Re: Efficient bit counting with a twist.

[quester]

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.)

[BrowserUk]

Why pack 'V'?

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

div class=Why

[quester]

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.