To run tests type: rspec

To run console type: bin/console

The app is the implementation of Cabify's Ruby coding exercise: https://gist.github.com/patriciagao/377dca8920ba3b1fc8da

Here you can find Elixir implementation of the same exercise.

Those examples:

Items: VOUCHER, TSHIRT, MUG
Total: 32.50€

Items: VOUCHER, TSHIRT, VOUCHER
Total: 25.00€

Items: TSHIRT, TSHIRT, TSHIRT, VOUCHER, TSHIRT
Total: 81.00€

Items: VOUCHER, TSHIRT, VOUCHER, VOUCHER, MUG, TSHIRT, TSHIRT
Total: 74.50€

are included in spec/integration/checkout_spec.rb

Code guidelines

Checkout interface

Object of Checkout class is the interface for #scan and #total actions.

co = Checkout.new
co.scan('VOUCHER')
co.total

Scan operation

Scan pipelines operations. Each operation responds to #call and takes as an input CheckoutStruct object and outputs new CheckoutStruct object. It's implemented in such manner, that it can easily be wrapped in a computational context (Either monad) for some neat error handling. For example tt can be refactored with ease into dry-transaction.

Pricing rules

Pricing rules are auto injected into Checkout as a dependency from application Container. They can also be passed explicitly:

my_pricing_rules = [
  Class.new(BaseRule) do
    def calculate_items(items)
      items
    end
  end
]
Checkout.new(pricing_rules: my_pricing_rules)

Defining new pricing rules

To add a new rule simply:

1. Define class inheriting from BaseRule that implements #calculate_items, which takes as input Array<Checkout::ItemStruct> and requires the same as output.
2. Include new rule in PricingRules::Manifest::ACTIVE.

Pricing rules preferences

PricingRuleRepo is the interface to pricing_rules ROM relation. It's resolved from app container on :pricing_rule_repo key. PricingRules::Manifest::KLASS_TO_CODE_MAP binds rule classes with persisted rule code.

Stubbing rules

Rules can also be easily stubbed in tests:

require 'dry/container/stub'
Container.enable_stubs!
Container.stub(:pricing_rules, [->(checkout_struct) { checkout_struct }])

Bulk purchase

This rule depends on code, discount and quantity preferences. For all checkout items within code scope, if their quantity is equal or greater than quantity preference, then for each item preferred discount is applied. For different type of items specific required volume and discount can be applied.

Buy X get one free

This rule depends on code and quantity preferences. Quantity preference defines for how many items of the same type buyer gets the same item for free. For different types of items, specific quantity can be required to fulfill promotion. E.g. you can set that for 3 mugs you get one free, and for 4 t-shirts you get one for free.

Persistance layer

In the real life app this would probably be some SQL RDBMS. Here we have simple YAML file (db/cabify_db.yml). What is important is that interface to the data is completely decoupled from the persistance layer, so it can be replaced with different storage. ROM library is used for mapping between stored data and Ruby objects.

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