Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!zaphod.mps.ohio-state.edu!usc!apple!portal!motcad!jtc
From: jtc@motcad.portal.com (J.T. Conklin)
Newsgroups: comp.lang.c
Subject: Re: How to reverse bits...
Message-ID: <1990Aug14.203212.16248@motcad.portal.com>
Date: 14 Aug 90 20:32:12 GMT
References: <2059@ux.acs.umn.edu>
Organization: CADnet Inc., San Ramon CA
Lines: 28

In article <2059@ux.acs.umn.edu> dhoyt@vw.acs.umn.edu writes:
>In article <1990Aug13.185757.3236@sti.fi>, ttl@sti.fi (Timo Lehtinen) writes...
>>This might be trivial, but here goes...
>>What's the most optimal way to reverse the bits in an unsigned char,
>>i.e. change from MSB to LSB ordering ?
>
>  You don't have to change anything.  Big and little endians only cause
>trouble when you look at different integer formats.  Bytes is bytes, as they
>say.

The question was how to flip the bits in an unsigned char, not a bytes
in a word.  Like has been indicated in previous postings, a 256 element
lookup table is the best way to perform this task.

For example, most fax machines/modems present the bits in lsb-to-msb bit
order.  If your G3 decompression code was written for msb-to-lsb order,
bit reversal is needed.

Another useful table is one that flips the bits 1-for-0,0-for-1.  This
is useful for inverting an image.

	--jtc

-- 
J.T. Conklin	UniFax Communications, Inc.

		CADnet Inc, San Ramon California
		jtc@motcad.portal.com, ...!portal!motcad!jtc