Taking on the Gilded Rose Kata

I was recently invited to attend the Chicago Software Craftsmanship presentation on “Untangling the Gilded Rose”.  While I was unfortunately unable to attend, I found the topic inspirational enough to write about.  The topic was a code kata, described as taking an existing code base and refactoring it to be more maintainable.  The  kata for this blog post is called the Gilded Rose Kata and is described here with a project template found here.

1.  Write the Specs

We are given the specs, but I’ve copied and consolidated them below.  They seem simple at first, but going through them multiple times reveals some subtleties.

  1. ‘Aged Brie’ increases quality increase by 1 each day.
  2.  ‘+5 Dexterity Vest’ quality increase by 1 each day.
  3. ‘Elixir of the Mongoose’ quality increase by 1 each day.
  4. ‘Sulfuras, Hand of Ragnaros’ quality and sellin stay the same.
  5. ‘Backstage passes to a TAFKAL80ETC concert’ quality goes up by 1 each day when the Sellin is greater than 10.  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.
  6. “Conjured” items degrade in Quality twice as fast as normal items.
  7. All items quality is never negative.
  8. All items quality may never go over 50.

Do we understand the specs?  In this case, a few things weren’t clear from the specs.

  1. Would the item being updated ever be in an invalid state?  If so, how would we handle it?  I’m going to assume no for the purposes of the refactoring since neither the original code nor the specs address this issue.
  2. Should the Quality being calculated before or after the SellIn is reduced?  We can derive this from the current code base, which indicate we should reduce the sellin before calculating the quality.
  3. Should we consider if multiple rules could apply to an item?  For example, what if there was a “Conjured Aged Brie” that required both the rules for “Conjured” and “Aged Brie” be applied?  We’ll assume no, as there no indication from the specs this should be considered.
  4. There are many other possible requirements that we’ll ignore and assume should be up to our discretion since they are not explicitly stated, e.g. performance, development methodology, testing methodology, etc.

2.  Write the tests

One of the purposes of the Kata is to practice TDD, or test driven development. So we’ll first write our unit tests.  However, this leads to a common chicken or egg problem with TDD.  How do I know what classes and methods to test when there is no class design yet? Typically, I will think through the overall class design and write the stubs before writing the units test, so at least there is a starting point.  There will be probably be changes and not all of our assumptions may turn out to be true, but its better to plot a course now and adjust later than to completely miss the mark.

3. What are the design goals?

The kata is pretty open ended as far as design goals. Given this, we’ll come up with our own set of goals to accomplish.

  1. Localize item processing to the item’s type –  The low cohesion of the items type to the calculation for that type’s quality is probably the most serious problem we see with the current design.  If you were trying to follow how the quality was calculated for “Backstage passes to a TAFKAL80ETC concert” it would be difficult as it is in 2 different places, separated by unrelated code, and deeply nested within if/else structures.  We want to have a single place for each items quality calculation.
  2. Reduce nested if/else structures.   Nested if/else constructs make code difficult to understand. Why?  Because you have to understand several different lines of code in conjunction to understand if a particular nested statement will be executed.  Favor flattening conditional statements whenever possible.  This will localize the conditional logic of whether a particular statement is executed.
  3. Make testable - We want all of our code to be easily testable with unit tests so any further refactoring will be easier and have code coverage.

4. The design

So the design I went with is to have a class for each unique update algorithm.  This is going to be represented by a strategy pattern.  This will allow us to iterate through each item and apply the update function to each item independently of knowing the type of each item.  This leaves us with a mapping problem of finding the correct strategy for the type.  We will solve this with a factory pattern to create and return the strategy instance for the unique type.

public interface IUpdateStrategyFactory
{
    IUpdateStrategy Create(string name);
}

public interface IUpdateStrategy
{
    void UpdateQuality(Item item);
}

5.  Write the code

I won’t go through all the trivial details, but here is the factory, 2 of the strategies, and the greatly simplifed UpdateQuality() loop.

public class UpdateStrategyFactory : IUpdateStrategyFactory
{
    public IUpdateStrategy Create(string name)
    {
        if (name == "Sulfuras, Hand of Ragnaros")
        {
            return new DoNothingUpdate();
        }
        else if (name == "Aged Brie")
        {
            return new AgedBrieUpdateStrategy();
        }
        else if (name == "Backstage passes to a TAFKAL80ETC concert")
        {
            return new BackstagePassesUpdateStrategy();
        }
        else if (name.Contains("Conjured"))
        {
            return new ConjuredUpdateStrategy();
        }
        else
        {
            return new StandardUpdateStrategy();
        }
    }
}
public class StandardUpdateStrategy : IUpdateStrategy
{
    public void UpdateQuality(Item item)
    {
        item.SellIn--;
        if (item.Quality <= 0)
        {
            return;
        }
        else if (item.Quality == 1)
        {
            item.Quality = 0;
        }
        else
        {
            item.Quality -= item.SellIn < 0 ? 2 : 1;
        }
    }
}

The final loop is now much easier to read and understand.

public void UpdateQuality()
{
    foreach (Item item in Items)
    {
        var updateStrategy = _strategyFactory.Create(item.Name);
        updateStrategy.UpdateQuality(item);
    }
}

That’s it!  If you have any feedback, please leave it below in the comments.  The source code is available for download.

Starting with SignalR and Knockout.js

After working in web development for many years its rare that you get to work with a technology that changes how you program a web app.  I recently had the opportunity to work with one such web technology generally known as Comet.  While it has been around for several years, its become more common as the demand for real time data and responsive applications has increased.

I’ll walk through creating a real time stock price tracker using SignalR, knockoutjs, and underscore to show how easy it is to feed real time data to a client browser without the typical heavy-handed async updates and DOM manipulation of most applications.

Setup Hub

For our simple example the client will only be listening for messages, and not sending them to the server.  So we only need an empty class that inherits from Hub to send updates to the client.

public class DemoHub : SignalR.Hubs.Hub
{
}

Add NuGet packages

  1. jQuery – Dependency for signalr
  2. knockoutjs – viewmodel ->view binding framework
  3. signalr – Both client and server libraries for sync communication
  4. underscore.js – collection querying

Reference signalR as well the other javascript libraries:

@Scripts.Render("~/bundles/jquery")
<script src="~/Scripts/jquery.signalR-1.1.3.js" type="text/javascript"></script>
<script src="~/signalr/hubs" type="text/javascript"></script>
<script src="~/Scripts/underscore.js" type="text/javascript"></script>
<script src="~/Scripts/knockout-2.3.0.js" type="text/javascript"></script>

Send data

We’ll create a separate thread to send price updates to the client via the Hub.

protected void Application_Start()
{
....
    var random = new Random();

    ThreadPool.QueueUserWorkItem(_ =>
    {
        var hubContext = GlobalHost.ConnectionManager.GetHubContext();

        while (true)
        {
            hubContext.Clients.UpdatePrice(new DemoHubPriceUpdateModel { Symbol = "AAPL", Price = (random.Next(620, 650) + random.NextDouble())});
            Thread.Sleep(500);
        }
    });
}

Create binding template

<table>
    <tbody>
        <tr>
            <th>Symbol</th>
            <th>Price</th>
            <th>Last Updated</th>
        </tr>
    </tbody>
    <tbody data-bind="foreach: stocks">
        <tr>
            <td data-bind="text: symbol"></td>
            <td data-bind="text: price"></td>
            <td data-bind="text: updated"></td>
        </tr>
    </tbody>
</table>

Create ViewModel

Here we will create the viewmodel, which is just a plain old javascript object.  We will also create the function listener for when price updates are sent. The UpdatePrice function will be passed the data from the server side as JSON, and update the price and last updated date of the relevant stock.

function Stock(symbol, price, updated) {
    this.symbol = symbol;
    this.price = ko.observable(price);
    this.updated = ko.observable(updated);
};

$(function () {
    var now = new Date();
    var viewModel = {
        stocks: [
            new Stock("AAPL", 648.11, now),
            new Stock("MSFT", 30.90, now),
            new Stock("FB", 19.05, now),
            new Stock("YHOO", 15.03, now)]
    };
$(function () {
    // Proxy created on the fly
    var demoHub = $.connection.demoHub;
    demoHub.UpdatePrice = function (priceUpdate) {
        _.chain(viewModel.stocks)
            .filter(function (stock) {return stock.symbol == priceUpdate.Symbol;})
            .each(function (stock) {
                stock.price(priceUpdate.Price.toFixed(2));
                stock.updated(new Date());
            });
        };

        // Start the connection
        $.connection.hub.start();
    });

    ko.applyBindings(viewModel);
});

Live Running App

running application

I hope this example demonstrate the simplicity of working with real time data with a few open source libraries.  The source code of this post is available for download here.

Project Euler Problem 10 in F#

This problem is very similar to several of the other prime generation problems, but being the dedicated blogger that I am here is problem 10:

The sum of the primes below 10 is 2 + 3 + 5 + 7 = 17.

Find the sum of all the primes below two million.

  1. Generate primes using Sieve of Eratosthenes.
  2. Stop before reaching 2 million.
  3. Find the sum of this set of primes.

type public PrimesGenerator() =

member this.getPrimesMax max =

let primes = new BitArray(max+1, true)

let result = new ResizeArray(max/10)

for n = 2 to max do

if primes.[n] then

let start = (int64 n * int64 n)

if start < int64 max then

let i = ref (int start)

while !i <= max do primes.[!i] <- false; i := !i + n

result.Add n

result

member this.Problem9() =

let primesGen = new PrimesGenerator()

primesGen.getPrimesMax 2000000 |> Seq.map (fun prime -> (int64)prime) |> Seq.sum

Download the source code with unit tests.

Project Euler Problem 9 in F#

In the continuing series of solving Project Euler problems with F#, here is problem 9 with my solution:

A Pythagorean triplet is a set of three natural numbers, a b c, for which,

a2 + b2 = c2
For example, 32 + 42 = 9 + 16 = 25 = 52.

There exists exactly one Pythagorean triplet for which a + b + c = 1000.
Find the product abc.

  1. Enumerate from 1 to 1000.  Create tuples of all sets (m, n) where m > n.
  2. Create Pythagorean triplets using Euclid’s formula from the tuples.
  3. Find the set where a + b + c = 1000.
  4. Find the product of this set.
let problem9 =
    let pythagorean_triplets(m:int, n:int) =
        let a = m*m-n*n
        let b = 2*m*n
        let c = m*m+n*n
        [a;b;c]
    let tops = 1000
    [for m in [1..tops] do
        for n in [1..m-1] do yield (m, n)] |> Seq.map (fun t -> pythagorean_triplets((fst t, snd t)))
            |> Seq.filter (fun x -> x |> Seq.sum = tops) |> Seq.head |> Seq.fold (fun acc elem -> acc * elem) 1

Download the source code with unit tests.

Project Euler Problem 7 in F#

In the continuing series of solving Project Euler problems with F#, here is problem 7:

By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13. What is the 10,001st prime number?

  1. Generate primes using Sieve of Eratosthenes.
  2. Skip the first ten thousand primes and take the next one.

type public PrimesGenerator() =

member this.PrimesInfinite () =

let rec nextPrime n p primes =

if primes |> Map.containsKey n then

nextPrime (n + p) p primes

else

primes.Add(n, p)

let rec prime n primes =

seq {

if primes |> Map.containsKey n then

let p = primes.Item n

yield! prime (n + 1) (nextPrime (n + p) p (primes.Remove n))

else

yield n

yield! prime (n + 1) (primes.Add(n * n, n))

}

prime 2 Map.empty

member this.Problem7() =

let primes = new PrimesGenerator()

primes.PrimesInfinite() |> Seq.skip 10000 |> Seq.take 1 |> Seq.head

Download the source code with unit tests.

Project Euler Problem 8 in F#

In the continuing series of solving Project Euler problems with F#, here is problem 8 and my solution:

Find the greatest product of five consecutive digits in the 1000-digit number.

73167176531330624919225119674426574742355349194934
96983520312774506326239578318016984801869478851843
85861560789112949495459501737958331952853208805511
12540698747158523863050715693290963295227443043557
66896648950445244523161731856403098711121722383113
62229893423380308135336276614282806444486645238749
30358907296290491560440772390713810515859307960866
70172427121883998797908792274921901699720888093776
65727333001053367881220235421809751254540594752243
52584907711670556013604839586446706324415722155397
53697817977846174064955149290862569321978468622482
83972241375657056057490261407972968652414535100474
82166370484403199890008895243450658541227588666881
16427171479924442928230863465674813919123162824586
17866458359124566529476545682848912883142607690042
24219022671055626321111109370544217506941658960408
07198403850962455444362981230987879927244284909188
84580156166097919133875499200524063689912560717606
05886116467109405077541002256983155200055935729725
71636269561882670428252483600823257530420752963450

  1. Parse the input string to a list of integers
  2. Create lists of each 5 consecutive digits.
  3. Find the product of each list.
  4. Find the max product.
open System

let Problem8 =
    let multiply lst = lst |> Seq.fold (fun acc elem -> acc * elem) 1
    let s = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450"
    let values = s.ToCharArray() |> Seq.map (fun x -> Int32.Parse(x.ToString())) |> Seq.toList
    [0..(values.Length - 5)]
        |> Seq.map (fun h -> [0..4] |> Seq.map (fun a -> values.[(h + a)]) |> multiply )
        |> Seq.max

Download the source code with unit tests.

Project Euler Problem 6 in F#

In the continuing series of solving Project Euler problems with F#, here is problem 6 and my solution:

The sum of the squares of the first ten natural numbers is,

12 + 22 + … + 102 = 385
The square of the sum of the first ten natural numbers is,

(1 + 2 + … + 10)2 = 552 = 3025
Hence the difference between the sum of the squares of the first ten natural numbers and the square of the sum is 3025 – 385 = 2640.

Find the difference between the sum of the squares of the first one hundred natural numbers and the square of the sum.

  1. Enumerate 1-100 and  square each value.  Add the results together
  2. Enumerate 1-100 and add each value together.  Square the resulting sum.
  3. Find the difference between the two.
let problem6 = 
    let values = [1..100]
    let sumOfSquares = values |> Seq.map (fun x -> pown x 2) |> Seq.sum
    let squareOfSums = values |> Seq.sum |> (fun x -> pown x 2)
    squareOfSums - sumOfSquares

Download the source code with unit tests.