public class DES
    {
        private readonly double[] _rawDataPoints;
        private readonly double[] _smoothedDataPoints;
        private readonly double[] _trends;

        public DES(double[] dataPoints, double smoothingFactor, double trendFactor)
        {
            _rawDataPoints = dataPoints;
            _smoothedDataPoints = new double[dataPoints.Length];

            _smoothedDataPoints[0] = double.NaN;
            _trends[0] = double.NaN;
            _smoothedDataPoints[1] = dataPoints[1];
            _trends[1] = dataPoints[1] - dataPoints[0];

            for (int i = 2; i < dataPoints.Length; i++)
            {
                _smoothedDataPoints[i] = smoothingFactor * dataPoints[i] + (1 - smoothingFactor) * (_smoothedDataPoints[i - 1] + _trends[i - 1]);
                _trends[i] = trendFactor * (_smoothedDataPoints[i] - _smoothedDataPoints[i - 1]) + (1 - trendFactor) * _trends[i - 1];
            }

        }

        public double Predict(int index)
        {
            if (index < _smoothedDataPoints.Length)
                return _smoothedDataPoints[index] + _trends[index];
            return _smoothedDataPoints.Last() + (index - _smoothedDataPoints.Length) * _trends.Last();
        }
    }