json_batchallocator.h

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// Copyright 2007-2010 Baptiste Lepilleur
// Distributed under MIT license, or public domain if desired and
// recognized in your jurisdiction.
// See file LICENSE for detail or copy at http://jsoncpp.sourceforge.net/LICENSE
 
#ifndef JSONCPP_BATCHALLOCATOR_H_INCLUDED
#define JSONCPP_BATCHALLOCATOR_H_INCLUDED
 
#include <stdlib.h>
#include <assert.h>
 
#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:
  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;
  }
 
private:
  struct BatchInfo {
    BatchInfo *next_;
    AllocatedType *used_;
    AllocatedType *end_;
    AllocatedType buffer_[objectPerAllocation];
  };
 
  // disabled copy constructor and assignement operator.
  BatchAllocator(const BatchAllocator &);
  void operator=(const BatchAllocator &);
 
  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
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