absl/strings/str_cat.cc

// 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.

#include "absl/strings/str_cat.h"

#include <assert.h>

#include <cstddef>
#include <cstdint>
#include <cstring>
#include <initializer_list>
#include <limits>
#include <string>

#include "absl/base/config.h"
#include "absl/base/internal/raw_logging.h"
#include "absl/base/nullability.h"
#include "absl/base/optimization.h"
#include "absl/base/throw_delegate.h"
#include "absl/strings/internal/append_and_overwrite.h"
#include "absl/strings/resize_and_overwrite.h"
#include "absl/strings/string_view.h"

namespace absl {
ABSL_NAMESPACE_BEGIN

// ----------------------------------------------------------------------
// StrCat()
//    This merges the given strings or integers, with no delimiter. This
//    is designed to be the fastest possible way to construct a string out
//    of a mix of raw C strings, string_views, strings, and integer values.
// ----------------------------------------------------------------------

namespace {
// Append is merely a version of memcpy that returns the address of the byte
// after the area just overwritten.
inline char* absl_nonnull Append(char* absl_nonnull out, const AlphaNum& x) {
  // memcpy is allowed to overwrite arbitrary memory, so doing this after the
  // call would force an extra fetch of x.size().
  char* after = out + x.size();
  if (x.size() != 0) {
    memcpy(out, x.data(), x.size());
  }
  return after;
}

// Safely adds size_t values, throwing std::length_error if overflow occurs.
inline size_t SafeAdd(size_t a, size_t b) {
  const uint64_t sum = static_cast<uint64_t>(a) + b;
  if (ABSL_PREDICT_FALSE(sum > (std::numeric_limits<size_t>::max)())) {
    ThrowStdLengthError("absl string append length overflow");
  }
  return static_cast<size_t>(sum);
}

inline size_t SafeAdd(std::initializer_list<size_t> sizes) {
  uint64_t sum = 0;
  for (size_t size : sizes) {
    sum += size;
  }
  if (ABSL_PREDICT_FALSE(sum > (std::numeric_limits<size_t>::max)())) {
    ThrowStdLengthError("absl string append length overflow");
  }
  return static_cast<size_t>(sum);
}

}  // namespace

std::string StrCat(const AlphaNum& a, const AlphaNum& b) {
  std::string result;
  // Use uint64_t to prevent size_t overflow. We assume it is not possible for
  // in memory strings to overflow a uint64_t.
  constexpr uint64_t kMaxSize = uint64_t{std::numeric_limits<size_t>::max()};
  const uint64_t result_size =
      static_cast<uint64_t>(a.size()) + static_cast<uint64_t>(b.size());
  ABSL_INTERNAL_CHECK(result_size <= kMaxSize, "size_t overflow");
  absl::StringResizeAndOverwrite(result, static_cast<size_t>(result_size),
                                 [&a, &b](char* const begin, size_t buf_size) {
                                   char* out = begin;
                                   out = Append(out, a);
                                   out = Append(out, b);
                                   assert(out == begin + buf_size);
                                   return buf_size;
                                 });
  return result;
}

std::string StrCat(const AlphaNum& a, const AlphaNum& b, const AlphaNum& c) {
  std::string result;
  // Use uint64_t to prevent size_t overflow. We assume it is not possible for
  // in memory strings to overflow a uint64_t.
  constexpr uint64_t kMaxSize = uint64_t{std::numeric_limits<size_t>::max()};
  const uint64_t result_size = static_cast<uint64_t>(a.size()) +
                               static_cast<uint64_t>(b.size()) +
                               static_cast<uint64_t>(c.size());
  ABSL_INTERNAL_CHECK(result_size <= kMaxSize, "size_t overflow");
  absl::StringResizeAndOverwrite(
      result, static_cast<size_t>(result_size),
      [&a, &b, &c](char* const begin, size_t buf_size) {
        char* out = begin;
        out = Append(out, a);
        out = Append(out, b);
        out = Append(out, c);
        assert(out == begin + buf_size);
        return buf_size;
      });
  return result;
}

std::string StrCat(const AlphaNum& a, const AlphaNum& b, const AlphaNum& c,
                   const AlphaNum& d) {
  std::string result;
  // Use uint64_t to prevent size_t overflow. We assume it is not possible for
  // in memory strings to overflow a uint64_t.
  constexpr uint64_t kMaxSize = uint64_t{std::numeric_limits<size_t>::max()};
  const uint64_t result_size =
      static_cast<uint64_t>(a.size()) + static_cast<uint64_t>(b.size()) +
      static_cast<uint64_t>(c.size()) + static_cast<uint64_t>(d.size());
  ABSL_INTERNAL_CHECK(result_size <= kMaxSize, "size_t overflow");
  absl::StringResizeAndOverwrite(
      result, static_cast<size_t>(result_size),
      [&a, &b, &c, &d](char* const begin, size_t buf_size) {
        char* out = begin;
        out = Append(out, a);
        out = Append(out, b);
        out = Append(out, c);
        out = Append(out, d);
        assert(out == begin + buf_size);
        return buf_size;
      });
  return result;
}

namespace strings_internal {

// Do not call directly - these are not part of the public API.
std::string CatPieces(std::initializer_list<absl::string_view> pieces) {
  std::string result;
  // Use uint64_t to prevent size_t overflow. We assume it is not possible for
  // in memory strings to overflow a uint64_t.
  constexpr uint64_t kMaxSize = uint64_t{std::numeric_limits<size_t>::max()};
  uint64_t total_size = 0;
  for (absl::string_view piece : pieces) {
    total_size += piece.size();
  }
  ABSL_INTERNAL_CHECK(total_size <= kMaxSize, "size_t overflow");
  absl::StringResizeAndOverwrite(result, static_cast<size_t>(total_size),
                                 [&pieces](char* const begin, size_t buf_size) {
                                   char* out = begin;
                                   for (absl::string_view piece : pieces) {
                                     const size_t this_size = piece.size();
                                     if (this_size != 0) {
                                       memcpy(out, piece.data(), this_size);
                                       out += this_size;
                                     }
                                   }
                                   assert(out == begin + buf_size);
                                   return buf_size;
                                 });
  return result;
}

// It's possible to call StrAppend with an absl::string_view that is itself a
// fragment of the string we're appending to.  However the results of this are
// random. Therefore, check for this in debug mode.  Use unsigned math so we
// only have to do one comparison. Note, there's an exception case: appending an
// empty string is always allowed.
#define ASSERT_NO_OVERLAP(dest, src) \
  assert(((src).size() == 0) ||      \
         (uintptr_t((src).data() - (dest).data()) > uintptr_t((dest).size())))

void AppendPieces(std::string* absl_nonnull dest,
                  std::initializer_list<absl::string_view> pieces) {
  size_t to_append = 0;
  for (absl::string_view piece : pieces) {
    ASSERT_NO_OVERLAP(*dest, piece);
    to_append = SafeAdd(to_append, piece.size());
  }
  StringAppendAndOverwrite(*dest, to_append,
                           [&pieces](char* const buf, size_t buf_size) {
                             char* out = buf;
                             for (absl::string_view piece : pieces) {
                               const size_t this_size = piece.size();
                               if (this_size != 0) {
                                 memcpy(out, piece.data(), this_size);
                                 out += this_size;
                               }
                             }
                             assert(out == buf + buf_size);
                             return buf_size;
                           });
}

}  // namespace strings_internal

void StrAppend(std::string* absl_nonnull dest, const AlphaNum& a) {
  ASSERT_NO_OVERLAP(*dest, a);
  strings_internal::StringAppendAndOverwrite(
      *dest, a.size(), [&a](char* const buf, size_t buf_size) {
        char* out = buf;
        out = Append(out, a);
        assert(out == buf + buf_size);
        return buf_size;
      });
}

void StrAppend(std::string* absl_nonnull dest, const AlphaNum& a,
               const AlphaNum& b) {
  ASSERT_NO_OVERLAP(*dest, a);
  ASSERT_NO_OVERLAP(*dest, b);
  strings_internal::StringAppendAndOverwrite(
      *dest, SafeAdd(a.size(), b.size()),
      [&a, &b](char* const buf, size_t buf_size) {
        char* out = buf;
        out = Append(out, a);
        out = Append(out, b);
        assert(out == buf + buf_size);
        return buf_size;
      });
}

void StrAppend(std::string* absl_nonnull dest, const AlphaNum& a,
               const AlphaNum& b, const AlphaNum& c) {
  ASSERT_NO_OVERLAP(*dest, a);
  ASSERT_NO_OVERLAP(*dest, b);
  ASSERT_NO_OVERLAP(*dest, c);
  strings_internal::StringAppendAndOverwrite(
      *dest, SafeAdd({a.size(), b.size(), c.size()}),
      [&a, &b, &c](char* const buf, size_t buf_size) {
        char* out = buf;
        out = Append(out, a);
        out = Append(out, b);
        out = Append(out, c);
        assert(out == buf + buf_size);
        return buf_size;
      });
}

void StrAppend(std::string* absl_nonnull dest, const AlphaNum& a,
               const AlphaNum& b, const AlphaNum& c, const AlphaNum& d) {
  ASSERT_NO_OVERLAP(*dest, a);
  ASSERT_NO_OVERLAP(*dest, b);
  ASSERT_NO_OVERLAP(*dest, c);
  ASSERT_NO_OVERLAP(*dest, d);
  strings_internal::StringAppendAndOverwrite(
      *dest, SafeAdd({a.size(), b.size(), c.size(), d.size()}),
      [&a, &b, &c, &d](char* const buf, size_t buf_size) {
        char* out = buf;
        out = Append(out, a);
        out = Append(out, b);
        out = Append(out, c);
        out = Append(out, d);
        assert(out == buf + buf_size);
        return buf_size;
      });
}

ABSL_NAMESPACE_END
}  // namespace absl