Path: utzoo!utgpu!attcan!uunet!steinmetz!davidsen
From: davidsen@steinmetz.ge.com (William E. Davidsen Jr)
Newsgroups: comp.binaries.ibm.pc.d
Subject: Re: SIMTEL20 to ban ARC files
Keywords: lzw, atob/btoa, 7 bit pure
Message-ID: <12266@steinmetz.ge.com>
Date: 29 Sep 88 18:14:48 GMT
References:  <12229@steinmetz.ge.com> <1221@ndmath.UUCP>
Reply-To: davidsen@crdos1.UUCP (bill davidsen)
Organization: General Electric CRD, Schenectady, NY
Lines: 39

In article <1221@ndmath.UUCP> cww@ndmath.UUCP (Clarence W. Wilkerson) writes:
| 
| I think the comparisons offered by Bill Davidsen are most
| indicative of the effect of 16 bit compress compresses on
| Unix system. With a 12 bit compress, I don't think you would
| see such a large difference.

  A good point, but... I tried creating a zoo archive using the "no
compression" flag, then compressing it. Same order of magnitude of
results, compressed with arc, zoo, 12 or 16 bit compress. The results
were all the same, but if the archive being compressed had a relatively
small number of discrete tokens the 12 bit compress was actually
smaller by 8 bytes.

  As a further test I created a file holding an "ls -lR" listing of a
small subdirectory, then catted three copies into a 2nd file, and eight
copies of the 2nd file into a 3rd. I then compressed the files with zoo,
and here are the results.


Archive foo.zoo:
Length    CF  Size Now  Date      Time
--------  --- --------  --------- --------
    3619  57%     1543  29 Sep 88 14:10:16     x
   10857  68%     3457  29 Sep 88 14:10:16     y
   86856  79%    18601  29 Sep 88 14:10:18     z
--------  --- --------  --------- --------
  101332  77%    23601     3 files


  The reason the compression gets better on the same data is that the
LVW algorithm "learns" about the data, and therefore does a better job
as long as the data are similar. Using more bits only makes a big
difference when a LOT of data is being processed, and the number of
tokens to be remembered becomes larger than will fit in 12 bits.
-- 
	bill davidsen		(wedu@ge-crd.arpa)
  {uunet | philabs}!steinmetz!crdos1!davidsen
"Stupidity, like virtue, is its own reward" -me