json_batchallocator.h

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#ifndef JSONCPP_BATCHALLOCATOR_H_INCLUDED
#define JSONCPP_BATCHALLOCATOR_H_INCLUDED

#include <cassert>
#include <cstdlib>

#ifndef JSONCPP_DOC_EXCLUDE_IMPLEMENTATION

namespace Json {

  /* Fast memory allocator.
 *
 * This memory allocator allocates memory for a batch of object (specified by
 * the page size, the number of object in each page).
 *
 * It does not allow the destruction of a single object. All the allocated objects
 * can be destroyed at once. The memory can be either released or reused for future
 * allocation.
 * 
 * The in-place new operator must be used to construct the object using the pointer
 * returned by allocate.
 */
  template <typename AllocatedType, const unsigned int objectPerAllocation>
  class BatchAllocator {
  public:
    typedef AllocatedType Type;

    BatchAllocator(unsigned int objectsPerPage = 255) : freeHead_(0), objectsPerPage_(objectsPerPage) {
      //      printf( "Size: %d => %s\n", sizeof(AllocatedType), typeid(AllocatedType).name() );
      assert(sizeof(AllocatedType) * objectPerAllocation >=
             sizeof(AllocatedType *));  // We must be able to store a slist in the object free space.
      assert(objectsPerPage >= 16);
      batches_ = allocateBatch(0);  // allocated a dummy page
      currentBatch_ = batches_;
    }

    ~BatchAllocator() {
      for (BatchInfo *batch = batches_; batch;) {
        BatchInfo *nextBatch = batch->next_;
        free(batch);
        batch = nextBatch;
      }
    }

    AllocatedType *allocate() {
      if (freeHead_)  // returns node from free list.
      {
        AllocatedType *object = freeHead_;
        freeHead_ = *(AllocatedType **)object;
        return object;
      }
      if (currentBatch_->used_ == currentBatch_->end_) {
        currentBatch_ = currentBatch_->next_;
        while (currentBatch_ && currentBatch_->used_ == currentBatch_->end_)
          currentBatch_ = currentBatch_->next_;

        if (!currentBatch_)  // no free batch found, allocate a new one
        {
          currentBatch_ = allocateBatch(objectsPerPage_);
          currentBatch_->next_ = batches_;  // insert at the head of the list
          batches_ = currentBatch_;
        }
      }
      AllocatedType *allocated = currentBatch_->used_;
      currentBatch_->used_ += objectPerAllocation;
      return allocated;
    }

    void release(AllocatedType *object) {
      assert(object != 0);
      *(AllocatedType **)object = freeHead_;
      freeHead_ = object;
    }

    // disabled copy constructor and assignement operator.
    BatchAllocator(const BatchAllocator &) = delete;
    void operator=(const BatchAllocator &) = delete;

  private:
    struct BatchInfo {
      BatchInfo *next_;
      AllocatedType *used_;
      AllocatedType *end_;
      AllocatedType buffer_[objectPerAllocation];
    };

    static BatchInfo *allocateBatch(unsigned int objectsPerPage) {
      const unsigned int mallocSize = sizeof(BatchInfo) - sizeof(AllocatedType) * objectPerAllocation +
                                      sizeof(AllocatedType) * objectPerAllocation * objectsPerPage;
      BatchInfo *batch = static_cast<BatchInfo *>(malloc(mallocSize));
      batch->next_ = 0;
      batch->used_ = batch->buffer_;
      batch->end_ = batch->buffer_ + objectsPerPage;
      return batch;
    }

    BatchInfo *batches_;
    BatchInfo *currentBatch_;
    AllocatedType *freeHead_;
    unsigned int objectsPerPage_;
  };

}  // namespace Json

#endif  // ifndef JSONCPP_DOC_INCLUDE_IMPLEMENTATION

#endif  // JSONCPP_BATCHALLOCATOR_H_INCLUDED