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Re: counting bits [message #34215 is a reply to message #34069]

Thu, 20 February 2003 07:24

JD Smith
Messages: 850
Registered: December 1999

Senior Member

On Wed, 19 Feb 2003 22:31:32 -0700, Craig Markwardt wrote:

> JD Smith <jdsmith@as.arizona.edu> writes:
>>
>> Here's a reasonably fast implementation of your proposed method for
>> arrays of unsigned longs, to count the highest bit (or rather, the
>> number of leading 0 bits).
>>
>> function leading_zeroes_reg,num
>> num=[num]
>> zeroes=make_array(/BYTE,DIMENSION=size(num,/DIMENSIONS),VALU E=255b)
>> for i=0,31 do begin
>> shft=ishft(num,-(31-i)) AND 1
>> zeroes=(zeroes ne 255b)*zeroes+(zeroes eq 255b)* $
>> ((shft eq 1)*i+(shft ne 1)*255b)
>> endfor
>> return, (zeroes eq 255b)*32+(zeroes ne 255b)*zeroes
>> end
>
> Thanks. Thinking about it further, one could probably do a pretty fast
> look up table on a byte-by-byte basis, similar to the counting-bits
> lookup that you presented initially.
>

Yes, but it wouldn't have the street cred of '7dcd629'XUL.

JD

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[Message index]

		Re: counting bits By: Craig Markwardt on Wed, 19 February 2003 21:31
		Re: counting bits By: Dick Jackson on Wed, 19 February 2003 07:47
		Re: counting bits By: Craig Markwardt on Tue, 18 February 2003 15:10
		Re: counting bits By: JD Smith on Tue, 18 February 2003 14:58
		Re: counting bits By: David Fanning on Tue, 18 February 2003 13:55
		Re: counting bits By: Rick Towler on Tue, 18 February 2003 13:27
		Re: counting bits By: JD Smith on Tue, 18 February 2003 11:56
		Re: counting bits By: eddie haskell on Tue, 18 February 2003 08:10
		Re: counting bits By: David Fanning on Mon, 17 February 2003 21:09
		Re: counting bits By: Craig Markwardt on Mon, 17 February 2003 20:45
		Re: counting bits By: JD Smith on Mon, 17 February 2003 15:54
		Re: counting bits By: David Fanning on Mon, 17 February 2003 12:41
		Re: counting bits By: David Fanning on Mon, 17 February 2003 12:38
		Re: counting bits By: JD Smith on Thu, 20 February 2003 07:43
		Re: counting bits By: JD Smith on Thu, 20 February 2003 07:24

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