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// Copyright 2020 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_STRINGS_STRCAT_INTERNAL_H_
#define BASE_STRINGS_STRCAT_INTERNAL_H_
#include <concepts>
#include <string>
#include "base/compiler_specific.h"
#include "base/containers/span.h"
namespace base {
namespace internal {
// Default to regular `std::basic_string::resize()`.
template <typename CharT>
void Resize(std::basic_string<CharT>& str, size_t total_size) {
str.resize(total_size);
}
// Optimized version of `std::basic_string::resize()` that skips zero
// initialization of appended characters. Reading from the newly allocated
// characters results in undefined behavior if they are not explicitly
// initialized afterwards. Available in C++23 as
// `std::basic_string::resize_and_overwrite()`:
// https://en.cppreference.com/w/cpp/string/basic_string/resize_and_overwrite
template <typename CharT>
requires requires(std::basic_string<CharT>& str, size_t total_size) {
{ str.__resize_default_init(total_size) } -> std::same_as<void>;
}
auto Resize(std::basic_string<CharT>& str, size_t total_size) {
str.__resize_default_init(total_size);
}
// Appends `pieces` to `dest`. Instead of simply calling `dest.append()`
// `pieces.size()` times, this method first resizes `dest` to be of the desired
// size, and then appends each piece via `std::char_traits::copy`. This achieves
// two goals:
// 1) Allocating the desired size all at once avoids other allocations that
// could happen if intermediate allocations did not reserve enough capacity.
// 2) Invoking std::char_traits::copy instead of std::basic_string::append
// avoids having to write the terminating '\0' character n times.
template <typename CharT, typename StringT>
void StrAppendT(std::basic_string<CharT>& dest, span<const StringT> pieces) {
const size_t initial_size = dest.size();
size_t total_size = initial_size;
for (const auto& cur : pieces) {
total_size += cur.size();
}
// Note: As opposed to `reserve()` calling `resize()` with an argument smaller
// than the current `capacity()` does not result in the string releasing spare
// capacity. Furthermore, common std::string implementations apply a geometric
// growth strategy if the current capacity is not sufficient for the newly
// added characters. Since this codepath is also triggered by `resize()`, we
// don't have to manage the std::string's capacity ourselves here to avoid
// performance hits in case `StrAppend()` gets called in a loop.
Resize(dest, total_size);
CharT* dest_char = &dest[initial_size];
for (const auto& cur : pieces) {
std::char_traits<CharT>::copy(dest_char, cur.data(), cur.size());
UNSAFE_TODO(dest_char += cur.size());
}
}
template <typename StringT>
auto StrCatT(span<const StringT> pieces) {
std::basic_string<typename StringT::value_type> result;
StrAppendT(result, pieces);
return result;
}
} // namespace internal
} // namespace base
#endif // BASE_STRINGS_STRCAT_INTERNAL_H_
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