reversing a mask

[Eryk Vershen]

Apropos of nothing here is a cute way to reverse bits due to C. Strachey.

int mask[] = {
	0x55555555,
	0x33333333,
	0x0F0F0F0F,
	0x00FF00FF,
	0x0000FFFF
}

long
bitrev(val)
long val;
{
	register long i;
	register int j ,k;

	i = val;
	j = 16;
	k = 4;
	while (j) {
		i = ((i & mask[k]) << j) | ((i >> j) & mask[k]);
		j >>= 1;
		k--;
	}
	return(i);
}

/*
 * Proof of the above (by example).
 *
 *  j=16  i=abcdefghijklmnopqrstuvwxyz012345
 *     8    qrstuvwxyz012345abcdefghijklmnop
 *     4    yz012345qrstuvwxijklmnopabcdefgh
 *     2    2345yz01uvwxqrstmnopijklefghabcd
 *     1    452301yzwxuvstqropmnklijghefcdab
 *     0    543210zyxwvutsrqponmlkjihgfedcba
 */
--
					- eryk vershen
					ucbvax!unisoft!eryk
					"overcoming computer literacy"