Question

I have small library i want to use for creating games. First, i tried to implement pixel perfect collision detection, but that did not went well, so i decided to use simple bounding box collision detection. It works fine, but after amount of objects exceeds around 20, it starts slowing down. Here is my code:
(Runs in loop, 25 times per second)

for (int i=0;i<sc.collGr.size();i++){
    CollisionGroup gr=sc.collGr.get(i);
    Collidable[] cc=gr.getCollidables();
    for (int l=0;l<cc.length;l++){
        for (int w=l+1;w<cc.length;w++){
            if (BorderBox.areColliding(cc[l].getBorderBox(), cc[w].getBorderBox()){
                addEventToHandler(sc.collGr.get(i),cc[l],cc[w]);
            }
        }
   }
}

part of BorderBox class:

public class BorderBox {
int top;
int down;
int left;
int right;

/**
 * Creates new BorderBox object
 * Arguments: (top, down, left, right);
 * */
public BorderBox(int topy,int downy, int leftx,int rightx){
    top=topy;
    down=downy;
    left=leftx;
    right=rightx;
}

/**
 * Checks if two provided BorderBoxes are colliding.
 * */
public static boolean areColliding(BorderBox a,BorderBox b){
    if (b.left<=a.right && b.right>=a.left && b.down>=a.top && b.top<=a.down){
        return true;
    }
    return false;
}

Answer 1

Checking this way is an O(n^2) operation. That is, as you add a new objects, the amount of work you are doing grows quadratic-ally.

The way I have mitigated this before, along with the other suggestions, is to have two types of collidable objects in two different arrays. One array has a small number of objects, primarily the character, and the other array has enemies. The idea is that you don’t care about enemies colliding with each-other. Thus you dramatically cut down on the amount of work you have to do, by only checking if elements from the smaller array collide with elements in the larger one.

This is of course game specific, and cannot work if everything has to collide with everything else. Although, I find that is often not the case with simple games.