Pinta.ImageManipulation.SplineInterpolator.PreCompute C# (CSharp) Method

        private void PreCompute()
        {
            int n = points.Count;
            double[] u = new double[n];
            IList<double> xa = points.Keys;
            IList<double> ya = points.Values;

            this.y2 = new double[n];

            u[0] = 0;
            this.y2[0] = 0;

            for (int i = 1; i < n - 1; ++i)
            {
                // This is the decomposition loop of the tridiagonal algorithm. 
                // y2 and u are used for temporary storage of the decomposed factors.
                double wx = xa[i + 1] - xa[i - 1];
                double sig = (xa[i] - xa[i - 1]) / wx;
                double p = sig * y2[i - 1] + 2.0;

                this.y2[i] = (sig - 1.0) / p;

                double ddydx = 
                    (ya[i + 1] - ya[i]) / (xa[i + 1] - xa[i]) - 
                    (ya[i] - ya[i - 1]) / (xa[i] - xa[i - 1]);

                u[i] = (6.0 * ddydx / wx - sig * u[i - 1]) / p;
            }

            this.y2[n - 1] = 0;

            // This is the backsubstitution loop of the tridiagonal algorithm
            for (int i = n - 2; i >= 0; --i)
            {
                this.y2[i] = this.y2[i] * this.y2[i + 1] + u[i];
            }
        }
    }