http://stackoverflow.com/q/11832196/6962

claudiu.rb

# http://stackoverflow.com/a/11832301/6962
# Translated from Python by Claudiu.

$tokens_in_slots = [0, 0, 0]
$slot_distributions = [0.5, 0.3, 0.2]

def pick_slot
  sum_tokens = $tokens_in_slots.inject(:+)
  if sum_tokens.zero?
    slot = rand($tokens_in_slots.length)
  else
    expected_tokens = $slot_distributions.map { |d| d * sum_tokens }
    errors = expected_tokens.zip($tokens_in_slots).map { |e, a| e - a }
    slot = errors.index(errors.max)
  end
  $tokens_in_slots[slot] += 1
end

100.times do
  pick_slot
  puts format("%s: %s", $tokens_in_slots.inject(:+), $tokens_in_slots)
end

claudiu_objectified.rb

class Picker
  def initialize(distributions)
    @distributions = distributions
    @tokens_in_slots = Array.new(@distributions.length, 0)
  end

  def pick_slot
    @tokens_in_slots[next_slot] += 1
  end

  def tokens_in_slots
    @tokens_in_slots
  end

  def total_tokens
    @tokens_in_slots.inject(:+)
  end

  private

  def next_slot
    sum_tokens = @tokens_in_slots.inject(:+)

    if sum_tokens.zero?
      rand(@tokens_in_slots.length)
    else
      expected_tokens = @distributions.map { |d| d * sum_tokens }
      errors = expected_tokens.zip(@tokens_in_slots).map { |e, a| e - a }
      errors.index(errors.max)
    end
  end
end

picker = Picker.new([0.5, 0.3, 0.2])

100.times do
  picker.pick_slot
  puts format("%s: %s", picker.total_tokens, picker.tokens_in_slots)
end