libcrc8.py

'''  libcrc8.py

    Library of routines to compute CRC-8 checksum over
    an array of unsigned bytes. The crc8 function uses a
    table to process the array byte-by-byte rather than
    bit-by-bit.  The division polynomial and initial
    value may both be changed by the user via function calls.

    To access,
       import libcrc8

    Use:
       cs = libcrc8.crc8(msg,LengthOfMsg,init)
    returns the unsigned byte representing the CRC-8
    checksum of the message "msg", a bytearray() of length "LengthOfMsg"
    bytes, using the initial remainder "init".

    By default, the polynomial used in the CRC-8 calculation
    is 0x97, also known as "C2"..  
      See https://users.ece.cmu.edu/~koopman/roses/dsn04/koopman04_crc_poly_embedded.pdf
    To set the crc8 function to use a different polynomial, invoke
    the function
       libcrc8.buildCRC8Table(poly)
    which resets the internally-defined polynomial to "poly" and 
    recomputes the table used for the CRC8 calculation.

    To retrieve the value of the polynomial currently being
    used in the crc8 function,
       poly = libcrc8.getCRC8Poly()

    To dump the table currently in use, 
       libcrc8.dumpCRC8Table()

    HD Todd, February, 2022
    Adapted from a number of other sources and references,
    but particularly note William's paper from
    http://ross.net/crc/download/crc_v3.txt, 
    Koopman's work (one citation is above), 
    and https://www.pololu.com/docs/0J44/6.7.6.

'''

#   Predefined table of CRC-8 lookup bytes computed using
#   the polynomial 0x97, also know as "C2". Per Koopman,
#   "arguably the best" for messages up to 119 bits long

#   This table can be recomputed for a different polynomial
#   using libcrc8.buildCRC8Table(poly) below.

CRC8POLY = 0x97
CRC8Table = bytearray([
	0x00, 0x97, 0xb9, 0x2e, 0xe5, 0x72, 0x5c, 0xcb, 
	0x5d, 0xca, 0xe4, 0x73, 0xb8, 0x2f, 0x01, 0x96, 
	0xba, 0x2d, 0x03, 0x94, 0x5f, 0xc8, 0xe6, 0x71, 
	0xe7, 0x70, 0x5e, 0xc9, 0x02, 0x95, 0xbb, 0x2c, 
	0xe3, 0x74, 0x5a, 0xcd, 0x06, 0x91, 0xbf, 0x28, 
	0xbe, 0x29, 0x07, 0x90, 0x5b, 0xcc, 0xe2, 0x75, 
	0x59, 0xce, 0xe0, 0x77, 0xbc, 0x2b, 0x05, 0x92, 
	0x04, 0x93, 0xbd, 0x2a, 0xe1, 0x76, 0x58, 0xcf, 
	0x51, 0xc6, 0xe8, 0x7f, 0xb4, 0x23, 0x0d, 0x9a, 
	0x0c, 0x9b, 0xb5, 0x22, 0xe9, 0x7e, 0x50, 0xc7, 
	0xeb, 0x7c, 0x52, 0xc5, 0x0e, 0x99, 0xb7, 0x20, 
	0xb6, 0x21, 0x0f, 0x98, 0x53, 0xc4, 0xea, 0x7d, 
	0xb2, 0x25, 0x0b, 0x9c, 0x57, 0xc0, 0xee, 0x79, 
	0xef, 0x78, 0x56, 0xc1, 0x0a, 0x9d, 0xb3, 0x24, 
	0x08, 0x9f, 0xb1, 0x26, 0xed, 0x7a, 0x54, 0xc3, 
	0x55, 0xc2, 0xec, 0x7b, 0xb0, 0x27, 0x09, 0x9e, 
	0xa2, 0x35, 0x1b, 0x8c, 0x47, 0xd0, 0xfe, 0x69, 
	0xff, 0x68, 0x46, 0xd1, 0x1a, 0x8d, 0xa3, 0x34, 
	0x18, 0x8f, 0xa1, 0x36, 0xfd, 0x6a, 0x44, 0xd3, 
	0x45, 0xd2, 0xfc, 0x6b, 0xa0, 0x37, 0x19, 0x8e, 
	0x41, 0xd6, 0xf8, 0x6f, 0xa4, 0x33, 0x1d, 0x8a, 
	0x1c, 0x8b, 0xa5, 0x32, 0xf9, 0x6e, 0x40, 0xd7, 
	0xfb, 0x6c, 0x42, 0xd5, 0x1e, 0x89, 0xa7, 0x30, 
	0xa6, 0x31, 0x1f, 0x88, 0x43, 0xd4, 0xfa, 0x6d, 
	0xf3, 0x64, 0x4a, 0xdd, 0x16, 0x81, 0xaf, 0x38, 
	0xae, 0x39, 0x17, 0x80, 0x4b, 0xdc, 0xf2, 0x65, 
	0x49, 0xde, 0xf0, 0x67, 0xac, 0x3b, 0x15, 0x82, 
	0x14, 0x83, 0xad, 0x3a, 0xf1, 0x66, 0x48, 0xdf, 
	0x10, 0x87, 0xa9, 0x3e, 0xf5, 0x62, 0x4c, 0xdb, 
	0x4d, 0xda, 0xf4, 0x63, 0xa8, 0x3f, 0x11, 0x86, 
	0xaa, 0x3d, 0x13, 0x84, 0x4f, 0xd8, 0xf6, 0x61, 
	0xf7, 0x60, 0x4e, 0xd9, 0x12, 0x85, 0xab, 0x3c ]);


#   Build an array with CRC values of all 256 possible bytes
#   using the polymomial provided
def buildCRC8Table(poly):
  global CRC8POLY
  global CRC8Table
  CRC8POLY = poly

  for i in range (0,256):
    c = i
    for j in range (0,8):
        c = c<<1 if ((c & 0x80) == 0) else (c<<1) ^ poly
        c &= 0xff
    CRC8Table[i] = c
  return

def getCRC8Poly():
  return CRC8POLY

def dumpCRC8Table():
  print("Dump of CRC8 table for polynomial 0x{:02x}\n".format(getCRC8Poly()))
  for i in range(0, 32):
    for j in range(0,8):
      print("0x{:02x}, ".format(CRC8Table[i*8+j]), end="")
    print("")
  return

def crc8(msg, lengthOfMsg, init):
  rem = init
  for i in range (0,lengthOfMsg):
    rem = CRC8Table[ (rem ^ msg[i])] & 0xff
  return rem