promotions.rb

require 'minitest/autorun'
# Money are in floats for simplicity, it should be bigint
# Promotions are prioritized in order [A,B] => A rule is applied first, B rule is applied second

class Item
  attr_accessor :name, :price, :product_code

  def initialize(name, price, product_code)
    @name = name
    @price = price
    @product_code = product_code
  end

end

class OverLimitPromoRule

  attr_reader :limit

  def initialize(limit = 60)
    @limit = limit
  end

  attr_accessor :cart

  def discount(total, cart)
    @cart = cart
    if total > @limit
      return total * 0.1
    end
  end

end


class TwoOrMorePromoRule

  attr_accessor :cart, :product_code, :unit_discount

  def initialize(product_code, unit_discount)
    @product_code = product_code
    @unit_discount = unit_discount
  end

  def discount(total, cart)
    promo_items = cart.select {|item| item.product_code == @product_code }
    if promo_items.size > 1
      return promo_items.size * @unit_discount
    end
  end

end


class Checkout

  attr_reader :promo_rules, :cart

  def initialize(promo_rules)
    @cart = []
    @promo_rules = promo_rules
  end

  def scan(item)
    @cart << item
  end

  def total
    raw_total = @cart.inject(0){|acc, item| acc += item.price }
    total = @promo_rules.inject(raw_total){|total, rule| total -= rule.discount(total, @cart).to_f }.round(2)
  end

end


describe Item do

  it "has name, price and product code" do
    [:name, :price, :product_code].each do |field|
      Item.new("Travel Card Holder", 9.25, "001" ).must_respond_to field
    end
  end

end

describe OverLimitPromoRule do

  it "can produce discount for a cart" do
    cart = [
      Item.new("Travel Card Holder", 40.0, "001"),
      Item.new("Travel Card Holder Holder", 60.0, "002")
    ]
    OverLimitPromoRule.new.discount(100, cart).must_equal 10.0
  end

  it "don't give any discount if you are below limit" do
    cart = [
      Item.new("Travel Card Holder", 4.0, "001"),
      Item.new("Travel Card Holder Holder", 6.0, "002")
    ]
    OverLimitPromoRule.new.discount(10, cart).must_be_nil
  end

end

describe TwoOrMorePromoRule do

  it "can produce discount for a cart" do
    cart = [
      Item.new("Travel Card Holder", 9.25, "001"),
      Item.new("Travel Card Holder", 9.25, "001"),
      Item.new("Travel Card Holder Holder", 60.0, "002")
    ]
    TwoOrMorePromoRule.new("001", 0.75).discount(100, cart).must_equal 1.50
  end

  it "don't give any discount if you are below limit" do
    cart = [
      Item.new("Travel Card Holder", 4.0, "001"),
      Item.new("Travel Card Holder Holder", 6.0, "002")
    ]
    TwoOrMorePromoRule.new("001", 0.75).discount(100, cart).must_be_nil
  end

end


describe Checkout do
  before do
    @promotional_rules = [
      TwoOrMorePromoRule.new("001", 0.75),
      OverLimitPromoRule.new
    ]
    @item1 = Item.new("Travel Card Holder", 9.25,      "001")
    @item2 = Item.new("Personalised cufflinks", 45.00, "002")
    @item3 = Item.new("Kids T-shirt", 19.95,           "003")
    @co = Checkout.new(@promotional_rules)
  end

  describe "it can checkout!" do
    it "must not explode and give you back value" do
      @co.scan(@item1)
      @co.scan(@item2)
      price = @co.total
      price.wont_be_nil
    end

    it "must give back for 66.78 basket 001,002,003" do
      @co.scan(@item1)
      @co.scan(@item2)
      @co.scan(@item3)
      @co.total.must_equal 66.78
    end

    it "must give back for 36.95 basket 001,003,001" do
      @co.scan(@item1)
      @co.scan(@item3)
      @co.scan(@item1)
      @co.total.must_equal 36.95
    end

    it "must give back for 73.76 basket 001,002,001,003" do
      @co.scan(@item1)
      @co.scan(@item2)
      @co.scan(@item1)
      @co.scan(@item3)
      @co.total.must_equal 73.76
    end

  end

end