Supermarket

This is a solution to a technical test for a senior position at Kantox.

The original test description can be found in the file Technical evaluation Elixir.pdf.

Assignment

You are the lead programmer for a small chain of supermarkets. You are required to make a simple cashier function that adds products to a cart and displays the total price.

You have the following test products registered:

Product code	Name	Price
GR1	Green tea	£3.11
SR1	Strawberries	£5.00
CF1	Coffee	£11.23

Special conditions:

The CEO is a big fan of buy-one-get-one-free offers and of green tea. He wants us to add a rule to do this.
The COO, though, likes low prices and wants people buying strawberries to get a price discount for bulk purchases. If you buy 3 or more strawberries, the price should drop to £4.50 per strawberry.
The CTO is a coffee addict. If you buy 3 or more coffees, the price of all coffees should drop to two thirds of the original price.

Our check-out can scan items in any order, and because the CEO and COO change their minds often, it needs to be flexible regarding our pricing rules.

Implement a checkout system that fulfills these requirements.

Test data:

Basket: GR1, SR1, GR1, GR1, CF1
Total price expected: £22.45

Basket: GR1, GR1
Total price expected: £3.11

Basket: SR1, SR1, GR1, SR1
Total price expected: £16.61

Basket: GR1, CF1, SR1, CF1, CF1
Total price expected: £30.57

A solution

This repository contains a solution to the assignment above. The code is split into the following contexts:

The Supermarket.Product module defines a struct that allows representing products.
The Supermarket.Basket module defines convenience methods for working with shopping baskets. This is where a basket can be created and where products can be added to it.
The Supermarket.Pricing module defines convenience functions for calculating the price of a list of products, and allows for discounts based on customizable pricing rules.

It is with the combination of these three that we are able to fullfill the requirements of the assignment above.

Assignment compliance

The test file test/supermarket/functional_test.exs contains tests that validate the assignment's "Test data".

Overview

The source code contains documentation and typespecs. The documentation is also available online here.

We will now go through a brief introduction as to how the implemented functionality is intended to be used.

Product definition

Here is an example of a product definition.

iex> green_tea = Supermarket.Product.new("GR1", "Green tea", Money.new(311))
%Supermarket.Product{
  code: "GR1",
  name: "Green tea",
  price: %Money{amount: 311, currency: :GBP}
}

Basket management

Here is an example of creating a basket and adding products to it.

iex> Supermarket.Basket.create_basket("my-cool-basket")
{:ok, #PID<0.210.0>}

iex> basket = Supermarket.Basket.get_basket("my-cool-basket")
#PID<0.210.0>

iex> Supermarket.Basket.get_basket_products(basket)
[]

iex> Supermarket.Basket.add_product_to_basket(basket, green_tea)
:ok

iex> Supermarket.Basket.get_basket_products(basket)
[
  %Supermarket.Product{
    code: "GR1",
    name: "Green tea",
    price: %Money{amount: 311, currency: :GBP}
  }
]

Basket pricing

Here is an example of calculating the baseline price of a basket, i.e. the price with no discounts.

iex> Supermarket.Basket.get_basket_price(basket)
%Money{amount: 311, currency: :GBP}

And here is an example of applying a "Buy one get one" discount offer.

iex> pricing_rule = Supermarket.Pricing.Rule.new(Supermarket.Pricing.Rule.BuyOneGetOne, "GR1")
%Supermarket.Pricing.Rule{
  module: Supermarket.Pricing.Rule.BuyOneGetOne,
  product_code: "GR1",
  options: []
}

iex> Supermarket.Basket.get_basket_products(basket)
[
  %Supermarket.Product{
    code: "GR1",
    name: "Green tea",
    price: %Money{amount: 311, currency: :GBP}
  }
]

iex> Supermarket.Basket.get_basket_price(basket, [pricing_rule])
%Money{amount: 311, currency: :GBP}

iex> Supermarket.Basket.add_product_to_basket(basket, green_tea)
:ok

iex> Supermarket.Basket.add_product_to_basket(basket, green_tea)
:ok

iex> Supermarket.Basket.get_basket_products(basket)
[
  %Supermarket.Product{
    code: "GR1",
    name: "Green tea",
    price: %Money{amount: 311, currency: :GBP}
  },
  %Supermarket.Product{
    code: "GR1",
    name: "Green tea",
    price: %Money{amount: 311, currency: :GBP}
  },
  %Supermarket.Product{
    code: "GR1",
    name: "Green tea",
    price: %Money{amount: 311, currency: :GBP}
  }
]

iex> Supermarket.Basket.get_basket_price(basket, [pricing_rule])
%Money{amount: 622, currency: :GBP}

Pricing rules

There is a Supermarket.Pricing.Rule behaviour that can be used to implement new custom pricing rules.

At the moment of writing there are the following two built-in pricing rules:

Supermarket.Pricing.Rule.BuyOneGetOne that allows for "Buy one get one" or "2 x 1" discounts. The CEO discount is an example of this.
Supermarket.Pricing.Rule.BuySomeDiscountAll that allows for "Buy X and get a discount on all items" discounts. Both the COO and CTO discounts are an example: the first one discounts a 10% over strawberries when buying a minimum of three units, and the second one too discounts a whopping 33.33% over coffee when buying a minimum of three units.

satom99 / supermarket