Birthday Paradox

birthday_paradox.rb

class Bdays
  attr_reader :birthdates

  def initialize n
    @birthdates = Array.new(n) { [*0..365].sample }.map do |s|
      birthday = Time.new(0) + s * 60 * 60 * 24
      [birthday.month, birthday.day]
    end

    shared_birthdates
  end

  def shared_birthdates
    @shared ||= @birthdates.each_with_object(Hash.new 0) do |date, result|
      result[date] += 1 if @birthdates.count(date) > 1
    end
  end
end

bdays = Bdays.new 50
bdays.shared_birthdates