include/__string

// -*- C++ -*-
//===-------------------------- __string ----------------------------------===//
//
// 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___STRING
#define _LIBCPP___STRING

/*
    string synopsis

namespace std
{

template <class charT>
struct char_traits
{
    typedef charT     char_type;
    typedef ...       int_type;
    typedef streamoff off_type;
    typedef streampos pos_type;
    typedef mbstate_t state_type;

    static constexpr void assign(char_type& c1, const char_type& c2) noexcept;
    static constexpr bool eq(char_type c1, char_type c2) noexcept;
    static constexpr bool lt(char_type c1, char_type c2) noexcept;

    static constexpr int    compare(const char_type* s1, const char_type* s2, size_t n);
    static constexpr size_t length(const char_type* s);
    static constexpr const char_type*
                            find(const char_type* s, size_t n, const char_type& a);

    static constexpr char_type* move(char_type* s1, const char_type* s2, size_t n); // constexpr in C++20
    static constexpr char_type* copy(char_type* s1, const char_type* s2, size_t n); // constexpr in C++20
    static constexpr char_type* assign(char_type* s, size_t n, char_type a);        // constexpr in C++20

    static constexpr int_type  not_eof(int_type c) noexcept;
    static constexpr char_type to_char_type(int_type c) noexcept;
    static constexpr int_type  to_int_type(char_type c) noexcept;
    static constexpr bool      eq_int_type(int_type c1, int_type c2) noexcept;
    static constexpr int_type  eof() noexcept;
};

template <> struct char_traits<char>;
template <> struct char_traits<wchar_t>;
template <> struct char_traits<char8_t>;  // c++20

}  // std

*/

#include <__config>
#include <algorithm>  // for search and min
#include <cstdio>     // For EOF.
#include <memory>     // for __murmur2_or_cityhash

#include <__debug>

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

_LIBCPP_PUSH_MACROS
#include <__undef_macros>


_LIBCPP_BEGIN_NAMESPACE_STD

// The the extern template ABI lists are kept outside of <string> to improve the
// readability of that header.

// The extern template ABI lists are kept outside of <string> to improve the
// readability of that header. We maintain 2 ABI lists:
// - _LIBCPP_STRING_V1_EXTERN_TEMPLATE_LIST
// - _LIBCPP_STRING_UNSTABLE_EXTERN_TEMPLATE_LIST
// As the name implies, the ABI lists define the V1 (Stable) and unstable ABI.
//
// For unstable, we may explicitly remove function that are external in V1,
// and add (new) external functions to better control inlining and compiler
// optimization opportunities.
//
// For stable, the ABI list should rarely change, except for adding new
// functions supporting new c++ version / API changes. Typically entries
// must never be removed from the stable list.
#define _LIBCPP_STRING_V1_EXTERN_TEMPLATE_LIST(_Func, _CharType) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, value_type const*, size_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::rfind(value_type const*, size_type, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__init(value_type const*, size_type, size_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::basic_string(basic_string const&)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, value_type const*)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::basic_string(basic_string const&, std::allocator<_CharType> const&)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find_last_not_of(value_type const*, size_type, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::~basic_string()) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find_first_not_of(value_type const*, size_type, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::insert(size_type, size_type, value_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::operator=(value_type)) \
  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__init(value_type const*, size_type)) \
  _Func(_LIBCPP_FUNC_VIS const _CharType& basic_string<_CharType>::at(size_type) const) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::insert(size_type, value_type const*, size_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find_first_of(value_type const*, size_type, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, size_type, value_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::assign(value_type const*, size_type)) \
  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::reserve(size_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::append(value_type const*, size_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::assign(basic_string const&, size_type, size_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::copy(value_type*, size_type, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::basic_string(basic_string const&, size_type, size_type, std::allocator<_CharType> const&)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find(value_type, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__init(size_type, value_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::insert(size_type, value_type const*)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find_last_of(value_type const*, size_type, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__grow_by(size_type, size_type, size_type, size_type, size_type, size_type)) \
  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__grow_by_and_replace(size_type, size_type, size_type, size_type, size_type, size_type, value_type const*)) \
  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::push_back(value_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::append(size_type, value_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::rfind(value_type, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS const basic_string<_CharType>::size_type basic_string<_CharType>::npos) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::assign(size_type, value_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::erase(size_type, size_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::append(basic_string const&, size_type, size_type)) \
  _Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(value_type const*) const) \
  _Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(size_type, size_type, value_type const*) const) \
  _Func(_LIBCPP_FUNC_VIS _CharType& basic_string<_CharType>::at(size_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::assign(value_type const*)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find(value_type const*, size_type, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(size_type, size_type, basic_string const&, size_type, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(size_type, size_type, value_type const*, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::operator=(basic_string const&)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::append(value_type const*)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, basic_string const&, size_type, size_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::iterator basic_string<_CharType>::insert(basic_string::const_iterator, value_type)) \
  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::resize(size_type, value_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::insert(size_type, basic_string const&, size_type, size_type))

#define _LIBCPP_STRING_UNSTABLE_EXTERN_TEMPLATE_LIST(_Func, _CharType) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, value_type const*, size_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::rfind(value_type const*, size_type, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__init(value_type const*, size_type, size_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, value_type const*)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find_last_not_of(value_type const*, size_type, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::~basic_string()) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find_first_not_of(value_type const*, size_type, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::insert(size_type, size_type, value_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::operator=(value_type)) \
  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__init(value_type const*, size_type)) \
  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__init_copy_ctor_external(value_type const*, size_type)) \
  _Func(_LIBCPP_FUNC_VIS const _CharType& basic_string<_CharType>::at(size_type) const) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::insert(size_type, value_type const*, size_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find_first_of(value_type const*, size_type, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, size_type, value_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::assign(value_type const*, size_type)) \
  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::reserve(size_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::append(value_type const*, size_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::assign(basic_string const&, size_type, size_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::copy(value_type*, size_type, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::basic_string(basic_string const&, size_type, size_type, std::allocator<_CharType> const&)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find(value_type, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__init(size_type, value_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::insert(size_type, value_type const*)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find_last_of(value_type const*, size_type, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__grow_by(size_type, size_type, size_type, size_type, size_type, size_type)) \
  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__grow_by_and_replace(size_type, size_type, size_type, size_type, size_type, size_type, value_type const*)) \
  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__assign_no_alias<false>(value_type const*, size_type)) \
  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__assign_no_alias<true>(value_type const*, size_type)) \
  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::push_back(value_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::append(size_type, value_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::rfind(value_type, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS const basic_string<_CharType>::size_type basic_string<_CharType>::npos) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::assign(size_type, value_type)) \
  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::__erase_external_with_move(size_type, size_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::append(basic_string const&, size_type, size_type)) \
  _Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(value_type const*) const) \
  _Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(size_type, size_type, value_type const*) const) \
  _Func(_LIBCPP_FUNC_VIS _CharType& basic_string<_CharType>::at(size_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::assign(value_type const*)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::size_type basic_string<_CharType>::find(value_type const*, size_type, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(size_type, size_type, basic_string const&, size_type, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS int basic_string<_CharType>::compare(size_type, size_type, value_type const*, size_type) const) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::append(value_type const*)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::replace(size_type, size_type, basic_string const&, size_type, size_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>::iterator basic_string<_CharType>::insert(basic_string::const_iterator, value_type)) \
  _Func(_LIBCPP_FUNC_VIS void basic_string<_CharType>::resize(size_type, value_type)) \
  _Func(_LIBCPP_FUNC_VIS basic_string<_CharType>& basic_string<_CharType>::insert(size_type, basic_string const&, size_type, size_type))


// char_traits

template <class _CharT>
struct _LIBCPP_TEMPLATE_VIS char_traits
{
    typedef _CharT    char_type;
    typedef int       int_type;
    typedef streamoff off_type;
    typedef streampos pos_type;
    typedef mbstate_t state_type;

    static inline void _LIBCPP_CONSTEXPR_AFTER_CXX14
        assign(char_type& __c1, const char_type& __c2) _NOEXCEPT {__c1 = __c2;}
    static inline _LIBCPP_CONSTEXPR bool eq(char_type __c1, char_type __c2) _NOEXCEPT
        {return __c1 == __c2;}
    static inline _LIBCPP_CONSTEXPR bool lt(char_type __c1, char_type __c2) _NOEXCEPT
        {return __c1 < __c2;}

    static _LIBCPP_CONSTEXPR_AFTER_CXX14
    int compare(const char_type* __s1, const char_type* __s2, size_t __n);
    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_AFTER_CXX14
    size_t length(const char_type* __s);
    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_AFTER_CXX14
    const char_type* find(const char_type* __s, size_t __n, const char_type& __a);
    static _LIBCPP_CONSTEXPR_AFTER_CXX17
    char_type*       move(char_type* __s1, const char_type* __s2, size_t __n);
    _LIBCPP_INLINE_VISIBILITY
    static _LIBCPP_CONSTEXPR_AFTER_CXX17
    char_type*       copy(char_type* __s1, const char_type* __s2, size_t __n);
    _LIBCPP_INLINE_VISIBILITY
    static _LIBCPP_CONSTEXPR_AFTER_CXX17
    char_type*       assign(char_type* __s, size_t __n, char_type __a);

    static inline _LIBCPP_CONSTEXPR int_type  not_eof(int_type __c) _NOEXCEPT
        {return eq_int_type(__c, eof()) ? ~eof() : __c;}
    static inline _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT
        {return char_type(__c);}
    static inline _LIBCPP_CONSTEXPR int_type  to_int_type(char_type __c) _NOEXCEPT
        {return int_type(__c);}
    static inline _LIBCPP_CONSTEXPR bool      eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
        {return __c1 == __c2;}
    static inline _LIBCPP_CONSTEXPR int_type  eof() _NOEXCEPT
        {return int_type(EOF);}
};

template <class _CharT>
_LIBCPP_CONSTEXPR_AFTER_CXX14 int
char_traits<_CharT>::compare(const char_type* __s1, const char_type* __s2, size_t __n)
{
    for (; __n; --__n, ++__s1, ++__s2)
    {
        if (lt(*__s1, *__s2))
            return -1;
        if (lt(*__s2, *__s1))
            return 1;
    }
    return 0;
}

template <class _CharT>
inline
_LIBCPP_CONSTEXPR_AFTER_CXX14 size_t
char_traits<_CharT>::length(const char_type* __s)
{
    size_t __len = 0;
    for (; !eq(*__s, char_type(0)); ++__s)
        ++__len;
    return __len;
}

template <class _CharT>
inline
_LIBCPP_CONSTEXPR_AFTER_CXX14 const _CharT*
char_traits<_CharT>::find(const char_type* __s, size_t __n, const char_type& __a)
{
    for (; __n; --__n)
    {
        if (eq(*__s, __a))
            return __s;
        ++__s;
    }
    return 0;
}

template <class _CharT>
_LIBCPP_CONSTEXPR_AFTER_CXX17 _CharT*
char_traits<_CharT>::move(char_type* __s1, const char_type* __s2, size_t __n)
{
    if (__n == 0) return __s1;
    char_type* __r = __s1;
    if (__s1 < __s2)
    {
        for (; __n; --__n, ++__s1, ++__s2)
            assign(*__s1, *__s2);
    }
    else if (__s2 < __s1)
    {
        __s1 += __n;
        __s2 += __n;
        for (; __n; --__n)
            assign(*--__s1, *--__s2);
    }
    return __r;
}

template <class _CharT>
inline _LIBCPP_CONSTEXPR_AFTER_CXX17
_CharT*
char_traits<_CharT>::copy(char_type* __s1, const char_type* __s2, size_t __n)
{
    _LIBCPP_ASSERT(__s2 < __s1 || __s2 >= __s1+__n, "char_traits::copy overlapped range");
    char_type* __r = __s1;
    for (; __n; --__n, ++__s1, ++__s2)
        assign(*__s1, *__s2);
    return __r;
}

template <class _CharT>
inline _LIBCPP_CONSTEXPR_AFTER_CXX17
_CharT*
char_traits<_CharT>::assign(char_type* __s, size_t __n, char_type __a)
{
    char_type* __r = __s;
    for (; __n; --__n, ++__s)
        assign(*__s, __a);
    return __r;
}

// constexpr versions of move/copy/assign.

template <class _CharT>
static inline _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_IS_CONSTANT_EVALUATED
_CharT* __move_constexpr(_CharT* __s1, const _CharT* __s2, size_t __n) _NOEXCEPT
{
    if (__n == 0) return __s1;
    if (__s1 < __s2) {
      _VSTD::copy(__s2, __s2 + __n, __s1);
    } else if (__s2 < __s1) {
      _VSTD::copy_backward(__s2, __s2 + __n, __s1 + __n);
    }
    return __s1;
}

template <class _CharT>
static inline _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_IS_CONSTANT_EVALUATED
_CharT* __copy_constexpr(_CharT* __s1, const _CharT* __s2, size_t __n) _NOEXCEPT
{
    _VSTD::copy_n(__s2, __n, __s1);
    return __s1;
}

template <class _CharT>
static inline _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_IS_CONSTANT_EVALUATED
_CharT* __assign_constexpr(_CharT* __s, size_t __n, _CharT __a) _NOEXCEPT
{
     _VSTD::fill_n(__s, __n, __a);
     return __s;
}

// char_traits<char>

template <>
struct _LIBCPP_TEMPLATE_VIS char_traits<char>
{
    typedef char      char_type;
    typedef int       int_type;
    typedef streamoff off_type;
    typedef streampos pos_type;
    typedef mbstate_t state_type;

    static inline _LIBCPP_CONSTEXPR_AFTER_CXX14
    void assign(char_type& __c1, const char_type& __c2) _NOEXCEPT {__c1 = __c2;}
    static inline _LIBCPP_CONSTEXPR bool eq(char_type __c1, char_type __c2) _NOEXCEPT
            {return __c1 == __c2;}
    static inline _LIBCPP_CONSTEXPR bool lt(char_type __c1, char_type __c2) _NOEXCEPT
        {return (unsigned char)__c1 < (unsigned char)__c2;}

    static _LIBCPP_CONSTEXPR_AFTER_CXX14
    int compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
    static inline size_t _LIBCPP_CONSTEXPR_AFTER_CXX14
    length(const char_type* __s)  _NOEXCEPT {return __builtin_strlen(__s);}
    static _LIBCPP_CONSTEXPR_AFTER_CXX14
    const char_type* find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT;
    static inline _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_IS_CONSTANT_EVALUATED
    char_type* move(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
        {
            return __libcpp_is_constant_evaluated()
                       ? __move_constexpr(__s1, __s2, __n)
                       : __n == 0 ? __s1 : (char_type*)memmove(__s1, __s2, __n);
        }
    static inline _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_IS_CONSTANT_EVALUATED
    char_type* copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
        {
            _LIBCPP_ASSERT(__s2 < __s1 || __s2 >= __s1+__n, "char_traits::copy overlapped range");
            return __libcpp_is_constant_evaluated()
                       ? __copy_constexpr(__s1, __s2, __n)
                       : __n == 0 ? __s1 : (char_type*)memcpy(__s1, __s2, __n);
        }
    static inline _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_IS_CONSTANT_EVALUATED
    char_type* assign(char_type* __s, size_t __n, char_type __a) _NOEXCEPT
        {
            return __libcpp_is_constant_evaluated()
                       ? __assign_constexpr(__s, __n, __a)
                       : __n == 0 ? __s : (char_type*)memset(__s, to_int_type(__a), __n);
        }

    static inline _LIBCPP_CONSTEXPR int_type  not_eof(int_type __c) _NOEXCEPT
        {return eq_int_type(__c, eof()) ? ~eof() : __c;}
    static inline _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT
        {return char_type(__c);}
    static inline _LIBCPP_CONSTEXPR int_type to_int_type(char_type __c) _NOEXCEPT
        {return int_type((unsigned char)__c);}
    static inline _LIBCPP_CONSTEXPR bool eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
        {return __c1 == __c2;}
    static inline _LIBCPP_CONSTEXPR int_type  eof() _NOEXCEPT
        {return int_type(EOF);}
};

inline _LIBCPP_CONSTEXPR_AFTER_CXX14
int
char_traits<char>::compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
{
    if (__n == 0)
        return 0;
#if __has_feature(cxx_constexpr_string_builtins)
    return __builtin_memcmp(__s1, __s2, __n);
#elif _LIBCPP_STD_VER <= 14
    return memcmp(__s1, __s2, __n);
#else
    for (; __n; --__n, ++__s1, ++__s2)
    {
        if (lt(*__s1, *__s2))
            return -1;
        if (lt(*__s2, *__s1))
            return 1;
    }
    return 0;
#endif
}

inline _LIBCPP_CONSTEXPR_AFTER_CXX14
const char*
char_traits<char>::find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT
{
    if (__n == 0)
        return nullptr;
#if __has_feature(cxx_constexpr_string_builtins)
    return __builtin_char_memchr(__s, to_int_type(__a), __n);
#elif _LIBCPP_STD_VER <= 14
    return (const char_type*) memchr(__s, to_int_type(__a), __n);
#else
    for (; __n; --__n)
    {
        if (eq(*__s, __a))
            return __s;
        ++__s;
    }
    return nullptr;
#endif
}


// char_traits<wchar_t>

template <>
struct _LIBCPP_TEMPLATE_VIS char_traits<wchar_t>
{
    typedef wchar_t   char_type;
    typedef wint_t    int_type;
    typedef streamoff off_type;
    typedef streampos pos_type;
    typedef mbstate_t state_type;

    static inline _LIBCPP_CONSTEXPR_AFTER_CXX14
    void assign(char_type& __c1, const char_type& __c2) _NOEXCEPT {__c1 = __c2;}
    static inline _LIBCPP_CONSTEXPR bool eq(char_type __c1, char_type __c2) _NOEXCEPT
        {return __c1 == __c2;}
    static inline _LIBCPP_CONSTEXPR bool lt(char_type __c1, char_type __c2) _NOEXCEPT
        {return __c1 < __c2;}

    static _LIBCPP_CONSTEXPR_AFTER_CXX14
    int compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
    static _LIBCPP_CONSTEXPR_AFTER_CXX14
    size_t length(const char_type* __s) _NOEXCEPT;
    static _LIBCPP_CONSTEXPR_AFTER_CXX14
    const char_type* find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT;
    static inline _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_IS_CONSTANT_EVALUATED
    char_type* move(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
        {
            return __libcpp_is_constant_evaluated()
                       ? __move_constexpr(__s1, __s2, __n)
                       : __n == 0 ? __s1 : wmemmove(__s1, __s2, __n);
        }
    static inline _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_IS_CONSTANT_EVALUATED
    char_type* copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
        {
            _LIBCPP_ASSERT(__s2 < __s1 || __s2 >= __s1+__n, "char_traits::copy overlapped range");
            return __libcpp_is_constant_evaluated()
                       ? __copy_constexpr(__s1, __s2, __n)
                       : __n == 0 ? __s1 : wmemcpy(__s1, __s2, __n);
        }
    static inline _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_IS_CONSTANT_EVALUATED
    char_type* assign(char_type* __s, size_t __n, char_type __a) _NOEXCEPT
        {
            return __libcpp_is_constant_evaluated()
                       ? __assign_constexpr(__s, __n, __a)
                       : __n == 0 ? __s : wmemset(__s, __a, __n);
        }
    static inline _LIBCPP_CONSTEXPR int_type  not_eof(int_type __c) _NOEXCEPT
        {return eq_int_type(__c, eof()) ? ~eof() : __c;}
    static inline _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT
        {return char_type(__c);}
    static inline _LIBCPP_CONSTEXPR int_type to_int_type(char_type __c) _NOEXCEPT
        {return int_type(__c);}
    static inline _LIBCPP_CONSTEXPR bool eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
        {return __c1 == __c2;}
    static inline _LIBCPP_CONSTEXPR int_type eof() _NOEXCEPT
        {return int_type(WEOF);}
};

inline _LIBCPP_CONSTEXPR_AFTER_CXX14
int
char_traits<wchar_t>::compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
{
    if (__n == 0)
        return 0;
#if __has_feature(cxx_constexpr_string_builtins)
    return __builtin_wmemcmp(__s1, __s2, __n);
#elif _LIBCPP_STD_VER <= 14
    return wmemcmp(__s1, __s2, __n);
#else
    for (; __n; --__n, ++__s1, ++__s2)
    {
        if (lt(*__s1, *__s2))
            return -1;
        if (lt(*__s2, *__s1))
            return 1;
    }
    return 0;
#endif
}


template <class _Traits>
_LIBCPP_INLINE_VISIBILITY
_LIBCPP_CONSTEXPR
inline size_t __char_traits_length_checked(const typename _Traits::char_type* __s) _NOEXCEPT {
#if _LIBCPP_DEBUG_LEVEL >= 1
  return __s ? _Traits::length(__s) : (_VSTD::__libcpp_debug_function(_VSTD::__libcpp_debug_info(__FILE__, __LINE__, "p == nullptr", "null pointer pass to non-null argument of char_traits<...>::length")), 0);
#else
  return _Traits::length(__s);
#endif
}

inline _LIBCPP_CONSTEXPR_AFTER_CXX14
size_t
char_traits<wchar_t>::length(const char_type* __s) _NOEXCEPT
{
#if __has_feature(cxx_constexpr_string_builtins)
    return __builtin_wcslen(__s);
#elif _LIBCPP_STD_VER <= 14
    return wcslen(__s);
#else
    size_t __len = 0;
    for (; !eq(*__s, char_type(0)); ++__s)
        ++__len;
    return __len;
#endif
}

inline _LIBCPP_CONSTEXPR_AFTER_CXX14
const wchar_t*
char_traits<wchar_t>::find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT
{
    if (__n == 0)
        return nullptr;
#if __has_feature(cxx_constexpr_string_builtins)
    return __builtin_wmemchr(__s, __a, __n);
#elif _LIBCPP_STD_VER <= 14
    return wmemchr(__s, __a, __n);
#else
    for (; __n; --__n)
    {
        if (eq(*__s, __a))
            return __s;
        ++__s;
    }
    return nullptr;
#endif
}


#ifndef _LIBCPP_NO_HAS_CHAR8_T

template <>
struct _LIBCPP_TEMPLATE_VIS char_traits<char8_t>
{
    typedef char8_t        char_type;
    typedef unsigned int   int_type;
    typedef streamoff      off_type;
    typedef u8streampos    pos_type;
    typedef mbstate_t      state_type;

    static inline constexpr void assign(char_type& __c1, const char_type& __c2) noexcept
        {__c1 = __c2;}
    static inline constexpr bool eq(char_type __c1, char_type __c2) noexcept
        {return __c1 == __c2;}
    static inline constexpr bool lt(char_type __c1, char_type __c2) noexcept
        {return __c1 < __c2;}

    static constexpr
    int              compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;

    static constexpr
    size_t           length(const char_type* __s) _NOEXCEPT;

    _LIBCPP_INLINE_VISIBILITY static constexpr
    const char_type* find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT;

    static _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_IS_CONSTANT_EVALUATED
    char_type*       move(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
        {
            return __libcpp_is_constant_evaluated()
                       ? __move_constexpr(__s1, __s2, __n)
                       : __n == 0 ? __s1 : (char_type*)memmove(__s1, __s2, __n);
        }

    static _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_IS_CONSTANT_EVALUATED
    char_type*       copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
       {
            _LIBCPP_ASSERT(__s2 < __s1 || __s2 >= __s1+__n, "char_traits::copy overlapped range");
            return __libcpp_is_constant_evaluated()
                       ? __copy_constexpr(__s1, __s2, __n)
                       : __n == 0 ? __s1 : (char_type*)memcpy(__s1, __s2, __n);
        }

    static _LIBCPP_CONSTEXPR_AFTER_CXX17_WITH_IS_CONSTANT_EVALUATED
    char_type*       assign(char_type* __s, size_t __n, char_type __a) _NOEXCEPT
        {
            return __libcpp_is_constant_evaluated()
                       ? __assign_constexpr(__s, __n, __a)
                       : __n == 0 ? __s : (char_type*)memset(__s, to_int_type(__a), __n);
        }

    static inline constexpr int_type  not_eof(int_type __c) noexcept
        {return eq_int_type(__c, eof()) ? ~eof() : __c;}
    static inline constexpr char_type to_char_type(int_type __c) noexcept
        {return char_type(__c);}
    static inline constexpr int_type to_int_type(char_type __c) noexcept
        {return int_type(__c);}
    static inline constexpr bool eq_int_type(int_type __c1, int_type __c2) noexcept
        {return __c1 == __c2;}
    static inline constexpr int_type eof() noexcept
        {return int_type(EOF);}
};

// TODO use '__builtin_strlen' if it ever supports char8_t ??
inline constexpr
size_t
char_traits<char8_t>::length(const char_type* __s) _NOEXCEPT
{
    size_t __len = 0;
    for (; !eq(*__s, char_type(0)); ++__s)
        ++__len;
    return __len;
}

inline constexpr
int
char_traits<char8_t>::compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
{
#if __has_feature(cxx_constexpr_string_builtins)
    return __builtin_memcmp(__s1, __s2, __n);
#else
    for (; __n; --__n, ++__s1, ++__s2)
    {
        if (lt(*__s1, *__s2))
            return -1;
        if (lt(*__s2, *__s1))
            return 1;
    }
    return 0;
#endif
}

// TODO use '__builtin_char_memchr' if it ever supports char8_t ??
inline constexpr
const char8_t*
char_traits<char8_t>::find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT
{
    for (; __n; --__n)
    {
        if (eq(*__s, __a))
            return __s;
        ++__s;
    }
    return 0;
}

#endif // #_LIBCPP_NO_HAS_CHAR8_T

#ifndef _LIBCPP_HAS_NO_UNICODE_CHARS

template <>
struct _LIBCPP_TEMPLATE_VIS char_traits<char16_t>
{
    typedef char16_t       char_type;
    typedef uint_least16_t int_type;
    typedef streamoff      off_type;
    typedef u16streampos   pos_type;
    typedef mbstate_t      state_type;

    static inline _LIBCPP_CONSTEXPR_AFTER_CXX14
    void assign(char_type& __c1, const char_type& __c2) _NOEXCEPT {__c1 = __c2;}
    static inline _LIBCPP_CONSTEXPR bool eq(char_type __c1, char_type __c2) _NOEXCEPT
        {return __c1 == __c2;}
    static inline _LIBCPP_CONSTEXPR bool lt(char_type __c1, char_type __c2) _NOEXCEPT
        {return __c1 < __c2;}

    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_AFTER_CXX14
    int              compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_AFTER_CXX14
    size_t           length(const char_type* __s) _NOEXCEPT;
    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_AFTER_CXX14
    const char_type* find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT;
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    static char_type*       move(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    static char_type*       copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    static char_type*       assign(char_type* __s, size_t __n, char_type __a) _NOEXCEPT;

    static inline _LIBCPP_CONSTEXPR int_type  not_eof(int_type __c) _NOEXCEPT
        {return eq_int_type(__c, eof()) ? ~eof() : __c;}
    static inline _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT
        {return char_type(__c);}
    static inline _LIBCPP_CONSTEXPR int_type to_int_type(char_type __c) _NOEXCEPT
        {return int_type(__c);}
    static inline _LIBCPP_CONSTEXPR bool eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
        {return __c1 == __c2;}
    static inline _LIBCPP_CONSTEXPR int_type eof() _NOEXCEPT
        {return int_type(0xFFFF);}
};

inline _LIBCPP_CONSTEXPR_AFTER_CXX14
int
char_traits<char16_t>::compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
{
    for (; __n; --__n, ++__s1, ++__s2)
    {
        if (lt(*__s1, *__s2))
            return -1;
        if (lt(*__s2, *__s1))
            return 1;
    }
    return 0;
}

inline _LIBCPP_CONSTEXPR_AFTER_CXX14
size_t
char_traits<char16_t>::length(const char_type* __s) _NOEXCEPT
{
    size_t __len = 0;
    for (; !eq(*__s, char_type(0)); ++__s)
        ++__len;
    return __len;
}

inline _LIBCPP_CONSTEXPR_AFTER_CXX14
const char16_t*
char_traits<char16_t>::find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT
{
    for (; __n; --__n)
    {
        if (eq(*__s, __a))
            return __s;
        ++__s;
    }
    return 0;
}

inline _LIBCPP_CONSTEXPR_AFTER_CXX17
char16_t*
char_traits<char16_t>::move(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
{
    if (__n == 0) return __s1;
    char_type* __r = __s1;
    if (__s1 < __s2)
    {
        for (; __n; --__n, ++__s1, ++__s2)
            assign(*__s1, *__s2);
    }
    else if (__s2 < __s1)
    {
        __s1 += __n;
        __s2 += __n;
        for (; __n; --__n)
            assign(*--__s1, *--__s2);
    }
    return __r;
}

inline _LIBCPP_CONSTEXPR_AFTER_CXX17
char16_t*
char_traits<char16_t>::copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
{
    _LIBCPP_ASSERT(__s2 < __s1 || __s2 >= __s1+__n, "char_traits::copy overlapped range");
    char_type* __r = __s1;
    for (; __n; --__n, ++__s1, ++__s2)
        assign(*__s1, *__s2);
    return __r;
}

inline _LIBCPP_CONSTEXPR_AFTER_CXX17
char16_t*
char_traits<char16_t>::assign(char_type* __s, size_t __n, char_type __a) _NOEXCEPT
{
    char_type* __r = __s;
    for (; __n; --__n, ++__s)
        assign(*__s, __a);
    return __r;
}

template <>
struct _LIBCPP_TEMPLATE_VIS char_traits<char32_t>
{
    typedef char32_t       char_type;
    typedef uint_least32_t int_type;
    typedef streamoff      off_type;
    typedef u32streampos   pos_type;
    typedef mbstate_t      state_type;

    static inline _LIBCPP_CONSTEXPR_AFTER_CXX14
    void assign(char_type& __c1, const char_type& __c2) _NOEXCEPT {__c1 = __c2;}
    static inline _LIBCPP_CONSTEXPR bool eq(char_type __c1, char_type __c2) _NOEXCEPT
        {return __c1 == __c2;}
    static inline _LIBCPP_CONSTEXPR bool lt(char_type __c1, char_type __c2) _NOEXCEPT
        {return __c1 < __c2;}

    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_AFTER_CXX14
    int              compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_AFTER_CXX14
    size_t           length(const char_type* __s) _NOEXCEPT;
    _LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR_AFTER_CXX14
    const char_type* find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT;
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    static char_type*       move(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    static char_type*       copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT;
    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
    static char_type*       assign(char_type* __s, size_t __n, char_type __a) _NOEXCEPT;

    static inline _LIBCPP_CONSTEXPR int_type  not_eof(int_type __c) _NOEXCEPT
        {return eq_int_type(__c, eof()) ? ~eof() : __c;}
    static inline _LIBCPP_CONSTEXPR char_type to_char_type(int_type __c) _NOEXCEPT
        {return char_type(__c);}
    static inline _LIBCPP_CONSTEXPR int_type to_int_type(char_type __c) _NOEXCEPT
        {return int_type(__c);}
    static inline _LIBCPP_CONSTEXPR bool eq_int_type(int_type __c1, int_type __c2) _NOEXCEPT
        {return __c1 == __c2;}
    static inline _LIBCPP_CONSTEXPR int_type eof() _NOEXCEPT
        {return int_type(0xFFFFFFFF);}
};

inline _LIBCPP_CONSTEXPR_AFTER_CXX14
int
char_traits<char32_t>::compare(const char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
{
    for (; __n; --__n, ++__s1, ++__s2)
    {
        if (lt(*__s1, *__s2))
            return -1;
        if (lt(*__s2, *__s1))
            return 1;
    }
    return 0;
}

inline _LIBCPP_CONSTEXPR_AFTER_CXX14
size_t
char_traits<char32_t>::length(const char_type* __s) _NOEXCEPT
{
    size_t __len = 0;
    for (; !eq(*__s, char_type(0)); ++__s)
        ++__len;
    return __len;
}

inline _LIBCPP_CONSTEXPR_AFTER_CXX14
const char32_t*
char_traits<char32_t>::find(const char_type* __s, size_t __n, const char_type& __a) _NOEXCEPT
{
    for (; __n; --__n)
    {
        if (eq(*__s, __a))
            return __s;
        ++__s;
    }
    return 0;
}

inline _LIBCPP_CONSTEXPR_AFTER_CXX17
char32_t*
char_traits<char32_t>::move(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
{
    if (__n == 0) return __s1;
    char_type* __r = __s1;
    if (__s1 < __s2)
    {
        for (; __n; --__n, ++__s1, ++__s2)
            assign(*__s1, *__s2);
    }
    else if (__s2 < __s1)
    {
        __s1 += __n;
        __s2 += __n;
        for (; __n; --__n)
            assign(*--__s1, *--__s2);
    }
    return __r;
}

inline _LIBCPP_CONSTEXPR_AFTER_CXX17
char32_t*
char_traits<char32_t>::copy(char_type* __s1, const char_type* __s2, size_t __n) _NOEXCEPT
{
    _LIBCPP_ASSERT(__s2 < __s1 || __s2 >= __s1+__n, "char_traits::copy overlapped range");
    char_type* __r = __s1;
    for (; __n; --__n, ++__s1, ++__s2)
        assign(*__s1, *__s2);
    return __r;
}

inline _LIBCPP_CONSTEXPR_AFTER_CXX17
char32_t*
char_traits<char32_t>::assign(char_type* __s, size_t __n, char_type __a) _NOEXCEPT
{
    char_type* __r = __s;
    for (; __n; --__n, ++__s)
        assign(*__s, __a);
    return __r;
}

#endif  // _LIBCPP_HAS_NO_UNICODE_CHARS

// helper fns for basic_string and string_view

// __str_find
template<class _CharT, class _SizeT, class _Traits, _SizeT __npos>
inline _SizeT _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY
__str_find(const _CharT *__p, _SizeT __sz,
             _CharT __c, _SizeT __pos) _NOEXCEPT
{
    if (__pos >= __sz)
        return __npos;
    const _CharT* __r = _Traits::find(__p + __pos, __sz - __pos, __c);
    if (__r == 0)
        return __npos;
    return static_cast<_SizeT>(__r - __p);
}

template <class _CharT, class _Traits>
inline _LIBCPP_CONSTEXPR_AFTER_CXX11 const _CharT *
__search_substring(const _CharT *__first1, const _CharT *__last1,
                   const _CharT *__first2, const _CharT *__last2) {
  // Take advantage of knowing source and pattern lengths.
  // Stop short when source is smaller than pattern.
  const ptrdiff_t __len2 = __last2 - __first2;
  if (__len2 == 0)
    return __first1;

  ptrdiff_t __len1 = __last1 - __first1;
  if (__len1 < __len2)
    return __last1;

  // First element of __first2 is loop invariant.
  _CharT __f2 = *__first2;
  while (true) {
    __len1 = __last1 - __first1;
    // Check whether __first1 still has at least __len2 bytes.
    if (__len1 < __len2)
      return __last1;

    // Find __f2 the first byte matching in __first1.
    __first1 = _Traits::find(__first1, __len1 - __len2 + 1, __f2);
    if (__first1 == 0)
      return __last1;

    // It is faster to compare from the first byte of __first1 even if we
    // already know that it matches the first byte of __first2: this is because
    // __first2 is most likely aligned, as it is user's "pattern" string, and
    // __first1 + 1 is most likely not aligned, as the match is in the middle of
    // the string.
    if (_Traits::compare(__first1, __first2, __len2) == 0)
      return __first1;

    ++__first1;
  }
}

template<class _CharT, class _SizeT, class _Traits, _SizeT __npos>
inline _SizeT _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY
__str_find(const _CharT *__p, _SizeT __sz,
       const _CharT* __s, _SizeT __pos, _SizeT __n) _NOEXCEPT
{
    if (__pos > __sz)
        return __npos;

    if (__n == 0) // There is nothing to search, just return __pos.
        return __pos;

    const _CharT *__r = __search_substring<_CharT, _Traits>(
        __p + __pos, __p + __sz, __s, __s + __n);

    if (__r == __p + __sz)
        return __npos;
    return static_cast<_SizeT>(__r - __p);
}


// __str_rfind

template<class _CharT, class _SizeT, class _Traits, _SizeT __npos>
inline _SizeT _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY
__str_rfind(const _CharT *__p, _SizeT __sz,
              _CharT __c, _SizeT __pos) _NOEXCEPT
{
    if (__sz < 1)
        return __npos;
    if (__pos < __sz)
        ++__pos;
    else
        __pos = __sz;
    for (const _CharT* __ps = __p + __pos; __ps != __p;)
    {
        if (_Traits::eq(*--__ps, __c))
            return static_cast<_SizeT>(__ps - __p);
    }
    return __npos;
}

template<class _CharT, class _SizeT, class _Traits, _SizeT __npos>
inline _SizeT _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY
__str_rfind(const _CharT *__p, _SizeT __sz,
        const _CharT* __s, _SizeT __pos, _SizeT __n) _NOEXCEPT
{
    __pos = _VSTD::min(__pos, __sz);
    if (__n < __sz - __pos)
        __pos += __n;
    else
        __pos = __sz;
    const _CharT* __r = _VSTD::__find_end(
                  __p, __p + __pos, __s, __s + __n, _Traits::eq,
                        random_access_iterator_tag(), random_access_iterator_tag());
    if (__n > 0 && __r == __p + __pos)
        return __npos;
    return static_cast<_SizeT>(__r - __p);
}

// __str_find_first_of
template<class _CharT, class _SizeT, class _Traits, _SizeT __npos>
inline _SizeT _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY
__str_find_first_of(const _CharT *__p, _SizeT __sz,
                const _CharT* __s, _SizeT __pos, _SizeT __n) _NOEXCEPT
{
    if (__pos >= __sz || __n == 0)
        return __npos;
    const _CharT* __r = _VSTD::__find_first_of_ce
        (__p + __pos, __p + __sz, __s, __s + __n, _Traits::eq );
    if (__r == __p + __sz)
        return __npos;
    return static_cast<_SizeT>(__r - __p);
}


// __str_find_last_of
template<class _CharT, class _SizeT, class _Traits, _SizeT __npos>
inline _SizeT _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY
__str_find_last_of(const _CharT *__p, _SizeT __sz,
               const _CharT* __s, _SizeT __pos, _SizeT __n) _NOEXCEPT
    {
    if (__n != 0)
    {
        if (__pos < __sz)
            ++__pos;
        else
            __pos = __sz;
        for (const _CharT* __ps = __p + __pos; __ps != __p;)
        {
            const _CharT* __r = _Traits::find(__s, __n, *--__ps);
            if (__r)
                return static_cast<_SizeT>(__ps - __p);
        }
    }
    return __npos;
}


// __str_find_first_not_of
template<class _CharT, class _SizeT, class _Traits, _SizeT __npos>
inline _SizeT _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY
__str_find_first_not_of(const _CharT *__p, _SizeT __sz,
                    const _CharT* __s, _SizeT __pos, _SizeT __n) _NOEXCEPT
{
    if (__pos < __sz)
    {
        const _CharT* __pe = __p + __sz;
        for (const _CharT* __ps = __p + __pos; __ps != __pe; ++__ps)
            if (_Traits::find(__s, __n, *__ps) == 0)
                return static_cast<_SizeT>(__ps - __p);
    }
    return __npos;
}


template<class _CharT, class _SizeT, class _Traits, _SizeT __npos>
inline _SizeT _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY
__str_find_first_not_of(const _CharT *__p, _SizeT __sz,
                          _CharT __c, _SizeT __pos) _NOEXCEPT
{
    if (__pos < __sz)
    {
        const _CharT* __pe = __p + __sz;
        for (const _CharT* __ps = __p + __pos; __ps != __pe; ++__ps)
            if (!_Traits::eq(*__ps, __c))
                return static_cast<_SizeT>(__ps - __p);
    }
    return __npos;
}


// __str_find_last_not_of
template<class _CharT, class _SizeT, class _Traits, _SizeT __npos>
inline _SizeT _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY
__str_find_last_not_of(const _CharT *__p, _SizeT __sz,
                   const _CharT* __s, _SizeT __pos, _SizeT __n) _NOEXCEPT
{
    if (__pos < __sz)
        ++__pos;
    else
        __pos = __sz;
    for (const _CharT* __ps = __p + __pos; __ps != __p;)
        if (_Traits::find(__s, __n, *--__ps) == 0)
            return static_cast<_SizeT>(__ps - __p);
    return __npos;
}


template<class _CharT, class _SizeT, class _Traits, _SizeT __npos>
inline _SizeT _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY
__str_find_last_not_of(const _CharT *__p, _SizeT __sz,
                         _CharT __c, _SizeT __pos) _NOEXCEPT
{
    if (__pos < __sz)
        ++__pos;
    else
        __pos = __sz;
    for (const _CharT* __ps = __p + __pos; __ps != __p;)
        if (!_Traits::eq(*--__ps, __c))
            return static_cast<_SizeT>(__ps - __p);
    return __npos;
}

template<class _Ptr>
inline _LIBCPP_INLINE_VISIBILITY
size_t __do_string_hash(_Ptr __p, _Ptr __e)
{
    typedef typename iterator_traits<_Ptr>::value_type value_type;
    return __murmur2_or_cityhash<size_t>()(__p, (__e-__p)*sizeof(value_type));
}

template <class _CharT, class _Iter, class _Traits=char_traits<_CharT> >
struct __quoted_output_proxy
{
    _Iter  __first;
    _Iter  __last;
    _CharT  __delim;
    _CharT  __escape;

    __quoted_output_proxy(_Iter __f, _Iter __l, _CharT __d, _CharT __e)
    : __first(__f), __last(__l), __delim(__d), __escape(__e) {}
    //  This would be a nice place for a string_ref
};

_LIBCPP_END_NAMESPACE_STD

_LIBCPP_POP_MACROS

#endif  // _LIBCPP___STRING