xref: /src/external/gpl3/gcc/dist/libstdc++-v3/include/std/numeric

// <numeric> -*- C++ -*-

// Copyright (C) 2001-2020 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library 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, or (at your option)
// any later version.

// This library 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.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/*
 *
 * Copyright (c) 1994
 * Hewlett-Packard Company
 *
 * Permission to use, copy, modify, distribute and sell this software
 * and its documentation for any purpose is hereby granted without fee,
 * provided that the above copyright notice appear in all copies and
 * that both that copyright notice and this permission notice appear
 * in supporting documentation.  Hewlett-Packard Company makes no
 * representations about the suitability of this software for any
 * purpose.  It is provided "as is" without express or implied warranty.
 *
 *
 * Copyright (c) 1996,1997
 * Silicon Graphics Computer Systems, Inc.
 *
 * Permission to use, copy, modify, distribute and sell this software
 * and its documentation for any purpose is hereby granted without fee,
 * provided that the above copyright notice appear in all copies and
 * that both that copyright notice and this permission notice appear
 * in supporting documentation.  Silicon Graphics makes no
 * representations about the suitability of this software for any
 * purpose.  It is provided "as is" without express or implied warranty.
 */

/** @file include/numeric
 *  This is a Standard C++ Library header.
 */

#ifndef _GLIBCXX_NUMERIC
#define _GLIBCXX_NUMERIC 1

#pragma GCC system_header

#include <bits/c++config.h>
#include <bits/stl_iterator_base_types.h>
#include <bits/stl_numeric.h>
#include <ext/numeric_traits.h>

#ifdef _GLIBCXX_PARALLEL
# include <parallel/numeric>
#endif

/**
 * @defgroup numerics Numerics
 *
 * Components for performing numeric operations. Includes support for
 * complex number types, random number generation, numeric (n-at-a-time)
 * arrays, generalized numeric algorithms, and mathematical special functions.
 */

#if __cplusplus >= 201402L
#include <type_traits>
#include <ext/numeric_traits.h>

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

namespace __detail
{
  // Like std::abs, but supports unsigned types and returns the specified type,
  // so |std::numeric_limits<_Tp>::min()| is OK if representable in _Res.
  template<typename _Res, typename _Tp>
    constexpr _Res
    __abs_r(_Tp __val)
    {
      static_assert(sizeof(_Res) >= sizeof(_Tp),
	  "result type must be at least as wide as the input type");

      if (__val >= 0)
	return __val;
#if defined _GLIBCXX_ASSERTIONS && defined _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
      if (!__builtin_is_constant_evaluated()) // overflow already detected in constexpr
	__glibcxx_assert(__val != __gnu_cxx::__int_traits<_Res>::__min);
#endif
      return -static_cast<_Res>(__val);
    }

  template<typename> void __abs_r(bool) = delete;

  // GCD implementation
  template<typename _Tp>
    constexpr _Tp
    __gcd(_Tp __m, _Tp __n)
    {
      static_assert(is_unsigned<_Tp>::value, "type must be unsigned");
      return __m == 0 ? __n
	: __n == 0 ? __m
	: __detail::__gcd(__n, _Tp(__m % __n));
    }
} // namespace __detail

#if __cplusplus >= 201703L

#define __cpp_lib_gcd_lcm 201606
// These were used in drafts of SD-6:
#define __cpp_lib_gcd 201606
#define __cpp_lib_lcm 201606

  /// Greatest common divisor
  template<typename _Mn, typename _Nn>
    constexpr common_type_t<_Mn, _Nn>
    gcd(_Mn __m, _Nn __n) noexcept
    {
      static_assert(is_integral_v<_Mn> && is_integral_v<_Nn>,
		    "std::gcd arguments must be integers");
      static_assert(_Mn(2) == 2 && _Nn(2) == 2,
		    "std::gcd arguments must not be bool");
      using _Ct = common_type_t<_Mn, _Nn>;
      const _Ct __m2 = __detail::__abs_r<_Ct>(__m);
      const _Ct __n2 = __detail::__abs_r<_Ct>(__n);
      return __detail::__gcd<make_unsigned_t<_Ct>>(__m2, __n2);
    }

  /// Least common multiple
  template<typename _Mn, typename _Nn>
    constexpr common_type_t<_Mn, _Nn>
    lcm(_Mn __m, _Nn __n) noexcept
    {
      static_assert(is_integral_v<_Mn> && is_integral_v<_Nn>,
		    "std::lcm arguments must be integers");
      static_assert(_Mn(2) == 2 && _Nn(2) == 2,
		    "std::lcm arguments must not be bool");
      using _Ct = common_type_t<_Mn, _Nn>;
      const _Ct __m2 = __detail::__abs_r<_Ct>(__m);
      const _Ct __n2 = __detail::__abs_r<_Ct>(__n);
      if (__m2 == 0 || __n2 == 0)
	return 0;
      _Ct __r = __m2 / __detail::__gcd<make_unsigned_t<_Ct>>(__m2, __n2);

#if defined _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
      if constexpr (is_signed_v<_Ct>)
	if (__builtin_is_constant_evaluated())
	  return __r * __n2; // constant evaluation can detect overflow here.
#endif

      bool __overflow = __builtin_mul_overflow(__r, __n2, &__r);
#if defined _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
      if (__builtin_is_constant_evaluated())
	if (__overflow)
	  _GLIBCXX_THROW_OR_ABORT("std::lcm result is out of range of type");
#endif
      __glibcxx_assert(!__overflow);
      return __r;
    }

#endif // C++17

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif // C++14

#if __cplusplus > 201703L
#include <limits>

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
  // midpoint
# define __cpp_lib_interpolate 201902L

  template<typename _Tp>
    constexpr
    enable_if_t<__and_v<is_arithmetic<_Tp>, is_same<remove_cv_t<_Tp>, _Tp>,
			__not_<is_same<_Tp, bool>>>,
		_Tp>
    midpoint(_Tp __a, _Tp __b) noexcept
    {
      if constexpr (is_integral_v<_Tp>)
	{
	  using _Up = make_unsigned_t<_Tp>;

	  int __k = 1;
	  _Up __m = __a;
	  _Up __M = __b;
	  if (__a > __b)
	    {
	      __k = -1;
	      __m = __b;
	      __M = __a;
	    }
	  return __a + __k * _Tp(_Up(__M - __m) / 2);
	}
      else // is_floating
	{
	  constexpr _Tp __lo = numeric_limits<_Tp>::min() * 2;
	  constexpr _Tp __hi = numeric_limits<_Tp>::max() / 2;
	  const _Tp __abs_a = __a < 0 ? -__a : __a;
	  const _Tp __abs_b = __b < 0 ? -__b : __b;
	  if (__abs_a <= __hi && __abs_b <= __hi) [[likely]]
	    return (__a + __b) / 2; // always correctly rounded
	  if (__abs_a < __lo) // not safe to halve __a
	    return __a + __b/2;
	  if (__abs_b < __lo) // not safe to halve __b
	    return __a/2 + __b;
	  return __a/2 + __b/2;	    // otherwise correctly rounded
	}
    }

  template<typename _Tp>
    constexpr enable_if_t<is_object_v<_Tp>, _Tp*>
    midpoint(_Tp* __a, _Tp* __b) noexcept
    {
      static_assert( sizeof(_Tp) != 0, "type must be complete" );
      return __a  + (__b - __a) / 2;
    }
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif // C++20

#if __cplusplus > 201402L
#include <bits/stl_function.h>

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

#if __cplusplus > 201703L
#define __cpp_lib_constexpr_numeric 201911L
#endif

  /// @addtogroup numeric_ops
  /// @{

  /**
   *  @brief  Calculate reduction of values in a range.
   *
   *  @param  __first  Start of range.
   *  @param  __last  End of range.
   *  @param  __init  Starting value to add other values to.
   *  @param  __binary_op A binary function object.
   *  @return  The final sum.
   *
   *  Reduce the values in the range `[first,last)` using a binary operation.
   *  The initial value is `init`.  The values are not necessarily processed
   *  in order.
   *
   *  This algorithm is similar to `std::accumulate` but is not required to
   *  perform the operations in order from first to last. For operations
   *  that are commutative and associative the result will be the same as
   *  for `std::accumulate`, but for other operations (such as floating point
   *  arithmetic) the result can be different.
   */
  template<typename _InputIterator, typename _Tp, typename _BinaryOperation>
    _GLIBCXX20_CONSTEXPR
    _Tp
    reduce(_InputIterator __first, _InputIterator __last, _Tp __init,
	   _BinaryOperation __binary_op)
    {
      using __ref = typename iterator_traits<_InputIterator>::reference;
      static_assert(is_invocable_r_v<_Tp, _BinaryOperation&, _Tp&, __ref>);
      static_assert(is_invocable_r_v<_Tp, _BinaryOperation&, __ref, _Tp&>);
      static_assert(is_invocable_r_v<_Tp, _BinaryOperation&, _Tp&, _Tp&>);
      static_assert(is_invocable_r_v<_Tp, _BinaryOperation&, __ref, __ref>);
      if constexpr (__is_random_access_iter<_InputIterator>::value)
	{
	  while ((__last - __first) >= 4)
	    {
	      _Tp __v1 = __binary_op(__first[0], __first[1]);
	      _Tp __v2 = __binary_op(__first[2], __first[3]);
	      _Tp __v3 = __binary_op(__v1, __v2);
	      __init = __binary_op(__init, __v3);
	      __first += 4;
	    }
	}
      for (; __first != __last; ++__first)
	__init = __binary_op(__init, *__first);
      return __init;
    }

 /**
   *  @brief  Calculate reduction of values in a range.
   *
   *  @param  __first  Start of range.
   *  @param  __last  End of range.
   *  @param  __init  Starting value to add other values to.
   *  @return  The final sum.
   *
   *  Reduce the values in the range `[first,last)` using addition.
   *  Equivalent to calling `std::reduce(first, last, init, std::plus<>())`.
   */
  template<typename _InputIterator, typename _Tp>
    _GLIBCXX20_CONSTEXPR
    inline _Tp
    reduce(_InputIterator __first, _InputIterator __last, _Tp __init)
    { return std::reduce(__first, __last, std::move(__init), plus<>()); }

  /**
   *  @brief  Calculate reduction of values in a range.
   *
   *  @param  __first  Start of range.
   *  @param  __last  End of range.
   *  @return  The final sum.
   *
   *  Reduce the values in the range `[first,last)` using addition, with
   *  an initial value of `T{}`, where `T` is the iterator's value type.
   *  Equivalent to calling `std::reduce(first, last, T{}, std::plus<>())`.
   */
  template<typename _InputIterator>
    _GLIBCXX20_CONSTEXPR
    inline typename iterator_traits<_InputIterator>::value_type
    reduce(_InputIterator __first, _InputIterator __last)
    {
      using value_type = typename iterator_traits<_InputIterator>::value_type;
      return std::reduce(__first, __last, value_type{}, plus<>());
    }

  /**
   *  @brief  Combine elements from two ranges and reduce
   *
   *  @param  __first1  Start of first range.
   *  @param  __last1  End of first range.
   *  @param  __first2  Start of second range.
   *  @param  __init  Starting value to add other values to.
   *  @param  __binary_op1 The function used to perform reduction.
   *  @param  __binary_op2 The function used to combine values from the ranges.
   *  @return  The final sum.
   *
   *  Call `binary_op2(first1[n],first2[n])` for each `n` in `[0,last1-first1)`
   *  and then use `binary_op1` to reduce the values returned by `binary_op2`
   *  to a single value of type `T`.
   *
   *  The range beginning at `first2` must contain at least `last1-first1`
   *  elements.
   */
  template<typename _InputIterator1, typename _InputIterator2, typename _Tp,
	   typename _BinaryOperation1, typename _BinaryOperation2>
    _GLIBCXX20_CONSTEXPR
    _Tp
    transform_reduce(_InputIterator1 __first1, _InputIterator1 __last1,
		     _InputIterator2 __first2, _Tp __init,
		     _BinaryOperation1 __binary_op1,
		     _BinaryOperation2 __binary_op2)
    {
      if constexpr (__and_v<__is_random_access_iter<_InputIterator1>,
			    __is_random_access_iter<_InputIterator2>>)
	{
	  while ((__last1 - __first1) >= 4)
	    {
	      _Tp __v1 = __binary_op1(__binary_op2(__first1[0], __first2[0]),
				      __binary_op2(__first1[1], __first2[1]));
	      _Tp __v2 = __binary_op1(__binary_op2(__first1[2], __first2[2]),
				      __binary_op2(__first1[3], __first2[3]));
	      _Tp __v3 = __binary_op1(__v1, __v2);
	      __init = __binary_op1(__init, __v3);
	      __first1 += 4;
	      __first2 += 4;
	    }
	}
      for (; __first1 != __last1; ++__first1, (void) ++__first2)
	__init = __binary_op1(__init, __binary_op2(*__first1, *__first2));
      return __init;
    }

  /**
   *  @brief  Combine elements from two ranges and reduce
   *
   *  @param  __first1  Start of first range.
   *  @param  __last1  End of first range.
   *  @param  __first2  Start of second range.
   *  @param  __init  Starting value to add other values to.
   *  @return  The final sum.
   *
   *  Call `first1[n]*first2[n]` for each `n` in `[0,last1-first1)` and then
   *  use addition to sum those products to a single value of type `T`.
   *
   *  The range beginning at `first2` must contain at least `last1-first1`
   *  elements.
   */
  template<typename _InputIterator1, typename _InputIterator2, typename _Tp>
    _GLIBCXX20_CONSTEXPR
    inline _Tp
    transform_reduce(_InputIterator1 __first1, _InputIterator1 __last1,
		     _InputIterator2 __first2, _Tp __init)
    {
      return std::transform_reduce(__first1, __last1, __first2,
				   std::move(__init),
				   plus<>(), multiplies<>());
    }

  /**
   *  @brief  Transform the elements of a range and reduce
   *
   *  @param  __first  Start of range.
   *  @param  __last  End of range.
   *  @param  __init  Starting value to add other values to.
   *  @param  __binary_op The function used to perform reduction.
   *  @param  __unary_op The function used to transform values from the range.
   *  @return  The final sum.
   *
   *  Call `unary_op(first[n])` for each `n` in `[0,last-first)` and then
   *  use `binary_op` to reduce the values returned by `unary_op`
   *  to a single value of type `T`.
   */
  template<typename _InputIterator, typename _Tp,
	   typename _BinaryOperation, typename _UnaryOperation>
    _GLIBCXX20_CONSTEXPR
    _Tp
    transform_reduce(_InputIterator __first, _InputIterator __last, _Tp __init,
		     _BinaryOperation __binary_op, _UnaryOperation __unary_op)
    {
      if constexpr (__is_random_access_iter<_InputIterator>::value)
	{
	  while ((__last - __first) >= 4)
	    {
	      _Tp __v1 = __binary_op(__unary_op(__first[0]),
				     __unary_op(__first[1]));
	      _Tp __v2 = __binary_op(__unary_op(__first[2]),
				     __unary_op(__first[3]));
	      _Tp __v3 = __binary_op(__v1, __v2);
	      __init = __binary_op(__init, __v3);
	      __first += 4;
	    }
	}
      for (; __first != __last; ++__first)
	__init = __binary_op(__init, __unary_op(*__first));
      return __init;
    }

  /** @brief Output the cumulative sum of one range to a second range
   *
   *  @param __first  Start of input range.
   *  @param __last   End of input range.
   *  @param __result Start of output range.
   *  @param __init   Initial value.
   *  @param __binary_op Function to perform summation.
   *  @return The end of the output range.
   *
   *  Write the cumulative sum (aka prefix sum, aka scan) of the input range
   *  to the output range. Each element of the output range contains the
   *  running total of all earlier elements (and the initial value),
   *  using `binary_op` for summation.
   *
   *  This function generates an "exclusive" scan, meaning the Nth element
   *  of the output range is the sum of the first N-1 input elements,
   *  so the Nth input element is not included.
   */
  template<typename _InputIterator, typename _OutputIterator, typename _Tp,
	   typename _BinaryOperation>
    _GLIBCXX20_CONSTEXPR
    _OutputIterator
    exclusive_scan(_InputIterator __first, _InputIterator __last,
		   _OutputIterator __result, _Tp __init,
		   _BinaryOperation __binary_op)
    {
      while (__first != __last)
	{
	  auto __v = __init;
	  __init = __binary_op(__init, *__first);
	  ++__first;
	  *__result++ = std::move(__v);
	}
      return __result;
    }

  /** @brief Output the cumulative sum of one range to a second range
   *
   *  @param __first  Start of input range.
   *  @param __last   End of input range.
   *  @param __result Start of output range.
   *  @param __init   Initial value.
   *  @return The end of the output range.
   *
   *  Write the cumulative sum (aka prefix sum, aka scan) of the input range
   *  to the output range. Each element of the output range contains the
   *  running total of all earlier elements (and the initial value),
   *  using `std::plus<>` for summation.
   *
   *  This function generates an "exclusive" scan, meaning the Nth element
   *  of the output range is the sum of the first N-1 input elements,
   *  so the Nth input element is not included.
   */
  template<typename _InputIterator, typename _OutputIterator, typename _Tp>
    _GLIBCXX20_CONSTEXPR
    inline _OutputIterator
    exclusive_scan(_InputIterator __first, _InputIterator __last,
		   _OutputIterator __result, _Tp __init)
    {
      return std::exclusive_scan(__first, __last, __result, std::move(__init),
				 plus<>());
    }

  /** @brief Output the cumulative sum of one range to a second range
   *
   *  @param __first  Start of input range.
   *  @param __last   End of input range.
   *  @param __result Start of output range.
   *  @param __binary_op Function to perform summation.
   *  @param __init   Initial value.
   *  @return The end of the output range.
   *
   *  Write the cumulative sum (aka prefix sum, aka scan) of the input range
   *  to the output range. Each element of the output range contains the
   *  running total of all earlier elements (and the initial value),
   *  using `binary_op` for summation.
   *
   *  This function generates an "inclusive" scan, meaning the Nth element
   *  of the output range is the sum of the first N input elements,
   *  so the Nth input element is included.
   */
  template<typename _InputIterator, typename _OutputIterator,
	   typename _BinaryOperation, typename _Tp>
    _GLIBCXX20_CONSTEXPR
    _OutputIterator
    inclusive_scan(_InputIterator __first, _InputIterator __last,
		   _OutputIterator __result, _BinaryOperation __binary_op,
		   _Tp __init)
    {
      for (; __first != __last; ++__first)
	*__result++ = __init = __binary_op(__init, *__first);
      return __result;
    }

  /** @brief Output the cumulative sum of one range to a second range
   *
   *  @param __first  Start of input range.
   *  @param __last   End of input range.
   *  @param __result Start of output range.
   *  @param __binary_op Function to perform summation.
   *  @return The end of the output range.
   *
   *  Write the cumulative sum (aka prefix sum, aka scan) of the input range
   *  to the output range. Each element of the output range contains the
   *  running total of all earlier elements, using `binary_op` for summation.
   *
   *  This function generates an "inclusive" scan, meaning the Nth element
   *  of the output range is the sum of the first N input elements,
   *  so the Nth input element is included.
   */
  template<typename _InputIterator, typename _OutputIterator,
	   typename _BinaryOperation>
    _GLIBCXX20_CONSTEXPR
    _OutputIterator
    inclusive_scan(_InputIterator __first, _InputIterator __last,
		   _OutputIterator __result, _BinaryOperation __binary_op)
    {
      if (__first != __last)
	{
	  auto __init = *__first;
	  *__result++ = __init;
	  ++__first;
	  if (__first != __last)
	    __result = std::inclusive_scan(__first, __last, __result,
					   __binary_op, std::move(__init));
	}
      return __result;
    }

  /** @brief Output the cumulative sum of one range to a second range
   *
   *  @param __first  Start of input range.
   *  @param __last   End of input range.
   *  @param __result Start of output range.
   *  @return The end of the output range.
   *
   *  Write the cumulative sum (aka prefix sum, aka scan) of the input range
   *  to the output range. Each element of the output range contains the
   *  running total of all earlier elements, using `std::plus<>` for summation.
   *
   *  This function generates an "inclusive" scan, meaning the Nth element
   *  of the output range is the sum of the first N input elements,
   *  so the Nth input element is included.
   */
  template<typename _InputIterator, typename _OutputIterator>
    _GLIBCXX20_CONSTEXPR
    inline _OutputIterator
    inclusive_scan(_InputIterator __first, _InputIterator __last,
		   _OutputIterator __result)
    { return std::inclusive_scan(__first, __last, __result, plus<>()); }

  /** @brief Output the cumulative sum of one range to a second range
   *
   *  @param __first  Start of input range.
   *  @param __last   End of input range.
   *  @param __result Start of output range.
   *  @param __init   Initial value.
   *  @param __binary_op Function to perform summation.
   *  @param __unary_op Function to transform elements of the input range.
   *  @return The end of the output range.
   *
   *  Write the cumulative sum (aka prefix sum, aka scan) of the input range
   *  to the output range. Each element of the output range contains the
   *  running total of all earlier elements (and the initial value),
   *  using `__unary_op` to transform the input elements
   *  and using `__binary_op` for summation.
   *
   *  This function generates an "exclusive" scan, meaning the Nth element
   *  of the output range is the sum of the first N-1 input elements,
   *  so the Nth input element is not included.
   */
  template<typename _InputIterator, typename _OutputIterator, typename _Tp,
	   typename _BinaryOperation, typename _UnaryOperation>
    _GLIBCXX20_CONSTEXPR
    _OutputIterator
    transform_exclusive_scan(_InputIterator __first, _InputIterator __last,
			     _OutputIterator __result, _Tp __init,
			     _BinaryOperation __binary_op,
			     _UnaryOperation __unary_op)
    {
      while (__first != __last)
	{
	  auto __v = __init;
	  __init = __binary_op(__init, __unary_op(*__first));
	  ++__first;
	  *__result++ = std::move(__v);
	}
      return __result;
    }

  /** @brief Output the cumulative sum of one range to a second range
   *
   *  @param __first  Start of input range.
   *  @param __last   End of input range.
   *  @param __result Start of output range.
   *  @param __binary_op Function to perform summation.
   *  @param __unary_op Function to transform elements of the input range.
   *  @param __init   Initial value.
   *  @return The end of the output range.
   *
   *  Write the cumulative sum (aka prefix sum, aka scan) of the input range
   *  to the output range. Each element of the output range contains the
   *  running total of all earlier elements (and the initial value),
   *  using `__unary_op` to transform the input elements
   *  and using `__binary_op` for summation.
   *
   *  This function generates an "inclusive" scan, meaning the Nth element
   *  of the output range is the sum of the first N input elements,
   *  so the Nth input element is included.
   */
  template<typename _InputIterator, typename _OutputIterator,
	   typename _BinaryOperation, typename _UnaryOperation, typename _Tp>
    _GLIBCXX20_CONSTEXPR
    _OutputIterator
    transform_inclusive_scan(_InputIterator __first, _InputIterator __last,
			     _OutputIterator __result,
			     _BinaryOperation __binary_op,
			     _UnaryOperation __unary_op,
			     _Tp __init)
    {
      for (; __first != __last; ++__first)
	*__result++ = __init = __binary_op(__init, __unary_op(*__first));
      return __result;
    }

  /** @brief Output the cumulative sum of one range to a second range
   *
   *  @param __first  Start of input range.
   *  @param __last   End of input range.
   *  @param __result Start of output range.
   *  @param __binary_op Function to perform summation.
   *  @param __unary_op Function to transform elements of the input range.
   *  @return The end of the output range.
   *
   *  Write the cumulative sum (aka prefix sum, aka scan) of the input range
   *  to the output range. Each element of the output range contains the
   *  running total of all earlier elements,
   *  using `__unary_op` to transform the input elements
   *  and using `__binary_op` for summation.
   *
   *  This function generates an "inclusive" scan, meaning the Nth element
   *  of the output range is the sum of the first N input elements,
   *  so the Nth input element is included.
   */
  template<typename _InputIterator, typename _OutputIterator,
	  typename _BinaryOperation, typename _UnaryOperation>
    _GLIBCXX20_CONSTEXPR
    _OutputIterator
    transform_inclusive_scan(_InputIterator __first, _InputIterator __last,
			     _OutputIterator __result,
			     _BinaryOperation __binary_op,
			     _UnaryOperation __unary_op)
    {
      if (__first != __last)
	{
	  auto __init = __unary_op(*__first);
	  *__result++ = __init;
	  ++__first;
	  if (__first != __last)
	    __result = std::transform_inclusive_scan(__first, __last, __result,
						     __binary_op, __unary_op,
						     std::move(__init));
	}
      return __result;
    }

  /// @} group numeric_ops

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

// Parallel STL algorithms
# if _PSTL_EXECUTION_POLICIES_DEFINED
// If <execution> has already been included, pull in implementations
#  include <pstl/glue_numeric_impl.h>
# else
// Otherwise just pull in forward declarations
#  include <pstl/glue_numeric_defs.h>
#  define _PSTL_NUMERIC_FORWARD_DECLARED 1
# endif

// Feature test macro for parallel algorithms
# define __cpp_lib_parallel_algorithm 201603L
#endif // C++17

#endif /* _GLIBCXX_NUMERIC */