modified rabin mueller decoder for NFC

mrm_decode.py

#! /usr/bin/env python2                                                                                                                                                              

import sys

class mrm_decoder():
    def __init__(self):
        self.last = "0"

    # Take one symbol and decode it.                                                                                                                                                 
    # Symbol can be: 1 (kurz = short), 2 (mittel = medium), 3 (lang = long)                                                                                                          
    def decode(self, sym):
        result = None
        if sym == "1":
            if self.last == "0":
                result = "0"
            else:
                result = "1"
        elif sym == "2":
            if self.last == "0":
                result = "1"
            else:
                result = "00"
        elif sym == "3":
            result = "01"
        else:
            raise Exception("Unknown symbol: %s" % sym)
        self.last = result[-1]
        return result

mrm = mrm_decoder()
bitstream = "0"
line = sys.argv[1].strip()
for symbol in line:
    bitstream += mrm.decode(symbol)
print "bits: ", bitstream

for idx in range(0, len(bitstream), 9):
    byte = bitstream[idx+8:idx:-1]
    if len(byte) < 8:
        print byte
    else:
        print byte, bitstream[idx+8], "=", '0x%02x' % int(byte, 2)