/*

 * 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_ALLOCATOR_H_

#define FLATBUFFERS_ALLOCATOR_H_

#include "flatbuffers/base.h"

namespace mapserver {

namespace flatbuffers {

// Allocator interface. This is flatbuffers-specific and meant only for

// `vector_downward` usage.

class Allocator {

 public:

  virtual ~Allocator() {}

  // Allocate `size` bytes of memory.

  virtual uint8_t *allocate(size_t size) = 0;

  // Deallocate `size` bytes of memory at `p` allocated by this allocator.

  virtual void deallocate(uint8_t *p, size_t size) = 0;

  // Reallocate `new_size` bytes of memory, replacing the old region of size

  // `old_size` at `p`. In contrast to a normal realloc, this grows downwards,

  // and is intended specifcally for `vector_downward` use.

  // `in_use_back` and `in_use_front` indicate how much of `old_size` is

  // actually in use at each end, and needs to be copied.

  virtual uint8_t *reallocate_downward(uint8_t *old_p, size_t old_size,

                                       size_t new_size, size_t in_use_back,

                                       size_t in_use_front) {

    FLATBUFFERS_ASSERT(new_size > old_size);  // vector_downward only grows

    uint8_t *new_p = allocate(new_size);

    memcpy_downward(old_p, old_size, new_p, new_size, in_use_back,

                    in_use_front);

    deallocate(old_p, old_size);

    return new_p;

  }

 protected:

  // Called by `reallocate_downward` to copy memory from `old_p` of `old_size`

  // to `new_p` of `new_size`. Only memory of size `in_use_front` and

  // `in_use_back` will be copied from the front and back of the old memory

  // allocation.

  void memcpy_downward(uint8_t *old_p, size_t old_size, uint8_t *new_p,

                       size_t new_size, size_t in_use_back,

                       size_t in_use_front) {

    memcpy(new_p + new_size - in_use_back, old_p + old_size - in_use_back,

           in_use_back);

    memcpy(new_p, old_p, in_use_front);

  }

};

}  // namespace flatbuffers

}  // namespace mapserver

#endif  // FLATBUFFERS_ALLOCATOR_H_