/*
* Copyright 2009-2019 The VOTCA Development Team
* (http://www.votca.org)
*
* Licensed under the Apache License, Version 2.0 (the "License")
*
* You may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*
*/
#pragma once
#ifndef XTP_VXC_NEWTON_RAPSON_H
#define XTP_VXC_NEWTON_RAPSON_H
#include <votca/tools/types.h>
#include <votca/xtp/eigen.h>
namespace votca {
namespace xtp {
/**
* \brief Newton Rapson rootfinder for 1d functions
*
* https://en.wikipedia.org/wiki/Newton%27s_method#Modified_Newton_methods
*
*/
template <class Func>
class NewtonRapson {
public:
enum Errors { success, smalldenom, notconverged };
NewtonRapson(Index max_iterations, double tolerance)
: _max_iterations(max_iterations), _tolerance(tolerance) {}
NewtonRapson(Index max_iterations, double tolerance, double alpha)
: _max_iterations(max_iterations), _tolerance(tolerance), _alpha(alpha) {}
double FindRoot(const Func& f, double x0) {
_info = Errors::notconverged;
double x = x0;
for (_iter = 0; _iter < _max_iterations; _iter++) {
std::pair<double, double> res = f(x);
if (std::abs(res.second) < 1e-12) {
_info = Errors::smalldenom;
break;
}
double step = -_alpha * res.first / res.second;
if (std::abs(step) < _tolerance) {
_info = Errors::success;
break;
}
x += step;
}
return x;
}
Errors getInfo() const { return _info; }
Index getIterations() const { return _iter; }
private:
Errors _info = Errors::notconverged;
Index _max_iterations;
Index _iter;
double _tolerance;
double _alpha = 1.0;
};
} // namespace xtp
} // namespace votca
#endif // XTP_VXC_NEWTON_RAPSON_H