item_alloc

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// -*- Mode: C++ -*-
// vim:ft=cpp
/*
 * (c) 2008-2009 Adam Lackorzynski <adam@os.inf.tu-dresden.de>,
 *               Alexander Warg <warg@os.inf.tu-dresden.de>
 *     economic rights: Technische Universität Dresden (Germany)
 *
 * This file is part of TUD:OS and distributed under the terms of the
 * GNU General Public License 2.
 * Please see the COPYING-GPL-2 file for details.
 *
 * As a special exception, you may use this file as part of a free software
 * library without restriction.  Specifically, if other files instantiate
 * templates or use macros or inline functions from this file, or you compile
 * this file and link it with other files to produce an executable, this
 * file does not by itself cause the resulting executable to be covered by
 * the GNU General Public License.  This exception does not however
 * invalidate any other reasons why the executable file might be covered by
 * the GNU General Public License.
 */
 
#pragma once
 
#include <l4/cxx/bitmap>
 
namespace L4Re { namespace Util {
 
using cxx::Bitmap_base;
using cxx::Bitmap;
 
class Item_alloc_base
{
private:
  long _capacity;
  long _free_hint;
  Bitmap_base _bits;
 
  void hint(long hint)
  { __atomic_store_n(&_free_hint, hint, __ATOMIC_RELAXED); }
 
public:
  bool is_allocated(long item) const noexcept
  { return _bits[item]; }
 
  long hint() const { return __atomic_load_n(&_free_hint, __ATOMIC_RELAXED); }
 
  bool alloc(long item) noexcept
  {
    return !_bits.atomic_get_and_set(item);
  }
 
  void free(long item) noexcept
  {
    if (item < hint())
      hint(item);
 
    _bits.atomic_clear_bit(item);
  }
 
  Item_alloc_base(long size, void *mem) noexcept
    : _capacity(size), _free_hint(0), _bits(mem)
  {}
 
  long alloc() noexcept
  {
    long free_hint = hint();
 
    for (long i = free_hint; i < _capacity; ++i)
      if (alloc(i))
        {
          hint(i + 1);
          return i;
        }
 
    // _free_hint is not necessarily correct in case of multi-threading! Make
    // sure we don't miss any potentially free slots.
    for (long i = 0; i < free_hint && i < _capacity; ++i)
      if (alloc(i))
        {
          hint(i + 1);
          return i;
        }
 
    return -1;
  }
 
  long size() const noexcept
  {
    return _capacity;
  }
};
 
template< long Bits >
class Item_alloc : public Item_alloc_base
{
private:
  typename Bitmap_base::Word<Bits>::Type _bits[Bitmap_base::Word<Bits>::Size];
 
public:
  Item_alloc() noexcept : Item_alloc_base(Bits, _bits) {}
};
 
}}