CubicSpline.h

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//
// Cubic Spline Interpolation to replace GSL spline
//
// Copyright (c) 2023, Paul Scherrer Institute, Villigen PSI, Switzerland
// All rights reserved
//
// This file is part of OPAL.
//
// OPAL is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// You should have received a copy of the GNU General License
// along with OPAL. If not, see <https://www.gnu.org/licenses/>.
//
 
#ifndef OPAL_CUBIC_SPLINE_HH
#define OPAL_CUBIC_SPLINE_HH
 
#include <vector>
#include "AbstractSpline.h"
 
// Simple cubic spline interpolation class to replace GSL spline
class CubicSpline final : public AbstractSpline {
public:
    CubicSpline() = default;
 
    CubicSpline(const std::vector<double>& x, const std::vector<double>& y);
 
    void init(const std::vector<double>& x, const std::vector<double>& y) override;
 
    double eval(double x, Accelerator& accel) const override;
 
    double evalIntegral(double xa, double xb, Accelerator& accel) const override;
 
private:
    void computeCoefficients();
 
    void computeIntegrals();
 
    [[nodiscard]] double integral(size_t i, double dx) const;
 
    [[nodiscard]] double extrapolateLeft(double x) const;
 
    [[nodiscard]] double extrapolateRight(double x) const;
 
    std::vector<double> b_;  // Linear coefficients
    std::vector<double> c_;  // Quadratic coefficients
    std::vector<double> d_;  // Cubic coefficients
    std::vector<double> integrals_;
};
 
#endif  // OPAL_CUBIC_SPLINE_HH