Count 1 bits in integer (Ruby)

count_ones.rb

# protip: use Ruby 1.9 for block support with Enumerable#count

def count_ones(num)
  return 0 if num == 0
  digits = Math.log2(num).floor + 1
  0.upto(digits).count do |i|
    num & (2 ** i) == (2 ** i)
  end
end