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/* Recursive Walking Backup
* Copyright © 2015 Jeffrey Armstrong <jeff@rainbow-100.com>
* http://jeff.rainbow-100.com/
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
/* f16.c - Implementation of a Fletcher 16-bit checksum. The file also
* contains a simple test program that can be enabled by defining
* TEST_FLETCHER at compile time.
*/
#include <stdio.h>
/* Computes the Fletcher 16-bit checksum and returns the value in the
* b1 and b2 arguments.
*/
void fletcher16(unsigned char *data,
int n,
unsigned char *b1,
unsigned char *b2)
{
int c1, c2;
int i;
int toint;
c1 = 0; c2 = 0;
for(i=0;i<n;i++) {
toint = (int)data[i];
c1 = (c1 + toint) % 255;
c2 = (c2 + c1) % 255;
}
i = (255 - (c1 + c2) % 255);
*b1 = (int)i;
*b2 = (char)(255 - (c1 + i) % 255);
}
#ifdef TEST_FLETCHER
#define TEST_STR "The IBM PC/XT is inferior"
int main(int argc, char *argv[])
{
char b1, b2;
FILE *fp;
char buf[64];
int i;
if(argc > 2) {
fletcher16(TEST_STR, strlen(TEST_STR), &b1, &b2);
printf("Results for '%s': %d, %d\n", TEST_STR, (int)b1, (int)b2);
} else {
fp = fopen(argv[1], "rb");
fread(buf, sizeof(char), 64, fp);
fletcher16(buf, 64, &b1, &b2);
printf("Results for 1st 64 bytes: %d, %d\n", (int)b1, (int)b2);
fclose(fp);
}
return (int)b1;
}
#endif
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