include/flatbuffers/buffer.h

/*
 * Copyright 2021 Google Inc. All rights reserved.
 *
 * 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
 *
 *     http://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.
 */

#ifndef FLATBUFFERS_BUFFER_H_
#define FLATBUFFERS_BUFFER_H_

#include <algorithm>

#include "flatbuffers/base.h"
#include "flatbuffers/stl_emulation.h"

namespace flatbuffers {

// Wrapper for uoffset_t to allow safe template specialization.
// Value is allowed to be 0 to indicate a null object (see e.g. AddOffset).
template <typename T = void>
struct Offset {
  // The type of offset to use.
  typedef uoffset_t offset_type;

  offset_type o;
  Offset() : o(0) {}
  Offset(const offset_type _o) : o(_o) {}
  Offset<> Union() const { return o; }
  bool IsNull() const { return !o; }
};

template <typename T>
struct is_specialisation_of_Offset : false_type {};
template <typename T>
struct is_specialisation_of_Offset<Offset<T>> : true_type {};

// Wrapper for uoffset64_t Offsets.
template <typename T = void>
struct Offset64 {
  // The type of offset to use.
  typedef uoffset64_t offset_type;

  offset_type o;
  Offset64() : o(0) {}
  Offset64(const offset_type offset) : o(offset) {}
  Offset64<> Union() const { return o; }
  bool IsNull() const { return !o; }
};

template <typename T>
struct is_specialisation_of_Offset64 : false_type {};
template <typename T>
struct is_specialisation_of_Offset64<Offset64<T>> : true_type {};

// Litmus check for ensuring the Offsets are the expected size.
static_assert(sizeof(Offset<>) == 4, "Offset has wrong size");
static_assert(sizeof(Offset64<>) == 8, "Offset64 has wrong size");

inline void EndianCheck() {
  int endiantest = 1;
  // If this fails, see FLATBUFFERS_LITTLEENDIAN above.
  FLATBUFFERS_ASSERT(*reinterpret_cast<char*>(&endiantest) ==
                     FLATBUFFERS_LITTLEENDIAN);
  (void)endiantest;
}

template <typename T>
FLATBUFFERS_CONSTEXPR size_t AlignOf() {
  // clang-format off
  #ifdef _MSC_VER
    return __alignof(T);
  #else
    #ifndef alignof
      return __alignof__(T);
    #else
      return alignof(T);
    #endif
  #endif
  // clang-format on
}

// Lexicographically compare two strings (possibly containing nulls), and
// return true if the first is less than the second.
static inline bool StringLessThan(const char* a_data, uoffset_t a_size,
                                  const char* b_data, uoffset_t b_size) {
  const auto cmp = memcmp(a_data, b_data, (std::min)(a_size, b_size));
  return cmp == 0 ? a_size < b_size : cmp < 0;
}

// When we read serialized data from memory, in the case of most scalars,
// we want to just read T, but in the case of Offset, we want to actually
// perform the indirection and return a pointer.
// The template specialization below does just that.
// It is wrapped in a struct since function templates can't overload on the
// return type like this.
// The typedef is for the convenience of callers of this function
// (avoiding the need for a trailing return decltype)
template <typename T, typename Enable = void>
struct IndirectHelper {
  typedef T return_type;
  typedef T mutable_return_type;
  static const size_t element_stride = sizeof(T);

  static return_type Read(const uint8_t* p, const size_t i) {
    return EndianScalar((reinterpret_cast<const T*>(p))[i]);
  }
  static mutable_return_type Read(uint8_t* p, const size_t i) {
    return reinterpret_cast<mutable_return_type>(
        Read(const_cast<const uint8_t*>(p), i));
  }
};

// For vector of Offsets.
template <typename T, template <typename> class OffsetT>
struct IndirectHelper<OffsetT<T>> {
  typedef const T* return_type;
  typedef T* mutable_return_type;
  typedef typename OffsetT<T>::offset_type offset_type;
  static const offset_type element_stride = sizeof(offset_type);

  static return_type Read(const uint8_t* const p, const offset_type i) {
    // Offsets are relative to themselves, so first update the pointer to
    // point to the offset location.
    const uint8_t* const offset_location = p + i * element_stride;

    // Then read the scalar value of the offset (which may be 32 or 64-bits) and
    // then determine the relative location from the offset location.
    return reinterpret_cast<return_type>(
        offset_location + ReadScalar<offset_type>(offset_location));
  }
  static mutable_return_type Read(uint8_t* const p, const offset_type i) {
    // Offsets are relative to themselves, so first update the pointer to
    // point to the offset location.
    uint8_t* const offset_location = p + i * element_stride;

    // Then read the scalar value of the offset (which may be 32 or 64-bits) and
    // then determine the relative location from the offset location.
    return reinterpret_cast<mutable_return_type>(
        offset_location + ReadScalar<offset_type>(offset_location));
  }
};

// For vector of structs.
template <typename T>
struct IndirectHelper<
    T, typename std::enable_if<
           !std::is_scalar<typename std::remove_pointer<T>::type>::value &&
           !is_specialisation_of_Offset<T>::value &&
           !is_specialisation_of_Offset64<T>::value>::type> {
 private:
  typedef typename std::remove_pointer<typename std::remove_cv<T>::type>::type
      pointee_type;

 public:
  typedef const pointee_type* return_type;
  typedef pointee_type* mutable_return_type;
  static const size_t element_stride = sizeof(pointee_type);

  static return_type Read(const uint8_t* const p, const size_t i) {
    // Structs are stored inline, relative to the first struct pointer.
    return reinterpret_cast<return_type>(p + i * element_stride);
  }
  static mutable_return_type Read(uint8_t* const p, const size_t i) {
    // Structs are stored inline, relative to the first struct pointer.
    return reinterpret_cast<mutable_return_type>(p + i * element_stride);
  }
};

/// @brief Get a pointer to the file_identifier section of the buffer.
/// @return Returns a const char pointer to the start of the file_identifier
/// characters in the buffer.  The returned char * has length
/// 'flatbuffers::FlatBufferBuilder::kFileIdentifierLength'.
/// This function is UNDEFINED for FlatBuffers whose schema does not include
/// a file_identifier (likely points at padding or the start of a the root
/// vtable).
inline const char* GetBufferIdentifier(const void* buf,
                                       bool size_prefixed = false) {
  return reinterpret_cast<const char*>(buf) +
         ((size_prefixed) ? 2 * sizeof(uoffset_t) : sizeof(uoffset_t));
}

// Helper to see if the identifier in a buffer has the expected value.
inline bool BufferHasIdentifier(const void* buf, const char* identifier,
                                bool size_prefixed = false) {
  return strncmp(GetBufferIdentifier(buf, size_prefixed), identifier,
                 flatbuffers::kFileIdentifierLength) == 0;
}

/// @cond FLATBUFFERS_INTERNAL
// Helpers to get a typed pointer to the root object contained in the buffer.
template <typename T>
T* GetMutableRoot(void* buf) {
  if (!buf) return nullptr;
  EndianCheck();
  return reinterpret_cast<T*>(reinterpret_cast<uint8_t*>(buf) +
                              EndianScalar(*reinterpret_cast<uoffset_t*>(buf)));
}

template <typename T, typename SizeT = uoffset_t>
T* GetMutableSizePrefixedRoot(void* buf) {
  return GetMutableRoot<T>(reinterpret_cast<uint8_t*>(buf) + sizeof(SizeT));
}

template <typename T>
const T* GetRoot(const void* buf) {
  return GetMutableRoot<T>(const_cast<void*>(buf));
}

template <typename T, typename SizeT = uoffset_t>
const T* GetSizePrefixedRoot(const void* buf) {
  return GetRoot<T>(reinterpret_cast<const uint8_t*>(buf) + sizeof(SizeT));
}

}  // namespace flatbuffers

#endif  // FLATBUFFERS_BUFFER_H_