Monthly Archives: May 2010

Tank Age: Movement

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I’ve been developing a top-down tank game, and I needed a way to accurately represent the movement of a vehicle with independently mobile treads. Obviously controls such as left/right/up/down needed to be polled from the keyboard, but their specific interaction with eachother and the rate of movement corresponding to a key being continuously held down needs to be representative of gas/brakes, etc. Additionally, when a tank turns, its speed is negatively affected by the nature of treads; To turn a tank slows or reverses one tread, thus causing a differential in the speed one tread is traveling in relation to another. Here’s what I came up with:

    private void gameLoopInput()
    {
        Tank tank = _state.getTank();

        if(_input.left())
            tank.rotateLeft();
        if(_input.right())
            tank.rotateRight();

        if((_input.left() == false) && (_input.right() == false))
            if(_input.up())
                tank.increaseDirectionalForce();

        if(_input.down())
            tank.decreaseDirectionalForce();

        if(_input.up() == false && _input.down() == false)
            tank.decayDirectionalForce();
    }

The specifics behind the rotation methods of the tank sprite:

    public void rotateLeft()
    {
        if(_velocity > 80)
            _angle = clampAngle(_angle - Constants.ROTATE_ANGLE_FAST);
        else
            _angle = clampAngle(_angle - Constants.ROTATE_ANGLE_SLOW);
    }

    public void rotateRight()
    {
        if(_velocity > 80)
            _angle = clampAngle(_angle + Constants.ROTATE_ANGLE_FAST);
        else
            _angle = clampAngle(_angle + Constants.ROTATE_ANGLE_SLOW);
    }

    private double clampAngle(double angle)
    {
        double result = angle % 360;
        if (result < 0)
            result = result + 360;
        return result;
    }

And the wrappers around those methods for the tank, as well as the other tank methods:

    public void increaseDirectionalForce()
    {
        long millis = System.currentTimeMillis();
        if((millis - Constants.SPEED_DELAY) < _lastDirectionalForceChangeMillis)
            return;

        if(_directionalForce < 10)
        {
            _directionalForce++;
            _lastDirectionalForceChangeMillis = System.currentTimeMillis();
        }
    }

    public void decreaseDirectionalForce()
    {
        long millis = System.currentTimeMillis();
        if((millis - Constants.SPEED_DELAY) < _lastDirectionalForceChangeMillis)
            return;

        if(_directionalForce > -10)
        {
            _directionalForce--;
            _lastDirectionalForceChangeMillis = System.currentTimeMillis();
        }
    }

    public void decayDirectionalForce()
    {
        long millis = System.currentTimeMillis();
        if(millis - Constants.SPEED_DELAY < _lastDirectionalForceChangeMillis)
            return;

        if(_directionalForce > 0)
        {
            _directionalForce--;
            _lastDirectionalForceChangeMillis = System.currentTimeMillis();
        }
        else if(_directionalForce < 0)
        {
            _directionalForce++;
            _lastDirectionalForceChangeMillis = System.currentTimeMillis();
        }
    }

    public void rotateLeft()
    {
        _sprite.rotateLeft();

        long millis = System.currentTimeMillis();
        if(millis - Constants.SPEED_DELAY < _lastDirectionalForceChangeMillis)
            return;

        if(_directionalForce > 5)
            _directionalForce--;

        _lastDirectionalForceChangeMillis = System.currentTimeMillis();
    }

    public void rotateRight()
    {
        _sprite.rotateRight();

        long millis = System.currentTimeMillis();
        if(millis - Constants.SPEED_DELAY < _lastDirectionalForceChangeMillis)
            return;

        if(_directionalForce > 5)
            _directionalForce--;

        _lastDirectionalForceChangeMillis = System.currentTimeMillis();
    }

Zorro!

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I’m doing some simplistic string exercises using some of the programming competition problems from years past at EWU. This one is called ‘Zorro’. It takes in an integer representing the width of a ‘Z’ which will be represented on the screen using the letters of the alphabet in sequence (circularly).

Example input:

4

Example output:

abcd
  e
 f
ghij

This is the main body of the code to do it:

        private static void OutputZ(int widthX)
        {
            Console.WriteLine();

            char c = 'z';
            int posX = 0;

            for (posX = 0; posX < widthX; posX++)
                Console.Write(c = NextChar(c));

            Console.WriteLine();

            for (posX = widthX - 2; posX > 0; posX--)
            {
                for (int i = 0; i < posX; i++)
                    Console.Write(" ");

                Console.WriteLine(c = NextChar(c));
            }

            for (posX = 0; posX < widthX; posX++)
                Console.Write(c = NextChar(c));

            Console.WriteLine();
        }

 

OpenID Vocabulary

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Most programmers have heard of OpenID, and have seen it implemented usefully at many internet sites, but unless you have to implement it yourself, you probably don’t care much about its internal workings. For the project I’m working on, I decided that the notion of authentication and user storage is a solved problem, so I opted to defer that responsibility to others. OpenID was the natural choice, so I set out to learn more about it. I could have just gone with a 3rd party library, but abstractions are leaky, and its important to understand them. I’ll share the vocabulary I’ve become familiar with here:

  • Identifier – This is the central idea behind OpenID; The URL that identifies a user.
  • Provider – The authenticating site who verifies that someone is who they say they are.
  • Relying Party – A site that depends on a Provider to verify a persons authenticity.

There are other terms but those are the important ones.