Finding MSB in bit string

[Guido van Rossum]

In article <11053 at cca.UUCP> g-rh at cca.UUCP (Richard Harter) writes:
>Oh yes, what is the MSB of 0?

This should cause an exception like 1/0 does.  If msb(x) is defined as
floor(2 log x), then it isn't defined for x==0.

>Here's another one -- suppose you want the number of 1's in a bit string.
>What's a good general method?  How would a real programmer do it?

This has been discussed in this group a while ago.  It goes something like

	return count[x&0xff] + count[(x>>8)&0xff] +
		count[(x>>16)&0xff] + count[(x>>32)&0xff;

where count is a 256-byte array of precomputed values.  Of course you
could use a smaller array and add more values, possibly in a loop.

(I hope this time I didn't forget some offsets like in my solution of
the MSB problem.)