SymmetricMatrix.cs

using System;
 
namespace Waher.Content.Semantic
{
    public class SymmetricMatrix<T>
    {
        private T[] elements;
        private int size;
        private int len;
 
        public SymmetricMatrix()
            : this(1)
        {
        }
 
        public SymmetricMatrix(int Size)
        {
            if (Size <= 0)
                throw new ArgumentOutOfRangeException(nameof(Size), "Must be positive.");
 
            this.size = Size;
            this.len = (this.size + 1) * this.size / 2;
            this.elements = new T[this.len];
        }
 
        public int Size => this.size;
 
        public T this[int X, int Y]
        {
            get
            {
                int i = this.GetIndex(X, Y);
                if (i >= this.len)
                    return default;
                else
                    return this.elements[i];
            }
 
            set
            {
                int i = this.GetIndex(X, Y);
                if (i >= this.len)
                {
                    this.size = Math.Max(X, Y) + 1;
                    this.len = i + 1;
                    Array.Resize(ref this.elements, this.len);
                }
 
                this.elements[i] = value;
            }
        }
 
        private int GetIndex(int X, int Y)
        {
            if (X < 0)
                throw new ArgumentOutOfRangeException(nameof(X), "Negative cooridates not allowed.");
 
            if (Y < 0)
                throw new ArgumentOutOfRangeException(nameof(Y), "Negative cooridates not allowed.");
 
            if (X > Y)
                return X * (X - 1) / 2 + Y;
            else if (Y == 0)
                return 0;
            else
                return Y * (Y - 1) / 2 + X;
        }
    }
}