src/base/strings/strcat.cc

// Copyright 2017 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.

#include "base/strings/strcat.h"

namespace base {

namespace {

// Reserves an additional amount of size in the given string, growing by at
// least 2x. Used by StrAppend().
//
// 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().
//
// If the string is empty, we assume that exponential growth is not necessary.
template <typename String>
void ReserveAdditional(String* str, typename String::size_type additional) {
  str->reserve(std::max(str->size() + additional, str->size() * 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();
  ReserveAdditional(dest, additional_size);

  for (const auto& cur : pieces)
    dest->append(cur.data(), cur.size());
}

}  // namespace

std::string StrCat(span<const StringPiece> pieces) {
  std::string result;
  StrAppendT(&result, pieces);
  return result;
}

string16 StrCat(span<const StringPiece16> pieces) {
  string16 result;
  StrAppendT(&result, pieces);
  return result;
}

std::string StrCat(span<const std::string> pieces) {
  std::string result;
  StrAppendT(&result, pieces);
  return result;
}

string16 StrCat(span<const string16> pieces) {
  string16 result;
  StrAppendT(&result, pieces);
  return result;
}

void StrAppend(std::string* dest, span<const StringPiece> pieces) {
  StrAppendT(dest, pieces);
}

void StrAppend(string16* dest, span<const StringPiece16> pieces) {
  StrAppendT(dest, pieces);
}

void StrAppend(std::string* dest, span<const std::string> pieces) {
  StrAppendT(dest, pieces);
}

void StrAppend(string16* dest, span<const string16> pieces) {
  StrAppendT(dest, pieces);
}

}  // namespace base