127 lines
3.6 KiB
C
127 lines
3.6 KiB
C
/*
|
|
* Library: libcrc
|
|
* File: src/crckrmit.c
|
|
* Author: Lammert Bies
|
|
*
|
|
* This file is licensed under the MIT License as stated below
|
|
*
|
|
* Copyright (c) 1999-2016 Lammert Bies
|
|
*
|
|
* Permission is hereby granted, free of charge, to any person obtaining a copy
|
|
* of this software and associated documentation files (the "Software"), to deal
|
|
* in the Software without restriction, including without limitation the rights
|
|
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
|
|
* copies of the Software, and to permit persons to whom the Software is
|
|
* furnished to do so, subject to the following conditions:
|
|
*
|
|
* The above copyright notice and this permission notice shall be included in all
|
|
* copies or substantial portions of the Software.
|
|
*
|
|
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
|
|
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
|
|
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
|
|
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
|
|
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
|
|
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
|
|
* SOFTWARE.
|
|
*
|
|
* Description
|
|
* -----------
|
|
* The source file src/crckrmit.c contains routines which calculate the CRC
|
|
* Kermit cyclic redundancy check value for an incomming byte string.
|
|
*/
|
|
|
|
#include <stdbool.h>
|
|
#include <stdlib.h>
|
|
#include "checksum.h"
|
|
|
|
static void init_crc_tab( void );
|
|
|
|
static bool crc_tab_init = false;
|
|
static uint16_t crc_tab[256];
|
|
|
|
/*
|
|
* uint16_t crc_kermit( const unsigned char *input_str, size_t num_bytes );
|
|
*
|
|
* The function crc_kermit() calculates the 16 bits Kermit CRC in one pass for
|
|
* a byte string of which the beginning has been passed to the function. The
|
|
* number of bytes to check is also a parameter.
|
|
*/
|
|
|
|
uint16_t crc_kermit( const unsigned char *input_str, size_t num_bytes ) {
|
|
|
|
uint16_t crc;
|
|
uint16_t low_byte;
|
|
uint16_t high_byte;
|
|
const unsigned char *ptr;
|
|
size_t a;
|
|
|
|
if ( ! crc_tab_init ) init_crc_tab();
|
|
|
|
crc = CRC_START_KERMIT;
|
|
ptr = input_str;
|
|
|
|
if ( ptr != NULL ) for (a=0; a<num_bytes; a++) {
|
|
|
|
crc = (crc >> 8) ^ crc_tab[ (crc ^ (uint16_t) *ptr++) & 0x00FF ];
|
|
}
|
|
|
|
low_byte = (crc & 0xff00) >> 8;
|
|
high_byte = (crc & 0x00ff) << 8;
|
|
crc = low_byte | high_byte;
|
|
|
|
return crc;
|
|
|
|
} /* crc_kermit */
|
|
|
|
/*
|
|
* uint16_t update_crc_kermit( uint16_t crc, unsigned char c );
|
|
*
|
|
* The function update_crc_kermit() calculates a new CRC Kermit value based on
|
|
* the previous value of the CRC and the next byte of data to be checked.
|
|
*/
|
|
|
|
uint16_t update_crc_kermit( uint16_t crc, unsigned char c ) {
|
|
|
|
if ( ! crc_tab_init ) init_crc_tab();
|
|
|
|
return (crc >> 8) ^ crc_tab[ (crc ^ (uint16_t) c) & 0x00FF ];
|
|
|
|
} /* update_crc_kermit */
|
|
|
|
/*
|
|
* static void init_crc_tab( void );
|
|
*
|
|
* For optimal performance, the CRC Kermit routine uses a lookup table with
|
|
* values that can be used directly in the XOR arithmetic in the algorithm.
|
|
* This lookup table is calculated by the init_crc_tab() routine, the first
|
|
* time the CRC function is called.
|
|
*/
|
|
|
|
static void init_crc_tab( void ) {
|
|
|
|
uint16_t i;
|
|
uint16_t j;
|
|
uint16_t crc;
|
|
uint16_t c;
|
|
|
|
for (i=0; i<256; i++) {
|
|
|
|
crc = 0;
|
|
c = i;
|
|
|
|
for (j=0; j<8; j++) {
|
|
|
|
if ( (crc ^ c) & 0x0001 ) crc = ( crc >> 1 ) ^ CRC_POLY_KERMIT;
|
|
else crc = crc >> 1;
|
|
|
|
c = c >> 1;
|
|
}
|
|
|
|
crc_tab[i] = crc;
|
|
}
|
|
|
|
crc_tab_init = true;
|
|
|
|
} /* init_crc_tab */
|