1 files changed, 90 insertions, 0 deletions
diff --git a/sample/sub.c b/sample/sub.c
new file mode 100644
index 0000000..a06cc9b
--- /dev/null
+++ b/sample/sub.c
@@ -0,0 +1,90 @@
+/* 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 <config.h>
+#include <stdio.h>
+#include <stdlib.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(EXIT_FAILURE);
+  }
+  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(EXIT_FAILURE);
+  }
+
+  /* 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(EXIT_SUCCESS);
+  return 0; /* avoid warning */
+}