Beefy Boxes and Bandwidth Generously Provided by pair Networks
P is for Practical

Re: Efficient bit counting with a twist.

by quester (Vicar)
on Jan 27, 2013 at 07:59 UTC ( #1015566=note: print w/replies, xml ) Need Help??

in reply to Efficient bit counting with a twist.

Build a bit mask for the byte that contains the position. Then count the bits in the bytes before the position, and do a bitwise and for the byte that contains the position.

my $mask = pack "V", 2 ** ($p % 8) - 1; my $c = unpack( '%32b*', substr( $v, 0, $p / 8 ) ) + unpack( '%32b*', substr( $v, $p / 8, 1 ) & $mask );

(Update: removed a superfluous pair of parens in the last line.)

Replies are listed 'Best First'.
Re^2: Efficient bit counting with a twist.
by BrowserUk (Pope) on Jan 27, 2013 at 08:33 UTC

    Why pack 'V'?

    substr( $v, $p / 8, 1 ) is a byte, why mask with 4 bytes?

    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
    div class=Why
      Oh. That. It's just force of habit, I adapted this bit of code from the way I code bitmasks for IPv4 subnets. It would make more sense to code it as pack 'C',... rather than pack 'V', although it works either way.

Log In?

What's my password?
Create A New User
Node Status?
node history
Node Type: note [id://1015566]
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others making s'mores by the fire in the courtyard of the Monastery: (4)
As of 2020-06-05 03:57 GMT
Find Nodes?
    Voting Booth?
    Do you really want to know if there is extraterrestrial life?

    Results (35 votes). Check out past polls.