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 #include <stdlib.h>
  12
  13 #define MAGIC1    'S' /* sub data */
  14 #define MAGIC2    26  /* ^Z */
  15 #define MAX_DIST  16384
  16
  17 char a[MAX_DIST];       /* last byte buffer for up to MAX_DIST differences */
  18
  19 int
  20 main ()
  21 {
  22   int n;                /* number of differences */
  23   int i;                /* difference counter */
  24   int c;                /* byte from input */
  25
  26   /* check magic word */
  27   if (getchar() != MAGIC1 || getchar() != MAGIC2)
  28   {
  29     fputs("add: input stream not made by sub\n", stderr);
  30     exit(EXIT_FAILURE);
  31   }
  32
  33   /* get number of differences from data */
  34   if ((n = getchar()) == EOF || (i = getchar()) == EOF) {
  35     fputs("add: unexpected end of file\n", stderr);
  36     exit(EXIT_FAILURE);
  37   }
  38   n += (i<<8);
  39   if (n <= 0 || n > MAX_DIST) {
  40     fprintf(stderr, "add: incorrect distance %d\n", n);
  41     exit(EXIT_FAILURE);
  42   }
  43
  44   /* initialize last byte */
  45   i = n;
  46   do {
  47     a[--i] = 0;
  48   } while (i);
  49
  50   /* read differenced data and restore original */
  51   while ((c = getchar()) != EOF)
  52   {
  53     c = (a[i++] += c) & 0xff;   /* restore data, save last byte */
  54     putchar(c);                 /* write original */
  55     if (i == n)                 /* cycle on n differences */
  56       i = 0;
  57   }
  58   exit(EXIT_SUCCESS);
  59   return 0;                     /* avoid warning */
  60 }