hyperbolic_secant_kernel.hpp

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/*
 * PoPS model - hyperbolic secant dispersal kernel
 *
 * Copyright (C) 2015-2020 by the authors.
 *
 * Authors: Margaret Lawrimore malawrim ncsu edu
 *
 * The code contained herein is licensed under the GNU General Public
 * License. You may obtain a copy of the GNU General Public License
 * Version 2 or later at the following locations:
 *
 * http://www.opensource.org/licenses/gpl-license.html
 * http://www.gnu.org/copyleft/gpl.html
 */
 
#ifndef POPS_HYPERBOLIC_SECANT_KERNEL_HPP
#define POPS_HYPERBOLIC_SECANT_KERNEL_HPP
 
#include "kernel_types.hpp"
#include "raster.hpp"
 
#include <random>
 
namespace pops {
 
using std::cosh;
using std::tan;
using std::log;
 
class HyperbolicSecantKernel
{
protected:
    double sigma;
    std::uniform_real_distribution<double> distribution;
 
public:
    HyperbolicSecantKernel(double s) : sigma(s), distribution(0.0, 1.0)
    {
        if (s == 0) {
            throw std::invalid_argument("s cannot be 0.0");
        }
    }
 
    template<class Generator>
    double random(Generator& generator)
    {
        double x = distribution(generator);
        return icdf(x);
    }
 
    double pdf(double x)
    {
        if (sigma == 1) {
            return 0.5 * (1.0 / cosh((M_PI * x) / 2));
        }
        return (1.0 / (2 * sigma)) * (1 / cosh((M_PI * x) / (2 * sigma)));
    }
 
    double icdf(double x)
    {
        if (x <= 0 || x >= 1) {
            throw std::invalid_argument("icdf: x must be between 0.0 and 1.0");
        }
        if (sigma == 1) {
            return (2.0 / M_PI) * log(tan(M_PI / 2.0 * x));
        }
        return ((log(tan((x * M_PI) / 2.0)) * (2.0 * sigma)) / M_PI);
    }
};
 
}  // namespace pops
 
#endif  // POPS_HYPERBOLIC_SECANT_KERNEL_HPP