lottery.rb

module Lottery
  class << self
    def draw(probablity_rates, upper_limit = 10000)
      chosen_value = rand(0...upper_limit)
      probablity_rates.find_index{|probablity_rate|
        chosen_value -= probablity_rate
        chosen_value < 0
      }
    end
  end
end

lotterycheck.rb

rates = [1000, 2000, 3000, 4000]

result = {}

10000.times do
  index = Lottery.draw(rates, rates.inject(:+))
  result[index] ||= 0
  result[index] += 1
end

result.keys.sort.each do |k|
  puts "#{k}, #{result[k]}"
end

__END__
0, 984
1, 1966
2, 3041
3, 4009

Prelude> import Data.List
Prelude> let rates = [1000, 2000, 3000, 4000]
Prelude> let chosen_value = 2222
Prelude> findIndex (> chosen_value) $ scanl1 (+) rates
#=> Just 1