Optional Reading

I cover the basics of this article in a multi-part blog series, which you should check out if you have trouble:

Part 1 - How Audio Data is Represented
Part 2 - Demystifying the WAV Format
Part 3 - Synthesizing Simple WAV Audio Using C#
Part 4 - Algorithms for Different Sound Waves in C#

What's An Oscillator?

An oscillator is a device or application that generates a waveform. In electrical engineering terms, it's a device that outputs an electrical current with varying voltage. If you plot the voltage over time, you get a regular wave in a particular form, such as a sine, square, triangle or sawtooth.

An oscillator is the most basic type of synthesizer. Analog synths use electrical circuits to output a sound wave. Digital synthesizers do the same thing, but with software.

You can create a pretty neat sounding instrument by combining the outputs of multiple oscillators. For example, if you have three oscillators oscillating at a frequency of 440Hz (concert A pitch), but each of them has a different waveform (saw, square, sine) you get a very interesting, layered sound.

But before we get too deep into this subject, let's briefly explore the physics of sound.

The Physics of Sound

Sound happens when air pressure changes on your ear drum. When you clap in an empty room, pressure waves bounce all over the place and dance on your eardrum. The changes in pressure are detected continuously by your ear.

Digitally, “pressure” is referred to by a scalar value called amplitude. The amplitude (loudness) of the wave is measured thousands of times per second (44,100 times per second on CDs). Every measurement of pressure (aka amplitude) is called a sample—CDs are recorded with 44,100 samples per second, each with a value between the minimum and maximum amplitude for the bit depth.

Think about 44,100 samples per second. That's a lot of stuff for your ear to detect. That's how we're able to hear so much stuff going on in the mix of a song, especially in stereo tracks where you have 44,100 samples per second, per ear.

It turns out that there is a horribly intense mathematical theorem which basically tells us that 44,100 samples per second is enough to accurately represent a pitch as high as 22 KHz. The human ear can really hear only up to 20KHz, so a 44.1KHz sampling rate is a more than high-enough sampling rate.

This whole section is expanded in detail on my blog:

Part 1 - How Audio Data is Represented

Terminology

So now you have a rather glancing overview of how sound works, and perhaps some clues as to how we should go about representing it in computers. Let's go over all this new terminology (plus some even newer terms) in delicious, bulleted format:

· Sample: A measurement of a sound wave at a very small point in time. 44,100 of these measurements in a row form a single channel of CD-quality audio.

· Amplitude: The value of a sample. Max and min values are dependent upon the bit depth.

· Bit depth: The number of bits used to represent a sample. 16-bit, 32-bit, etc. Max amplitude is (2^depth) / 2 – 1.

· Sample rate (aka sampling rate, aka bit rate): The number of samples per second of audio. 44,100 is standard for CD-quality audio.

How sound is represented

By now, you've probably surmised that a second of audio data is somehow represented by an array of some integer data type, which has a length of 44,100. You would be correct in that assumption. However, if you want sound to play from a computer's sound card, that data has to be accompanied with a bunch of format information. WAV is probably the easiest format to deal with.

See more in the following article:

Part 2 - Demystifying the WAV Format

You can also see how to build out a WAV file, old school and binary style, in the 3^rd part of that series:

Part 3 - Synthesizing Simple WAV Audio Using C#

However, we are taking a slightly easier route, by using DirectSound. DirectSound gives us a lot of nice classes for all the format information, abstracting all that stuff away and allowing us to pump a stream of data into a DirectSound object and play it. Perfect for a synthesizer app!

So, let's get started!

Building the app

I learned some Blend while working with this app, since it's built on WPF. The image buttons are just radio buttons. I had to differentiate the group number per instance of the user control at runtime (in the constructor of the Oscillator class).

I'm a terrible UI designer for the most part, so this is about as sexy as I'm willing to make this application. But feel free to make it look and act better!

Designing the UI

There's a dirty little secret in this application. It says it can oscillate 3 waves, but in truth, there's a constant (set to 3) that you can modify. You could have six if you wanted. How did I accomplish this? Each synth that you see is an instance of a WPF user control called Oscillator.xaml:

I have a StackPanel called Oscs in the main window. In the Window_Loaded event handler of the main window, I use this bit of code to add instances of the usercontrol:

C#

// Add 3 oscillators
Oscillator tmp;
for (int i = 0; i < NUM_GENERATORS; i++)
{
    tmp = new Oscillator();
    Oscs.Children.Add(tmp);
    mixer.Oscillators.Add(tmp);
}

VB

' Add 3 oscillators
Dim tmp As Oscillator
Dim i As Integer = 0
While i < NUM_GENERATORS
    tmp = New Oscillator()
    Oscs.Children.Add(tmp)
    mixer.Oscillators.Add(tmp)
    System.Math.Max(System.Threading.Interlocked.Increment(i),i - 1)
End While

The long rectangular canvas is used to plot the values of the generated wave, so you can visualize the wave as it's played. It is scaled along the X axis so you can see the general shape of the wave, which would be impossible without scaling it with 44,100 samples per second.

Earlier in the article, I noted that a sound file is basically a really, really long array of 16- or 32-bit floating point numbers between -1 and 1. We use this data to plot the graph as well. More on that later.

Now that we have the UI figured out (dynamic addition of oscillators), let's take a look at exactly how the sound is produced.

Bzzzzt! Making Sounds and the Mixer

One of the many cool things about DirectSound is that it basically wraps the WAV format for you. You set the buffering/format options and then shove a bunch of data into it, and it will play. Magic.

The way I've architected the solution is a little more modular. None of the oscillators has the ability to play itself—rather, uses its UI to control some values such as frequency, amplitude and wave type. These values are tied to public properties. The Oscillator component does virtually no audio work at all.

The generation of audio data is handled by the custom Mixer class, which takes a collection of Oscillators and, based on their properties, creates a composite of all the generators. This is done by averaging the samples in every oscillator and putting them into a new array of data.

The Mixer class looks like this:

One of the workhorses of the Mixer class is the method GenerateOscillatorSampleData. This takes an Oscillator as an argument to give access to the public properties set in the UI. From there, the algorithm generates 1 second of sample data (specified by the member bufferDurationSeconds) based on the wave type that has been selected in the UI. This is where the mathy stuff comes in to play. Check out this method and the different cases in the switch statement that determine what kind of wave to create below.

C#

public short[] GenerateOscillatorSampleData(Oscillator osc)
{
    // Creates a looping buffer based on the params given
    // Fill the buffer with whatever waveform at the specified frequency            
    int numSamples = Convert.ToInt32(bufferDurationSeconds * 
        waveFormat.SamplesPerSecond);
    short[] sampleData = new short[numSamples];
    double frequency = osc.Frequency;
    int amplitude = osc.Amplitude;
    double angle = (Math.PI * 2 * frequency) / 
        (waveFormat.SamplesPerSecond * waveFormat.Channels);            

    switch (osc.WaveType)
    {
        case WaveType.Sine:
            {
                for (int i = 0; i < numSamples; i++)
                    // Generate a sine wave in both channels.
                    sampleData[i] = Convert.ToInt16(amplitude * 
        Math.Sin(angle * i));
            }
            break;
        case WaveType.Square:
            {
                for (int i = 0; i < numSamples; i++)
                {
                    // Generate a square wave in both channels.
                    if (Math.Sin(angle * i) > 0)
                        sampleData[i] = Convert.ToInt16(amplitude);
                    else
                        sampleData[i] = Convert.ToInt16(-amplitude);
                }
            }
            break;
        case WaveType.Sawtooth:
            {
                int samplesPerPeriod = Convert.ToInt32(
        waveFormat.SamplesPerSecond / 
        (frequency / waveFormat.Channels));
                short sampleStep = Convert.ToInt16(
        (amplitude * 2) / samplesPerPeriod);
                short tempSample = 0;

                int i = 0;
                int totalSamplesWritten = 0;
                while (totalSamplesWritten < numSamples)
                {
                    tempSample = (short)-amplitude;
                    for (i = 0; i < samplesPerPeriod && 
        totalSamplesWritten < numSamples; i++)
                    {
                        tempSample += sampleStep;
                        sampleData[totalSamplesWritten] = tempSample;

                        totalSamplesWritten++;
                    }
                }
            }
            break;
        case WaveType.Noise:
            {
                Random rnd = new Random();
                for (int i = 0; i < numSamples; i++)
                {
                    sampleData[i] = Convert.ToInt16(
        rnd.Next(-amplitude, amplitude));
                }
            }
            break;
    }
    return sampleData;
}

VB.Net

Public Function GenerateOscillatorSampleData(ByVal osc As Oscillator) As Short()
    ' Creates a looping buffer based on the params given
    ' Fill the buffer with whatever waveform at the specified frequency 
    Dim numSamples As Integer = Convert.ToInt32(
     bufferDurationSeconds * waveFormat.SamplesPerSecond)
    Dim sampleData As Short() = New Short(numSamples - 1) {}
    Dim frequency As Double = osc.Frequency
    Dim amplitude As Integer = osc.Amplitude
    Dim angle As Double = (Math.PI * 2 * frequency) /
     (waveFormat.SamplesPerSecond * waveFormat.Channels)

    Select Case osc.WaveType
        Case WaveType.Sine
            If True Then
                For i As Integer = 0 To numSamples - 1
                    ' Generate a sine wave in both channels.
                    sampleData(i) =
                     Convert.ToInt16(amplitude * Math.Sin(angle * i))
                Next
            End If
            Exit Select
        Case WaveType.Square
            If True Then
                For i As Integer = 0 To numSamples - 1
                    ' Generate a square wave in both channels.
                    If Math.Sin(angle * i) > 0 Then
                        sampleData(i) = Convert.ToInt16(amplitude)
                    Else
                        sampleData(i) = Convert.ToInt16(-amplitude)
                    End If
                Next
            End If
            Exit Select
        Case WaveType.Sawtooth
            If True Then
                Dim samplesPerPeriod As Integer =
                 Convert.ToInt32(waveFormat.SamplesPerSecond /
                      (frequency / waveFormat.Channels))
                Dim sampleStep As Short =
                 Convert.ToInt16((amplitude * 2) / samplesPerPeriod)
                Dim tempSample As Short = 0

                Dim i As Integer = 0
                Dim totalSamplesWritten As Integer = 0
                While totalSamplesWritten < numSamples
                    tempSample = CShort(-amplitude)
                    i = 0
                    While i < samplesPerPeriod AndAlso totalSamplesWritten < numSamples
                        tempSample += sampleStep
                        sampleData(totalSamplesWritten) = tempSample

                        totalSamplesWritten += 1
                        i += 1
                    End While
                End While
            End If
            Exit Select
        Case WaveType.Noise
            If True Then
                Dim rnd As New Random()
                For i As Integer = 0 To numSamples - 1
                    sampleData(i) = Convert.ToInt16(
                     rnd.[Next](-amplitude, amplitude))
                Next
            End If
            Exit Select
    End Select
    Return sampleData
End Function

The Mixer is the heart of the app, and it's a beautiful example of object orientation and cohesion. Give it three things (oscillators) and it spits out a new thing you can use (an array of sample data).

Now that we have the sample data, all we have to do is play it back using DirectSound.

Sound Playback with DirectSound

As I mentioned, DirectSound provides a wrapper over the WAV format. You set up your buffer and format information and then feed it a bunch of data in the form of an array of shorts (arrays of trousers are known to cause errors).

First, we initialize the format information and buffer in the Window_Loaded event handler of the main form. The values below are not really arbitrary; there is an explanation of them in the Optional Reading section above (see Demystifying the WAV Format). This code also contains the code to add the oscillators, as shown earlier in the article.

C#

private void Window_Loaded(object sender, System.Windows.RoutedEventArgs e)
{
    WindowInteropHelper helper = 
        new WindowInteropHelper(Application.Current.MainWindow);
    device.SetCooperativeLevel(helper.Handle, CooperativeLevel.Normal);

    waveFormat = new Microsoft.DirectX.DirectSound.WaveFormat();
    waveFormat.SamplesPerSecond = 44100;
    waveFormat.Channels = 2;
    waveFormat.FormatTag = WaveFormatTag.Pcm;
    waveFormat.BitsPerSample = 16;
    waveFormat.BlockAlign = 4;
    waveFormat.AverageBytesPerSecond = 176400;

    bufferDesc = new BufferDescription(waveFormat);
    bufferDesc.DeferLocation = true;
    bufferDesc.BufferBytes = Convert.ToInt32(
        bufferDurationSeconds * waveFormat.AverageBytesPerSecond / waveFormat.Channels);

    // Add 3 oscillators
    Oscillator tmp;
    for (int i = 0; i < NUM_GENERATORS; i++)
    {
        tmp = new Oscillator();
        Oscs.Children.Add(tmp);
        mixer.Oscillators.Add(tmp);
    }            
}

VB

Private Sub Window_Loaded(ByVal sender As Object, ByVal e As System.Windows.RoutedEventArgs)
    Dim helper As New WindowInteropHelper(Application.Current.MainWindow)
    device.SetCooperativeLevel(helper.Handle, CooperativeLevel.Normal)

    waveFormat = New Microsoft.DirectX.DirectSound.WaveFormat()
    waveFormat.SamplesPerSecond = 44100
    waveFormat.Channels = 2
    waveFormat.FormatTag = WaveFormatTag.Pcm
    waveFormat.BitsPerSample = 16
    waveFormat.BlockAlign = 4
    waveFormat.AverageBytesPerSecond = 176400

    bufferDesc = New BufferDescription(waveFormat)
    bufferDesc.DeferLocation = True
    bufferDesc.BufferBytes = Convert.ToInt32(
     bufferDurationSeconds * waveFormat.AverageBytesPerSecond / waveFormat.Channels)

    ' Add 3 oscillators
    Dim tmp As Oscillator
    For i As Integer = 0 To NUM_GENERATORS - 1
        tmp = New Oscillator()
        Oscs.Children.Add(tmp)
        mixer.Oscillators.Add(tmp)
    Next
End Sub

When you click the Play button, the application takes its collection of oscillators and passes the values of the UI controls to the Mixer (which is initialized on each click with a reference to the main form window, so it can grab the Oscillator user controls).

The mixer outputs an array of shorts, which we write to a DirectSound buffer.

Here is the code for the Play button's click event handler:

C#

private void btnPlay_Click(object sender, System.Windows.RoutedEventArgs e)
{                                    
    mixer.Initialize(Application.Current.MainWindow);

    short[] sampleData = mixer.MixToStream();
    buffer = new SecondaryBuffer(bufferDesc, device);            
    buffer.Write(0, sampleData, LockFlag.EntireBuffer);
    buffer.Play(0, BufferPlayFlags.Default);            

    GraphWaveform(sampleData);
}

VB

Private Sub btnPlay_Click(sender As Object, e As System.Windows.RoutedEventArgs)
    mixer.Initialize(Application.Current.MainWindow)

    Dim sampleData As Short() = mixer.MixToStream()
    buffer = New SecondaryBuffer(bufferDesc, device)
    buffer.Write(0, sampleData, LockFlag.EntireBuffer)
    buffer.Play(0, BufferPlayFlags.[Default])

    GraphWaveform(sampleData)
End Sub

Drawing Pretty Graphs

All that's left is to draw the graph of the waveform on the canvas. Below is the GraphWaveform method. This method could graph anything it wanted to, as long as it was an array of shorts (not trousers). It's reminiscent of trying to graph things using Flash back in the day, when you had to actually figure out points and lines (most likely on paper), but WPF's Polyline object makes this rather trivial.

C#

private void GraphWaveform(short[] data)
{
    cvDrawingArea.Children.Clear();

    double canvasHeight = cvDrawingArea.Height;
    double canvasWidth = cvDrawingArea.Width;

    int observablePoints = 1800;
            
    double xScale = canvasWidth / observablePoints;
    double yScale = (canvasHeight / 
        (double)(amplitude * 2)) * ((double)amplitude / MAX_AMPLITUDE);            

    Polyline graphLine = new Polyline();
    graphLine.Stroke = Brushes.Black;
    graphLine.StrokeThickness = 1;

    for (int i = 0; i < observablePoints; i++)
    {
        graphLine.Points.Add(
            new Point(i * xScale, (canvasHeight / 2) - (data[i] * yScale) ));
    }

    cvDrawingArea.Children.Add(graphLine);            
}

VB

Private Sub GraphWaveform(ByVal data As Short())
    cvDrawingArea.Children.Clear()

    Dim canvasHeight As Double = cvDrawingArea.Height
    Dim canvasWidth As Double = cvDrawingArea.Width

    Dim observablePoints As Integer = 1800

    Dim xScale As Double = canvasWidth / observablePoints
    Dim yScale As Double = (canvasHeight / CDbl((amplitude * 2))) * (CDbl(amplitude) / MAX_AMPLITUDE)

    Dim graphLine As New Polyline()
    graphLine.Stroke = Brushes.Black
    graphLine.StrokeThickness = 1

    For i As Integer = 0 To observablePoints - 1
        graphLine.Points.Add(
         New Point(i * xScale, (canvasHeight / 2) - (data(i) * yScale)))
    Next

    cvDrawingArea.Children.Add(graphLine)
End Sub

Conclusion

This was a really fun little project that took way less time to code than it does to explain. It's a great exercise because it requires you to think about an ancillary field of science before you can sit down and code, which is really what coding for fun's all about, anyway!

If you want to try this out, the download link for the source code is at the top of the article.

About The Author

Dan Waters is an Academic Evangelist at Microsoft, covering schools in the Pacific Northwest, Alaska, and Hawaii. He is based in Bellevue, WA. Dan has way too many guitars at home and tries to entice both of his young daughters to learn how to play them. Music, technology, and music+technology are among his favorite hobbies, along with snowboarding and trying to maintain cool dad status. You can find his blog at www.danwaters.com or follow him on Twitter at www.twitter.com/danwaters.

The Zune firmware version 3.1 brought us a professionally built incarnation of Texas Holdem that supports network play. Understanding how to send and receive data with the Zune can be a little daunting at first, but once you understand the pattern, it's easy.

To build out the entire game, you probably need about a week, but you can build some simpler examples in far less time.

This is an earlier project from before the release of my Zune game development book. Accordingly, some of the code samples you see in this article may be inconsistent with what you find in the download. The code in the article is the “correct” way to do things. The code in the download is still a work in progress.

The Workflow

Developing multiplayer games for the Zune is interesting because you have to deploy to each device individually. Once you have a build that works for you, it's helpful to run-deploy (Control+F5) to one device, leave it running there, and then plug in the other Zune and debug-deploy (F5) to it. This way you have one debuggable instance of the game running. Make sure to set the appropriate Zune device as the default in the XNA Game Studio Device Center (accessible from the Tools menu).

Starting A Network Session

Because the Zunes connect over an ad-hoc, peer-to-peer connection rather than through an access point, you will have to designate one Zune as the host device. The host is usually determined to be the one that creates a new game. Therefore, all Zunes that join the host's network session are simply peers. The difference between the host and the peers is that the host usually maintains the game state on top of executing the game as well, because the game data has to be centralized somewhere. Keep that in mind, because if one Zune is doing substantially more processing, it can lag behind and mess up your network session. Also, remember that all Zunes are running the exact same copy of the game, so the game must support both host and peer scenarios.

Create / Join / Lobby Model

Most peer-to-peer connected games allow a user to create or join a game. After doing so, the player is funneled into an area called the Lobby where they can specify their readiness. When all players are ready, the host can start the game. Some of this functionality is provided directly by the XNA Framework's Net and GamerServices libraries.

I normally create three separate screens based on the Network Game State Management sample from the Creators Club website. The first is the Create screen, which looks exactly like a lobby. It starts up a network session and waits for players to join and become ready. The code to create a network session looks like this (note that I have employed some abstraction to make my code a little more cohesive):

C#

public void CreateZuneSession(int maxNetworkPlayers)
{
    KillSession();

    try
    {
        Session = NetworkSession.Create(NetworkSessionType.SystemLink, 1,      
            maxNetworkPlayers); 
        Me = Session.LocalGamers[0];
    }
    catch (NetworkNotAvailableException)
    {
        throw new NetworkNotAvailableException("Zune wireless is not 
            enabled.");
    }
    catch (NetworkException ne)
    {
        throw ne;
    }

    if (Session == null)
        throw new NetworkException("The network session could not be 
            created.");
}

All Zune network sessions are of the type SystemLink, much like LAN-networked Xbox consoles. The ‘1' parameter specifies the number of local players – of course, on a Zune, there can only be one.

This line of code sets the Session property to a newly created network session. The Join screen, running on another Zune, will find this session asynchronously as an available network session and attempt to join it. The first step is to enumerate available network sessions:

C#

/// <summary>
/// Asynchronous: Begins to discover network sessions.
/// </summary>
public void BeginGetAvailableSessions()
{
    // Destroy any existing connections
    KillSession();

    NetworkSession.BeginFind(NetworkSessionType.SystemLink, 1, null, new 
        AsyncCallback(SessionsFoundCallback), null);
}

This code looks for SystemLink sessions and calls the callback method SessionsFoundCallback when the operation completes (successfully or unsuccessfully). If sessions are found, an event is fired. Other screens can subscribe to this event and transition to other screens or do other processing with the network session.

C#

/// <summary>
/// Called when network sessions are found.
/// </summary>
/// <param name="result"></param>
public void SessionsFoundCallback(IAsyncResult result)
{
    AvailableNetworkSessionCollection availableSessions = null;
    availableSessions = NetworkSession.EndFind(result);

    if (NetworkSessionsFound != null)
        NetworkSessionsFound(availableSessions);
}        

When all players are ready and the host presses the middle button, the network session's StartGame() method is called, which will cause all connected peers to receive a GameStarted event. This loads up the playing screen.

C#

if (allPlayersReady && atLeastTwoPlayers)
{
    if (ScreenManager.Network.Session != null)
        ScreenManager.Network.Session.StartGame();
}

That's basically how to connect up two Zunes in a network session. See my book for a much more detailed explanation.

Dealing With Cards

As Poker is a card game, you might want to develop a testable, standalone library that you can use not only to house your objects, but also to write out all the intense logic required for a game like poker. The task of determining what a player's best hand is out of seven cards (and whether it beats another player's hand) is more in-depth than you might expect.

An easy way to go about this is to create a Zune game library. I called mine CardLib. CardLib has objects such as these:

• Card which has a suit, a value, some comparers and some utility methods)
• Deck, which has a collection of cards and methods like Shuffle, ResetDeck, etc.
• Dealer, which has a deck, a list of the five community cards, and methods like Shuffle, DealCard, Burn, DealFlop, DealTurn, etc.
• HoldemHand, which contains all of the logic for determining what a hand is (always 7 cards)
• BestHand, which takes a HoldemHand and determines what the best possible hand is within that hand

Shuffling Cards

Shuffling cards is surprisingly easy. I use the Knuth-Fisher-Yates shuffling algorithm, which takes every card in the deck and randomly swaps it with another.

C#

/// <summary>
/// Knuth-Fisher-Yates shuffling algorithm:
/// http://www.codinghorror.com/blog/archives/001015.html
/// </summary>
public void Shuffle()
{
    Random rand = new Random();
    for (int cardIndex = _cards.Count - 1; cardIndex > 0; cardIndex--)
    {
        int randomIndex = rand.Next(cardIndex + 1);
        SwapCardsByIndex(cardIndex, randomIndex);
    }
    _dealIndex = 0;
}

Poker Logic

An example method you'll see in this library is this snippet to check if the hand is a royal flush:

C#

public bool IsRoyalFlush(out List<Card> winningCards)
{
    List<Card> straightFlushCards;

    if (IsStraightFlush(out straightFlushCards))
    {                
        // Check to make sure the straight flush cards are: 10 J Q K A
        straightFlushCards.Sort();

        if (straightFlushCards[0].Value == 10 &&
            straightFlushCards[1].Value == Card.Jack &&
            straightFlushCards[2].Value == Card.Queen &&
            straightFlushCards[3].Value == Card.King &&
            straightFlushCards[4].Value == Card.Ace)
        {
            winningCards = straightFlushCards;
            return true;
        }

        else
        {
            winningCards = null;
            return false;
        }
    }
    else
    {
        winningCards = null;
        return false;
    }
}

There is a ton of similar and even more disgusting logic to peruse in the HoldemHand.cs file in the region entitled “Poker Logic Fun Times.” This logic covers every possible poker hand and could probably be refactored to be way more elegant.

When it comes time to evaluate the winner, each player's best hand is determined and they are evaluated against each other to determine the winner. This code returns a list of WinnerInfo objects, which returns the player info and a list of the cards they won with.

C#

public List<WinnerInfo> DetermineWinners()
{
    BestHand overallBestHand = null;
    HoldemPlayer winner = null;

    List<WinnerInfo> winners = new List<WinnerInfo>();

    foreach (HoldemPlayer player in _players)
    {
        if (player.Status != PlayerStatus.Folded)
        {
            HoldemHand tempHand = new HoldemHand(player.Pocket, _communityCards);
            BestHand bestHand = tempHand.GetBestHand();

            if (overallBestHand == null) // if there is no best hand yet
            {
                overallBestHand = bestHand;
                winner = player;
                winners.Add(new WinnerInfo(overallBestHand, winner, Me));
            }
            else
            {
                // if this hand beats the current best hand
                if (BestHand.Beats(bestHand, overallBestHand))
                {
                    winners.Clear();
                    overallBestHand = bestHand;
                    winner = player;
                    winners.Add(new WinnerInfo(overallBestHand, winner, Me));
                }
                else
                {
                    if (BestHand.IsEquivalentTo(bestHand, overallBestHand))
                    {
                        winners.Add(new WinnerInfo(bestHand, player, Me));
                    }
                }
            }
        }
    }

    _winners = winners;
    return winners;
}

Drawing Cards On The Screen

Rather than have 52 different sprites that represent each possible card, I just made my own sheet of Zune-sized playing cards. Horizontally, they are ordered by value (1-13) and vertically they are ordered by suit (both alphabetically and corresponding to the numeric value of the Suit enumeration in the project, 1-4).

When a specific card is requested to be drawn, a formula is used to calculate the source rectangle from this larger image. This is very similar to how fonts worked before spritefonts were introduced into XNA. This allows us to strip out the graphic of a specific card and draw it onscreen at some other location.

C#

private void DrawCard(Card card, Vector2 position)
{
    if (card != Card.Undefined && card.Suit != Suit.Unassigned)
        ScreenManager.SpriteBatch.Draw(_texDeck, position, GetCardSourceRect(card), Color.White);
}

private Rectangle GetCardSourceRect(Card card)
{
    if (card.Suit == Suit.Unassigned)
        throw new ArgumentException("Unassigned cards cannot be drawn.");

    int cardColumn = card.Value - 1;
    int cardRow = (int)card.Suit - 1;

    int x = cardColumn * MainScreenElements.CARD_WIDTH;
    int y = cardRow * MainScreenElements.CARD_HEIGHT;

    return new Rectangle(x, y, MainScreenElements.CARD_WIDTH, MainScreenElements.CARD_HEIGHT);
}

Managing Network Data

One thing that increases the complexity of this game (in no small way) is the management of network data. First of all, the host's actions are a superset of the peer's actions (a host is also a peer, but it also has to manage the game and network state). For example, when a peer decides to bet, it has to send a message to the host saying “I would like to bet,” and then the host will process that message and relay it to the other peers. The host is also responsible for managing whose turn it is and determining who the winner is. Theoretically, you could do a lot of this with every peer acting equally but it feels safer to me to have the host responsible for important activities like dealing cards. In fact, the host has to be the only one who can deal cards, because if each client maintained its own deck, it would be randomized for every peer when the deck is shuffled.

How Data Is Sent And Received

I will usually create a static class called NetworkMessageSender that is responsible for sending various messages. I keep an enumeration of type byte that holds the possible network messages.

I always send the byte indicating the message type first, so that when the peer receives a byte, it knows how to respond. For example, if the peer receives a CardDealt message, it can pop off a string and a card from the incoming packets. If the card is not intended for the peer, it can simply discard the message.

Data you want to send is written to a packet writer object. When the data is ready to be sent, you call the SendData method of the LocalGamer object. Depending on how important packet receipt is, you can specify the type of transmission. I use ReliableInOrder during poker because network data is exchanged relatively infrequently. Although there is only one local network gamer, you can use a foreach loop to ensure this code will work on other platforms.

C#

private static void SendData()
{
    foreach (LocalNetworkGamer gamer in NetworkSessionManager.Session.LocalGamers)
        gamer.SendData(NetworkSessionManager.PacketWriter, SendDataOptions.ReliableInOrder);
}

Specific chunks of data are then sent using various static methods. Each piece of information is written sequentially. The card is serialized into a string format before being sent. The following chunk of code is used to send a card to a player.

C#

public static void DealCards(HoldemPlayer player)
{
    NetworkSessionManager.PacketWriter.Write((byte)NetworkMessageType.Deal);
    NetworkSessionManager.PacketWriter.Write(player.Name);
    NetworkSessionManager.PacketWriter.Write(player.Pocket.Card1.Serialize());
    NetworkSessionManager.PacketWriter.Write(player.Pocket.Card2.Serialize());
    SendData();
}

On the other side of that, whenever the screen updates, the game is constantly checking for new network data. The following code is part of a method that is called from the game screen's Update loop.

C#

private void UpdateNetworkSession()
{
    NetworkSessionManager.Update();

    foreach (LocalNetworkGamer localGamer in 
        NetworkSessionManager.Session.LocalGamers)
    {
        while (localGamer.IsDataAvailable)
        {
            NetworkGamer sender;
            localGamer.ReceiveData(NetworkSessionManager.PacketReader, 
                out sender);

            // Interpret the first piece of information, the message.
            NetworkMessageType message = (NetworkMessageType)NetworkSessionManager.PacketReader.ReadByte();

            // Determine what to read and what to do based on this message type
            switch (message)
            {
                // major snippage
                case NetworkMessageType.Deal: // Happens when a card is dealt
                {
                    string playerName = NetworkSessionManager.PacketReader.ReadString();
                    string card1 = NetworkSessionManager.PacketReader.ReadString();
                    string card2 = NetworkSessionManager.PacketReader.ReadString();

                 _gameplayManager.Players[playerName].Pocket.Set(PlayerPocket.FIRST_CARD_INDEX, new 
                    Card(card1));
                 _gameplayManager.Players[playerName].Pocket.Set(PlayerPocket.SECOND_CARD_INDEX, new 
                    Card(card2));
                }
                break;
            }
        }
    }
}

There is a whole bunch of similar code that underlies how the game executes. When a message needs to be sent, the NetworkMessageSender class is used. To determine what happens when a message is received, we look at that ginormous switch statement that gets called in the screen update loop.

Remember that the host is also a peer and it will receive the very same messages it sends (unless you specify otherwise). Be careful not to double-process messages if they are sent by the host. In some cases you will need to check that the current device is or is not the host. In the sample download, this is achieved just by checking a Boolean value that is set early on in the game.

Conclusion

Building Poker for the Zune, from the ground up, is no small feat! Networked Zune games are far simpler when you have a much more limited set of possible data and messages that could be sent. For example, networked Pong, Battleship or Tetris would be pretty easy compared to poker. Turn-based games on the Zune also provide an interesting challenge in terms of what happens when a Zune drops from the network session.

This is just a small dip into networked Zune game development. For a deep dive, check out my book, Zune Game Development using XNA 3.0, also available on Apress.com as an eBook. The final chapter is a sprawling 120 pages covering how to build Crazy Eights for the Zune from the ground up.

About The Author

Dan Waters is an Academic Developer Evangelist at Microsoft, based in Tampa, FL. When he's not out showing the latest and greatest MS technology to students and faculty, he's spending time with his wife and young daughter or rocking out on one of his (far too numerous) guitars. Follow Dan on Twitter or check out his blog.