sample/add.c

   1 /* add.c   not copyrighted (n) 1993 by Mark Adler */
   2 /* version 1.1   11 Jun 1993 */
   3
   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
   7    good for. */
   8
   9 #include <config.h>
  10 #include <stdio.h>
  11
  12 #define MAGIC1    'S' /* sub data */
  13 #define MAGIC2    26  /* ^Z */
  14 #define MAX_DIST  16384
  15
  16 char a[MAX_DIST];       /* last byte buffer for up to MAX_DIST differences */
  17
  18 int main()
  19 {
  20   int n;                /* number of differences */
  21   int i;                /* difference counter */
  22   int c;                /* byte from input */
  23
  24   /* check magic word */
  25   if (getchar() != MAGIC1 || getchar() != MAGIC2)
  26   {
  27     fputs("add: input stream not made by sub\n", stderr);
  28     exit(1);
  29   }
  30
  31   /* get number of differences from data */
  32   if ((n = getchar()) == EOF || (i = getchar()) == EOF) {
  33     fputs("add: unexpected end of file\n", stderr);
  34     exit(1);
  35   }
  36   n += (i<<8);
  37   if (n <= 0 || n > MAX_DIST) {
  38     fprintf(stderr, "add: incorrect distance %d\n", n);
  39     exit(1);
  40   }
  41
  42   /* initialize last byte */
  43   i = n;
  44   do {
  45     a[--i] = 0;
  46   } while (i);
  47
  48   /* read differenced data and restore original */
  49   while ((c = getchar()) != EOF)
  50   {
  51     c = (a[i++] += c) & 0xff;   /* restore data, save last byte */
  52     putchar(c);                 /* write original */
  53     if (i == n)                 /* cycle on n differences */
  54       i = 0;
  55   }
  56   exit(0);
  57   return 0;                     /* avoid warning */
  58 }