allergies.rb

# Example usage:
#
# pry(main)> o = Allergies.new(0)
#=> #<Allergies:0x007f8a67b88190 @allergy_list=[]>
#[3] pry(main)> o.list
#=> []
#[4] pry(main)> o = Allergies.new(6)
#=> #<Allergies:0x007f8a660210a0 @allergy_list=[4, 2]>
#[5] pry(main)> o.allergic_to?('shellfish')
#=> true
#[6] pry(main)> o.allergic_to?('work')
#=> false
#[7] pry(main)> o.list
#=> ["peanuts", "shellfish"]
#
class Allergies
  ALLERGY_TABLE = {
    'eggs' => 1,
    'peanuts' => 2,
    'shellfish' => 4,
    'strawberries' => 8,
    'tomatoes' => 16,
    'chocolate' => 32,
    'pollen' => 64,
    'cats' => 128
  }

  def initialize(allergy_score)
    @allergy_list = get_allergy_list(allergy_score)
  end

  def allergic_to?(item)
    (ALLERGY_TABLE.keys & list).include?(item)
  end

  def list
    @allergy_list.reverse.sort.each_with_object([]) do |value, array|
      array.push(ALLERGY_TABLE.key(value)) if ALLERGY_TABLE.key(value)
    end
  end

  private

  def get_allergy_list(score)
    results = []
    return results if score.zero?
    case power_of_two?(score)
    when true
      results.push(score)
    else
      get_remainder_and_other(score).each do |number|
        get_allergy_list(number).each { |result| results.push(result) }
      end
      results
    end
    results
  end

  def power_of_two?(number)
    number & (number - 1) == 0
  end

  def get_remainder_and_other(number)
    remainder = (number & (number - 1))
    other = number - remainder
    [remainder, other]
  end
end