Re^2: Efficient bit counting with a twist.

[BrowserUk]

in reply to Re: Efficient bit counting with a twist.
in thread Efficient bit counting with a twist.

Using it to count set bits in the whole bytes before your position and then, if necessary, those bits in the partial byte up to but not including it via a mask might be viable.

Yes. That works and is the same scheme questor came up with (rather more compactly:) in Re: Efficient bit counting with a twist..

But the best answer is the one AnomalousMonk pointed out in Re: Efficient bit counting with a twist..

Ie. To recognise that the unpack template '%32b*' is not a indivisible token saying 'count the bits', but actually contains 3 parts:

1. %32 accumulate a 32-bit count ...
2. b of the binary bits set ...
3. * for all the bits in the string.

And that by simply interpolating $p into the template, in place of *, it counts the set bits within the first $p bits of the string.

Perl had the problem solved; I just didn't recognise it :) (Hence my: D'oh! D'oh! D'oh!..... moment. :)

[davido]

Thanks for the explanation. I wasn't getting his solution at first until I saw your description of the steps. For those who need a little more (as I did): perlpacktut. It's funny, I'm sure we've all read that document numerous times, and yet before this I never noticed the % feature.

Update: Fixed perlunpacktut: sb perlpacktut.

[AnomalousMonk]

The summing function is discussed in the Doing Sums section of perlpacktut (not perlunpacktut, which does not exist) and in unpack, although it must be admitted that examples of counted-field summation in both these sources are a bit rare.

[BrowserUk]

It's funny, I'm sure we've all read that document numerous times, and yet before this I never noticed the % feature.

I've read it many times, and use the bit-counting form regularly; but always as '%32b*'.

As soon as I half read AnomalousMonk's post, the significance/simplicity of using a count rather than '*' became immediately obvious to me, and I physically blushed red in the realisation.

The only factor that slightly lessened my embarrassment was that I was not the only one who missed it. But only slightly :)