xref: /freebsd-src/contrib/llvm-project/libcxx/include/__random/binomial_distribution.h (revision 4824e7fd18a1223177218d4aec1b3c6c5c4a444e)

//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef _LIBCPP___RANDOM_BINOMIAL_DISTRIBUTION_H
#define _LIBCPP___RANDOM_BINOMIAL_DISTRIBUTION_H

#include <__config>
#include <__random/uniform_real_distribution.h>
#include <cmath>
#include <iosfwd>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#pragma GCC system_header
#endif

_LIBCPP_PUSH_MACROS
#include <__undef_macros>

_LIBCPP_BEGIN_NAMESPACE_STD

template<class _IntType = int>
class _LIBCPP_TEMPLATE_VIS binomial_distribution
{
public:
    // types
    typedef _IntType result_type;

    class _LIBCPP_TEMPLATE_VIS param_type
    {
        result_type __t_;
        double __p_;
        double __pr_;
        double __odds_ratio_;
        result_type __r0_;
    public:
        typedef binomial_distribution distribution_type;

        explicit param_type(result_type __t = 1, double __p = 0.5);

        _LIBCPP_INLINE_VISIBILITY
        result_type t() const {return __t_;}
        _LIBCPP_INLINE_VISIBILITY
        double p() const {return __p_;}

        friend _LIBCPP_INLINE_VISIBILITY
            bool operator==(const param_type& __x, const param_type& __y)
            {return __x.__t_ == __y.__t_ && __x.__p_ == __y.__p_;}
        friend _LIBCPP_INLINE_VISIBILITY
            bool operator!=(const param_type& __x, const param_type& __y)
            {return !(__x == __y);}

        friend class binomial_distribution;
    };

private:
    param_type __p_;

public:
    // constructors and reset functions
#ifndef _LIBCPP_CXX03_LANG
    _LIBCPP_INLINE_VISIBILITY
    binomial_distribution() : binomial_distribution(1) {}
    _LIBCPP_INLINE_VISIBILITY
    explicit binomial_distribution(result_type __t, double __p = 0.5)
        : __p_(param_type(__t, __p)) {}
#else
    _LIBCPP_INLINE_VISIBILITY
    explicit binomial_distribution(result_type __t = 1, double __p = 0.5)
        : __p_(param_type(__t, __p)) {}
#endif
    _LIBCPP_INLINE_VISIBILITY
    explicit binomial_distribution(const param_type& __p) : __p_(__p) {}
    _LIBCPP_INLINE_VISIBILITY
    void reset() {}

    // generating functions
    template<class _URNG>
        _LIBCPP_INLINE_VISIBILITY
        result_type operator()(_URNG& __g)
        {return (*this)(__g, __p_);}
    template<class _URNG> result_type operator()(_URNG& __g, const param_type& __p);

    // property functions
    _LIBCPP_INLINE_VISIBILITY
    result_type t() const {return __p_.t();}
    _LIBCPP_INLINE_VISIBILITY
    double p() const {return __p_.p();}

    _LIBCPP_INLINE_VISIBILITY
    param_type param() const {return __p_;}
    _LIBCPP_INLINE_VISIBILITY
    void param(const param_type& __p) {__p_ = __p;}

    _LIBCPP_INLINE_VISIBILITY
    result_type min() const {return 0;}
    _LIBCPP_INLINE_VISIBILITY
    result_type max() const {return t();}

    friend _LIBCPP_INLINE_VISIBILITY
        bool operator==(const binomial_distribution& __x,
                        const binomial_distribution& __y)
        {return __x.__p_ == __y.__p_;}
    friend _LIBCPP_INLINE_VISIBILITY
        bool operator!=(const binomial_distribution& __x,
                        const binomial_distribution& __y)
        {return !(__x == __y);}
};

#ifndef _LIBCPP_MSVCRT_LIKE
extern "C" double lgamma_r(double, int *);
#endif

inline _LIBCPP_INLINE_VISIBILITY double __libcpp_lgamma(double __d) {
#if defined(_LIBCPP_MSVCRT_LIKE)
  return lgamma(__d);
#else
  int __sign;
  return lgamma_r(__d, &__sign);
#endif
}

template<class _IntType>
binomial_distribution<_IntType>::param_type::param_type(result_type __t, double __p)
    : __t_(__t), __p_(__p)
{
    if (0 < __p_ && __p_ < 1)
    {
        __r0_ = static_cast<result_type>((__t_ + 1) * __p_);
        __pr_ = _VSTD::exp(__libcpp_lgamma(__t_ + 1.) -
                           __libcpp_lgamma(__r0_ + 1.) -
                           __libcpp_lgamma(__t_ - __r0_ + 1.) + __r0_ * _VSTD::log(__p_) +
                           (__t_ - __r0_) * _VSTD::log(1 - __p_));
        __odds_ratio_ = __p_ / (1 - __p_);
    }
}

// Reference: Kemp, C.D. (1986). `A modal method for generating binomial
//           variables', Commun. Statist. - Theor. Meth. 15(3), 805-813.
template<class _IntType>
template<class _URNG>
_IntType
binomial_distribution<_IntType>::operator()(_URNG& __g, const param_type& __pr)
{
    if (__pr.__t_ == 0 || __pr.__p_ == 0)
        return 0;
    if (__pr.__p_ == 1)
        return __pr.__t_;
    uniform_real_distribution<double> __gen;
    double __u = __gen(__g) - __pr.__pr_;
    if (__u < 0)
        return __pr.__r0_;
    double __pu = __pr.__pr_;
    double __pd = __pu;
    result_type __ru = __pr.__r0_;
    result_type __rd = __ru;
    while (true)
    {
        bool __break = true;
        if (__rd >= 1)
        {
            __pd *= __rd / (__pr.__odds_ratio_ * (__pr.__t_ - __rd + 1));
            __u -= __pd;
            __break = false;
            if (__u < 0)
                return __rd - 1;
        }
        if ( __rd != 0 )
            --__rd;
        ++__ru;
        if (__ru <= __pr.__t_)
        {
            __pu *= (__pr.__t_ - __ru + 1) * __pr.__odds_ratio_ / __ru;
            __u -= __pu;
            __break = false;
            if (__u < 0)
                return __ru;
        }
        if (__break)
            return 0;
    }
}

template <class _CharT, class _Traits, class _IntType>
basic_ostream<_CharT, _Traits>&
operator<<(basic_ostream<_CharT, _Traits>& __os,
           const binomial_distribution<_IntType>& __x)
{
    __save_flags<_CharT, _Traits> __lx(__os);
    typedef basic_ostream<_CharT, _Traits> _OStream;
    __os.flags(_OStream::dec | _OStream::left | _OStream::fixed |
               _OStream::scientific);
    _CharT __sp = __os.widen(' ');
    __os.fill(__sp);
    return __os << __x.t() << __sp << __x.p();
}

template <class _CharT, class _Traits, class _IntType>
basic_istream<_CharT, _Traits>&
operator>>(basic_istream<_CharT, _Traits>& __is,
           binomial_distribution<_IntType>& __x)
{
    typedef binomial_distribution<_IntType> _Eng;
    typedef typename _Eng::result_type result_type;
    typedef typename _Eng::param_type param_type;
    __save_flags<_CharT, _Traits> __lx(__is);
    typedef basic_istream<_CharT, _Traits> _Istream;
    __is.flags(_Istream::dec | _Istream::skipws);
    result_type __t;
    double __p;
    __is >> __t >> __p;
    if (!__is.fail())
        __x.param(param_type(__t, __p));
    return __is;
}

_LIBCPP_END_NAMESPACE_STD

_LIBCPP_POP_MACROS

#endif // _LIBCPP___RANDOM_BINOMIAL_DISTRIBUTION_H