text: /*
   * Copyright (c) 2020, Andreas Kling <kling@seguebaseos.org>
   *
   * SPDX-License-Identifier: BSD-2-Clause
   */
  
  #pragma once
  
  #include <AK/IntrusiveList.h>
  #include <AK/Platform.h>
  #include <AK/Types.h>
  #include <LibJS/Forward.h>
  #include <LibJS/Heap/Cell.h>
  
  #ifdef HAS_ADDRESS_SANITIZER
  #    include <sanitizer/asan_interface.h>
  #endif
  
  namespace JS {
  
  class HeapBlock {
      AK_MAKE_NONCOPYABLE(HeapBlock);
      AK_MAKE_NONMOVABLE(HeapBlock);
  
  public:
      static constexpr size_t block_size = 16 * KiB;
      static NonnullOwnPtr<HeapBlock> create_with_cell_size(Heap&, size_t);
  
      size_t cell_size() const { return m_cell_size; }
      size_t cell_count() const { return (block_size - sizeof(HeapBlock)) / m_cell_size; }
      bool is_full() const { return !has_lazy_freelist() && !m_freelist; }
  
      ALWAYS_INLINE Cell* allocate()
      {
          Cell* allocated_cell = nullptr;
          if (m_freelist) {
              VERIFY(is_valid_cell_pointer(m_freelist));
              allocated_cell = exchange(m_freelist, m_freelist->next);
          } else if (has_lazy_freelist()) {
              allocated_cell = cell(m_next_lazy_freelist_index++);
          }
  
          if (allocated_cell) {
              ASAN_UNPOISON_MEMORY_REGION(allocated_cell, m_cell_size);
          }
          return allocated_cell;
      }
  
      void deallocate(Cell*);
  
      template<typename Callback>
      void for_each_cell(Callback callback)
      {
          auto end = has_lazy_freelist() ? m_next_lazy_freelist_index : cell_count();
          for (size_t i = 0; i < end; ++i)
              callback(cell(i));
      }
  
      template<Cell::State state, typename Callback>
      void for_each_cell_in_state(Callback callback)
      {
          for_each_cell([&](auto* cell) {
              if (cell->state() == state)
                  callback(cell);
          });
      }
  
      Heap& heap() { return m_heap; }
  
      static HeapBlock* from_cell(const Cell* cell)
      {
          return reinterpret_cast<HeapBlock*>((FlatPtr)cell & ~(block_size - 1));
      }
  
      Cell* cell_from_possible_pointer(FlatPtr pointer)
      {
          if (pointer < reinterpret_cast<FlatPtr>(m_storage))
              return nullptr;
          size_t cell_index = (pointer - reinterpret_cast<FlatPtr>(m_storage)) / m_cell_size;
          auto end = has_lazy_freelist() ? m_next_lazy_freelist_index : cell_count();
          if (cell_index >= end)
              return nullptr;
          return cell(cell_index);
      }
  
      bool is_valid_cell_pointer(const Cell* cell)
      {
          return cell_from_possible_pointer((FlatPtr)cell);
      }
  
      IntrusiveListNode<HeapBlock> m_list_node;
  
  private:
      HeapBlock(Heap&, size_t cell_size);
  
      bool has_lazy_freelist() const { return m_next_lazy_freelist_index < cell_count(); }
  
      struct FreelistEntry final : public Cell {
          FreelistEntry* next { nullptr };
  
          virtual const char* class_name() const override { return "FreelistEntry"; }
      };
  
      Cell* cell(size_t index)
      {
          return reinterpret_cast<Cell*>(&m_storage[index * cell_size()]);
      }
  
      Heap& m_heap;
      size_t m_cell_size { 0 };
      size_t m_next_lazy_freelist_index { 0 };
      FreelistEntry* m_freelist { nullptr };
      alignas(Cell) u8 m_storage[];
  
  public:
      static constexpr size_t min_possible_cell_size = sizeof(FreelistEntry);
  };
  
  }
  