lottery.rb

# file spec/libs/lottery_spec.rb
require 'spec_helper'

RSpec.describe 'lib/lottery' do
  describe 'Lottery.pick' do
    let(:characters) { create_list(:character, 2) }
    let(:num_samples) { 10000 }
    subject do
      num_chosen = 0
      num_samples.times do
        num_chosen += 1 if Lottery.pick(characters) == characters.first
      end
      { num_chosen: num_chosen, real_probability: Lottery.probabilities(characters).first }
    end

    it 'returns a model with probability within the tolerance interval' do
      n = num_samples
      p = subject[:real_probability]
      q = 1.0 - p

      # 3.89 = inverse of normal distribution function with alpha=0.9999
      expect(subject[:num_chosen]).to(
        be_within( 3.89 * Math.sqrt(n * p * q) ).of(n * p),
        'this test will fail once in 10000 times. don\'t freak out if it does, you\'re lucky :)')
    end
  end
end

# file lib/lottery.rb
class Lottery
  def self.pick(models, weight_key = :weight)
    weight_total = models.to_a.sum(&weight_key)
    random_number = rand(0...weight_total)
    sum = 0
    models.each do |model|
      return model if random_number < sum += model[weight_key]
    end
  end

  def self.probabilities(models, weight_key = :weight)
    weight_total = models.to_a.sum(&weight_key)
    models.map { |model| model[weight_key].to_f / weight_total.to_f }
  end
end