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/*
* Copyright (C) 2014 Tomasz Kramkowski <tk@the-tk.com>
*
* This program is free software. It is licensed under version 3 of the
* GNU General Public License.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see [http://www.gnu.org/licenses/].
*/
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
int main(int argc, char **argv)
{
unsigned char subst[4][256];
unsigned char lastval[] = {249, 20, 174, 199}; /* Offsets */
/* Generate substitution tables. */
for (int i = 0; i < 256; i++) {
for (int ii = 0; ii < 4; ii++)
subst[ii][i] = lastval[ii];
if (i % 8 == 7) lastval[0] -= 13;
else lastval[0] += i & 1 ? 3 : -1;
if (i % 8 == 7) lastval[1] += 9;
else if ((i + 4) % 8 == 7) lastval[1] -=7;
else lastval[1] += 1;
if (i % 16 == 15) lastval[2] += 29;
else lastval[2] += i & 1 ? -3 : 1;
if (i % 8 == 7) lastval[3] += 15;
else lastval[3] -= 1;
}
/* Decode */
int c;
int pos = argc > 1 ? strtol(argv[1], NULL, 10) : 0;
while ((c = getchar()) != EOF) {
for (int i = 0; i < 256; i++)
if (subst[pos%4][i] == c) {
if (isascii(i)) putchar(i);
break;
}
pos++;
}
return 0;
}
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