exponential_kernel.hpp

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/*
 * PoPS model - exponential 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_EXPONENTIAL_KERNEL_HPP
#define POPS_EXPONENTIAL_KERNEL_HPP
 
#include "kernel_types.hpp"
#include "raster.hpp"
#include "utils.hpp"
 
#include <random>
 
namespace pops {
 
using std::exp;
using std::log;
 
class ExponentialKernel
{
protected:
    double beta;
    std::exponential_distribution<double> exponential_distribution;
 
public:
    ExponentialKernel(double b) : beta(b), exponential_distribution(1.0 / beta)
    {
        if (beta <= 0) {
            throw std::invalid_argument("beta must be greater than 0.0");
        }
    }
 
    template<class Generator>
    double random(Generator& generator)
    {
        return std::abs(exponential_distribution(generator));
    }
 
    double pdf(double x)
    {
        if (x < 0) {
            throw std::invalid_argument("x must be greater than or equal to zero");
        }
        return (1.0 / beta) * (exp(-x / beta));
    }
 
    double icdf(double x)
    {
        if (x <= 0 || x >= 1) {
            throw std::invalid_argument("icdf: x must be between 0.0 and 1.0");
        }
        else {
            return -beta * log(1 - x);
        }
    }
};
 
}  // namespace pops
 
#endif  // POPS_EXPONENTIAL_KERNEL_HPP