Multithread safe

multi_safe.rb

def add_range_safe
  threads = (1..256).each_slice(16).map do |range|
    Thread.new do
      Thread.current[:sum] = range.inject(0) do |memo, n|
        StatelessCalculator.add memo, n
      end
    end
  end

  result = threads.map { |t| t.join[:sum] } .reduce(:+)
end

puts add_range_safe # 32896
puts add_range_safe # 32896
puts add_range_safe # 32896