The Gilded Rose is a classic refactoring kata, available in many languages athttps://github.

com/emilybache/GildedRose​-Refactoring-Kata. In this video series we start with the Java version and show how to migrate it to Kotlin as well as refactoring the procedural code into both object-oriented and functional styles.The rules of the exercise are here.
Part 1 takes the procedural Java code and ends with a nice OO solution, using inheritance to represent the different types of item. Part 2 replaces the inheritance with composition, getting perhaps closer to the heart of the domain. Part 3 removes the mutation inherent in the procedural code to give a nice functional solution. Part 4 is a bonus refactoring, starting from the bning to see how far the update code can be improved while making no changes to any other files.
The code is available at

https://github.com/dmcg/gilded-rose-kata