courgette/third_party/paged_array.h

// Copyright (c) 2010 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// PagedArray implements an array stored using many fixed-size pages.
//
// PagedArray is a work-around to allow large arrays to be allocated when there
// is too much address space fragmentation for allocating the large arrays as
// contigous arrays.
#ifndef COURGETTE_BSDIFF_PAGED_ARRAY_H_
#define COURGETTE_BSDIFF_PAGED_ARRAY_H_

// For std::nothrow:
#include <new>

#include "base/basictypes.h"

namespace courgette {

// PagedArray implements an array stored using many fixed-size pages.
template<typename T>
class PagedArray {
  enum {
    // Page size in elements.  Page size of 2^18 * sizeof(T) is 1MB for T = int.
    kLogPageSize = 18,
    kPageSize = 1 << kLogPageSize
  };

 public:
  PagedArray() : pages_(NULL), page_count_(0) {}

  ~PagedArray() { clear(); }

  T& operator[](size_t i) {
    size_t page = i >> kLogPageSize;
    size_t offset = i & (kPageSize - 1);
    // It is tempting to add a DCHECK(page < page_count_), but that makes
    // bsdiff_create run 2x slower (even when compiled optimized.)
    return pages_[page][offset];
  }

  // Allocates storage for |size| elements. Returns true on success and false if
  // allocation fails.
  bool Allocate(size_t size) {
    clear();
    size_t pages_needed = (size + kPageSize - 1) >> kLogPageSize;
    pages_ = new(std::nothrow) T*[pages_needed];
    if (pages_ == NULL)
      return false;

    for (page_count_ = 0; page_count_ < pages_needed; ++page_count_) {
      T* block = new(std::nothrow) T[kPageSize];
      if (block == NULL) {
        clear();
        return false;
      }
      pages_[page_count_] = block;
    }
    return true;
  }

  // Releases all storage.  May be called more than once.
  void clear() {
    if (pages_ != NULL) {
      while (page_count_ != 0) {
        --page_count_;
        delete[] pages_[page_count_];
      }
      delete[] pages_;
      pages_ = NULL;
    }
  }

 private:
  T** pages_;
  size_t page_count_;

  DISALLOW_COPY_AND_ASSIGN(PagedArray);
};
}  // namespace
#endif  // COURGETTE_BSDIFF_PAGED_ARRAY_H_