class Urn

  def initialize(n_balls=2,black_proportion=0.5)
    @black = n_balls * black_proportion
    @white = n_balls * ( 1 - black_proportion )
  end

  def draw_balls(n_draws=1)
    n_draws.times {
      prob_black = proportion
      rand < prob_black ? @black += 1 : @white += 1
      # The statement above is equivalent to:
      # if rand < prob_black
      #   @black += 1
      # else
      #   @while += 1
      # end
    }
  end

  def proportion
    return @black.to_f / ( @black + @white )
  end

  # n_balls   = Number of balls initially in the urn
  # theta     = Initial proportion of black balls
  # n_draws   = Number of times to draw balls
  # precision = Number of test statistics to collect
  def Urn.sample(n_balls,theta,n_draws,precision=1000)
    draws = Array.new(precision)
    for i in 0...precision
      urn = Urn.new(n_balls,theta)
      urn.draw_balls(n_draws)
      draws[i] = urn.proportion
    end
    draws
  end

end



# The statement "__FILE__ == $0" is explained on page 337 of the pickaxe book.
if __FILE__ == $0

  theta = 0.5
  draws = Urn.sample(10,theta,100,10_000)

  observed_test_statistic = 0.60
  tail_count = 0
  for i in 0...(draws.length)
    tail_count += ( draws[i] >= observed_test_statistic ? 1 : 0 )
  end
  p_value = tail_count.to_f / draws.length
  puts "Ho: \\theta = #{theta}"
  puts "Test statistic is #{observed_test_statistic}"
  puts "p-value is #{p_value}"

end