Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.2 9/12/84; site desint.UUCP
Path: utzoo!watmath!clyde!bonnie!akgua!sdcsvax!sdcrdcf!trwrb!desint!geoff
From: geoff@desint.UUCP (Geoff Kuenning)
Newsgroups: net.lang.c
Subject: Re: reversing a mask
Message-ID: <242@desint.UUCP>
Date: Sun, 25-Nov-84 18:14:41 EST
Article-I.D.: desint.242
Posted: Sun Nov 25 18:14:41 1984
Date-Received: Wed, 28-Nov-84 01:41:07 EST
References: <1@imd.UUCP> <469@ncoast.UUCP> <691@gloria.UUCP>
Organization: his home computer, Manhattan Beach, CA
Lines: 24

In article <691@gloria.UUCP> colonel@gloria.UUCP (George Sicherman) writes:

>What's the fastest way to reverse a 16-bit mask in C?  In 68000 asl?
>(Maybe this should be in net.puzzle!)

Try this:

    static unsigned char revtab[256] = {0x80, 0x40, 0xC0, 0x20, 0xA0,...};

    short bitrev (mask)
	register unsigned short	mask;
	{
	return (revtab[mask & 0xFF] << 8) | revtab[mask >> 8];
	}

Note:  I haven't actually tried compiling this program.  It depends on the
compiler generating a zero-filling right-shift for unsigned shorts.  If yours
does not, you will need to use:

	return (revtab[mask & 0xFF] << 8) | revtab[(mask >> 8) & 0xFF];
-- 

	Geoff Kuenning
	...!ihnp4!trwrb!desint!geoff