         header = CCSDSPacket._parse_header(header_bytes)  # pylint: disable=protected-access
 
         # per the CCSDS spec
         # 4.1.3.5.3 The length count C shall be expressed as:
         #   C = (Total Number of Octets in the Packet Data Field) – 1
         n_bytes_data = header['PKT_LEN'] + 1
-        n_bytes_packet = CCSDS_HEADER_LENGTH_BYTES + n_bytes_data
+        n_bytes_packet = CCSDSPacket.HEADER_LENGTH + n_bytes_data
 
         # Based on PKT_LEN fill buffer enough to read a full packet
         while len(read_buffer) - current_pos < n_bytes_packet: