TDD Kata: Risk Card Sets

by Jeff Langr

July 17, 2017


“IXS_4046,” courtesy Leon Brocard. License.

Tired of the same old programming katas? Give this one a try!

In the game of Risk, you receive a card at the end of your turn if you captured at least one territory during that turn. If you end up with a proper set (to be defined shortly), you can redeem in on a later turn for bonus armies.

Cards either contain one of three symbols–horseman (H), cannon (C), soldier (S)–or are Jokers (J) that can act as any one of the three symbols.

This Kata has two parts. Please do Part 1 imagining that you know nothing about Part 2, i.e. minimally like a good TDDer should.

Part 1: Is the collection of cards a valid set?

Determine whether or not a collection of three (selected) cards represents a valid set that can be turned in. A valid set contains three cards either all with the same symbol, or each will different symbols.

The user has selected exactly three valid cards–the UI constrains how many cards can be selected–so your solution should concern itself with no other possibilities.

Examples of valid sets:

C-S-J (with the Joker acting as an H)
S-S-J (with the Joker acting as an S)

Examples of invalid sets:


Part 2: Does the collection of cards contain a valid set?

Determine whether or not a collection of 0 through 5+ cards (i.e. all the cards that a user holds) contains a valid set.

Five cards logically must always contain a valid set. The only case where a four card set is invalid is when it contains only two symbols, e.g. H-H-C-C.

Discussion / Suggestions

This is not a long kata: My first implementation (in Java, I’ll try another language next time) that covers both parts is essentially a complex conditional with four conditional expressions, probably 20 minutes of effort. It did bring up a couple interesting questions as I ran through it.

  • How long did it take the first time through? The second time?

  • If you lead others through this kata: How long does it take for someone new to TDD?

  • Was this too difficult? Too easy?

  • Functional approaches (LINQ, Java 8 streams, etc) seem to be the way to go. If you produce a more imperative solution, how many lines was your code? How readable by another developer?

  • How much do your implementation choices leak into the test names? Do your test names describe it more from an implementation stance, in other words, or are they essentially a restatement of the Risk rules?

  • Did you extract your conditional expressions into intention-declaring functions, or do you assume the ability of a developer to understand their intent? If you extracted conditionals, to what extent did you sacrifice performance to improve readability / composition?

  • How many tests did you end up with for each part? Does it make sense to eliminate any (for the sake of keeping documentation simple) now that you have a complete solution?

  • In Part 1, were there tests you felt compelled to write for confidence, but that passed immediately (i.e. you were unable to follow R-G-Refactor)? How might you have avoided this?

  • Did some of your tests for Part 2 pass immediately? If so, what would have been a simpler solution for Part 1?

  • When you built Part 2, did you factor it to take advantage of the method created in Part 1, or did you have the method created in Part 1 take advantage of the method created in Part 2? Or neither?

I’d love to hear from you if you try this kata–please report back any interesting findings!

I was triggered to create this Kata after I saw a complex open source implementation (in a working Risk app) that required several dozens of lines of Java code. You can do much better.

My first implementation appears here.

Pingback: TDD Katas / Exercises: Stock Portfolio (1 / 5+)

Pingback: TDD Katas / Exercises: Multimap (2 / 5+)

Pingback: TDD Katas / Exercises: Name Normalizer (3 / 5+)

Pingback: DD Katas / Exercises: Soundex (4 / 5+)

Pingback: TDD Katas / Exercises: Risk Card Sets (5 / 5+)

Share your comment

Jeff Langr

About the Author

Jeff Langr has been building software for 40 years and writing about it heavily for 20. You can find out more about Jeff, learn from the many helpful articles and books he's written, or read one of his 1000+ combined blog (including Agile in a Flash) and public posts.