[core] implemented CRC-16 computation methods

Methods for CRC-16/ARC specifically.

Author: MrClock8163

src/geocompy/communication.py

@@ -556,3 +556,128 @@ def timeout_override(
         finally:
             self._port.timeout = saved_timeout
             self._logger.debug(f"Restored timeout to {saved_timeout}")
+
+
+# The GeoCOM protocol supports CRC-16 checksums in message exchanges.
+# From testing it appears it uses the ARC/IBM parameter set.
+#
+# Parameters and other info on various CRC versions:
+# https://reveng.sourceforge.io/crc-catalogue/16.htm#disclaimer
+#
+# Good reference article and calculator:
+# https://www.sunshine2k.de/articles/coding/crc/understanding_crc.html
+# https://www.sunshine2k.de/coding/javascript/crc/crc_js.html
+#
+# The implementations below are for this specific variant of CRC-16, not
+# general solutions. The methods were modified to take into account the
+# input and output reflection.
+def crc16_bitwise(value: str | bytes) -> int:
+    """
+    Bitwise CRC-16/ARC calculation method.
+
+    Adapted from: https://stackoverflow.com/a/68095008
+
+    Parameters
+    ----------
+    value : str | bytes
+        Data to digest.
+
+    Returns
+    -------
+    int
+        CRC-16 checksum
+    """
+    crc = 0
+
+    if len(value) == 0:
+        return crc
+
+    if isinstance(value, bytes):
+        data = value
+    else:
+        data = value.encode("ascii")
+
+    for char in data:
+        crc ^= char
+
+        for _ in range(8):
+            crc = (crc >> 1) ^ 0xa001 if (crc & 1) else (crc >> 1)
+
+    return crc
+
+
+_crc16_table_reflected: list[int] = [
+    0x0000, 0xc0c1, 0xc181, 0x0140, 0xc301, 0x03c0, 0x0280, 0xc241,
+    0xc601, 0x06c0, 0x0780, 0xc741, 0x0500, 0xc5c1, 0xc481, 0x0440,
+    0xcc01, 0x0cc0, 0x0d80, 0xcd41, 0x0f00, 0xcfc1, 0xce81, 0x0e40,
+    0x0a00, 0xcac1, 0xcb81, 0x0b40, 0xc901, 0x09c0, 0x0880, 0xc841,
+    0xd801, 0x18c0, 0x1980, 0xd941, 0x1b00, 0xdbc1, 0xda81, 0x1a40,
+    0x1e00, 0xdec1, 0xdf81, 0x1f40, 0xdd01, 0x1dc0, 0x1c80, 0xdc41,
+    0x1400, 0xd4c1, 0xd581, 0x1540, 0xd701, 0x17c0, 0x1680, 0xd641,
+    0xd201, 0x12c0, 0x1380, 0xd341, 0x1100, 0xd1c1, 0xd081, 0x1040,
+    0xf001, 0x30c0, 0x3180, 0xf141, 0x3300, 0xf3c1, 0xf281, 0x3240,
+    0x3600, 0xf6c1, 0xf781, 0x3740, 0xf501, 0x35c0, 0x3480, 0xf441,
+    0x3c00, 0xfcc1, 0xfd81, 0x3d40, 0xff01, 0x3fc0, 0x3e80, 0xfe41,
+    0xfa01, 0x3ac0, 0x3b80, 0xfb41, 0x3900, 0xf9c1, 0xf881, 0x3840,
+    0x2800, 0xe8c1, 0xe981, 0x2940, 0xeb01, 0x2bc0, 0x2a80, 0xea41,
+    0xee01, 0x2ec0, 0x2f80, 0xef41, 0x2d00, 0xedc1, 0xec81, 0x2c40,
+    0xe401, 0x24c0, 0x2580, 0xe541, 0x2700, 0xe7c1, 0xe681, 0x2640,
+    0x2200, 0xe2c1, 0xe381, 0x2340, 0xe101, 0x21c0, 0x2080, 0xe041,
+    0xa001, 0x60c0, 0x6180, 0xa141, 0x6300, 0xa3c1, 0xa281, 0x6240,
+    0x6600, 0xa6c1, 0xa781, 0x6740, 0xa501, 0x65c0, 0x6480, 0xa441,
+    0x6c00, 0xacc1, 0xad81, 0x6d40, 0xaf01, 0x6fc0, 0x6e80, 0xae41,
+    0xaa01, 0x6ac0, 0x6b80, 0xab41, 0x6900, 0xa9c1, 0xa881, 0x6840,
+    0x7800, 0xb8c1, 0xb981, 0x7940, 0xbb01, 0x7bc0, 0x7a80, 0xba41,
+    0xbe01, 0x7ec0, 0x7f80, 0xbf41, 0x7d00, 0xbdc1, 0xbc81, 0x7c40,
+    0xb401, 0x74c0, 0x7580, 0xb541, 0x7700, 0xb7c1, 0xb681, 0x7640,
+    0x7200, 0xb2c1, 0xb381, 0x7340, 0xb101, 0x71c0, 0x7080, 0xb041,
+    0x5000, 0x90c1, 0x9181, 0x5140, 0x9301, 0x53c0, 0x5280, 0x9241,
+    0x9601, 0x56c0, 0x5780, 0x9741, 0x5500, 0x95c1, 0x9481, 0x5440,
+    0x9c01, 0x5cc0, 0x5d80, 0x9d41, 0x5f00, 0x9fc1, 0x9e81, 0x5e40,
+    0x5a00, 0x9ac1, 0x9b81, 0x5b40, 0x9901, 0x59c0, 0x5880, 0x9841,
+    0x8801, 0x48c0, 0x4980, 0x8941, 0x4b00, 0x8bc1, 0x8a81, 0x4a40,
+    0x4e00, 0x8ec1, 0x8f81, 0x4f40, 0x8d01, 0x4dc0, 0x4c80, 0x8c41,
+    0x4400, 0x84c1, 0x8581, 0x4540, 0x8701, 0x47c0, 0x4680, 0x8641,
+    0x8201, 0x42c0, 0x4380, 0x8341, 0x4100, 0x81c1, 0x8081, 0x4040
+]
+"""
+Reflected CRC-16/ARC lookup table for bytewise computation.
+
+https://www.sunshine2k.de/coding/javascript/crc/crc_js.html
+"""
+
+
+def crc16_bytewise(value: str | bytes) -> int:
+    """
+    Bytewise CRC-16/ARC calculation method.
+
+    The method uses a precomputed lookup table to reduce the number of
+    required operations to calculate a checksum.
+
+    Parameters
+    ----------
+    value : str | bytes
+        Data to digest.
+
+    Returns
+    -------
+    int
+        CRC-16 checksum
+    """
+    crc = 0
+
+    if len(value) == 0:
+        return crc
+
+    if isinstance(value, bytes):
+        data: bytes = value
+    else:
+        data = value.encode("ascii")
+
+    for char in data:
+        # Due to the arbirarily sized integers in Python, the index must be
+        # masked to 8 bit
+        idx = (char ^ (crc)) & 0xff
+        crc = _crc16_table_reflected[idx] ^ (crc >> 8)
+
+    return crc

tests/test_communication.py

@@ -4,7 +4,9 @@
 from geocompy.communication import (
     get_dummy_logger,
     open_serial,
-    SerialConnection
+    SerialConnection,
+    crc16_bitwise,
+    crc16_bytewise
 )
 
 
@@ -86,3 +88,10 @@ def test_messaging(self) -> None:
                     com.receive()
 
                 assert com._timeout_counter == 2
+
+
+class TestCrc:
+    def test_crc(self) -> None:
+        # Verify CRC-16/ARC check value of "123456789" string
+        assert crc16_bitwise("123456789") == 0xbb3d
+        assert crc16_bytewise("123456789") == 0xbb3d