Re: Dr. Dobbs Dev. Update 1/5 July 94 & Schneier

[Eli Brandt <ebrandt@jarthur.cs.hmc.edu>]

> From: tim werner <werner@mc.ab.com>
> The loop was really a waste when you consider that it could
> have been done in 1 instruction.

You can do better than a bit-serial loop -- though not down to
one instruction!  There are a lot of very cool approaches, only
one of which I remember.

Look at the problem as that of finding the sum of n 1-bit blocks.
Well, we can easily find the sum of a single n-bit block.  The
intermediate conversions are the magic part.

Let's look at an 8-bit word.  How shall we get, for example, from a
sum of 4 2-bit blocks to a sum of 2 4-bit blocks?  What we do is add
adjacent blocks.  The block-pair sums will actually fit in three
bits, so they'll certainly fit in four without overflowing.  And all
of this can be done bit-parallel using logic ops.

In C, this looks like:

int byte_ones(int a)
// hope this is correct...
{
	a = (a & 0x55) + (a & 0xAA)/2;		// 0x55 == 01010101b
	a = (a & 0x33) + (a & 0xCC)/4;		// 0x33 == 00110011b
	a = (a & 0x0F) + (a & 0xF0)/16;		// 0x0F == 00001111b
	return  a;
}

Oh, and one AND in the third line is superfluous.  This is not the
fastest algorithm for this, but it's the only one I understand and
remember.

   Eli   ebrandt@hmc.edu
(I won't ask why you needed a one-hot encoding in the first place...)