sample/sub.c

   1 /* sub.c   not copyrighted (n) 1993 by Mark Adler */
   2 /* version 1.1   11 Jun 1993 */
   3
   4 /* sub is a simple filter to preprocess a data file before compression.
   5    It can increase compression for data whose points tend to be close to
   6    the last point.  The output is the difference of successive bytes of
   7    the input.  The add filter is used to undo what sub does.  This could
   8    be used on 8-bit sound or graphics data.
   9
  10    sub can also take an argument to apply this to interleaved sets of
  11    bytes.  For example, if the data are 16-bit sound samples, then you
  12    can use "sub 2" to take differences on the low-byte stream and the
  13    high-byte stream.  (This gives nearly the same effect as subtracting
  14    the 16-bit values, but avoids the complexities of endianess of the
  15    data.)  The concept extends to RGB image data (sub 3), 16-bit stereo
  16    data (sub 4), floating point data (sub 4 or sub 8), etc.
  17
  18    add takes no options, since the number of interleaved byte streams
  19    is put in the first two bytes of the output stream for add to use
  20    (in little-endian format).
  21
  22    Examples:
  23
  24       sub < graph.vga | gzip -9 > graph.vga.sgz
  25       sub < phone.snd | gzip -9 > phone.snd.sgz
  26       sub 2 < audio.snd | gzip -9 > audio.snd.sgz
  27       sub 3 < picture.rgb | gzip -9 > picture.rgb.sgz
  28       sub 4 < stereo.snd | gzip -9 > stereo.snd.sgz
  29       sub 8 < double.data | gzip -9 > double.data.sgz
  30
  31    To expand, use the reverse operation, as in:
  32
  33       gunzip < double.data.sgz | add > double.data
  34 */
  35
  36 #include <config.h>
  37 #include <stdio.h>
  38 #include <stdlib.h>
  39
  40 #define MAGIC1    'S' /* sub data */
  41 #define MAGIC2    26  /* ^Z */
  42 #define MAX_DIST  16384
  43
  44 char a[MAX_DIST];       /* last byte buffer for up to MAX_DIST differences */
  45
  46 int main(argc, argv)
  47   int argc;
  48   char **argv;
  49 {
  50   int n = 1;            /* number of differences */
  51   int i;                /* difference counter */
  52   int c;                /* byte from input */
  53   int atoi();           /* (avoid including stdlib for portability) */
  54
  55   /* process arguments */
  56   if (argc > 2)
  57   {
  58     fputs("sub: only one argument needed--# of differences\n", stderr);
  59     exit(EXIT_FAILURE);
  60   }
  61   if (argc > 1)
  62     n = atoi(argv[1]);
  63
  64   if (n < 0) n = -n;    /* tolerate "sub -2" */
  65   if (n == 0 || n > MAX_DIST) {
  66     fputs("sub: incorrect distance\n", stderr);
  67     exit(EXIT_FAILURE);
  68   }
  69
  70   /* initialize last byte */
  71   i = n;
  72   do {
  73     a[--i] = 0;
  74   } while (i);
  75
  76   /* write differenced data */
  77   putchar(MAGIC1);  putchar(MAGIC2);    /* magic word for add */
  78   putchar(n & 0xff);                    /* so add knows what to do */
  79   putchar((n>>8) & 0xff);
  80
  81   while ((c = getchar()) != EOF)
  82   {
  83     putchar((c - a[i]) & 0xff);         /* write difference */
  84     a[i++] = c;                         /* save last byte */
  85     if (i == n)                         /* cycle on n differences */
  86       i = 0;
  87   }
  88   exit(EXIT_SUCCESS);
  89   return 0; /* avoid warning */
  90 }