gistfile1.rb

class Fairish
  def initialize(probability, unfair_low, unfair_high, min_rolls)
    @probability, @unfair_low, @unfair_high, @min_rolls = probability, unfair_low, unfair_high, min_rolls
    @rolls, @hits = 0, 0
  end

  def fire!
    hit = if @rolls >= @min_rolls && observed_probability > @unfair_high
      false
    elsif @rolls >= @min_rolls && observed_probability < @unfair_low
      true
    else
      rand <= @probability
    end
    @hits += 1 if hit
    @rolls += 1
    return hit
  end

private
  def observed_probability
    @hits.to_f / @rolls
  end
end

class Random
  def initialize(probability)
    @probability = probability
  end

  def fire!
    rand <= @probability
  end
end

trials = 100000
trial_size = 10
random_out_of_bounds_low = 0
random_out_of_bounds_high = 0
fairish_out_of_bounds_low = 0
fairish_out_of_bounds_high = 0

probability = 0.2
lower_bound = 0.1
upper_bound = 0.4
min_rolls = trial_size / 2

trials.times do |n|
  fairish = Fairish.new probability, lower_bound, upper_bound, min_rolls
  fairish_hits = 0
  random = Random.new probability
  random_hits = 0
  
  trial_size.times do |x|
    fairish_hits += 1 if fairish.fire!
    random_hits += 1 if random.fire!
  end

  fairish_out_of_bounds_low += 1 if fairish_hits.to_f / trial_size  < lower_bound
  fairish_out_of_bounds_high += 1 if fairish_hits.to_f / trial_size  > upper_bound
  random_out_of_bounds_low += 1 if random_hits.to_f / trial_size  < lower_bound
  random_out_of_bounds_high += 1 if random_hits.to_f / trial_size  > upper_bound
end

puts "Fairish was out of bounds #{(fairish_out_of_bounds_low + fairish_out_of_bounds_high).to_f/trials * 100}% of the time (#{fairish_out_of_bounds_low.to_f/trials * 100}% low and #{fairish_out_of_bounds_high.to_f/trials * 100}% high)."
puts "Random was out of bounds #{(random_out_of_bounds_low + random_out_of_bounds_high).to_f/trials * 100}% of the time (#{random_out_of_bounds_low.to_f/trials * 100}% low and #{random_out_of_bounds_high.to_f/trials * 100}% high)."