Solution to http://codekata.pragprog.com/2007/01/kata_nine_back_.html

kata9.rb

require 'spec'
require 'active_support'

class Rule < Struct.new(:quantity, :price)
  def apply(number)
    [ number / quantity * price, number % quantity ]
  end
end

class CheckOut < Struct.new(:rules, :items)
  def initialize(rules)
    super(rules, Hash.new)
  end

  def scan(item)
    items[item] ||= 0
    items[item] += 1
  end

  def total
    items.inject(0) { |acc, (item, quantity)|
      acc += total_for(quantity, item)
    }
  end

  private
  def total_for(quantity, item)
    remaining = quantity
    rules[item].inject(0) do |acc, rule|
      subtotal, remaining = rule.apply(remaining)
      acc += subtotal
    end
  end
end

describe CheckOut do
  subject do
    rules = {
      "A" => [ Rule.new(4, 140), Rule.new(3, 130), Rule.new(1, 50) ],
      "B" => [ Rule.new(2, 45), Rule.new(1, 30) ],
      "C" => [ Rule.new(1, 20) ],
      "D" => [ Rule.new(1, 15) ]
    }
    described_class.new(rules)
  end

  describe '#total' do
    { ""       => 0,
      "A"      => 50,
      "AB"     => 80,
      "CDBA"   => 115,
      "AA"     => 100,
      "AAA"    => 130,
      "AAAA"   => 140,
      "AAAAA"  => 190,
      "AAAAAA" => 240,
      "AAAB"   => 160,
      "AAABB"  => 175,
      "AAABBD" => 190,
      "DABABA" => 190
    }.each do |items, total|
      context "after scanning #{items.inspect}" do
        it "returns #{total}" do
          items.split(//).each { |i| subject.scan(i) }
          subject.total.should == total
        end
      end
    end

    it "changes after scanning an item" do
      subject.total.should == 0
      subject.scan("A")
      subject.total.should == 50
      subject.scan("B")
      subject.total.should == 80
      subject.scan("A")
      subject.total.should == 130
      subject.scan("A")
      subject.total.should == 160
      subject.scan("B")
      subject.total.should == 175
    end
  end
end