math.distribution.h

/*
    Lightmetrica : A research-oriented renderer

    Copyright (c) 2014 Hisanari Otsu

    This program 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.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#pragma once
#ifndef LIB_LIGHTMETRICA_MATH_DISTRIBUTION_H
#define LIB_LIGHTMETRICA_MATH_DISTRIBUTION_H

#include "math.types.h"
#include "logger.h"
#include <vector>

LM_NAMESPACE_BEGIN
LM_MATH_NAMESPACE_BEGIN

class DiscreteDistribution1D
{
public:

    DiscreteDistribution1D() { Clear(); }

public:

    void Add(const Math::Float& v)
    {
        cdf.push_back(cdf.back() + v);
    }

    void Normalize()
    {
        auto sum = cdf.back();
        if (sum > Math::Float(0))
        {
            auto invSum = Math::Float(1) / sum;
            for (auto& v : cdf)
            {
                v *= invSum;
            }
        }
        else
        {
            LM_LOG_WARN("Unable to normalize. Sum is zero.")
        }
    }

    size_t Sample(const Math::Float& u) const
    {
        size_t v = static_cast<size_t>(std::upper_bound(cdf.begin(), cdf.end(), u) - cdf.begin()) - 1;
        return Math::Clamp<size_t>(v, 0, cdf.size() - 2);
    }
    
    Math::Float EvaluatePDF(int i) const
    {
        return (i < 0 || i + 1 >= static_cast<int>(cdf.size())) ? Math::Float(0) : cdf[i + 1] - cdf[i];
    }

    void Clear()
    {
        cdf.clear();
        cdf.push_back(Math::Float(0));
    }

    bool Empty() const
    {
        return cdf.size() == 1;
    }

private:

    std::vector<Math::Float> cdf;

};

LM_MATH_NAMESPACE_END
LM_NAMESPACE_END

#endif // LIB_LIGHTMETRICA_MATH_DISTRIBUTION_H