Coding Dojo

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by Matthew D Moss, blatantly stolen from

If you want to try this Kata for yourself or at your dojo meeting, read the problem description and see if the Kata appeals to you. The rest is commentary and helpful clues for if you get stuck solving it. I would recommend trying the Kata for yourself before reading too much of it. For an easier kata, try KataPokerHands first.

Difficulty: hard

Problem description

You work for a cable network; specifically, you are the resident hacker for a Texas Hold’Em Championship show.

The show’s producer has come to you for a favor. It seems the play-by-play announcers just can’t think very fast. All beauty, no brains. The announcers could certainly flap their jaws well enough, if they just knew what hands the players were holding and which hand won the round. Since this is live TV, they need those answers quick. Time to step up to the plate. Bob, the producer, explains what you need to do.

BOB: Each player’s cards for the round will be on a separate line of the input. Each card is a pair of characters, the first character represents the face, the second is the suit. Cards are separated by exactly one space. Here’s a sample hand.

  Kc 9s Ks Kd 9d 3c 6d
  9c Ah Ks Kd 9d 3c 6d
  Ac Qc Ks Kd 9d 3c
  9h 5s
  4d 2d Ks Kd 9d 3c 6d
  7s Ts Ks Kd 9d

YOU: Okay, I was going ask what character to use for 10, but I guess ’T’ is it. And ‘c’, ’d’, ‘h’ and ’s’ for the suits, makes sense. Why aren’t seven cards listed for every player?

BOB: Well, if a player folds, only his hole cards and the community cards he’s seen so far are shown.

YOU: Right. And why did the fifth player play with a 4 and 2? They’re suited, but geez, talk about risk…

BOB: Stay on topic. Now, the end result of your code should generate output that looks like this:

  Kc 9s Ks Kd 9d 3c 6d Full House (winner)
  9c Ah Ks Kd 9d 3c 6d Two Pair
  Ac Qc Ks Kd 9d 3c 
  9h 5s 
  4d 2d Ks Kd 9d 3c 6d Flush
  7s Ts Ks Kd 9d 

YOU: Okay, so I repeat the cards, list the rank or nothing if the player folded, and the word “winner” in parenthesis next to the winning hand. Do you want the cards rearranged at all?

BOB: Hmmm… we can get by without it, but if you have the time, do it. Don’t bother for folded hands, but for ranked hands, move the cards used to the front of the line, sorted by face. Kickers follow that, and the two unused cards go at the end, just before the rank is listed.

YOU: Sounds good. One other thing, I need to brush up on the hand ranks. You have any good references for Texas Hold’Em?

BOB: Yeah, do an internet search on Poker Hand Rankings. And if you need it, the Rules of Texas Hold’Em. While ranking, don’t forget the kicker, the next highest card in their hand if player’s are tied. And of course, if – even after the kicker – player’s are still tied, put “(winner)” on each appropriate line of output.

YOU: Ok. I still don’t understand one thing…

BOB: What’s that?

YOU: Why he stayed in with only the 4 and 2 of diamonds? That’s just…

BOB: Hey! Show’s on in ten minutes! Get to work!


There are two dimensions to this problem. Firstly how to rank a particular hand (“Flush” or “Two Pairs” etc) and secondly how to compare hands and determine which will win. A very easy way to get going with this Kata is just to concentrate on the first dimension, and write lots of code that can successfully assigns a rank to all the different kinds of hand. This is all well and good, but I think you’ll find some refactoring in order when you come to tackle the second dimension. My recommendation is to write just enough code to identify one or two ranks, then start working on comparing hands. You can fill in the details of all the different ranks when you have a basic structure that can both rank and compare hands.

One interesting aspect of this Kata is the OO design that you come up with. What classes do you need? What responsibilities do they have? Have you removed all duplication?

Another interesting aspect which you may or may not want to go into, is how you express the rules of Poker. These rules are both repetitive and arbitrary and in many ways resemble the kinds of business rules you meet in the real world. Can you find a way of expressing them that will leave an expert poker player able to verify that they are implemented correctly, even if they are not themselves a programmer?