// -*- c++ -*-
/******************************************************************************
 **  arenaallocimpl.h
 **  
 **  Internal implementation types of the arena allocator
 **  MIT license
 *****************************************************************************/

#ifndef _ARENA_ALLOC_IMPL_H
#define _ARENA_ALLOC_IMPL_H

#ifdef ARENA_ALLOC_DEBUG
#include <stdio.h>
#endif

namespace ArenaAlloc
{

  template< typename T, typename A, typename M >
  class Alloc;
  
  // internal structure for tracking memory blocks
  template < typename AllocImpl >
  struct _memblock
  {
    // allocations are rounded up to a multiple of the size of this
      // struct to maintain proper alignment for any pointer and double
      // values stored in the allocation.
      // A future goal is to support even stricter alignment for example
      // to support cache alignment, special device  dependent mappings,
      // or GPU ops.
      union _roundsize {
          double d;
          void* p;
      };

      _memblock* m_next{nullptr};  // blocks kept link listed for cleanup at end
      std::size_t m_bufferSize;  // size of the buffer
      std::size_t m_index;  // index of next allocatable byte in the block
      char* m_buffer;  // pointer to large block to allocate from

      _memblock(std::size_t bufferSize, AllocImpl& allocImpl)
          : m_bufferSize(roundSize(bufferSize)), m_index(0),
            m_buffer(reinterpret_cast<char*>(allocImpl.allocate(
                bufferSize)))  // this works b/c of order of decl
      {
      }

      std::size_t roundSize( std::size_t numBytes )
    {
      // this is subject to overflow.  calling logic should not permit
      // an attempt to allocate a really massive size.
      // i.e. an attempt to allocate 10s of terabytes should be an error      
      return ( ( numBytes + sizeof( _roundsize ) - 1 ) / 
	       sizeof( _roundsize ) ) * sizeof( _roundsize );
    }

    char * allocate( std::size_t numBytes )
    {
      std::size_t roundedSize = roundSize( numBytes );
      if( roundedSize + m_index > m_bufferSize )
	return 0;

      char * ptrToReturn = &m_buffer[ m_index ];
      m_index += roundedSize;
      return ptrToReturn;
    }
  
    void dispose( AllocImpl& impl )
    {
      impl.deallocate( m_buffer );
    }

    ~_memblock()
    {
    }    
  };
  
  template< typename AllocatorImpl, typename Derived >
  struct _memblockimplbase
  {
    AllocatorImpl m_alloc;
    std::size_t m_refCount; // when refs -> 0 delete this
    std::size_t m_defaultSize;
        
    std::size_t m_numAllocate; // number of times allocate called
    std::size_t m_numDeallocate; // number of time deallocate called
    std::size_t m_numBytesAllocated; // A good estimate of amount of space used
    
    _memblock<AllocatorImpl> * m_head;
    _memblock<AllocatorImpl> * m_current;

    // round up 2 next power of 2 if not already
    // a power of 2
    std::size_t roundpow2( std::size_t value )
    {
      // note this works because subtracting 1 is equivalent to
      // inverting the lowest set bit and complementing any
      // bits lower than that.  only a power of 2
      // will yield 0 in the following check
      if( 0 == ( value & ( value - 1 ) ) )
	return value; // already a power of 2
      
      // fold t over itself. This will set all bits after the highest set bit of t to 1
      // who said bit twiddling wasn't practical?
      value |= value >> 1;
      value |= value >> 2;
      value |= value >> 4;
      value |= value >> 8;
      value |= value >> 16;
      value |= value >> 32;

      return value + 1;            
    }

    _memblockimplbase( std::size_t defaultSize, AllocatorImpl& allocator ):
      m_alloc( allocator ),
      m_refCount( 1 ),
      m_defaultSize( defaultSize ),
      m_numAllocate( 0 ),
      m_numDeallocate( 0 ),
      m_numBytesAllocated( 0 ),
      m_head( 0 ),
      m_current( 0 )
    {      
      if( m_defaultSize < 256 )
      {
	m_defaultSize = 256; // anything less is academic. a more practical size is 4k or more
      }
      else if ( m_defaultSize > 1024UL*1024*1024*16 ) 
      {
	// when this becomes a problem, this package has succeeded beyond my wildest expectations
	m_defaultSize = 1024UL*1024*1024*16;
      }
      
      // for convenience block size should be a power of 2
      // round up to next power of 2
      m_defaultSize = roundpow2( m_defaultSize );
      allocateNewBlock( m_defaultSize );      
    }
        
    char * allocate( std::size_t numBytes )
    {
      char * ptrToReturn = m_current->allocate( numBytes );
      if( !ptrToReturn )
      {
	allocateNewBlock( numBytes > m_defaultSize / 2 ? roundpow2( numBytes*2 ) : 
			  m_defaultSize );
	
	ptrToReturn = m_current->allocate( numBytes );	
      }
      
#ifdef ARENA_ALLOC_DEBUG
      fprintf( stdout, "_memblockimpl=%p allocated %ld bytes at address=%p\n", this, numBytes, ptrToReturn );
#endif

      ++ m_numAllocate;
      m_numBytesAllocated += numBytes; // does not account for the small overhead in tracking the allocation
      
      return ptrToReturn;
    }
    
    void allocateNewBlock( std::size_t blockSize )
    {      
      _memblock<AllocatorImpl> * newBlock = new ( m_alloc.allocate( sizeof( _memblock<AllocatorImpl> ) ) )
	_memblock<AllocatorImpl>( blockSize, m_alloc );
						  
#ifdef ARENA_ALLOC_DEBUG
      fprintf( stdout, "_memblockimplbase=%p allocating a new block of size=%ld\n", this, blockSize );
#endif      
      
      if( m_head == 0 )
      {
	m_head = m_current = newBlock;
      }
      else
      {
	m_current->m_next = newBlock;
	m_current = newBlock;
      }      
    }    
    
    void deallocate( void * ptr )
    {
      ++ m_numDeallocate;
    }
    
    size_t getNumAllocations() { return m_numAllocate; }
    size_t getNumDeallocations() { return m_numDeallocate; }
    size_t getNumBytesAllocated() { return m_numBytesAllocated; }
  
    void clear()
    {
      _memblock<AllocatorImpl> * block = m_head;
      while( block )
      {
	_memblock<AllocatorImpl> * curr = block;
	block = block->m_next;
	curr->dispose( m_alloc );
	curr->~_memblock<AllocatorImpl>();
	m_alloc.deallocate( curr );
      }      
    }    

    // The ref counting model does not permit the sharing of 
    // this object across multiple threads unless an external locking mechanism is applied 
    // to ensure the atomicity of the reference count.  
    void incrementRefCount() 
    { 
      ++m_refCount; 
#ifdef ARENA_ALLOC_DEBUG
      fprintf( stdout, "ref count on _memblockimplbase=%p incremented to %ld\n", this, m_refCount );
#endif      
    }

    void decrementRefCount()
    {
      --m_refCount;
#ifdef ARENA_ALLOC_DEBUG
      fprintf( stdout, "ref count on _memblockimplbase=%p decremented to %ld\n", this, m_refCount );
#endif      
      
      if( m_refCount == 0 )
      {
	Derived::destroy( static_cast<Derived*>(this) );
      }
    }                      
  };


  // Each allocator points to an instance of _memblockimpl which
  // contains the list of _memblock objects and other tracking info
  // including a refcount.
  // This object is instantiated in space obtained from the allocator
  // implementation. The allocator implementation is the component
  // on which allocate/deallocate are called to obtain storage from.
  template< typename AllocatorImpl >
  struct _memblockimpl : public _memblockimplbase<AllocatorImpl, _memblockimpl<AllocatorImpl> >
  {     
  private:

    typedef struct _memblockimplbase< AllocatorImpl, _memblockimpl<AllocatorImpl> > base_t;
    friend struct _memblockimplbase< AllocatorImpl, _memblockimpl<AllocatorImpl> >;
    
    // to get around some sticky access issues between Alloc<T1> and Alloc<T2> when sharing
    // the implementation.
    template <typename U, typename A, typename M >
    friend class Alloc;
    
    template< typename T >
    static void assign( const Alloc<T,AllocatorImpl, _memblockimpl<AllocatorImpl> >& src, 
			  _memblockimpl *& dest );
        
    static _memblockimpl<AllocatorImpl> * create( size_t defaultSize, AllocatorImpl& alloc )
    {
      return new ( alloc.allocate( sizeof( _memblockimpl ) ) ) _memblockimpl<AllocatorImpl>( defaultSize, 
											     alloc );
    }
   
    static void destroy( _memblockimpl<AllocatorImpl> * objToDestroy )
    {      
      AllocatorImpl allocImpl = objToDestroy->m_alloc;
      objToDestroy-> ~_memblockimpl<AllocatorImpl>();
      allocImpl.deallocate( objToDestroy );      
    }
    
    _memblockimpl( std::size_t defaultSize, AllocatorImpl& allocImpl ):
      _memblockimplbase<AllocatorImpl, _memblockimpl<AllocatorImpl> >( defaultSize, allocImpl )
    {
#ifdef ARENA_ALLOC_DEBUG
      fprintf( stdout, "_memblockimpl=%p constructed with default size=%ld\n", this, 
	       base_t::m_defaultSize );
#endif
    }
  
    ~_memblockimpl( )
    {
#ifdef ARENA_ALLOC_DEBUG
      fprintf( stdout, "~memblockimpl() called on _memblockimpl=%p\n", this );
#endif      
      base_t::clear();
    }  
  };
}

#endif
