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// Copyright 2020 The Chromium Authors. All rights reserved.
// 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 <string>
#include "base/containers/span.h"
namespace base {
namespace internal {
// Reserves an additional amount of capacity in the given string, growing by at
// least 2x if necessary. Used by StrAppendT().
//
// The "at least 2x" growing rule duplicates the exponential growth of
// std::string. The problem is that most implementations of reserve() will grow
// exactly to the requested amount instead of exponentially growing like would
// happen when appending normally. If we didn't do this, an append after the
// call to StrAppend() would definitely cause a reallocation, and loops with
// StrAppend() calls would have O(n^2) complexity to execute. Instead, we want
// StrAppend() to have the same semantics as std::string::append().
template <typename String>
void ReserveAdditionalIfNeeded(String* str,
typename String::size_type additional) {
const size_t required = str->size() + additional;
// Check whether we need to reserve additional capacity at all.
if (required <= str->capacity())
return;
str->reserve(std::max(required, str->capacity() * 2));
}
template <typename DestString, typename InputString>
void StrAppendT(DestString* dest, span<const InputString> pieces) {
size_t additional_size = 0;
for (const auto& cur : pieces)
additional_size += cur.size();
ReserveAdditionalIfNeeded(dest, additional_size);
for (const auto& cur : pieces)
dest->append(cur.data(), cur.size());
}
template <typename StringT>
auto StrCatT(span<const StringT> pieces) {
std::basic_string<typename StringT::value_type, typename StringT::traits_type>
result;
StrAppendT(&result, pieces);
return result;
}
} // namespace internal
} // namespace base
#endif // BASE_STRINGS_STRCAT_INTERNAL_H_
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