Crossover chalange sort

sort.rb

def rearrange(elements)
  number_of_ones = Hash.new {|h, n|  h[n] = n.to_s(2).each_char.count('1')}

  elements.map(&:to_i).uniq.sort do |a,b|
    noo_a = number_of_ones[a]
    noo_b = number_of_ones[b]
    if noo_a==noo_b
      a<=>b
    else
      noo_a<=>noo_b
    end
  end
end

Originally second line was like:

number_of_ones = ->(n){n.to_s(2).each_char.count('1')}

but I had lot of spare time so I've added cashing in hash