_          _         _
                            ( )_       ( )     _ ( )_
   ___    _    _   _   ___  | ,_)      | |_   (_)| ,_)  ___
 /'___) /'_`\ ( ) ( )/' _ `\| |        | '_`\ | || |  /',__)
( (___ ( (_) )| (_) || ( ) || |_       | |_) )| || |_ \__, \
`\____)`\___/'`\___/'(_) (_)`\__)      (_,__/'(_)`\__)(____/
                                 ______
                                (______)

Why?

Because it's fast. https://gist.github.com/knugie/3865903

How?

$ git clone git@github.com:knugie/count_bits.git
$ cd count_bits
$ gem build count_bits.gemspec
$ gem install count_bits-0.1.4.gem
$ irb
:001 > require 'count_bits'
=> true
:002 > count_bits(1234567890)
=> 12