Making use of ruby-2.0 syntax to test weighted rule selection

def weighted_random(rules)
  total = rules.values.reduce(&:+)
  srand
  chance = rand(0..total)
  rules.each do |item, weight|
    return item unless chance > weight
    chance -= weight
  end
end

symbols = %i(bird fish turtle)        # ruby 2.0
weights = [0.5, 0.4, 0.1]             # for convenience sum = 1.0
rules = symbols.zip(weights).to_h     # ruby 2.0

count_bird = 0
count_fish = 0
count_turtle = 0

10_000.times do
  case weighted_random(rules)
  when :bird
      count_bird += 1.0
  when :fish
      count_fish += 1.0
  when :turtle
      count_turtle += 1.0
  end
end

puts format('bird   %0.4f', count_bird / 10_000)
puts format('fish   %0.4f', count_fish / 10_000)
puts format('turtle %0.4f', count_turtle / 10_000)

Result

bird   0.5001
fish   0.3976
turtle 0.1023