//

// Copyright 2017 The Abseil Authors.

//

// Licensed under the Apache License, Version 2.0 (the "License");

// you may not use this file except in compliance with the License.

// You may obtain a copy of the License at

//

//      https://www.apache.org/licenses/LICENSE-2.0

//

// Unless required by applicable law or agreed to in writing, software

// distributed under the License is distributed on an "AS IS" BASIS,

// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

// See the License for the specific language governing permissions and

// limitations under the License.

//

// -----------------------------------------------------------------------------

// File: string_view.h

// -----------------------------------------------------------------------------

//

// This file contains the definition of the `absl::string_view` class. A

// `string_view` points to a contiguous span of characters, often part or all of

// another `std::string`, double-quoted string literal, character array, or even

// another `string_view`.

//

// This `absl::string_view` abstraction is designed to be a drop-in

// replacement for the C++17 `std::string_view` abstraction.

#ifndef ABSL_STRINGS_STRING_VIEW_H_

#define ABSL_STRINGS_STRING_VIEW_H_

#include <algorithm>

#include <cassert>

#include <cstddef>

#include <cstring>

#include <iosfwd>

#include <iterator>

#include <limits>

#include <string>

#include "absl/base/attributes.h"

#include "absl/base/nullability.h"

#include "absl/base/config.h"

#include "absl/base/internal/throw_delegate.h"

#include "absl/base/macros.h"

#include "absl/base/optimization.h"

#include "absl/base/port.h"

#ifdef ABSL_USES_STD_STRING_VIEW

#include <string_view>  // IWYU pragma: export

namespace absl {

ABSL_NAMESPACE_BEGIN

using string_view = std::string_view;

ABSL_NAMESPACE_END

}  // namespace absl

#else  // ABSL_USES_STD_STRING_VIEW

#if ABSL_HAVE_BUILTIN(__builtin_memcmp) ||        \

    (defined(__GNUC__) && !defined(__clang__)) || \

    (defined(_MSC_VER) && _MSC_VER >= 1928)

#define ABSL_INTERNAL_STRING_VIEW_MEMCMP __builtin_memcmp

#else  // ABSL_HAVE_BUILTIN(__builtin_memcmp)

#define ABSL_INTERNAL_STRING_VIEW_MEMCMP memcmp

#endif  // ABSL_HAVE_BUILTIN(__builtin_memcmp)

namespace absl {

ABSL_NAMESPACE_BEGIN

// absl::string_view

//

// A `string_view` provides a lightweight view into the string data provided by

// a `std::string`, double-quoted string literal, character array, or even

// another `string_view`. A `string_view` does *not* own the string to which it

// points, and that data cannot be modified through the view.

//

// You can use `string_view` as a function or method parameter anywhere a

// parameter can receive a double-quoted string literal, `const char*`,

// `std::string`, or another `absl::string_view` argument with no need to copy

// the string data. Systematic use of `string_view` within function arguments

// reduces data copies and `strlen()` calls.

//

// Because of its small size, prefer passing `string_view` by value:

//

//   void MyFunction(absl::string_view arg);

//

// If circumstances require, you may also pass one by const reference:

//

//   void MyFunction(const absl::string_view& arg);  // not preferred

//

// Passing by value generates slightly smaller code for many architectures.

//

// In either case, the source data of the `string_view` must outlive the

// `string_view` itself.

//

// A `string_view` is also suitable for local variables if you know that the

// lifetime of the underlying object is longer than the lifetime of your

// `string_view` variable. However, beware of binding a `string_view` to a

// temporary value:

//

//   // BAD use of string_view: lifetime problem

//   absl::string_view sv = obj.ReturnAString();

//

//   // GOOD use of string_view: str outlives sv

//   std::string str = obj.ReturnAString();

//   absl::string_view sv = str;

//

// Due to lifetime issues, a `string_view` is sometimes a poor choice for a

// return value and usually a poor choice for a data member. If you do use a

// `string_view` this way, it is your responsibility to ensure that the object

// pointed to by the `string_view` outlives the `string_view`.

//

// A `string_view` may represent a whole string or just part of a string. For

// example, when splitting a string, `std::vector<absl::string_view>` is a

// natural data type for the output.

//

// For another example, a Cord is a non-contiguous, potentially very

// long string-like object.  The Cord class has an interface that iteratively

// provides string_view objects that point to the successive pieces of a Cord

// object.

//

// When constructed from a source which is NUL-terminated, the `string_view`

// itself will not include the NUL-terminator unless a specific size (including

// the NUL) is passed to the constructor. As a result, common idioms that work

// on NUL-terminated strings do not work on `string_view` objects. If you write

// code that scans a `string_view`, you must check its length rather than test

// for nul, for example. Note, however, that nuls may still be embedded within

// a `string_view` explicitly.

//

// You may create a null `string_view` in two ways:

//

//   absl::string_view sv;

//   absl::string_view sv(nullptr, 0);

//

// For the above, `sv.data() == nullptr`, `sv.length() == 0`, and

// `sv.empty() == true`. Also, if you create a `string_view` with a non-null

// pointer then `sv.data() != nullptr`. Thus, you can use `string_view()` to

// signal an undefined value that is different from other `string_view` values

// in a similar fashion to how `const char* p1 = nullptr;` is different from

// `const char* p2 = "";`. However, in practice, it is not recommended to rely

// on this behavior.

//

// Be careful not to confuse a null `string_view` with an empty one. A null

// `string_view` is an empty `string_view`, but some empty `string_view`s are

// not null. Prefer checking for emptiness over checking for null.

//

// There are many ways to create an empty string_view:

//

//   const char* nullcp = nullptr;

//   // string_view.size() will return 0 in all cases.

//   absl::string_view();

//   absl::string_view(nullcp, 0);

//   absl::string_view("");

//   absl::string_view("", 0);

//   absl::string_view("abcdef", 0);

//   absl::string_view("abcdef" + 6, 0);

//

// All empty `string_view` objects whether null or not, are equal:

//

//   absl::string_view() == absl::string_view("", 0)

//   absl::string_view(nullptr, 0) == absl::string_view("abcdef"+6, 0)

class ABSL_ATTRIBUTE_VIEW string_view {

 public:

  using traits_type = std::char_traits<char>;

  using value_type = char;

  using pointer = char* absl_nullable;

  using const_pointer = const char* absl_nullable;

  using reference = char&;

  using const_reference = const char&;

  using const_iterator = const char* absl_nullable;

  using iterator = const_iterator;

  using const_reverse_iterator = std::reverse_iterator<const_iterator>;

  using reverse_iterator = const_reverse_iterator;

  using size_type = size_t;

  using difference_type = std::ptrdiff_t;

  using absl_internal_is_view = std::true_type;

  static constexpr size_type npos = static_cast<size_type>(-1);

  // Null `string_view` constructor

  constexpr string_view() noexcept : ptr_(nullptr), length_(0) {}

  // Implicit constructors

  template <typename Allocator>

  string_view(  // NOLINT(runtime/explicit)

      const std::basic_string<char, std::char_traits<char>, Allocator>& str

          ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept

      // This is implemented in terms of `string_view(p, n)` so `str.size()`

      // doesn't need to be reevaluated after `ptr_` is set.

      // The length check is also skipped since it is unnecessary and causes

      // code bloat.

      : string_view(str.data(), str.size(), SkipCheckLengthTag{}) {}

  // Implicit constructor of a `string_view` from NUL-terminated `str`. When

  // accepting possibly null strings, use `absl::NullSafeStringView(str)`

  // instead (see below).

  // The length check is skipped since it is unnecessary and causes code bloat.

  constexpr string_view(  // NOLINT(runtime/explicit)

      const char* absl_nonnull str)

      : ptr_(str), length_(str ? StrlenInternal(str) : 0) {

    assert(str != nullptr);

  }

  // Constructor of a `string_view` from a `const char*` and length.

  constexpr string_view(const char* absl_nullable data, size_type len)

      : ptr_(data), length_(CheckLengthInternal(len)) {

    ABSL_ASSERT(data != nullptr || len == 0);

  }

  constexpr string_view(const string_view&) noexcept = default;

  string_view& operator=(const string_view&) noexcept = default;

  // Iterators

  // string_view::begin()

  //

  // Returns an iterator pointing to the first character at the beginning of the

  // `string_view`, or `end()` if the `string_view` is empty.

  constexpr const_iterator begin() const noexcept { return ptr_; }

  // string_view::end()

  //

  // Returns an iterator pointing just beyond the last character at the end of

  // the `string_view`. This iterator acts as a placeholder; attempting to

  // access it results in undefined behavior.

  constexpr const_iterator end() const noexcept { return ptr_ + length_; }

  // string_view::cbegin()

  //

  // Returns a const iterator pointing to the first character at the beginning

  // of the `string_view`, or `end()` if the `string_view` is empty.

  constexpr const_iterator cbegin() const noexcept { return begin(); }

  // string_view::cend()

  //

  // Returns a const iterator pointing just beyond the last character at the end

  // of the `string_view`. This pointer acts as a placeholder; attempting to

  // access its element results in undefined behavior.

  constexpr const_iterator cend() const noexcept { return end(); }

  // string_view::rbegin()

  //

  // Returns a reverse iterator pointing to the last character at the end of the

  // `string_view`, or `rend()` if the `string_view` is empty.

  const_reverse_iterator rbegin() const noexcept {

    return const_reverse_iterator(end());

  }

  // string_view::rend()

  //

  // Returns a reverse iterator pointing just before the first character at the

  // beginning of the `string_view`. This pointer acts as a placeholder;

  // attempting to access its element results in undefined behavior.

  const_reverse_iterator rend() const noexcept {

    return const_reverse_iterator(begin());

  }

  // string_view::crbegin()

  //

  // Returns a const reverse iterator pointing to the last character at the end

  // of the `string_view`, or `crend()` if the `string_view` is empty.

  const_reverse_iterator crbegin() const noexcept { return rbegin(); }

  // string_view::crend()

  //

  // Returns a const reverse iterator pointing just before the first character

  // at the beginning of the `string_view`. This pointer acts as a placeholder;

  // attempting to access its element results in undefined behavior.

  const_reverse_iterator crend() const noexcept { return rend(); }

  // Capacity Utilities

  // string_view::size()

  //

  // Returns the number of characters in the `string_view`.

  constexpr size_type size() const noexcept { return length_; }

  // string_view::length()

  //

  // Returns the number of characters in the `string_view`. Alias for `size()`.

  constexpr size_type length() const noexcept { return size(); }

  // string_view::max_size()

  //

  // Returns the maximum number of characters the `string_view` can hold.

  constexpr size_type max_size() const noexcept { return kMaxSize; }

  // string_view::empty()

  //

  // Checks if the `string_view` is empty (refers to no characters).

  constexpr bool empty() const noexcept { return length_ == 0; }

  // string_view::operator[]

  //

  // Returns the ith element of the `string_view` using the array operator.

  // Note that this operator does not perform any bounds checking.

  constexpr const_reference operator[](size_type i) const {

    ABSL_HARDENING_ASSERT(i < size());

    return ptr_[i];

  }

  // string_view::at()

  //

  // Returns the ith element of the `string_view`. Bounds checking is performed,

  // and an exception of type `std::out_of_range` will be thrown on invalid

  // access.

  constexpr const_reference at(size_type i) const {

    if (ABSL_PREDICT_FALSE(i >= size())) {

      base_internal::ThrowStdOutOfRange("absl::string_view::at");

    }

    return ptr_[i];

  }

  // string_view::front()

  //

  // Returns the first element of a `string_view`.

  constexpr const_reference front() const {

    ABSL_HARDENING_ASSERT(!empty());

    return ptr_[0];

  }

  // string_view::back()

  //

  // Returns the last element of a `string_view`.

  constexpr const_reference back() const {

    ABSL_HARDENING_ASSERT(!empty());

    return ptr_[size() - 1];

  }

  // string_view::data()

  //

  // Returns a pointer to the underlying character array (which is of course

  // stored elsewhere). Note that `string_view::data()` may contain embedded nul

  // characters, but the returned buffer may or may not be NUL-terminated;

  // therefore, do not pass `data()` to a routine that expects a NUL-terminated

  // string.

  constexpr const_pointer data() const noexcept { return ptr_; }

  // Modifiers

  // string_view::remove_prefix()

  //

  // Removes the first `n` characters from the `string_view`. Note that the

  // underlying string is not changed, only the view.

  constexpr void remove_prefix(size_type n) {

    ABSL_HARDENING_ASSERT(n <= length_);

    ptr_ += n;

    length_ -= n;

  }

  // string_view::remove_suffix()

  //

  // Removes the last `n` characters from the `string_view`. Note that the

  // underlying string is not changed, only the view.

  constexpr void remove_suffix(size_type n) {

    ABSL_HARDENING_ASSERT(n <= length_);

    length_ -= n;

  }

  // string_view::swap()

  //

  // Swaps this `string_view` with another `string_view`.

  constexpr void swap(string_view& s) noexcept {

    auto t = *this;

    *this = s;

    s = t;

  }

  // Explicit conversion operators

  // Converts to `std::basic_string`.

  template <typename A>

  explicit operator std::basic_string<char, traits_type, A>() const {

    if (!data()) return {};

    return std::basic_string<char, traits_type, A>(data(), size());

  }

  // string_view::copy()

  //

  // Copies the contents of the `string_view` at offset `pos` and length `n`

  // into `buf`.

  size_type copy(char* absl_nonnull buf, size_type n, size_type pos = 0) const {

    if (ABSL_PREDICT_FALSE(pos > length_)) {

      base_internal::ThrowStdOutOfRange("absl::string_view::copy");

    }

    size_type rlen = (std::min)(length_ - pos, n);

    if (rlen > 0) {

      const char* start = ptr_ + pos;

      traits_type::copy(buf, start, rlen);

    }

    return rlen;

  }

  // string_view::substr()

  //

  // Returns a "substring" of the `string_view` (at offset `pos` and length

  // `n`) as another string_view. This function throws `std::out_of_bounds` if

  // `pos > size`.

  // Use absl::ClippedSubstr if you need a truncating substr operation.

  constexpr string_view substr(size_type pos = 0, size_type n = npos) const {

    if (ABSL_PREDICT_FALSE(pos > length_)) {

      base_internal::ThrowStdOutOfRange("absl::string_view::substr");

    }

    return string_view(ptr_ + pos, (std::min)(n, length_ - pos));

  }

  // string_view::compare()

  //

  // Performs a lexicographical comparison between this `string_view` and

  // another `string_view` `x`, returning a negative value if `*this` is less

  // than `x`, 0 if `*this` is equal to `x`, and a positive value if `*this`

  // is greater than `x`.

  constexpr int compare(string_view x) const noexcept {

    return CompareImpl(length_, x.length_,

                       (std::min)(length_, x.length_) == 0

                           ? 0

                           : ABSL_INTERNAL_STRING_VIEW_MEMCMP(

                                 ptr_, x.ptr_, (std::min)(length_, x.length_)));

  }

  // Overload of `string_view::compare()` for comparing a substring of the

  // 'string_view` and another `absl::string_view`.

  constexpr int compare(size_type pos1, size_type count1, string_view v) const {

    return substr(pos1, count1).compare(v);

  }

  // Overload of `string_view::compare()` for comparing a substring of the

  // `string_view` and a substring of another `absl::string_view`.

  constexpr int compare(size_type pos1, size_type count1, string_view v,

                        size_type pos2, size_type count2) const {

    return substr(pos1, count1).compare(v.substr(pos2, count2));

  }

  // Overload of `string_view::compare()` for comparing a `string_view` and a

  // a different C-style string `s`.

  constexpr int compare(const char* absl_nonnull s) const {

    return compare(string_view(s));

  }

  // Overload of `string_view::compare()` for comparing a substring of the

  // `string_view` and a different string C-style string `s`.

  constexpr int compare(size_type pos1, size_type count1,

                        const char* absl_nonnull s) const {

    return substr(pos1, count1).compare(string_view(s));

  }

  // Overload of `string_view::compare()` for comparing a substring of the

  // `string_view` and a substring of a different C-style string `s`.

  constexpr int compare(size_type pos1, size_type count1,

                        const char* absl_nonnull s, size_type count2) const {

    return substr(pos1, count1).compare(string_view(s, count2));

  }

  // Find Utilities

  // string_view::find()

  //

  // Finds the first occurrence of the substring `s` within the `string_view`,

  // returning the position of the first character's match, or `npos` if no

  // match was found.

  size_type find(string_view s, size_type pos = 0) const noexcept;

  // Overload of `string_view::find()` for finding the given character `c`

  // within the `string_view`.

  size_type find(char c, size_type pos = 0) const noexcept;

  // Overload of `string_view::find()` for finding a substring of a different

  // C-style string `s` within the `string_view`.

  size_type find(const char* absl_nonnull s, size_type pos,

                 size_type count) const {

    return find(string_view(s, count), pos);

  }

  // Overload of `string_view::find()` for finding a different C-style string

  // `s` within the `string_view`.

  size_type find(const char* absl_nonnull s, size_type pos = 0) const {

    return find(string_view(s), pos);

  }

  // string_view::rfind()

  //

  // Finds the last occurrence of a substring `s` within the `string_view`,

  // returning the position of the first character's match, or `npos` if no

  // match was found.

  size_type rfind(string_view s, size_type pos = npos) const noexcept;

  // Overload of `string_view::rfind()` for finding the last given character `c`

  // within the `string_view`.

  size_type rfind(char c, size_type pos = npos) const noexcept;

  // Overload of `string_view::rfind()` for finding a substring of a different

  // C-style string `s` within the `string_view`.

  size_type rfind(const char* absl_nonnull s, size_type pos,

                  size_type count) const {

    return rfind(string_view(s, count), pos);

  }

  // Overload of `string_view::rfind()` for finding a different C-style string

  // `s` within the `string_view`.

  size_type rfind(const char* absl_nonnull s, size_type pos = npos) const {

    return rfind(string_view(s), pos);

  }

  // string_view::find_first_of()

  //

  // Finds the first occurrence of any of the characters in `s` within the

  // `string_view`, returning the start position of the match, or `npos` if no

  // match was found.

  size_type find_first_of(string_view s, size_type pos = 0) const noexcept;

  // Overload of `string_view::find_first_of()` for finding a character `c`

  // within the `string_view`.

  size_type find_first_of(char c, size_type pos = 0) const noexcept {

    return find(c, pos);

  }

  // Overload of `string_view::find_first_of()` for finding a substring of a

  // different C-style string `s` within the `string_view`.

  size_type find_first_of(const char* absl_nonnull s, size_type pos,

                          size_type count) const {

    return find_first_of(string_view(s, count), pos);

  }

  // Overload of `string_view::find_first_of()` for finding a different C-style

  // string `s` within the `string_view`.

  size_type find_first_of(const char* absl_nonnull s, size_type pos = 0) const {

    return find_first_of(string_view(s), pos);

  }

  // string_view::find_last_of()

  //

  // Finds the last occurrence of any of the characters in `s` within the

  // `string_view`, returning the start position of the match, or `npos` if no

  // match was found.

  size_type find_last_of(string_view s, size_type pos = npos) const noexcept;

  // Overload of `string_view::find_last_of()` for finding a character `c`

  // within the `string_view`.

  size_type find_last_of(char c, size_type pos = npos) const noexcept {

    return rfind(c, pos);

  }

  // Overload of `string_view::find_last_of()` for finding a substring of a

  // different C-style string `s` within the `string_view`.

  size_type find_last_of(const char* absl_nonnull s, size_type pos,

                         size_type count) const {

    return find_last_of(string_view(s, count), pos);

  }

  // Overload of `string_view::find_last_of()` for finding a different C-style

  // string `s` within the `string_view`.

  size_type find_last_of(const char* absl_nonnull s,

                         size_type pos = npos) const {

    return find_last_of(string_view(s), pos);

  }

  // string_view::find_first_not_of()

  //

  // Finds the first occurrence of any of the characters not in `s` within the

  // `string_view`, returning the start position of the first non-match, or

  // `npos` if no non-match was found.

  size_type find_first_not_of(string_view s, size_type pos = 0) const noexcept;

  // Overload of `string_view::find_first_not_of()` for finding a character

  // that is not `c` within the `string_view`.

  size_type find_first_not_of(char c, size_type pos = 0) const noexcept;

  // Overload of `string_view::find_first_not_of()` for finding a substring of a

  // different C-style string `s` within the `string_view`.

  size_type find_first_not_of(const char* absl_nonnull s, size_type pos,

                              size_type count) const {

    return find_first_not_of(string_view(s, count), pos);

  }

  // Overload of `string_view::find_first_not_of()` for finding a different

  // C-style string `s` within the `string_view`.

  size_type find_first_not_of(const char* absl_nonnull s,

                              size_type pos = 0) const {

    return find_first_not_of(string_view(s), pos);

  }

  // string_view::find_last_not_of()

  //

  // Finds the last occurrence of any of the characters not in `s` within the

  // `string_view`, returning the start position of the last non-match, or

  // `npos` if no non-match was found.

  size_type find_last_not_of(string_view s,

                             size_type pos = npos) const noexcept;

  // Overload of `string_view::find_last_not_of()` for finding a character

  // that is not `c` within the `string_view`.

  size_type find_last_not_of(char c, size_type pos = npos) const noexcept;

  // Overload of `string_view::find_last_not_of()` for finding a substring of a

  // different C-style string `s` within the `string_view`.

  size_type find_last_not_of(const char* absl_nonnull s, size_type pos,

                             size_type count) const {

    return find_last_not_of(string_view(s, count), pos);

  }

  // Overload of `string_view::find_last_not_of()` for finding a different

  // C-style string `s` within the `string_view`.

  size_type find_last_not_of(const char* absl_nonnull s,

                             size_type pos = npos) const {

    return find_last_not_of(string_view(s), pos);

  }

#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L

  // string_view::starts_with()

  //

  // Returns true if the `string_view` starts with the prefix `s`.

  //

  // This method only exists when targeting at least C++20.

  // If support for C++ prior to C++20 is required, use `absl::StartsWith()`

  // from `//absl/strings/match.h` for compatibility.

  constexpr bool starts_with(string_view s) const noexcept {

    return s.empty() ||

           (size() >= s.size() &&

            ABSL_INTERNAL_STRING_VIEW_MEMCMP(data(), s.data(), s.size()) == 0);

  }

  // Overload of `string_view::starts_with()` that returns true if `c` is the

  // first character of the `string_view`.

  constexpr bool starts_with(char c) const noexcept {

    return !empty() && front() == c;

  }

  // Overload of `string_view::starts_with()` that returns true if the

  // `string_view` starts with the C-style prefix `s`.

  constexpr bool starts_with(const char* absl_nonnull s) const {

    return starts_with(string_view(s));

  }

  // string_view::ends_with()

  //

  // Returns true if the `string_view` ends with the suffix `s`.

  //

  // This method only exists when targeting at least C++20.

  // If support for C++ prior to C++20 is required, use `absl::EndsWith()`

  // from `//absl/strings/match.h` for compatibility.

  constexpr bool ends_with(string_view s) const noexcept {

    return s.empty() || (size() >= s.size() && ABSL_INTERNAL_STRING_VIEW_MEMCMP(

                                                   data() + (size() - s.size()),

                                                   s.data(), s.size()) == 0);

  }

  // Overload of `string_view::ends_with()` that returns true if `c` is the

  // last character of the `string_view`.

  constexpr bool ends_with(char c) const noexcept {

    return !empty() && back() == c;

  }

  // Overload of `string_view::ends_with()` that returns true if the

  // `string_view` ends with the C-style suffix `s`.

  constexpr bool ends_with(const char* absl_nonnull s) const {

    return ends_with(string_view(s));

  }

#endif  // ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L

 private:

  // The constructor from std::string delegates to this constructor.

  // See the comment on that constructor for the rationale.

  struct SkipCheckLengthTag {};

  string_view(const char* absl_nullable data, size_type len,

              SkipCheckLengthTag) noexcept

      : ptr_(data), length_(len) {}

  static constexpr size_type kMaxSize =

      (std::numeric_limits<difference_type>::max)();

  static constexpr size_type CheckLengthInternal(size_type len) {

    ABSL_HARDENING_ASSERT(len <= kMaxSize);

    return len;

  }

  static constexpr size_type StrlenInternal(const char* absl_nonnull str) {

#if defined(_MSC_VER) && !defined(__clang__)

    // MSVC 2017+ can evaluate this at compile-time.

    const char* begin = str;

    while (*str != '\0') ++str;

    return str - begin;

#elif ABSL_HAVE_BUILTIN(__builtin_strlen) || \

    (defined(__GNUC__) && !defined(__clang__))

    // GCC has __builtin_strlen according to

    // https://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Other-Builtins.html, but

    // ABSL_HAVE_BUILTIN doesn't detect that, so we use the extra checks above.

    // __builtin_strlen is constexpr.

    return __builtin_strlen(str);

#else

    return str ? strlen(str) : 0;

#endif

  }

  static constexpr int CompareImpl(size_type length_a, size_type length_b,

                                   int compare_result) {

    return compare_result == 0 ? static_cast<int>(length_a > length_b) -

                                     static_cast<int>(length_a < length_b)

                               : (compare_result < 0 ? -1 : 1);

  }

  const char* absl_nullable ptr_;

  size_type length_;

};

// This large function is defined inline so that in a fairly common case where

// one of the arguments is a literal, the compiler can elide a lot of the

// following comparisons.

constexpr bool operator==(string_view x, string_view y) noexcept {

  return x.size() == y.size() &&

         (x.empty() ||

          ABSL_INTERNAL_STRING_VIEW_MEMCMP(x.data(), y.data(), x.size()) == 0);

}

constexpr bool operator!=(string_view x, string_view y) noexcept {

  return !(x == y);

}

constexpr bool operator<(string_view x, string_view y) noexcept {

  return x.compare(y) < 0;

}

constexpr bool operator>(string_view x, string_view y) noexcept {

  return y < x;

}

constexpr bool operator<=(string_view x, string_view y) noexcept {

  return !(y < x);

}

constexpr bool operator>=(string_view x, string_view y) noexcept {

  return !(x < y);

}

// IO Insertion Operator

std::ostream& operator<<(std::ostream& o, string_view piece);

ABSL_NAMESPACE_END

}  // namespace absl

#undef ABSL_INTERNAL_STRING_VIEW_MEMCMP

#endif  // ABSL_USES_STD_STRING_VIEW

namespace absl {

ABSL_NAMESPACE_BEGIN

// ClippedSubstr()

//

// Like `s.substr(pos, n)`, but clips `pos` to an upper bound of `s.size()`.

// Provided because std::string_view::substr throws if `pos > size()`

inline string_view ClippedSubstr(string_view s, size_t pos,

                                 size_t n = string_view::npos) {

  pos = (std::min)(pos, static_cast<size_t>(s.size()));

  return s.substr(pos, n);

}

// NullSafeStringView()

//

// Creates an `absl::string_view` from a pointer `p` even if it's null-valued.

// This function should be used where an `absl::string_view` can be created from

// a possibly-null pointer.

constexpr string_view NullSafeStringView(const char* absl_nullable p) {

  return p ? string_view(p) : string_view();

}

ABSL_NAMESPACE_END

}  // namespace absl

#endif  // ABSL_STRINGS_STRING_VIEW_H_