Accord.Math.Decompositions.NonnegativeFactorization.normdiff C# (CSharp) Method

        private static unsafe double normdiff(double[,] matrixA, double[,] matrixB)
        {
            int length = matrixA.Length;
            double dnorm = 0.0, d;

            fixed (double* ptrA = matrixA, ptrB = matrixB)
            {
                double* a = ptrA, b = ptrB;
                for (int i = 0; i < length; i++, a++, b++)
                {
                    d = *a - *b;
                    dnorm += d*d;
                }

                return System.Math.Sqrt(dnorm/length);
            }
        }