chromium/third_party/abseil-cpp/absl/strings/str_cat.h

//
// 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: str_cat.h
// -----------------------------------------------------------------------------
//
// This package contains functions for efficiently concatenating and appending
// strings: `StrCat()` and `StrAppend()`. Most of the work within these routines
// is actually handled through use of a special AlphaNum type, which was
// designed to be used as a parameter type that efficiently manages conversion
// to strings and avoids copies in the above operations.
//
// Any routine accepting either a string or a number may accept `AlphaNum`.
// The basic idea is that by accepting a `const AlphaNum &` as an argument
// to your function, your callers will automagically convert bools, integers,
// and floating point values to strings for you.
//
// NOTE: Use of `AlphaNum` outside of the //absl/strings package is unsupported
// except for the specific case of function parameters of type `AlphaNum` or
// `const AlphaNum &`. In particular, instantiating `AlphaNum` directly as a
// stack variable is not supported.
//
// Conversion from 8-bit values is not accepted because, if it were, then an
// attempt to pass ':' instead of ":" might result in a 58 ending up in your
// result.
//
// Bools convert to "0" or "1". Pointers to types other than `char *` are not
// valid inputs. No output is generated for null `char *` pointers.
//
// Floating point numbers are formatted with six-digit precision, which is
// the default for "std::cout <<" or printf "%g" (the same as "%.6g").
//
// You can convert to hexadecimal output rather than decimal output using the
// `Hex` type contained here. To do so, pass `Hex(my_int)` as a parameter to
// `StrCat()` or `StrAppend()`. You may specify a minimum hex field width using
// a `PadSpec` enum.
//
// User-defined types can be formatted with the `AbslStringify()` customization
// point. The API relies on detecting an overload in the user-defined type's
// namespace of a free (non-member) `AbslStringify()` function as a definition
// (typically declared as a friend and implemented in-line.
// with the following signature:
//
// class MyClass { ... };
//
// template <typename Sink>
// void AbslStringify(Sink& sink, const MyClass& value);
//
// An `AbslStringify()` overload for a type should only be declared in the same
// file and namespace as said type.
//
// Note that `AbslStringify()` also supports use with `absl::StrFormat()` and
// `absl::Substitute()`.
//
// Example:
//
// struct Point {
//   // To add formatting support to `Point`, we simply need to add a free
//   // (non-member) function `AbslStringify()`. This method specifies how
//   // Point should be printed when absl::StrCat() is called on it. You can add
//   // such a free function using a friend declaration within the body of the
//   // class. The sink parameter is a templated type to avoid requiring
//   // dependencies.
//   template <typename Sink> friend void AbslStringify(Sink&
//   sink, const Point& p) {
//     absl::Format(&sink, "(%v, %v)", p.x, p.y);
//   }
//
//   int x;
//   int y;
// };
// -----------------------------------------------------------------------------

#ifndef ABSL_STRINGS_STR_CAT_H_
#define ABSL_STRINGS_STR_CAT_H_

#include <algorithm>
#include <array>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <cstring>
#include <initializer_list>
#include <limits>
#include <string>
#include <type_traits>
#include <utility>
#include <vector>

#include "absl/base/attributes.h"
#include "absl/base/nullability.h"
#include "absl/base/port.h"
#include "absl/meta/type_traits.h"
#include "absl/strings/has_absl_stringify.h"
#include "absl/strings/internal/resize_uninitialized.h"
#include "absl/strings/internal/stringify_sink.h"
#include "absl/strings/numbers.h"
#include "absl/strings/string_view.h"

namespace absl {
ABSL_NAMESPACE_BEGIN

namespace strings_internal {
// AlphaNumBuffer allows a way to pass a string to StrCat without having to do
// memory allocation.  It is simply a pair of a fixed-size character array, and
// a size.  Please don't use outside of absl, yet.
template <size_t max_size>
struct AlphaNumBuffer { … };

}  // namespace strings_internal

// Enum that specifies the number of significant digits to return in a `Hex` or
// `Dec` conversion and fill character to use. A `kZeroPad2` value, for example,
// would produce hexadecimal strings such as "0a","0f" and a 'kSpacePad5' value
// would produce hexadecimal strings such as "    a","    f".
enum PadSpec : uint8_t { … };

// -----------------------------------------------------------------------------
// Hex
// -----------------------------------------------------------------------------
//
// `Hex` stores a set of hexadecimal string conversion parameters for use
// within `AlphaNum` string conversions.
struct Hex { … };

// -----------------------------------------------------------------------------
// Dec
// -----------------------------------------------------------------------------
//
// `Dec` stores a set of decimal string conversion parameters for use
// within `AlphaNum` string conversions.  Dec is slower than the default
// integer conversion, so use it only if you need padding.
struct Dec { … };

// -----------------------------------------------------------------------------
// AlphaNum
// -----------------------------------------------------------------------------
//
// The `AlphaNum` class acts as the main parameter type for `StrCat()` and
// `StrAppend()`, providing efficient conversion of numeric, boolean, decimal,
// and hexadecimal values (through the `Dec` and `Hex` types) into strings.
// `AlphaNum` should only be used as a function parameter. Do not instantiate
//  `AlphaNum` directly as a stack variable.

class AlphaNum { … };

// -----------------------------------------------------------------------------
// StrCat()
// -----------------------------------------------------------------------------
//
// Merges given strings or numbers, using no delimiter(s), returning the merged
// result as a string.
//
// `StrCat()` is designed to be the fastest possible way to construct a string
// out of a mix of raw C strings, string_views, strings, bool values,
// and numeric values.
//
// Don't use `StrCat()` for user-visible strings. The localization process
// works poorly on strings built up out of fragments.
//
// For clarity and performance, don't use `StrCat()` when appending to a
// string. Use `StrAppend()` instead. In particular, avoid using any of these
// (anti-)patterns:
//
//   str.append(StrCat(...))
//   str += StrCat(...)
//   str = StrCat(str, ...)
//
// The last case is the worst, with a potential to change a loop
// from a linear time operation with O(1) dynamic allocations into a
// quadratic time operation with O(n) dynamic allocations.
//
// See `StrAppend()` below for more information.

namespace strings_internal {

// Do not call directly - this is not part of the public API.
std::string CatPieces(std::initializer_list<absl::string_view> pieces);
void AppendPieces(absl::Nonnull<std::string*> dest,
                  std::initializer_list<absl::string_view> pieces);

template <typename Integer>
std::string IntegerToString(Integer i) { … }
template <typename Float>
std::string FloatToString(Float f) { … }

// `SingleArgStrCat` overloads take built-in `int`, `long` and `long long` types
// (signed / unsigned) to avoid ambiguity on the call side. If we used int32_t
// and int64_t, then at least one of the three (`int` / `long` / `long long`)
// would have been ambiguous when passed to `SingleArgStrCat`.
inline std::string SingleArgStrCat(int x) { … }
inline std::string SingleArgStrCat(unsigned int x) { … }
// NOLINTNEXTLINE
inline std::string SingleArgStrCat(long x) { … }
// NOLINTNEXTLINE
inline std::string SingleArgStrCat(unsigned long x) { … }
// NOLINTNEXTLINE
inline std::string SingleArgStrCat(long long x) { … }
// NOLINTNEXTLINE
inline std::string SingleArgStrCat(unsigned long long x) { … }
inline std::string SingleArgStrCat(float x) { … }
inline std::string SingleArgStrCat(double x) { … }

// As of September 2023, the SingleArgStrCat() optimization is only enabled for
// libc++. The reasons for this are:
// 1) The SSO size for libc++ is 23, while libstdc++ and MSSTL have an SSO size
// of 15. Since IntegerToString unconditionally resizes the string to 22 bytes,
// this causes both libstdc++ and MSSTL to allocate.
// 2) strings_internal::STLStringResizeUninitialized() only has an
// implementation that avoids initialization when using libc++. This isn't as
// relevant as (1), and the cost should be benchmarked if (1) ever changes on
// libstc++ or MSSTL.
#ifdef _LIBCPP_VERSION
#define ABSL_INTERNAL_STRCAT_ENABLE_FAST_CASE …
#else
#define ABSL_INTERNAL_STRCAT_ENABLE_FAST_CASE …
#endif

EnableIfFastCase;

#undef ABSL_INTERNAL_STRCAT_ENABLE_FAST_CASE

}  // namespace strings_internal

ABSL_MUST_USE_RESULT inline std::string StrCat() { … }

template <typename T>
ABSL_MUST_USE_RESULT inline std::string StrCat(
    strings_internal::EnableIfFastCase<T> a) { … }
ABSL_MUST_USE_RESULT inline std::string StrCat(const AlphaNum& a) { … }

ABSL_MUST_USE_RESULT std::string StrCat(const AlphaNum& a, const AlphaNum& b);
ABSL_MUST_USE_RESULT std::string StrCat(const AlphaNum& a, const AlphaNum& b,
                                        const AlphaNum& c);
ABSL_MUST_USE_RESULT std::string StrCat(const AlphaNum& a, const AlphaNum& b,
                                        const AlphaNum& c, const AlphaNum& d);

// Support 5 or more arguments
template <typename... AV>
ABSL_MUST_USE_RESULT inline std::string StrCat(
    const AlphaNum& a, const AlphaNum& b, const AlphaNum& c, const AlphaNum& d,
    const AlphaNum& e, const AV&... args) { … }

// -----------------------------------------------------------------------------
// StrAppend()
// -----------------------------------------------------------------------------
//
// Appends a string or set of strings to an existing string, in a similar
// fashion to `StrCat()`.
//
// WARNING: `StrAppend(&str, a, b, c, ...)` requires that none of the
// a, b, c, parameters be a reference into str. For speed, `StrAppend()` does
// not try to check each of its input arguments to be sure that they are not
// a subset of the string being appended to. That is, while this will work:
//
//   std::string s = "foo";
//   s += s;
//
// This output is undefined:
//
//   std::string s = "foo";
//   StrAppend(&s, s);
//
// This output is undefined as well, since `absl::string_view` does not own its
// data:
//
//   std::string s = "foobar";
//   absl::string_view p = s;
//   StrAppend(&s, p);

inline void StrAppend(absl::Nonnull<std::string*>) { … }
void StrAppend(absl::Nonnull<std::string*> dest, const AlphaNum& a);
void StrAppend(absl::Nonnull<std::string*> dest, const AlphaNum& a,
               const AlphaNum& b);
void StrAppend(absl::Nonnull<std::string*> dest, const AlphaNum& a,
               const AlphaNum& b, const AlphaNum& c);
void StrAppend(absl::Nonnull<std::string*> dest, const AlphaNum& a,
               const AlphaNum& b, const AlphaNum& c, const AlphaNum& d);

// Support 5 or more arguments
template <typename... AV>
inline void StrAppend(absl::Nonnull<std::string*> dest, const AlphaNum& a,
                      const AlphaNum& b, const AlphaNum& c, const AlphaNum& d,
                      const AlphaNum& e, const AV&... args) { … }

// Helper function for the future StrCat default floating-point format, %.6g
// This is fast.
inline strings_internal::AlphaNumBuffer<
    numbers_internal::kSixDigitsToBufferSize>
SixDigits(double d) { … }

ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_STRINGS_STR_CAT_H_