book_orderer.rb

class BookOrderer
  attr_accessor :copies, :price

  DEFAULT_PRICE = 99.freeze

  def initialize(args={})
    @copies = args[:copies].to_i
    @price = (args[:price].to_i.zero?) ? DEFAULT_PRICE : args[:price].to_i
  end

  def total_price
    raise "you can't sell no copies, stupid."  if copies < 1
    (price * copies) * discount
  end

  def discount_percent
    100 - (discount * 100).to_i
  end

  private

  def discount
    DISCOUNTS.select { |range| range === copies }.values.first || DISCOUNTS[:max]
  end

  DISCOUNTS = {
    0...10 => 1,
    10...20=> 0.8,
    20...50 => 0.7,
    50...100 => 0.6,
    :max => 0.5
  }

end

while true do
  puts "How many copies would you like to order?"
  copies = gets.chomp.to_i
  if copies.zero?
    print "Number must be more than zero. "
  else
    break
  end
end

puts "Enter the copy price, or hit enter to use the default price of #{BookOrderer::DEFAULT_PRICE}: "
price = gets.chomp.to_i

order = BookOrderer.new({copies: copies, price: price})
puts "Total price for #{order.copies} copies at $#{order.price} each"
puts "   with #{order.discount_percent}% discount = $#{order.total_price}"