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 *   Copyright (c) 2005-2026 by                                             *
 *       - Pierre-Henri WUILLEMIN(_at_LIP6)                                 *
 *       - Christophe GONZALES(_at_AMU)                                     *
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 *   SPDX-FileCopyrightText: Copyright 2005-2026                            *
 *       - Pierre-Henri WUILLEMIN(_at_LIP6)                                 *
 *       - Christophe GONZALES(_at_AMU)                                     *
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/**
 * @file
 * @brief Headers of the ChiSquare class.
 *
 * @author Pierre-Henri WUILLEMIN(_at_LIP6) and Jean-Christophe MAGNAN and Christophe
 * GONZALES(_at_AMU)
 */


#ifndef GUM_CHI_SQUARE_H
#  define GUM_CHI_SQUARE_H

#  include <agrum/base/core/hashTable.h>

#  include <agrum/base/core/math/math_utils.h>

namespace gum {

  /**
   * @class ChiSquare chiSquare.h
   * <agrum/FMDP/learning/core/testPolicy/chiSquare.h>
   * @brief
   * @ingroup fmdp_group
   *
   */

  class ChiSquare {
    public:
    static double probaChi2(double x, Size df);

    private:
    /// Required constant to compute the cdf
    /// @{

    /// maximum meaningful z value
    static constexpr double _Z_MAX_ = 6.0;

    /// accuracy of critchi approximation
    static constexpr double _CHI_EPSILON_ = 0.000001;

    /// maximum chi square value
    static constexpr double _CHI_MAX_ = 99999.0;

    /// log (sqrt (pi))
    static constexpr double _LOG_SQRT_PI_ = 0.5723649429247000870717135;

    /// 1 / sqrt (pi)
    static constexpr double _I_SQRT_PI_ = 0.5641895835477562869480795;

    /// max value to represent exp (x)
    static constexpr double _BIGX_ = 20.0;

    /// @}


    /// Required constant to compute the cdf
    /// @{

    static double _exp_(double x);

    static double _probaZValue_(double z);
    /// @}
  };

} /* namespace gum */

#  ifndef GUM_NO_INLINE
#    include <agrum/FMDP/learning/core/chiSquare_inl.h>
#  endif
#endif   // GUM_CHI_SQUARE_H


// static HashTable<std::pair<double, unsigned long>, double>  _chi2Cache_;
// static HashTable<double, double>  _ZCache_;
// static Idx nbZ;
// static Idx nbChi;
// static Idx nbZt;
// static Idx nbChit;
