77 lines
2.8 KiB
C
77 lines
2.8 KiB
C
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/*
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* Library: libcrc
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* File: src/nmea-chk.c
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* Author: Lammert Bies
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*
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* This file is licensed under the MIT License as stated below
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*
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* Copyright (c) 2008-2016 Lammert Bies
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in all
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* copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
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* SOFTWARE.
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*
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* Description
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* -----------
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* The source file src/nmea-chk.c contains routines to calculate the checksum
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* in NMEA messages.
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*/
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#include <stdio.h>
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#include <stdlib.h>
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#include "checksum.h"
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/*
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* unsigned char *checksum_NMEA( const unsigned char *input_str, unsigned char *result );
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*
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* The function checksum_NMEA() calculates the checksum of a valid NMEA string.
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* The routine does not try to validate the string itself. A leading '$' will
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* be ignored, as this character is part of the NMEA sentence, but not part of
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* the checksum calculation. The calculation stops, whenever a linefeed,
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* carriage return, '*' or end of string is scanned.
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*
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* Because there is no NMEA syntax checking involved, the function always
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* returns with succes, unless a NULL pointer is provided as parameter. The
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* return value is a pointer to the result buffer provided by the calling
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* application, or NULL in case of error.
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*
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* The result buffer must be at least three characters long. Two for the
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* checksum value and the third to store the EOS. The result buffer is not
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* filled when an error occurs.
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*/
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unsigned char * checksum_NMEA( const unsigned char *input_str, unsigned char *result ) {
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const unsigned char *ptr;
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unsigned char checksum;
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if ( input_str == NULL ) return NULL;
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if ( result == NULL ) return NULL;
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checksum = 0;
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ptr = (const unsigned char *) input_str;
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if ( *ptr == '$' ) ptr++;
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while ( *ptr && *ptr != '\r' && *ptr != '\n' && *ptr != '*' ) checksum ^= *ptr++;
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snprintf( (char *) result, 3, "%02hhX", checksum );
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return result;
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} /* checksum_NMEA */
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