class String
  def integer? 
    return true if self =~ /^[1-9]\d*(\.\d+)?$/
    false
  end
end

module Tippy

  class Builder
    def initialize total:, gratuity:
      @total = total
      @gratuity = gratuity
    end

    def generate
      return calculation if number_based?
      string_based
    end

    def number_based?
      (@gratuity.is_a? Numeric) || (@gratuity.integer?)
    end

    def string_based
      case @gratuity.downcase
      when 'high'     then calculation 25
      when 'standard' then calculation 18
      when 'low'      then calculation 15
      end
    end

    def calculation gratuity = @gratuity
      @total += @total * (gratuity.to_f / 100)
    end
  end

end