1 /* add.c not copyrighted (n) 1993 by Mark Adler */
2 /* version 1.1 11 Jun 1993 */
4 /* This filter reverses the effect of the sub filter. It requires no
5 arguments, since sub puts the information necessary for extraction
6 in the stream. See sub.c for what the filtering is and what it's
11 #define MAGIC1 'S' /* sub data */
12 #define MAGIC2 26 /* ^Z */
13 #define MAX_DIST 16384
15 char a[MAX_DIST]; /* last byte buffer for up to MAX_DIST differences */
19 int n; /* number of differences */
20 int i; /* difference counter */
21 int c; /* byte from input */
23 /* check magic word */
24 if (getchar() != MAGIC1 || getchar() != MAGIC2)
26 fputs("add: input stream not made by sub\n", stderr);
30 /* get number of differences from data */
31 if ((n = getchar()) == EOF || (i = getchar()) == EOF) {
32 fputs("add: unexpected end of file\n", stderr);
36 if (n <= 0 || n > MAX_DIST) {
37 fprintf(stderr, "add: incorrect distance %d\n", n);
41 /* initialize last byte */
47 /* read differenced data and restore original */
48 while ((c = getchar()) != EOF)
50 c = (a[i++] += c) & 0xff; /* restore data, save last byte */
51 putchar(c); /* write original */
52 if (i == n) /* cycle on n differences */
56 return 0; /* avoid warning */