89 lines
2.7 KiB
C
89 lines
2.7 KiB
C
/* sub.c not copyrighted (n) 1993 by Mark Adler */
|
|
/* version 1.1 11 Jun 1993 */
|
|
|
|
/* sub is a simple filter to preprocess a data file before compression.
|
|
It can increase compression for data whose points tend to be close to
|
|
the last point. The output is the difference of successive bytes of
|
|
the input. The add filter is used to undo what sub does. This could
|
|
be used on 8-bit sound or graphics data.
|
|
|
|
sub can also take an argument to apply this to interleaved sets of
|
|
bytes. For example, if the data are 16-bit sound samples, then you
|
|
can use "sub 2" to take differences on the low-byte stream and the
|
|
high-byte stream. (This gives nearly the same effect as subtracting
|
|
the 16-bit values, but avoids the complexities of endianess of the
|
|
data.) The concept extends to RGB image data (sub 3), 16-bit stereo
|
|
data (sub 4), floating point data (sub 4 or sub 8), etc.
|
|
|
|
add takes no options, since the number of interleaved byte streams
|
|
is put in the first two bytes of the output stream for add to use
|
|
(in little-endian format).
|
|
|
|
Examples:
|
|
|
|
sub < graph.vga | gzip -9 > graph.vga.sgz
|
|
sub < phone.snd | gzip -9 > phone.snd.sgz
|
|
sub 2 < audio.snd | gzip -9 > audio.snd.sgz
|
|
sub 3 < picture.rgb | gzip -9 > picture.rgb.sgz
|
|
sub 4 < stereo.snd | gzip -9 > stereo.snd.sgz
|
|
sub 8 < double.data | gzip -9 > double.data.sgz
|
|
|
|
To expand, use the reverse operation, as in:
|
|
|
|
gunzip < double.data.sgz | add > double.data
|
|
*/
|
|
|
|
#include <stdio.h>
|
|
|
|
#define MAGIC1 'S' /* sub data */
|
|
#define MAGIC2 26 /* ^Z */
|
|
#define MAX_DIST 16384
|
|
|
|
char a[MAX_DIST]; /* last byte buffer for up to MAX_DIST differences */
|
|
|
|
int main(argc, argv)
|
|
int argc;
|
|
char **argv;
|
|
{
|
|
int n = 1; /* number of differences */
|
|
int i; /* difference counter */
|
|
int c; /* byte from input */
|
|
int atoi(); /* (avoid including stdlib for portability) */
|
|
|
|
/* process arguments */
|
|
if (argc > 2)
|
|
{
|
|
fputs("sub: only one argument needed--# of differences\n", stderr);
|
|
exit(1);
|
|
}
|
|
if (argc > 1)
|
|
n = atoi(argv[1]);
|
|
|
|
if (n < 0) n = -n; /* tolerate "sub -2" */
|
|
if (n == 0 || n > MAX_DIST) {
|
|
fputs("sub: incorrect distance\n", stderr);
|
|
exit(1);
|
|
}
|
|
|
|
/* initialize last byte */
|
|
i = n;
|
|
do {
|
|
a[--i] = 0;
|
|
} while (i);
|
|
|
|
/* write differenced data */
|
|
putchar(MAGIC1); putchar(MAGIC2); /* magic word for add */
|
|
putchar(n & 0xff); /* so add knows what to do */
|
|
putchar((n>>8) & 0xff);
|
|
|
|
while ((c = getchar()) != EOF)
|
|
{
|
|
putchar((c - a[i]) & 0xff); /* write difference */
|
|
a[i++] = c; /* save last byte */
|
|
if (i == n) /* cycle on n differences */
|
|
i = 0;
|
|
}
|
|
exit(0);
|
|
return 0; /* avoid warning */
|
|
}
|