binomial_distribution.h

//===----------------------------------------------------------------------===//
//
// 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/is_valid.h>
#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 {
  static_assert(__libcpp_random_is_valid_inttype<_IntType>::value, "IntType must be a supported integer type");

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;

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

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

    friend _LIBCPP_HIDE_FROM_ABI bool operator==(const param_type& __x, const param_type& __y) {
      return __x.__t_ == __y.__t_ && __x.__p_ == __y.__p_;
    }
    friend _LIBCPP_HIDE_FROM_ABI 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_HIDE_FROM_ABI binomial_distribution() : binomial_distribution(1) {}
  _LIBCPP_HIDE_FROM_ABI explicit binomial_distribution(result_type __t, double __p = 0.5)
      : __p_(param_type(__t, __p)) {}
#else
  _LIBCPP_HIDE_FROM_ABI explicit binomial_distribution(result_type __t = 1, double __p = 0.5)
      : __p_(param_type(__t, __p)) {}
#endif
  _LIBCPP_HIDE_FROM_ABI explicit binomial_distribution(const param_type& __p) : __p_(__p) {}
  _LIBCPP_HIDE_FROM_ABI void reset() {}

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

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

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

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

  friend _LIBCPP_HIDE_FROM_ABI bool operator==(const binomial_distribution& __x, const binomial_distribution& __y) {
    return __x.__p_ == __y.__p_;
  }
  friend _LIBCPP_HIDE_FROM_ABI 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_HIDE_FROM_ABI 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_ = std::exp(
        std::__libcpp_lgamma(__t_ + 1.) - std::__libcpp_lgamma(__r0_ + 1.) - std::__libcpp_lgamma(__t_ - __r0_ + 1.) +
        __r0_ * std::log(__p_) + (__t_ - __r0_) * std::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) {
  static_assert(__libcpp_random_is_valid_urng<_URNG>::value, "");
  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>
_LIBCPP_HIDE_FROM_ABI 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>
_LIBCPP_HIDE_FROM_ABI 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