Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!mit-eddie!ll-xn!husc6!cmcl2!adm!smoke!gwyn
From: gwyn@smoke.BRL.MIL (Doug Gwyn )
Newsgroups: comp.unix.questions
Subject: Re: Bit Switching - How?
Keywords: bits, flip
Message-ID: <9977@smoke.BRL.MIL>
Date: 4 Apr 89 08:40:57 GMT
References: <1138@uvm-gen.UUCP>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 20

In article <1138@uvm-gen.UUCP> wirthlin@uvm-gen.UUCP (Ralph Wirthlin) writes:
>Does anyone know of a *very fast* way to swap bits at some location n 
>between two separate bytes b1 and b2?

The answer must of course depend on architectural and language implementation
details.  One very fast way is to directly index the result in a table a la
	b1_swapped = b_table[b1][b2][offset];
	b2_swapped = b_table[b2][b1][offset];
where of course the tables have been preinitialized with the correct answers.

If you don't have space for such a large table (512Kbytes, assuming 8-bit
bytes) then a compromise solution that generally takes longer but uses a
tiny table is
	b1_bit = b1 & mask[offset];
	b2_bit = b2 & mask[offset];
	b1 = b1 & ~mask[offset] | b2_bit;
	b2 = b2 & ~mask[offset] | b1_bit;
with suitable optimization.  If your processor has a very speedy shift
operator then replace mask[offset] with (1<<offset), but first check
that 1<<0 works (I once ran into a processor where it didn't).