code/breadcrumb/common/checksum.py

   1 # -*- coding: utf8 -*-
   2 """ Functions for creating and validating NMEA style checksums. """
   3
   4 # Copyright (C) 2008 Laurens Van Houtven <lvh at laurensvh.be>
   5 # This program is free software: you can redistribute it and/or modify
   6 # it under the terms of the GNU General Public License as published by
   7 # the Free Software Foundation, either version 3 of the License, or
   8 # (at your option) any later version.
   9 #
  10 # This program is distributed in the hope that it will be useful,
  11 # but WITHOUT ANY WARRANTY; without even the implied warranty of
  12 # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13 # GNU General Public License for more details.
  14 #
  15 # You should have received a copy of the GNU General Public License
  16 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
  17
  18 def is_valid_nmea_sentence(sentence):
  19     """Validates the checksum of an NMEA sentence.
  20
  21     Returns True if the sentence has either a valid checksum or no checkum.
  22     Returns False if the checksum was bad.
  23     """
  24     if sentence[-3] == '*': # sentence has a checksum
  25         reference, source = sentence[-2:], sentence[1:-3]
  26         return is_valid_hex_csum(source, reference)
  27     else: # sentence does not have a checksum
  28         return True
  29
  30 def is_valid_hex_csum(string, reference):
  31     """Calculates a hex checksum and compares it to the reference."""
  32     return generate_hex_checksum(string) == reference
  33
  34 def generate_checksum(string):
  35     """Generates an NMEA style checksum (in decimal) of a given string."""
  36     checksum = 0
  37     for char in string:
  38         checksum = checksum ^ ord(char)
  39     return checksum
  40
  41 def generate_hex_checksum(string):
  42     """Generates the hexadecimal form of the checksum of a given string."""
  43     hex_csum = "%02x" % generate_checksum(string)
  44     return hex_csum.upper()
  45
  46 def generate_byte_checksum(string):
  47     """Generates the byte form of the checksum of a given string."""
  48     return chr(generate_checksum(string))