Question

I’m writing a quadtree-like data structure which contains matrices of generic objects T. If four subnodes all contain defined matrices of T, I’m going to aggregate them into a single, larger matrix, then delete the subnodes. Is there a more efficient way to do this than looping through every reference and copying it over? Can I copy chunks of memory instead?

---

Example:

T[,] _leaf1 = new T[64,64];
T[,] _leaf2 = new T[64,64];
T[,] _leaf3 = new T[64,64];
T[,] _leaf4 = new T[64,64];

// Populate leafs

T[,] _root = new T[128,128];

CopyInto(ref _root, ref _leaf1, 64, 64);
CopyInto(ref _root, ref _leaf2, 0, 64);
CopyInto(ref _root, ref _leaf3, 0, 0);
CopyInto(ref _root, ref _leaf4, 64, 0);

Answer 1

If you can make the structure immutable, you can perhaps save yourself from having to make lots of copies. Eric Lippert has some great posts about immutable structures.

Edit:
Again, I don’t know if it will improve performance in your case, but here is an example of a possible design with immutable objects:

abstract class QuadTree<T>
{
    public QuadTree(int width, int height)
    {
        this.Width = width;
        this.Heigth = heigth;
    }

    public int Width { get; private set; }
    public int Height { get; private set; }

    public abstract T Get(int x, int y); 
}

class MatrixQuadTree<T> : QuadTree<T>
{
    private readonly T[,] matrix;

    public QuadTree(T[,] matrix, int width, int heigth)
        : base(width, heigth)
    {
        this.matrix = matrix;
    }

    public override T Get(int x, int y)
    {
       return this.matrix[x, y];
    }
}

class CompositeQuadTree<T> : QuadTree<T>
{
    private readonly QuadTree<T> topLeft;
    private readonly QuadTree<T> topRight;
    private readonly QuadTree<T> bottomLeft;
    private readonly QuadTree<T> bottomRight;

    public CompositeQuadTree(QuadTree<T> topLeft,
        QuadTree<T> topRight, QuadTree<T> bottomLeft,
        QuadTree<T> bottomRight)
        : base(topLeft.Width + topRight.Width, 
            topLeft.Height + bottomLeft.Heigth)
    {
        // TODO: Do proper checks.
        if (this.Width != topLeft.Width + bottomRight.Width)
            throw Exception();

        this.topLeft = topLeft;
        this.topRight = topRight;
        this.bottomLeft = bottomLeft;
        this.bottomRight = bottomRight;
    }

    public override T Get(int x, int y)
    {
        if (x <= this.topLeft.Width)
        {
            if (y <= this.topLeft.Width)
            {
                return this.topLeft.Get(x, y);
            }
            else
            {
                return this.topLeft.Get(x, y + this.topLeft.Heigth);
            }
        }
        else
        {
            if (y <= this.topLeft.Width)
            {
                return this.topRight.Get(x + this.topLeft.Width, y);
            }
            else
            {
                return this.topRight.Get(x + this.topLeft.Width, 
                    y + this.topLeft.Heigth);
            }
        }
    }
}

Now you would be able to use it as follows:

T[,] _leaf1 = new T[64,64];
T[,] _leaf2 = new T[64,64];
T[,] _leaf3 = new T[64,64];
T[,] _leaf4 = new T[64,64];

// Populate leafs

QuadTree<T> l1 = new MatrixQuadTree<T>(_leaf1,64,64);
QuadTree<T> l2 = new MatrixQuadTree<T>(_leaf2,64,64);
QuadTree<T> l3 = new MatrixQuadTree<T>(_leaf3,64,64);
QuadTree<T> l4 = new MatrixQuadTree<T>(_leaf4,64,64);

// Instead of copying, you can no do this:
QuadTree<T> c = CompositeQuadTree<T>(l1,l2,l3,l4);

// And you can even make composites, of other composites:
QuadTree<T> c2 = CompositeQuadTree<T>(c,c,c,c);

// And you can read a value as follows:
T value = c2[30, 50];

Again, I don’t know if it is appropriate in your situation or whether it gives an performance improvement, because you have a level of indirection when getting the value. However, there are several ways to improve this, but it depends on what you really need to do.

Good luck.