Class QuadtreeNode

src/main/java/mudmap2/backend/prquadtree/Quadtree.java:233–518

A quadtree node with up to four children, but no own data @param element data type

     * @param <T> element data type
     */
    private class QuadtreeNode<T> implements QuadtreeElement<T>{

        QuadtreeElement<T> parent;
        int length; // size in each direction
        // max amount of children below this node: (2 * length)^2
        final int x, y;
        private final QuadtreeElement<T> elements[] = new QuadtreeElement[4];

        // fake enum (no not-static enums in Java?)
        private static final int NW = 0, NE = 1, SW = 2, SE = 3;

        /**
         * Constructs a new node
         * @param _parent parent node or null (root)
         * @param _x coordinate next to the center (-0.5, -0.5)
         * @param _y coordinate next to the center (-0.5, -0.5)
         * @param _length size in each direction
         */
        public QuadtreeNode(QuadtreeNode<T> parent, int x, int y, int length) {
            this.parent = parent;
            this.x = x;
            this.y = y;
            this.length = length;
            for(int i = 0; i < 4; ++i){
                elements[i] = null;
            }
        }

        /**
         * Gets the index of the child at x, y, or -1 if outside of this node
         * @param x
         * @param y
         * @return index or -1
         */
        private int getChildNum(int x, int y){
            // check whether the child is in this node's range
            if(x < (this.x - length + 1) || y < (this.y - length + 1) ||
                x > (this.x + length) || y > (this.y + length)){
                return -1;
            }
            // calculate child num
            int id = 0;
            if(x > this.x){
                id = 1;
            }
            if(y > this.y){
                id |= 2;
            }
            return id;
        }

        /**
         * Gets the index of the child at x, y, or -1 if outside of this node
         * for "static class simulation"
         * @param x
         * @param y
         * @return index or -1
         */