Bezier Curves

Bezier Curves are very useful and have many applications, but what do we need to know to implement one ourselves? Well, you can keep reading, or you can just download the unity package that I made.

Lerp (Linear Interpolation)

Before we start we need to know what Lerp is. If you want to understand it more mathematically you can check the description in wikipedia. It’s very simple. Lerp is a function which takes three parameters – a, b and t, where a is start value, b is end value, and t is time (between 0 and 1). The function is defined like this

Lerp

In code it looks like this

float Lerp(float a, float b, float t)
{
    return (1f - t) * a + t * b;
}

Note that we can use the same logic if we want to lerp vectors

Vector3 Lerp(Vector3 a, Vector3 b, float t)
{
    return (1f - t) * a + t * b;
}

Bezier Curves

Now that we know what lerp is we can start.
A bezier curve is also defined by a function, but a function of higher degree (cubic to be precise). For a cubic curve we need 4 points (control points). These 4 points control the shape of the curve. Lets call the points p0, p1, p2 and p3. p0 is called start point, p1start tangent, p2end tangent, and p3end point. Lets imagine that the points are positioned like this:

Control Points

The slider at the bottom represents the t value.
In order to construct the function we are going to go through some steps. In each step we are going to make some lerps, and at the end we will combine these lerps.

Step One

We are going to make 3 lerps – between p0 and p1, between p1 and p2, and between p2 and p3.

Vector3 a = Lerp(p0, p1, t);
Vector3 b = Lerp(p1, p2, t);
Vector3 c = Lerp(p2, p3, t);

It looks like this

Lerp One

Now lets draw lines between a and b, and between b and c

Lerp One Plus Lines

Step Two

Again we are going to make some lerps – this time between a and b, and between b and c

Vector3 d = Lerp(a, b, t);
Vector3 e = Lerp(b, c, t);

Lerp Two

Now lets draw a line between d and e

Lerp Two Plus Lines

Step Three

Now all we need to do is to make one last lerp between d and e, so we can find the point on the curve at any given time t.

Vector3 pointOnCurve = Lerp(d, e, t);

Lets draw the point

Lerp Three

And now let’s draw the bezier curve

Draw Curve

The Function

Now if we combine all these lerps, we get the final function that defines any bezier curve by 4 points and a t value

Vector3 GetPointOnBezierCurve(Vector3 p0, Vector3 p1, Vector3 p2, Vector3 p3, float t)
{
    Vector3 a = Lerp(p0, p1, t);
    Vector3 b = Lerp(p1, p2, t);
    Vector3 c = Lerp(p2, p3, t);
    Vector3 d = Lerp(a, b, t);
    Vector3 e = Lerp(b, c, t);
    Vector3 pointOnCurve = Lerp(d, e, t);

    return pointOnCurve;
}

However this function is not very cheap. With a little math we can simplify the function and thus improve the performance.

Optimizations

Mathematically the function looks like this

Point On Curve

With a little calculations on paper we can simplify it to this

Optimized

So the final function in code can be written like this

Vector3 GetPointOnBezierCurve(Vector3 p0, Vector3 p1, Vector3 p2, Vector3 p3, float t)
{
    float u = 1f - t;
    float t2 = t * t;
    float u2 = u * u;
    float u3 = u2 * u;
    float t3 = t2 * t;

    Vector3 result =
        (u3) * p0 +
        (3f * u2 * t) * p1 +
        (3f * u * t2) * p2 +
        (t3) * p3;

    return result;
}

Bezier Splines (Bezier Paths)

What if we want to create a more complex curve?
There are two options:

  • Use a higher degree function
  • Combine (chain) cubic bezier curves

The first option is not very good, because every time we increase the degree of the function, we create more job for the CPU (we increase the number of calculations).
The second one is more widely used, and it can be explained like this:

Bezier Spline

p3 is the end of the first bezier curve and the start of the second one, and so on.

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Observer Pattern

What is the observer pattern used for, and why do we need it? If you’ve heard of the Model-View-Controller architectural pattern, let me tell you that underlying that is the Observer pattern. Observer is so widely used that java put it in its core library (java.util.Observer) and C# baked it right into the language (the event keyword).

In game development we have so many classes that do very different things. It’s good to keep them as decoupled as possible, but sometimes that is very hard. The observer pattern is a great tool to keep your code decoupled very easily.

Achievement System

Let’s say that we want to implement an achievement system. We can have so many different achievements like “Make 20 consecutive headshots”, “Beat the game under 3 hours”, “Fall from high height without dying”. Unlocking these achievements means that we have to make calls to some methods in many different places in our code. Let’s take for example the “Fall from high height without dying”. As you can guess this achievement is somehow tied to the Physics Engine, but do we really want to see a call to AchievementsManager.Unclock(Achievement.FallFromHighHeightWithoutDying) right in the middle of our collision detection or something? How do we avoid that?

That’s what the observer pattern is for. It lets one piece of code announce that something interesting happened without actually caring who receives the notification.

Imagine that our game world consists of entities. The interface for our entities is something like this:

public interface IEntity
{
    void PhysicsUpdate();

    // Other methods
}

The Physics Engine is responsible to update the physics of all entities in the game world. Our character is also an entity, and his PhysicsUpdate() method may look something like this:

public class Character : IEntity
{
    public virtual void PhysicsUpdate()
    {
        // Apply gravity and stuff...

        if (this.FellFromHighHeightWithoutDying())
        {
            AchievementsManager.Unlock(Achievement.FallFromHighHeightWithoutDying);
        }
    }

    // Other code...
}

We want to get rid of the call that unlocks the achievement through our AchievementsManager, because this way we couple the achievements system with our character. We are going to replace it with a call to another method:

NotifyObservers(this, Event.FallFromHighHeightWithoutDying);

This single line of code just says: “Look, I don’t know if anybody cares, but this entity just fell from high height without dying”. If someone cares however, he knows what to do. That someone is actually the observer. There can also be more than one observer, but let’s not get ahead of ourselves. Now we are decoupled from the AchievementsManager, but where did we actually get the NotifyObservers method from? All I can say is that it’s part of the big picture. Now lets draw that picture.

The Observer

Let’s start with the observer class. All observers observe for something interesting to happen. The interface for all observers is as follows.

public interface IObserver
{
    void OnNotify(IEntity entity, Event ev);
}

In our case the observer is the AchievementsManager.

public class AchievementsManager : IObserver
{
    public void OnNotify(IEntity entity, Event ev)
    {
        switch (ev)
        {
            case Event.FallFromHighHeightWithoutDying:
                bool isEntityCharacter = (entity as Character) != null;
                if (isEntityCharacter)
                {
                    this.UnlockAchievement(Achievement.FallFromHighHeightWithoutDying);
                }

                break;

            // Other cases...
        }
    }

    private void UnlockAchievement(Achievement achievement)
    {
        // Unlock if not already unlocked...
    }
}

The Observable Objects

The observable objects are the objects that are being observed by observers. These objects must also notify the observers if anything interesting happened. Let’s implement the interface for all observable objects. We need to be able to add, remove and notify any observers.

public interface IObservable
{
    void AddObserver(IObserver observer);

    void RemoveObserver(IObserver observer);

    void NotifyObservers(IEntity entity, Event ev);
}

In our case the observable object is our Character.

public class Character : IEntity, IObservable
{
    private List<IObserver> observers;

    // IEntity interface implementation
    public virtual void PhysicsUpdate()
    {
        // Apply gravity and stuff...

        if (this.FellFromHighHeightWithoutDying())
        {
            this.NotifyObservers(this, Event.FallFromHighHeightWithoutDying);
        }
    }
    // END IEntity interface implementation

    // IObservable interface implementation
    public virtual void AddObserver(IObserver observer)
    {
        this.observers.Add(observer);
    }

    public virtual void RemoveObserver(IObserver observer)
    {
        this.observers.Remove(observer);
    }

    public virtual void NotifyObservers(IEntity entity, Event ev)
    {
        foreach (var observer in this.observers)
        {
            observer.OnNotify(entity, ev);
        }
    }
    // END IObservable interface implementation
}

Now you see how it works. Our character is being observed by the achievement system. However, he actually doesn’t know that. He only knows that maybe he is being or is not being observed (sounds spooky :D). Either way, he must notify his observers (if any) when something interesting happens. The good thing is that he actually doesn’t know the true faces of his observers. He only has a list of references to instances of some interface. In this list our AchivementsManager is present, because the manager needs to know when the character falls from high height without dying. There is no problem for the AchievementsManager to know about the existence of our character, the problem is when our character knows about the manager. However, that problem is no more thanks to the observer pattern.

Deleting Observers

If you are using C# for a scripting language, or any language with garbage collection you must be wary of memory leaks. What do I mean? Let’s say for example that we have an observer (UI Screen) that observers some stats of our main character. When the player displays this UI screen, it’s added to the list of observers of the character, so when the stats change the UI screen is notified and updated. Let’s say the player closes this screen. The screen is not visible anymore. However the garbage collector will not delete it. You know why? Because we didn’t remove the observer from the list of observers. Our main character has a reference to the UI screen and the GC will not delete it.

If some observer has done it’s job and is not needed anymore, make sure that not a single observable object has a reference to it. Otherwise the observer won’t be garbage collected.

State Pattern

Why do we need this pattern and when to use it? In order to show you we are first going to see some bad code, and then we are going to rewrite the code with the state pattern.

Bad Code

Imagine that we are working on a third person stealth game, and we need to implement the character and make him respond to user input. He can jump and duck.

public class Character
{
    private bool isGrounded;
    private bool isDucking;
    private bool isJumping;

    public void Update()
    {
        // Update the player depending on the private fields
    }

    public void HandleInput(Input input)
    {
        if (input == Input.PRESS_SPACE)
        {
            if (this.isGrounded && !this.isDucking)
            {
                this.isJumping = true;
                this.isGrounded = false;
            }
        }
        else if (input == Input.PRESS_CTRL)
        {
            if (this.isGrounded)
            {
                this.isDucking = true;
            }
        }
        else if (input == Input.RELEASE_CTRL)
        {
            if (this.isDucking)
            {
                this.isDucking = false;
            }
        }
    }
}

At first glance this code is not written badly. When we press “Space” the character can jump only if he is grounded and not ducking. If we press “Ctrl” the character can duck only if he is grounded, not when he is jumping, and so on, and so on. Now imagine that the character can also swim, or even skydive like in Just Cause. Maybe he can also enter different vehicles (cars, jet planes). For all these states of the character we need different booleans – isSwimming, isSkydiving, isInCar, isInJetPlane. In all of these states the character has different input handling. If he is grounded for example and we press “Space” he will jump, but what if he is swimming, or he is in a car and we press “Space” again. Maybe while swimming  the character will ascent, and while he is in a car he will activate the brakes of the car. Now imagine that the character can do many other things. Soon our HandleInput(Input input) and Update() methods will be a nightmare to extent and maintain. In such cases the use of state machines can save our lives.

Finite State Machines

Finite State Machines are very simple. They only sound fancy. We can visualize our current character logic like that

State Machine

  1. The machine has a fixed set of states – Grounded, Ducking and Jumping in our case
  2. Only one state can be active at a time
  3. We can send input to the machine – in our case this is raw input from the user
  4. Each state has a set of transitions – each associated with an input and pointing to another state

Good Code – The State Pattern

Basically, all we need to do is to implement a finite state machine in code. How do we do that? If you are doing it for the first time – it’s hard. It’s actually very easy. Once you see how to do it, you won’t forget it.


Let us first start by implementing an interface for all states. Lets name it ICharacterState

public interface ICharacterState
{
    void OnEnter(Character character);

    void OnExit(Character character);

    void ToState(Character character, ICharacterState targetState);

    void Update(Character character);

    void HandleInput(Character character, Input input);
}

OnEnter and OnExit are very useful, because we can execute some code when the character enters/exits a specific state – play a sound, change the animation of the character, etc. These methods will be called automatically when we make a call to the ToState method. In order to make it automated we need a base abstract class for all of the states. Lets name it CharacterStateBase. But first lets change out Character class like this.

public class Character
{
    private ICharacterState state;

    public ICharacterState State
    {
        get
        {
            return this.state;
        }
        set
        {
            this.state = value;
        }
    }

    public void Update()
    {
        this.State.Update(this);
    }

    public void HandleInput(Input input)
    {
        this.State.HandleInput(this, input);
    }

    // Other code
}

The character has a state. In his Update() method we update his current state. Every state also has a different input handling. In the HandleInput(Input input) method we just make a call to the HandleInput(Character character, Input input) method of the current state.


And now the CharacterStateBase class.

public abstract class CharacterStateBase : ICharacterState
{
    public virtual void OnEnter(Character character) { }

    public virtual void OnExit(Character character) { }

    public virtual void ToState(Character character, ICharacterState targetState)
    {
        character.State.OnExit(character);
        character.State = targetState;
        character.State.OnEnter(character);
    }

    public abstract void Update(Character character);

    public abstract void HandleInput(Character character, Input input);
}

All that is left is to implement the “Grounded”, “Jumping” and “Ducking” states.


public class GroundedCharacterState : CharacterStateBase
{
    // Some code

    public override void HandleInput(Character character, Input input)
    {
        if (input == Input.PRESS_SPACE)
        {
            this.ToState(character, STATE_JUMPING);
        }
        else if (input == Input.PRESS_CTRL)
        {
            this.ToState(character, STATE_DUCKING);
        }

        // More code
    }

    public override void Update(Character character)
    {
        // Some code
    }
}

public class JumpingCharacterState : CharacterStateBase
{
    // Some code

    public override void HandleInput(Character character, Input input)
    {
        // Some code
    }

    public override void Update(Character character)
    {
        if (character.IsGrounded())
        {
            this.ToState(character, STATE_GROUNDED);
        }

        // More code
    }
}

public class DuckingCharacterState : CharacterStateBase
{
    // Some code

    public override void HandleInput(Character character, Input input)
    {
        if (input == Input.RELEASE_CTRL)
        {
            this.ToState(character, STATE_GROUNDED);
        }

        // More code
    }

    public override void Update(Character character)
    {
        // Some code
    }
}

The STATE_GROUNDED, STATE_JUMPING and STATE_DUCKING are just references to instances of GroundedCharacterState, JumpingCharacterState and DuckingCharacterState. You can create them anywhere you want. I personally like to create static readonly instances in the CharacterStateBase class.


I personally love this pattern. It’s very easy to extent the behavior of the character, we just need to create a bunch of different states. Once created we don’t touch them anymore. But as good as it is, there are also problems.

Concurrent State Machines

Lets imagine that the character is running. He is in a running state. But can he run and shoot at the same time? The problem is that only one state can be active at a time. The character can’t run and shoot at the same time, but we want him to be able to do so. One solution is to create a state that combines both the running and the shooting states. That however is not a very good solution. Imagine that the character can shoot while jumping, ducking, walking. Even worse – the character can shoot with different types of weapons (pistols, rifles, grenade launchers). The input handling for these weapons is a little different from one another. If we are to create that many combined states, we will end up overkilling the architecture of our code. A better solution is to create a secondary state, that can be updated separately from the main state. The character class will look like this.

public class Character
{
    private ICharacterState state;
    private ICharacterState equipmentState;

    public void Update()
    {
        this.State.Update(this);
        this.EquipmentState.Update(this);
    }

    public void HandleInput(Input input)
    {
        this.State.HandleInput(this, input);
        this.EquipmentState.HandleInput(this, input);
    }

    // Other code
}

When to Use

There is not a strict rule when to use it. This applies to all patterns in general. You need to ask yourself three questions:

  • Do you have an entity whose behavior changes based on some internal state?
  • Can that state be rigidly divided into one of a relatively small number of distinct options?
  • Does the entity respond to a series of inputs or events over time?

If the answer to all questions is a “YES” then you might consider using this pattern.

I’ve used it for simple AI behaviors. You can actually check this Video Tutorial from Unity Technologies. A guy shows how to implement a basic state machine for AI. However if you want to create a more complex AI, I advise you to use Behavior Trees. I’ve actually never used them myself, but I know that they are the right way to do more advanced AI.

I’ve also used the state pattern when I was implementing the UI in the main menu of a certain game. I had a bunch of screens and you can go from one screen to another, based on some user input. Each state is a different UI Screen. Based on the user input I just make transitions between the screens.

Abstract Input System in Unity

Right now I am creating a game. For this game I wanted to create a good input system. The thing is I didn’t know where to start. I am a big fan of Ori and the Blind Forest, so I decided to decompile the game and see how the big guys did it. As expected, their code was huge, but I took only the parts for my needs.

Interfaces

There are 2 basic interfaces for taking raw input: IAxisInput and IButtonInput.

public interface IAxisInput
{
    float GetAxis();
}

public interface IButtonInput
{
    bool GetButton(); // When the button is held down
    bool GetButtonDown(); // Executed once on button down
    bool GetButtonUp(); // Executed once on button up
}

For the next interface you need to know exactly what the character can do. In my case the character can only move horizontally and jump, so the interface looks like this.

public interface IInputProvider
{
    IAxisInput HorizontalAxisInput { get; }
    IButtonInput JumpButtonInput { get; }
}

This interface provides us with input from different sources. For example, we can create a KeyboardAndMouseInputProvider, or a MobileInputProvider. Lets actually implement those two.

KeyboardAndMouseInputProvider

public class KeyboardAndMouseInputProvider : IInputProvider
{
    private IAxisInput horizontalAxisInput;
    private IButtonInput jumpButtonInput;

    public KeyboardAndMouseInputProvider()
    {
        this.horizontalAxisInput = new CHorizontalAxisInput();
        this.jumpButtonInput = new CJumpButtonInput();
    }

    public virtual IAxisInput HorizontalAxisInput
    {
        get
        {
            return this.horizontalAxisInput;
        }
    }

    public virtual IButtonInput JumpButtonInput
    {
        get
        {
            return this.jumpButtonInput;
        }
    }

    private class CHorizontalAxisInput : IAxisInput
    {
        public float GetAxis()
        {
            return Input.GetAxis("Horizontal");
        }
    }

    private class CJumpButtonInput : IButtonInput
    {
        public bool GetButton()
        {
            return Input.GetButton("Jump");
        }

        public bool GetButtonDown()
        {
            return Input.GetButtonDown("Jump");
        }

        public bool GetButtonUp()
        {
            return Input.GetButtonUp("Jump");
        }
    }
}

MobileInputProvider

public class MobileInputProvider : MonoBehaviour, IInputProvider
{
    [SerializeField]
    private VirtualJoystick leftJoystick = null;

    private IAxisInput horizontalAxisInput;
    private IButtonInput jumpButtonInput;

    public virtual IAxisInput HorizontalAxisInput
    {
        get
        {
            return this.horizontalAxisInput;
        }
    }

    public virtual IButtonInput JumpButtonInput
    {
        get
        {
            return this.jumpButtonInput;
        }
    }

    public void Init()
    {
        this.horizontalAxisInput = new CHorizontalAxisInput(this.leftJoystick);
        this.jumpButtonInput = new CJumpButtonInput();
    }

    private class CHorizontalAxisInput : IAxisInput
    {
        private VirtualJoystick leftJoystick;

        public CHorizontalAxisInput(VirtualJoystick leftJoystick)
        {
            this.leftJoystick = leftJoystick;
        }

        public float GetAxis()
        {
            return this.leftJoystick.GetAxes().x;
        }
    }

    private class CJumpButtonInput : IButtonInput
    {
        private int lastJumpTouchId;

        public bool GetButton()
        {
            if (Input.touchCount > 0)
            {
                foreach (var touch in Input.touches)
                {
                    if ((touch.phase == TouchPhase.Stationary || touch.phase == TouchPhase.Moved) &&
                        (touch.position.x > Screen.width / 2f) &&
                        (touch.fingerId == this.lastJumpTouchId))
                    {
                        return true;
                    }
                }
            }

            return false;
        }

        public bool GetButtonDown()
        {
            if (Input.touchCount > 0)
            {
                foreach (var touch in Input.touches)
                {
                    if (touch.phase == TouchPhase.Began &&
                        touch.position.x > Screen.width / 2f)
                    {
                        this.lastJumpTouchId = touch.fingerId;
                        return true;
                    }
                }
            }

            return false;
        }

        public bool GetButtonUp()
        {
            if (Input.touchCount > 0)
            {
                foreach (var touch in Input.touches)
                {
                    if (touch.phase == TouchPhase.Ended &&
                        touch.position.x > Screen.width / 2f &&
                        touch.fingerId == this.lastJumpTouchId)
                    {
                        return true;
                    }
                }
            }

            return false;
        }
    }
}

Another layer of abstraction

In Unity if we want to get an axis we just do it like this.

float horizontalAxis = Input.GetAxis("Horizontal");

But we can’t get the the horizontal axis like that if we are on a mobile device. We have a MobileInputProvider, we just need a class that uses it.

public static class PlayerInput
{
    public static IInputProvider InputProvider { get; set; }

    public static IAxisInput HorizontalAxisInput
    {
        get
        {
            return InputProvider.HorizontalAxisInput;
        }
    }

    public static IButtonInput JumpButtonInput
    {
        get
        {
            return InputProvider.JumpButtonInput;
        }
    }
}

 
Now we can get the horizontal axis from a mobile device like this.

PlayerInput.InputProvider = mobileInputProvider;
float horizontalAxis = PlayerInput.HorizontalAxisInput.GetAxis();

Compound Input Provider

But what if we want to be able to get input both from a keyboard and from a mobile device. Well we need a provider that gets input from both, but that will be kinda stupid, because we have them in separate, why create a third provider that is copy-paste from the first two? We just need to think of a smart way to combine both input providers. That is how we do it.
 
First we create an axis input that can get axes from many sources. We will call this one CompoundAxisInput. Here is the implementation.

public class CompoundAxisInput : IAxisInput
{
    private const float AXIS_DEAD_ZONE = 0.2f;

    private IAxisInput[] axisInputs;
    private int lastPressedIndex;

    public CompoundAxisInput() { }

    public CompoundAxisInput(params IAxisInput[] axisInputs)
    {
        this.axisInputs = axisInputs;
    }

    public virtual float GetAxis()
    {
        float positiveAxis = 0f;
        float negativeAxis = 0f;
        if (this.axisInputs != null)
        {
            for (int i = 0; i < this.axisInputs.Length; i++)
            {
                float value = this.axisInputs[i].GetAxis();
                if (Mathf.Abs(value) > AXIS_DEAD_ZONE)
                {
                    this.lastPressedIndex = i;
                }
                else
                {
                    continue;
                }

                if (value < 0f)
                {
                    negativeAxis = Mathf.Min(negativeAxis, value);
                }
                else
                {
                    positiveAxis = Mathf.Max(positiveAxis, value);
                }
            }
        }

        return positiveAxis + negativeAxis;
    }

    public IAxisInput GetLastPressed()
    {
        return this.axisInputs[this.lastPressedIndex];
    }

    public void AddAxisInput(IAxisInput axisInput)
    {
        if (this.axisInputs == null)
        {
            this.axisInputs = new IAxisInput[1];
            this.axisInputs[0] = axisInput;
        }
        else
        {
            Array.Resize(ref this.axisInputs, this.axisInputs.Length + 1);
            this.axisInputs[this.axisInputs.Length - 1] = axisInput;
        }
    }

    public void ClearAxisInputs()
    {
        this.axisInputs = null;
    }
}

It’s an axis input that internally has an array of axis inputs. When we call the GetAxis() method, we find the most negative one and most positive one from all of the axis inputs and return the sum of them. That way if we press left on a keyboard and right on a joystick for example, the character will stay in one place, because the horizontal axis will be zero.


Now we have to do the same for the button input. Lets call the class CompoundButtonInput.

public class CompoundButtonInput : IButtonInput
{
    private IButtonInput[] buttonInputs;
    private int lastPressedIndex;

    public CompoundButtonInput() { }

    public CompoundButtonInput(params IButtonInput[] buttonInputs)
    {
        this.buttonInputs = buttonInputs;
    }

    public virtual bool GetButton()
    {
        if (this.buttonInputs != null)
        {
            for (int i = 0; i < this.buttonInputs.Length; i++)
            {
                if (this.buttonInputs[i].GetButton())
                {
                    this.lastPressedIndex = i;
                    return true;
                }
            }
        }

        return false;
    }

    public virtual bool GetButtonDown()
    {
        if (this.buttonInputs != null)
        {
            for (int i = 0; i < this.buttonInputs.Length; i++)
            {
                if (this.buttonInputs[i].GetButtonDown())
                {
                    this.lastPressedIndex = i;
                    return true;
                }
            }
        }

        return false;
    }

    public virtual bool GetButtonUp()
    {
        if (this.buttonInputs != null)
        {
            for (int i = 0; i < this.buttonInputs.Length; i++)
            {
                if (this.buttonInputs[i].GetButtonUp())
                {
                    this.lastPressedIndex = i;
                    return true;
                }
            }
        }

        return false;
    }

    public IButtonInput GetLastPressed()
    {
        return this.buttonInputs[this.lastPressedIndex];
    }

    public void AddButtonInput(IButtonInput buttonInput)
    {
        if (this.buttonInputs == null)
        {
            this.buttonInputs = new IButtonInput[1];
            this.buttonInputs[0] = buttonInput;
        }
        else
        {
            Array.Resize(ref this.buttonInputs, this.buttonInputs.Length + 1);
            this.buttonInputs[this.buttonInputs.Length - 1] = buttonInput;
        }
    }

    public void ClearButtonInputs()
    {
        this.buttonInputs = null;
    }
}

Finally we create a CompoundInputProvider

public class CompoundInputProvider : IInputProvider
{
    private IInputProvider[] inputProviders;
    private CompoundAxisInput horizontalAxisInput;
    private CompoundButtonInput jumpButtonInput;

    public CompoundInputProvider()
        : this(null)
    { }

    public CompoundInputProvider(params IInputProvider[] inputProviders)
    {
        this.horizontalAxisInput = new CompoundAxisInput();
        this.jumpButtonInput = new CompoundButtonInput();

        if (inputProviders != null)
        {
            for (int i = 0; i < inputProviders.Length; i++)
            {
                this.AddInputProvider(inputProviders[i]);
            }
        }
    }

    public virtual IAxisInput HorizontalAxisInput
    {
        get
        {
            return this.horizontalAxisInput;
        }
    }

    public virtual IButtonInput JumpButtonInput
    {
        get
        {
            return this.jumpButtonInput;
        }
    }

    public void AddInputProvider(IInputProvider inputProvider)
    {
        if (this.inputProviders == null)
        {
            this.inputProviders = new IInputProvider[1];
            this.inputProviders[0] = inputProvider;
        }
        else
        {
            Array.Resize(ref this.inputProviders, this.inputProviders.Length + 1);
            this.inputProviders[this.inputProviders.Length - 1] = inputProvider;
        }

        this.horizontalAxisInput.AddAxisInput(inputProvider.HorizontalAxisInput);
        this.jumpButtonInput.AddButtonInput(inputProvider.JumpButtonInput);
    }

    public void ClearInputProviders()
    {
        this.inputProviders = null;
        this.horizontalAxisInput.ClearAxisInputs();
        this.jumpButtonInput.ClearButtonInputs();
    }
}

All that’s left is to provide our PlayerInput class with the right CompoundInputProvider. I do this in an InputManager script.

public class InputManager : MonoBehaviour
{
    [SerializeField]
    private MobileInputProvider mobileInputProvider = null;

    protected virtual void Awake()
    {
        this.InitInputProvider();
    }

    private void InitInputProvider()
    {
        CompoundInputProvider compoundInputProvider = new CompoundInputProvider();
        if (this.mobileInputProvider != null)
        {
            this.mobileInputProvider.Init();
            compoundInputProvider.AddInputProvider(this.mobileInputProvider);
        }

#if UNITY_EDITOR
        compoundInputProvider.AddInputProvider(new KeyboardAndMouseInputProvider());
#endif

        PlayerInput.InputProvider = compoundInputProvider;
    }
}

Now we are ready to use our PlayerInput class instead of the Input class that Unity gives us.

Dynamic Queue Implementation – C#

A dynamic queue is a queue that is implemented with nodes. Nodes consist of two things: value and link to another node.

The following image illustrates the implementation:

dynamic-queue

The most left node is the front of the queue, and the most right one is the back of the queue. Every node is connected to the one right of it. The back of the queue points to null.

You can see the implementation of the queue here.
Lets jump to the explanation.


First lets see how the Node is implemented.

private class Node
{
	public T Item { get; set; }
	public Node Next { get; set; }

	public Node(T item)
	{
		this.Item = item;
		this.Next = null;
	}

	public Node(T item, Node previous)
	{
		this.Item = item;
		this.Next = null;
		previous.Next = this;
	}
}

We have an Item which represents the value of the node. The link is a simple reference to a node and is called Next. We also have two constructors. One that creates a node with some item and has a pointer to null. The other one creates the same node, but the difference is that a previous node is linked to the one that was just created. The Node class is a private class in the Queue class which I called CustomQueue.


Now lets jump to the CustomQueue.
Lets see what fields, constructors and properties we have.

private Node front;
private Node back;
private int count;

public CustomQueue()
{
	this.front = null;
	this.back = null;
	this.count = 0;
}

public int Count
{
	get
	{
		return this.count;
	}
}

We have references(pointers) to the front and the back of the queue. There is a count field that has a Count property with getter only (we don’t want to mess up the count from the outside). We also have only a single default constructor that makes an empty queue by initializing the front and the back with null and count with 0.


Now lets see the basic methods.

public void Enqueue(T item)
{
	if (this.front == null)
	{
		// We have empty queue
		this.front = new Node(item);
		this.back = this.front;
	}
	else
	{
		Node newNode = new Node(item, this.back);
		this.back = newNode;
	}

	this.count++;
}

The “Enqueue” method adds a new element to the front of the queue. First we check if the queue is empty (if so we make a queue with only one node that points to null). If the queue is not empty, we create a new node and tell the back of the queue to start pointing at the node we’ve just created. This is accomplished with the second constructor. We have to tell that the new back of the queue is the new node. Last we increment the count.

Consider the following code:

CustomQueue<int> queue = new CustomQueue<int>();
queue.Enqueue(1);
queue.Enqueue(2);
queue.Enqueue(3);

In the memory that looks much like this:

enqueue


public T Dequeue()
{
	if (this.count == 0)
	{
		// We have empty queue
		throw new InvalidOperationException("The queue is empty");
	}

	T result = this.front.Item;
	this.front = this.front.Next;
	this.count--;

	return result;
}

The “Dequeue” method removes the front of the queue and returns the value of it. We first make a check if the queue is empty (if so we throw an InvalidOperationExceltion). If the queue is not empty, then we remove the front. It’s simple, we just initialize the front with it’s link. The garbage collector will take care of the node that is discarded from the queue. We also have to decrement the count and return the result (the value of the removed element).

Consider the following code:

CustomQueue<int> queue = new CustomQueue<int>();
queue.Enqueue(1);
queue.Enqueue(2);
queue.Enqueue(3);

queue.Dequeue();
queue.Dequeue();
queue.Dequeue();

The dequeue process should look something like this:

dequeue


public T Peek()
{
	if (this.count == 0)
	{
		// We have empty queue
		throw new InvalidOperationException("The queue is empty");
	}

	return this.front.Item;
}

The “Peek” method gets the value of the front of the queue. Again there is a check for empty queue and an exception throw if it’s empty.


public void Clear()
{
	this.front = null;
	this.back = null;
	this.count = 0;
}

The “Clear” method clears the queue (makes it empty). It’s just like the constructor. We initialize the front and the back with null and the count with 0.


CustomQueue implements the ICloneable interface. It’s useful to clone the queue sometimes.

public object Clone()
{
	CustomQueue<T> clone = new CustomQueue<T>();

	Node currentNode = this.front;
	while (currentNode != null)
	{
		// We have non-empty queue
		clone.Enqueue(currentNode.Item);
		currentNode = currentNode.Next;
	}

	return clone;
}

The “Clone” method creates a deep copy of the queue. How do we do that? We create an empty queue named “clone”. If the original queue is empty we just return the empty clone queue. If it is not empty however, we have to make the deep copy. It’s very simple. We have a reference(node) called “currentNode” that first points to the front of the original queue. This node helps us to walk through the original queue from the front to the back. We say this: “while currentNode != null (we haven’t reached the end of the queue) – enqueue the value of the current node we’ve reached into the clone queue and go to the next node of the original queue”. Then we return the clone.

It looks like this:

queue-clone-process


I’ve also added some extension methods in the CustomQueue class. They are helpful, plus helped me for the unit tests.

public override string ToString()
{
	StringBuilder result = new StringBuilder();

	Node currentNode = this.front;
	while (currentNode != null)
	{
		result.Append(currentNode.Item);
		currentNode = currentNode.Next;
	}

	return result.ToString();
}

public T[] ToArray()
{
	T[] result = new T[this.count];

	Node currentNode = this.front;
	for (int i = 0; i < result.Length; i++)
	{
		result[i] = currentNode.Item;
		currentNode = currentNode.Next;
	}

	return result;
}

“ToString” and “ToArray” are much like the clone method. We are walking through the original queue and the value of the current node we’ve reached is appended in a StringBuilder in the first case, and put in an array in the second case.


The CustomQueue also implements the IEnumerable<T> interface so that the queue can be walked with a foreach loop.

public IEnumerator<T> GetEnumerator()
{
	Node currentNode = this.front;
	while (currentNode != null)
	{
		T result = currentNode.Item;
		currentNode = currentNode.Next;

		yield return result;
	}
}

IEnumerator IEnumerable.GetEnumerator()
{
	return this.GetEnumerator();
}

The IEnumerable and IEnumerator interfaces are hard to explain and this post does not concern them, so I will stop here. Maybe I will make a post on how to implement these interfaces the hard way and the easy way (here I’ve used the easy way). That’s all. Hope it was useful.

Dynamic Stack Implementation – C#

A dynamic stack is a stack that is implemented not with an array but with nodes. A node consists of two things (a value and a link to another node).

The next image illustrates the stack.

dynamic-stack

You can find the full source here.

Lets see how the Node is implemented.

private class Node
{
    public T Item { get; set; }
    public Node Next { get; set; }

    public Node(T item)
    {
        this.Item = item;
        this.Next = null;
    }

    public Node(T item, Node next)
    {
        this.Item = item;
        this.Next = next;
    }
}

In my implementation the value is called Item and the link to the next node is a Node reference called Next. We also have two constructors. One that creates a node with a given item and a link that points to null, and one that creates a node with a given item and a link that points to a node different than null.


What do we have?

private Node top;
private int count;

public CustomStack()
{
    this.top = null;
    this.count = 0;
}

public int Count
{
    get
    {
        return this.count;
    }
}

We have a node that points to the top of the stack. The count is optional but it’s always better to know how many items we have in a stack. We have only a simple default constructor that initializes our stack. The top is null and the count is zero when the stack is created. We also have a property for the count with getter only.


Not lets see how the basic methods are implemented.

public void Push(T item)
{
    if (this.top == null)
    {
        // We have empty stack
        this.top = new Node(item);
    }
    else
    {
        Node newNode = new Node(item, this.top);
        this.top = newNode;
    }

    this.count++;
}

The “Push” method adds a new node to the stack. If the stack is empty, then we create a new node that points to null. This new node is the only element in the stack (the stack top). If the stack is not empty however, a new node is created. This node has to be the top of the stack so we point it’s link to the stack top. Then the top node starts to point the new node (the new node becomes the top of the stack).

Lets see it with our eyes.
Consider the following code:

CustomStack<int> stack = new CustomStack<int>();
stack.Push(1);
stack.Push(2);
stack.Push(3);

In the memory it should look something like this:

stack-push


public T Peek()
{
    if (this.count == 0)
    {
        // We have empty stack
        throw new InvalidOperationException("The stack is empty");
    }

    return this.top.Item;
}

The “Peek” method gets the value of the top without removing the top from the stack. If the stack is empty however, an InvalidOperationException is thrown.


public T Pop()
{
    if (this.count == 0)
    {
        // We have empty stack
        throw new InvalidOperationException("The stack is empty");
    }

    T result = this.top.Item;
    this.top = this.top.Next;
    this.count--;

    return result;
}

The “Pop” is almost the same as “Peek”. The difference is that it not only gets the value of the top but it also removes the top from the stack. The value is taken and the top is assigned to it’s link (next node). The count is decremented. Finally the value is returned.

Consider the following code:

CustomStack<int> stack = new CustomStack<int>();
stack.Push(1);
stack.Push(2);
stack.Push(3);

stack.Pop();
stack.Pop();
stack.Pop();

The popping process looks like this:

stack-pop


public void Clear()
{
    this.top = null;
    this.count = 0;
}

“Clear” clears the stack. It’s just like the constructor. We initialize the top with null and the count with 0.


public object Clone()
{
    CustomStack clone = new CustomStack();

    if (this.count != 0)
    {
        // We have non-empty stack
        clone.top = new Node(this.top.Item);
        Node currentNode = this.top.Next;
        Node cloneLastNode = clone.top;

        while (currentNode != null)
        {
            Node newNode = new Node(currentNode.Item);
            cloneLastNode.Next = newNode;
            cloneLastNode = newNode;
            currentNode = currentNode.Next;
        }

        cloneLastNode.Next = null;
        clone.count = this.count;
    }

    return clone;
}

This is maybe the most complex method of all. If the stack is empty the job is easy. We just make an empty stack and returned it. If it is not, then we have to make the actual copy. This method makes a deep copy. That means that the two stacks don’t depend on each other. They are equal, but not the same (don’t share the same reference). How do we do that? First we make the clone a copy of the top. The “currentNode” is a helper node that will walk through the original stack from the top to the end. The “cloneLastNode” is the last node of the clone stack at any state of the copying progress. At first the “cloneLastNode” is the top of the clone stack. While “currentNode” is not null (we haven’t reached the original stack’s end) we make a new node exactly the same as the current node we’ve reached. Then the last node of the clone stack is linked to the new node and then is assigned to it. That way in the next iteration this can be repeated. The “currentNode” is also assigned to it’s link. Last we link the last node of the clone stack to null and copy the count of the original stack. I can say it like that: “while we haven’t reached the end of the stack, create a new node with value equal to the value of the current node we’ve reached, link the last node of the clone stack with the new node and make the last node of the clone to be the new node, then go to the next node of the original stack. Last the link of the last node of the clone stack is initialized with null”.

You just have to think it through and try to write it in a paper.
It’s very hard to be explained, maybe a picture will help:

stack-clone


The next methods are not basic for Stack implementation but are helpful sometimes. You can consider them as extensions.

public override string ToString()
{
    StringBuilder result = new StringBuilder();

    Node currentNode = this.top;
    while (currentNode != null)
    {
        result.Append(currentNode.Item.ToString());
        currentNode = currentNode.Next;
    }

    return result.ToString();
}

I’ve overrided the ToString method. It’s very simple. The stack is walked through from the top to the end and the values of the nodes are appended to a StringBuilder.


public T[] ToArray()
{
    T[] result = new T[this.count];

    Node currentNode = this.top;
    for (int i = 0; i < this.count; i++)
    {
        result[i] = currentNode.Item;
        currentNode = currentNode.Next;
    }

    return result;
}

Exactly the same as ToString. The difference is that the values of the nodes are put into an array.


public IEnumerator GetEnumerator()
{
    Node currentNode = this.top;
    while (currentNode != null)
    {
        T result = currentNode.Item;
        currentNode = currentNode.Next;

        yield return result;
    }
}

IEnumerator IEnumerable.GetEnumerator()
{
    return this.GetEnumerator();
}

This one is hard to be explained. My stack implements IEnumerable<T>. That way it can be walked through with a foreach loop. If you want to learn about it you can find a lot of information in MSDN or other resources. Maybe I will make a different post to explain how to implement this interface the hard way and the easy way.