function addToQuadTree(tree, entity)
{
    if(!rectCollidesRect(tree.pos, tree.size, entity.pos, entity.size)) {
        return false;
    }
    if(tree.elements.length < tree.capacity) {
        tree.elements.push(entity);
        return true;
    }
    else {
        if(!tree.children) {
            tree.children = [];
            let child0 = new QuadTree();
            let child1 = new QuadTree();
            let child2 = new QuadTree();
            let child3 = new QuadTree();

            child0.pos.x = tree.pos.x;
            child0.pos.y = tree.pos.y;
            child0.pos.size.x = tree.size.x / 2;
            child0.pos.size.y = tree.size.y / 2;

            child1.pos.x = tree.pos.x + tree.size.x / 2;
            child1.pos.y = tree.pos.y;
            child1.pos.size.x = tree.size.x / 2;
            child1.pos.size.y = tree.size.y / 2;

            child2.pos.x = tree.pos.x;
            child2.pos.y = tree.pos.y + tree.size.y / 2;
            child2.pos.size.x = tree.size.x / 2;
            child2.pos.size.y = tree.size.y / 2;

            child3.pos.x = tree.pos.x + tree.size.x / 2;
            child3.pos.y = tree.pos.y + tree.size.y / 2;
            child3.pos.size.x = tree.size.x / 2;
            child3.pos.size.y = tree.size.y / 2;

            tree.children.push(child0);
            tree.children.push(child1);
            tree.children.push(child2);
            tree.children.push(child3);
        }
        for(let i = 0; i < 4; i += 1) {
            if(addToQuadTree(tree.children[i], entity)) {
                return true;
            }
        }
    }
    return false;
}