60 lines
2.0 KiB
Python
60 lines
2.0 KiB
Python
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###############################################################
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# Copyright (C) 2015 Junzi Sun (TU Delft)
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# This program is free software: you can redistribute it and/or modify
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# it under the terms of the GNU General Public License as published by
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# the Free Software Foundation, either version 3 of the License, or
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# (at your option) any later version.
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# This program is distributed in the hope that it will be useful,
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# but WITHOUT ANY WARRANTY; without even the implied warranty of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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# GNU General Public License for more details.
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# You should have received a copy of the GNU General Public License
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# along with this program. If not, see <http://www.gnu.org/licenses/>.
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# the polynominal generattor code for CRC
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import math
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GENERATOR = "1111111111111010000001001"
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def hex2bin(hexstr):
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"""Convert a hexdecimal string to binary string, with zero fillings. """
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scale = 16
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num_of_bits = len(hexstr) * math.log(scale, 2)
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binstr = bin(int(hexstr, scale))[2:].zfill(int(num_of_bits))
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return binstr
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def bin2int(binstr):
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"""Convert a binary string to integer. """
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return int(binstr, 2)
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def crc(msg, encode=False):
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"""Mode-S Cyclic Redundancy Check
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Detect if bit error occurs in the Mode-S message
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Args:
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msg (string): 28 bytes hexadecimal message string
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encode (bool): True to encode the date only and return the checksum
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Returns:
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string: message checksum, or partity bits (encoder)
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"""
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msgbin = list(hex2bin(msg))
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if encode:
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msgbin[-24:] = ['0'] * 24
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# loop all bits, except last 24 piraty bits
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for i in range(len(msgbin)-24):
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# if 1, perform modulo 2 multiplication,
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if msgbin[i] == '1':
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for j in range(len(GENERATOR)):
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# modulo 2 multiplication = XOR
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msgbin[i+j] = str((int(msgbin[i+j]) ^ int(GENERATOR[j])))
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# last 24 bits
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reminder = ''.join(msgbin[-24:])
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return reminder
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