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module DiscreteDistribution
  
  # pick_random is useful for spliting up a collection by its specified discrete distribution
  # def test(runtime=1000, pdf=[0.5, 0.15, 0.35])
  #   sum = pdf.inject(0){|fraction, sum| sum += fraction}
  #   raise "cdf = #{sum}; it doesn't add up to 1; delta: #{sum > 1 ? sum-1 : 1-sum }" if sum != 1
  #   histo = [0] * pdf.size
  #   collection = [] 
  #   pdf.each_with_index{|fraction, i| collection << [i, fraction]}
  #   runtime.times do 
  #     e = DiscreteDistribution.pick_random( collection, :last )
  #     histo[e.first] += 1
  #   end
  #   runtime = runtime.to_f
  #   histo.map{ |count| count / runtime }
  # end
  # test(100000, [0.1, 0.2, 0.124, 0.1235, 0.234, 0.2, 0.0185]) => [0.09967, 0.2015, 0.12334, 0.12323, 0.23321, 0.2008, 0.01825]
  
  def self.pick_random(collection, pdf_method=:to_f)
    rand_fraction = rand
    sum = prev = 0
    collection.each { |element|
      return element if prev <= rand_fraction && rand_fraction < (sum += element.send(pdf_method) )
      prev = sum
    }
  end
end