Tips for resolving the Gilded Rose with Elixir

In this text, I will show difficulties that I went through and how I solved them, unfortunately, I will not add the resolution but show some tips to resolve.

Kata

Well, first you must have seen Kata several times, but if not, I will give you a brief introduction. Kata code, are exercises that help developers to practice their skills, I recommend you look at https://github.com/gamontal/awesome-katas to find out more similar exercises.

Okay, after that brief explanation, let’s move on to the problem.

Problem

======================================Gilded Rose Requirements Specification====================================== Hi and welcome to team Gilded Rose. As you know, we are a small inn with a prime location in aprominent city ran by a friendly innkeeper named Allison. We also buy and sell only the finest goods.Unfortunately, our goods are constantly degrading in quality as they approach their sell by date. Wehave a system in place that updates our inventory for us. It was developed by a no-nonsense type namedLeeroy, who has moved on to new adventures. Your task is to add the new feature to our system so thatwe can begin selling a new category of items. First an introduction to our system: — All items have a SellIn value which denotes the number of days we have to sell the item — All items have a Quality value which denotes how valuable the item is — At the end of each day our system lowers both values for every item Pretty simple, right? Well this is where it gets interesting: — Once the sell by date has passed, Quality degrades twice as fast — The Quality of an item is never negative — “Aged Brie” actually increases in Quality the older it gets — The Quality of an item is never more than 50 — “Sulfuras”, being a legendary item, never has to be sold or decreases in Quality — “Backstage passes”, like aged brie, increases in Quality as its SellIn value approaches; Quality increases by 2 when there are 10 days or less and by 3 when there are 5 days or less but Quality drops to 0 after the concert We have recently signed a supplier of conjured items. This requires an update to our system: — “Conjured” items degrade in Quality twice as fast as normal items Feel free to make any changes to the UpdateQuality method and add any new code as long as everythingstill works correctly. However, do not alter the Item class or Items property as those belong to thegoblin in the corner who will insta-rage and one-shot you as he doesn’t believe in shared codeownership (you can make the UpdateQuality method and Items property static if you like, we’ll coverfor you). Just for clarification, an item can never have its Quality increase above 50, however “Sulfuras” is alegendary item and as such its Quality is 80 and it never alters.

After understanding the problems, I will demonstrate my steps for solving the problem.

Let’s look at the code.

After looking at the code, we can see that in a way it is simple, containing only two main files, item.ex and gilded_rose.ex, one of the requirements is not to change item.ex, so let’s focus only on gilded_rose.ex.

Where to start?

You can see the difficulty of observing that within gilded_rose.ex we have a lot of control flows, and it would be at least impracticable to try to change anything within that function before we are sure that the existing result will not be affected, with that, we will run the tests … and … to our surprise, there are no tests, what to do? In this case, we will observe the specifications in the project and create the test cases according to your need, a tip I give here is to use two approaches after you have created your tests based on the specifications and I only added it after starting the resolution:

1. Use https://github.com/parroty/excoveralls to validate that all cases are 100% tested;
2. Create a mock with the existing function to make sure that your changes do not change the existing contract.

Tip:

After creating the tests, my recommendation is, create a function with the same name as the existing function, update_quality / 1, so you can take a look at possible improvements, create a new function for different use cases, with that use lots of patterns matching, after that, tips:

1. Use a lot of @tag: skip, so you can have peace of mind when changing your tests, and they won’t break, yes, they will break many tests;
2. Update your tests, removing the cases with update_quality / 1 for the pattern matching cases that you created;
3. I always like to create factories to help with test cases, so I recommend creating them;

With these tips, I believe you can solve Gilded Rose’s problem in a simpler way.