Hello,

I've got some code for a 2d game that orients an NPCs graphic towards the direction he is walking to. There are 4 directions (down, left, right, up), that have the values (0, 1, 2, 3) respectively (so 3 - direction will yield the other direction on that axis). It is possible for the NPC to walk straight, diagonally, and unevenly-diagonally (2 pixel on the x-axis, 1 on the y-axis, hope you know what I'm talking about). Now the code for mapping this movement to the direction is rather clumsy, just a big mess of if-statements:

unsigned int DirectionFromMove(const math::Vector2& vMove)
{
	if(vDir.x > 0)
	{
		if(vDir.y > 0)
		{
			if(vDir.x > vDir.y)
				return 2;
			else
				return 0;
		}
		else if(vDir.y < 0)
		{
			if(vDir.x > abs(vDir.y))
				return 2;
			else
				return 3;
		}
		else
			return 2;
	}
	else if(vDir.x < 0)
	{
		if(vDir.y > 0)
		{
			if(abs(vDir.x) > vDir.y)
				return 1;
			else
				return 0;
		}
		else if(vDir.y < 0)
		{
			if(vDir.x < vDir.y)
				return 1;
			else
				return 3;
		}
		else
			return 1;
	}
	else
	{
		if(vDir.y > 0)
			return 3;
		else if(vDir.y < 0)
			return 0;
	}
}

I thought about it for a while, but didn't find any better way myself. I'm really curious, since in "production"-code I always end up with stuff like this. Is there some clever solution here to get rid of all the ifs?

unsigned int DirectionFromMove(const math::Vector2& vMove)
{
    if(vDir.x < vDir.y) {
        // left or up
        if (vDir.x > -vDir.y){
            return 3;
        }
        else {
            return 1;
        }
    }
    else {
        // right or down
        if (vDir.x > -vDir.y){
            return 2;
        }
        else {
            return 0;
        }
    }
}

I hope this works... gone testing...

EDIT: Seems to work. What this does is separating the space by the function f(x) = x and after that by the function f(x) = -x testing which side of the half-plane the point lies in.

I hope this works... gone testing...

Yep, this does indeed work, thanks a lot, thats a whole lot easier. Except that I somehow got up and down mixed up, at least its easier to fix in your version :D So the general advice for such monstrous if-statements would be "look for an easier algorithmn", or is there some recommandation in case there is no easier algorithmn scheme?

unsigned int DirectionFromMove(const math::Vector2& vMove)
{
    if(vDir.x < vDir.y) {
        // left or up
        if (vDir.x > -vDir.y){
            return 3;
        }
        else {
            return 1;
        }
    }
    else {
        // right or down
        if (vDir.x > -vDir.y){
            return 2;
        }
        else {
            return 0;
        }
    }
}

I hope this works... gone testing...

EDIT: Seems to work. What this does is separating the space by the function f(x) = y and after that by the function f(x) = -y testing which side of the half-plane the point lies in.

Assuming that works you can be even more fancy if you want and do

unsigned int DirectionFromMove(const math::Vector2& vMove)
{
    return (vDir.x < vDir.y) + 2*(vDir.x > -vDir.y);
}

Also untested...

Yep, this does indeed work, thanks a lot, thats a whole lot easier. Except that I somehow got up and down mixed up, at least its easier to fix in your version So the general advice for such monstrous if-statements would be "look for an easier algorithmn", or is there some recommandation in case there is no easier algorithmn scheme?

I started by drawing a crude diagram and looking for a mathematical solution. Sometimes it's easier to visualize that way.

I want to add something to my last post.

If you have a function that returns 4 distinct values but there are some code paths that return the same value this is a very good indicator that the function can be refactored into a simpler solution.

When doing calculations in vector space I'd always recommended to scribble up a diagram except for the simplest of cases.