text: /*
   * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
   * All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions are met:
   *
   * 1. Redistributions of source code must retain the above copyright notice, this
   *    list of conditions and the following disclaimer.
   *
   * 2. Redistributions in binary form must reproduce the above copyright notice,
   *    this list of conditions and the following disclaimer in the documentation
   *    and/or other materials provided with the distribution.
   *
   * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
   * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
   * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
   * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
   * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
   * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
   * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
   * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
   */
  
  #pragma once
  
  #include <AK/Assertions.h>
  #include <AK/Atomic.h>
  #include <AK/Checked.h>
  #include <AK/Noncopyable.h>
  #include <AK/Platform.h>
  #include <AK/StdLibExtras.h>
  
  namespace AK {
  
  template<class T>
  constexpr auto call_will_be_destroyed_if_present(const T* object) -> decltype(const_cast<T*>(object)->will_be_destroyed(), TrueType {})
  {
      const_cast<T*>(object)->will_be_destroyed();
      return {};
  }
  
  constexpr auto call_will_be_destroyed_if_present(...) -> FalseType
  {
      return {};
  }
  
  template<class T>
  constexpr auto call_one_ref_left_if_present(const T* object) -> decltype(const_cast<T*>(object)->one_ref_left(), TrueType {})
  {
      const_cast<T*>(object)->one_ref_left();
      return {};
  }
  
  constexpr auto call_one_ref_left_if_present(...) -> FalseType
  {
      return {};
  }
  
  class RefCountedBase {
      AK_MAKE_NONCOPYABLE(RefCountedBase);
      AK_MAKE_NONMOVABLE(RefCountedBase);
  
  public:
      using RefCountType = unsigned int;
      using AllowOwnPtr = FalseType;
  
      ALWAYS_INLINE void ref() const
      {
          auto old_ref_count = m_ref_count.fetch_add(1, AK::MemoryOrder::memory_order_relaxed);
          ASSERT(old_ref_count > 0);
          ASSERT(!Checked<RefCountType>::addition_would_overflow(old_ref_count, 1));
      }
  
      ALWAYS_INLINE RefCountType ref_count() const
      {
          return m_ref_count.load(AK::MemoryOrder::memory_order_relaxed);
      }
  
  protected:
      RefCountedBase() { }
      ALWAYS_INLINE ~RefCountedBase()
      {
          ASSERT(m_ref_count.load(AK::MemoryOrder::memory_order_relaxed) == 0);
      }
  
      ALWAYS_INLINE RefCountType deref_base() const
      {
          auto old_ref_count = m_ref_count.fetch_sub(1, AK::MemoryOrder::memory_order_acq_rel);
          ASSERT(old_ref_count > 0);
          return old_ref_count - 1;
      }
  
      mutable Atomic<RefCountType> m_ref_count { 1 };
  };
  
  template<typename T>
  class RefCounted : public RefCountedBase {
  public:
      void unref() const
      {
          auto new_ref_count = deref_base();
          if (new_ref_count == 0) {
              call_will_be_destroyed_if_present(static_cast<const T*>(this));
              delete static_cast<const T*>(this);
          } else if (new_ref_count == 1) {
              call_one_ref_left_if_present(static_cast<const T*>(this));
          }
      }
  };
  
  }
  
  using AK::RefCounted;
  