mattercontrol/Community.CsharpSqlite/src/pcache1_c.cs

using System.Diagnostics;

using Pgno = System.UInt32;

namespace Community.CsharpSqlite
{
	using sqlite3_pcache = Sqlite3.PCache1;

	public partial class Sqlite3
	{
		/*
		** 2008 November 05
		**
		** The author disclaims copyright to this source code.  In place of
		** a legal notice, here is a blessing:
		**
		**    May you do good and not evil.
		**    May you find forgiveness for yourself and forgive others.
		**    May you share freely, never taking more than you give.
		**
		*************************************************************************
		**
		** This file implements the default page cache implementation (the
		** sqlite3_pcache interface). It also contains part of the implementation
		** of the SQLITE_CONFIG_PAGECACHE and sqlite3_release_memory() features.
		** If the default page cache implementation is overriden, then neither of
		** these two features are available.
		*************************************************************************
		**  Included in SQLite3 port to C#-SQLite;  2008 Noah B Hart
		**  C#-SQLite is an independent reimplementation of the SQLite software library
		**
		**  SQLITE_SOURCE_ID: 2011-06-23 19:49:22 4374b7e83ea0a3fbc3691f9c0c936272862f32f2
		**
		*************************************************************************
		*/

		//#include "sqliteInt.h"

		//typedef struct PCache1 PCache1;
		//typedef struct PgHdr1 PgHdr1;
		//typedef struct PgFreeslot PgFreeslot;
		//typedef struct PGroup PGroup;

		/* Each page cache (or PCache) belongs to a PGroup.  A PGroup is a set
		** of one or more PCaches that are able to recycle each others unpinned
		** pages when they are under memory pressure.  A PGroup is an instance of
		** the following object.
		**
		** This page cache implementation works in one of two modes:
		**
		**   (1)  Every PCache is the sole member of its own PGroup.  There is
		**        one PGroup per PCache.
		**
		**   (2)  There is a single global PGroup that all PCaches are a member
		**        of.
		**
		** Mode 1 uses more memory (since PCache instances are not able to rob
		** unused pages from other PCaches) but it also operates without a mutex,
		** and is therefore often faster.  Mode 2 requires a mutex in order to be
		** threadsafe, but is able recycle pages more efficient.
		**
		** For mode (1), PGroup.mutex is NULL.  For mode (2) there is only a single
		** PGroup which is the pcache1.grp global variable and its mutex is
		** SQLITE_MUTEX_STATIC_LRU.
		*/

		public class PGroup
		{
			public sqlite3_mutex mutex;           /* MUTEX_STATIC_LRU or NULL */
			public int nMaxPage;                  /* Sum of nMax for purgeable caches */
			public int nMinPage;                  /* Sum of nMin for purgeable caches */
			public int mxPinned;                  /* nMaxpage + 10 - nMinPage */
			public int nCurrentPage;              /* Number of purgeable pages allocated */
			public PgHdr1 pLruHead, pLruTail;     /* LRU list of unpinned pages */

			// C#
			public PGroup()
			{
				mutex = new sqlite3_mutex();
			}
		};

		/* Each page cache is an instance of the following object.  Every
		** open database file (including each in-memory database and each
		** temporary or transient database) has a single page cache which
		** is an instance of this object.
		**
		** Pointers to structures of this type are cast and returned as
		** opaque sqlite3_pcache* handles.
		*/

		public class PCache1
		{
			/* Cache configuration parameters. Page size (szPage) and the purgeable
			** flag (bPurgeable) are set when the cache is created. nMax may be
			** modified at any time by a call to the pcache1CacheSize() method.
			** The PGroup mutex must be held when accessing nMax.
			*/
			public PGroup pGroup;              /* PGroup this cache belongs to */
			public int szPage;                 /* Size of allocated pages in bytes */
			public bool bPurgeable;            /* True if cache is purgeable */
			public int nMin;                   /* Minimum number of pages reserved */
			public int nMax;                   /* Configured "cache_size" value */
			public int n90pct;                 /* nMax*9/10 */

			/* Hash table of all pages. The following variables may only be accessed
			** when the accessor is holding the PGroup mutex.
			*/
			public int nRecyclable;             /* Number of pages in the LRU list */
			public int nPage;                   /* Total number of pages in apHash */
			public int nHash;                   /* Number of slots in apHash[] */
			public PgHdr1[] apHash;             /* Hash table for fast lookup by key */

			public Pgno iMaxKey;                /* Largest key seen since xTruncate() */

			public void Clear()
			{
				nRecyclable = 0;
				nPage = 0;
				nHash = 0;
				apHash = null;
				iMaxKey = 0;
			}
		};

		/*
		** Each cache entry is represented by an instance of the following
		** structure. A buffer of PgHdr1.pCache.szPage bytes is allocated
		** directly before this structure in memory (see the PGHDR1_TO_PAGE()
		** macro below).
		*/

		public class PgHdr1
		{
			public Pgno iKey;                   /* Key value (page number) */
			public PgHdr1 pNext;                /* Next in hash table chain */
			public PCache1 pCache;              /* Cache that currently owns this page */
			public PgHdr1 pLruNext;             /* Next in LRU list of unpinned pages */
			public PgHdr1 pLruPrev;             /* Previous in LRU list of unpinned pages */

			// For C#
			public PgHdr pPgHdr = new PgHdr();   /* Pointer to Actual Page Header */

			public void Clear()
			{
				this.iKey = 0;
				this.pNext = null;
				this.pCache = null;
				this.pPgHdr.Clear();
			}
		};

		/*
		** Free slots in the allocator used to divide up the buffer provided using
		** the SQLITE_CONFIG_PAGECACHE mechanism.
		*/

		public class PgFreeslot
		{
			public PgFreeslot pNext;  /* Next free slot */
			public PgHdr _PgHdr;      /* Next Free Header */
		};

		/*
		** Global data used by this cache.
		*/

		public class PCacheGlobal
		{
			public PGroup grp;                    /* The global PGroup for mode (2) */

			/* Variables related to SQLITE_CONFIG_PAGECACHE settings.  The
			** szSlot, nSlot, pStart, pEnd, nReserve, and isInit values are all
			** fixed at sqlite3_initialize() time and do not require mutex protection.
			** The nFreeSlot and pFree values do require mutex protection.
			*/
			public bool isInit;                   /* True if initialized */
			public int szSlot;                    /* Size of each free slot */
			public int nSlot;                     /* The number of pcache slots */
			public int nReserve;                  /* Try to keep nFreeSlot above this */
			public object pStart, pEnd;           /* Bounds of pagecache malloc range */
			/* Above requires no mutex.  Use mutex below for variable that follow. */
			public sqlite3_mutex mutex;          /* Mutex for accessing the following: */
			public int nFreeSlot;                 /* Number of unused pcache slots */
			public PgFreeslot pFree;             /* Free page blocks */
			/* The following value requires a mutex to change.  We skip the mutex on
			** reading because (1) most platforms read a 32-bit integer atomically and
			** (2) even if an incorrect value is read, no great harm is done since this
			** is really just an optimization. */
			public bool bUnderPressure;            /* True if low on PAGECACHE memory */

			// C#
			public PCacheGlobal()
			{
				grp = new PGroup();
			}
		}

		private static PCacheGlobal pcache = new PCacheGlobal();

		/*
		** All code in this file should access the global structure above via the
		** alias "pcache1". This ensures that the WSD emulation is used when
		** compiling for systems that do not support real WSD.
		*/

		//#define pcache1 (GLOBAL(struct PCacheGlobal, pcache1_g))
		private static PCacheGlobal pcache1 = pcache;

		/*
		** When a PgHdr1 structure is allocated, the associated PCache1.szPage
		** bytes of data are located directly before it in memory (i.e. the total
		** size of the allocation is sizeof(PgHdr1)+PCache1.szPage byte). The
		** PGHDR1_TO_PAGE() macro takes a pointer to a PgHdr1 structure as
		** an argument and returns a pointer to the associated block of szPage
		** bytes. The PAGE_TO_PGHDR1() macro does the opposite: its argument is
		** a pointer to a block of szPage bytes of data and the return value is
		** a pointer to the associated PgHdr1 structure.
		**
		**   Debug.Assert( PGHDR1_TO_PAGE(PAGE_TO_PGHDR1(pCache, X))==X );
		*/

		//#define PGHDR1_TO_PAGE(p)    (void)(((char)p) - p.pCache.szPage)
		private static PgHdr PGHDR1_TO_PAGE(PgHdr1 p)
		{
			return p.pPgHdr;
		}

		//#define PAGE_TO_PGHDR1(c, p) (PgHdr1)(((char)p) + c.szPage)
		private static PgHdr1 PAGE_TO_PGHDR1(PCache1 c, PgHdr p)
		{
			return p.pPgHdr1;
		}

		/*
		** Macros to enter and leave the PCache LRU mutex.
		*/

		//#define pcache1EnterMutex(X) sqlite3_mutex_enter((X).mutex)
		private static void pcache1EnterMutex(PGroup X)
		{
			sqlite3_mutex_enter(X.mutex);
		}

		//#define pcache1LeaveMutex(X) sqlite3_mutex_leave((X).mutex)
		private static void pcache1LeaveMutex(PGroup X)
		{
			sqlite3_mutex_leave(X.mutex);
		}

		/******************************************************************************/
		/******** Page Allocation/SQLITE_CONFIG_PCACHE Related Functions **************/

		/*
		** This function is called during initialization if a static buffer is
		** supplied to use for the page-cache by passing the SQLITE_CONFIG_PAGECACHE
		** verb to sqlite3_config(). Parameter pBuf points to an allocation large
		** enough to contain 'n' buffers of 'sz' bytes each.
		**
		** This routine is called from sqlite3_initialize() and so it is guaranteed
		** to be serialized already.  There is no need for further mutexing.
		*/

		private static void sqlite3PCacheBufferSetup(object pBuf, int sz, int n)
		{
			if (pcache1.isInit)
			{
				PgFreeslot p;
				sz = ROUNDDOWN8(sz);
				pcache1.szSlot = sz;
				pcache1.nSlot = pcache1.nFreeSlot = n;
				pcache1.nReserve = n > 90 ? 10 : (n / 10 + 1);
				pcache1.pStart = null;
				pcache1.pEnd = null;
				pcache1.pFree = null;
				pcache1.bUnderPressure = false;
				while (n-- > 0)
				{
					p = new PgFreeslot();// (PgFreeslot)pBuf;
					p._PgHdr = new PgHdr();
					p.pNext = pcache1.pFree;
					pcache1.pFree = p;
					//pBuf = (void)&((char)pBuf)[sz];
				}
				pcache1.pEnd = pBuf;
			}
		}

		/*
		** Malloc function used within this file to allocate space from the buffer
		** configured using sqlite3_config(SQLITE_CONFIG_PAGECACHE) option. If no
		** such buffer exists or there is no space left in it, this function falls
		** back to sqlite3Malloc().
		**
		** Multiple threads can run this routine at the same time.  Global variables
		** in pcache1 need to be protected via mutex.
		*/

		private static PgHdr pcache1Alloc(int nByte)
		{
			PgHdr p = null;
			Debug.Assert(sqlite3_mutex_notheld(pcache1.grp.mutex));
			sqlite3StatusSet(SQLITE_STATUS_PAGECACHE_SIZE, nByte);
			if (nByte <= pcache1.szSlot)
			{
				sqlite3_mutex_enter(pcache1.mutex);
				p = pcache1.pFree._PgHdr;
				if (p != null)
				{
					pcache1.pFree = pcache1.pFree.pNext;
					pcache1.nFreeSlot--;
					pcache1.bUnderPressure = pcache1.nFreeSlot < pcache1.nReserve;
					Debug.Assert(pcache1.nFreeSlot >= 0);
					sqlite3StatusAdd(SQLITE_STATUS_PAGECACHE_USED, 1);
				}
				sqlite3_mutex_leave(pcache1.mutex);
			}
			if (p == null)
			{
				/* Memory is not available in the SQLITE_CONFIG_PAGECACHE pool.  Get
				** it from sqlite3Malloc instead.
				*/
				p = new PgHdr();// sqlite3Malloc( nByte );
				//if ( p != null )
				{
					int sz = nByte;//sqlite3MallocSize( p );
					sqlite3_mutex_enter(pcache1.mutex);
					sqlite3StatusAdd(SQLITE_STATUS_PAGECACHE_OVERFLOW, sz);
					sqlite3_mutex_leave(pcache1.mutex);
				}
				sqlite3MemdebugSetType(p, MEMTYPE_PCACHE);
			}
			return p;
		}

		/*
		** Free an allocated buffer obtained from pcache1Alloc().
		*/

		private static void pcache1Free(ref PgHdr p)
		{
			if (p == null)
				return;
			if (p.CacheAllocated)//if ( p >= pcache1.pStart && p < pcache1.pEnd )
			{
				PgFreeslot pSlot = new PgFreeslot();
				sqlite3_mutex_enter(pcache1.mutex);
				sqlite3StatusAdd(SQLITE_STATUS_PAGECACHE_USED, -1);
				pSlot._PgHdr = p;// pSlot = (PgFreeslot)p;
				pSlot.pNext = pcache1.pFree;
				pcache1.pFree = pSlot;
				pcache1.nFreeSlot++;
				pcache1.bUnderPressure = pcache1.nFreeSlot < pcache1.nReserve;
				Debug.Assert(pcache1.nFreeSlot <= pcache1.nSlot);
				sqlite3_mutex_leave(pcache1.mutex);
			}
			else
			{
				int iSize;
				Debug.Assert(sqlite3MemdebugHasType(p, MEMTYPE_PCACHE));
				sqlite3MemdebugSetType(p, MEMTYPE_HEAP);
				iSize = sqlite3MallocSize(p.pData);
				sqlite3_mutex_enter(pcache1.mutex);
				sqlite3StatusAdd(SQLITE_STATUS_PAGECACHE_OVERFLOW, -iSize);
				sqlite3_mutex_leave(pcache1.mutex);
				sqlite3_free(ref p.pData);
			}
		}

#if SQLITE_ENABLE_MEMORY_MANAGEMENT
/*
** Return the size of a pcache allocation
*/
static int pcache1MemSize(object p){
  if( p>=pcache1.pStart && p<pcache1.pEnd ){
    return pcache1.szSlot;
  }else{
    int iSize;
    Debug.Assert( sqlite3MemdebugHasType(p, MEMTYPE_PCACHE) );
    sqlite3MemdebugSetType(p, MEMTYPE_HEAP);
    iSize = sqlite3MallocSize(p);
    sqlite3MemdebugSetType(p, MEMTYPE_PCACHE);
    return iSize;
  }
}
#endif //* SQLITE_ENABLE_MEMORY_MANAGEMENT */

		/*
** Allocate a new page object initially associated with cache pCache.
*/

		private static PgHdr1 pcache1AllocPage(PCache1 pCache)
		{
			//int nByte = sizeof( PgHdr1 ) + pCache.szPage;
			PgHdr pPg = pcache1Alloc(pCache.szPage);//nByte );
			PgHdr1 p = null;
			//if ( pPg !=null)
			{
				//PAGE_TO_PGHDR1( pCache, pPg );
				p = new PgHdr1();
				p.pCache = pCache;
				p.pPgHdr = pPg;
				if (pCache.bPurgeable)
				{
					pCache.pGroup.nCurrentPage++;
				}
			}
			//else
			//{
			//  p = 0;
			//}
			return p;
		}

		/*
		** Free a page object allocated by pcache1AllocPage().
		**
		** The pointer is allowed to be NULL, which is prudent.  But it turns out
		** that the current implementation happens to never call this routine
		** with a NULL pointer, so we mark the NULL test with ALWAYS().
		*/

		private static void pcache1FreePage(ref PgHdr1 p)
		{
			if (ALWAYS(p))
			{
				PCache1 pCache = p.pCache;
				if (pCache.bPurgeable)
				{
					pCache.pGroup.nCurrentPage--;
				}
				pcache1Free(ref p.pPgHdr);//PGHDR1_TO_PAGE( p );
			}
		}

		/*
		** Malloc function used by SQLite to obtain space from the buffer configured
		** using sqlite3_config(SQLITE_CONFIG_PAGECACHE) option. If no such buffer
		** exists, this function falls back to sqlite3Malloc().
		*/

		private static PgHdr sqlite3PageMalloc(int sz)
		{
			return pcache1Alloc(sz);
		}

		/*
		** Free an allocated buffer obtained from sqlite3PageMalloc().
		*/

		private static void sqlite3PageFree(ref byte[] p)
		{
			if (p != null)
			{
				sqlite3_free(ref p);
				p = null;
			}
		}

		private static void sqlite3PageFree(ref PgHdr p)
		{
			pcache1Free(ref p);
		}

		/*
		** Return true if it desirable to avoid allocating a new page cache
		** entry.
		**
		** If memory was allocated specifically to the page cache using
		** SQLITE_CONFIG_PAGECACHE but that memory has all been used, then
		** it is desirable to avoid allocating a new page cache entry because
		** presumably SQLITE_CONFIG_PAGECACHE was suppose to be sufficient
		** for all page cache needs and we should not need to spill the
		** allocation onto the heap.
		**
		** Or, the heap is used for all page cache memory put the heap is
		** under memory pressure, then again it is desirable to avoid
		** allocating a new page cache entry in order to avoid stressing
		** the heap even further.
		*/

		private static bool pcache1UnderMemoryPressure(PCache1 pCache)
		{
			if (pcache1.nSlot != 0 && pCache.szPage <= pcache1.szSlot)
			{
				return pcache1.bUnderPressure;
			}
			else
			{
				return sqlite3HeapNearlyFull();
			}
		}

		/******************************************************************************/
		/******** General Implementation Functions ************************************/

		/*
		** This function is used to resize the hash table used by the cache passed
		** as the first argument.
		**
		** The PCache mutex must be held when this function is called.
		*/

		private static int pcache1ResizeHash(PCache1 p)
		{
			PgHdr1[] apNew;
			int nNew;
			int i;

			Debug.Assert(sqlite3_mutex_held(p.pGroup.mutex));

			nNew = p.nHash * 2;
			if (nNew < 256)
			{
				nNew = 256;
			}

			pcache1LeaveMutex(p.pGroup);
			if (p.nHash != 0)
			{
				sqlite3BeginBenignMalloc();
			}
			apNew = new PgHdr1[nNew];//(PgHdr1 *)sqlite3_malloc(sizeof(PgHdr1 )*nNew);
			if (p.nHash != 0)
			{
				sqlite3EndBenignMalloc();
			}
			pcache1EnterMutex(p.pGroup);
			if (apNew != null)
			{
				//memset(apNew, 0, sizeof(PgHdr1 )*nNew);
				for (i = 0; i < p.nHash; i++)
				{
					PgHdr1 pPage;
					PgHdr1 pNext = p.apHash[i];
					while ((pPage = pNext) != null)
					{
						Pgno h = (Pgno)(pPage.iKey % nNew);
						pNext = pPage.pNext;
						pPage.pNext = apNew[h];
						apNew[h] = pPage;
					}
				}
				//sqlite3_free( p.apHash );
				p.apHash = apNew;
				p.nHash = nNew;
			}

			return (p.apHash != null ? SQLITE_OK : SQLITE_NOMEM);
		}

		/*
		** This function is used internally to remove the page pPage from the
		** PGroup LRU list, if is part of it. If pPage is not part of the PGroup
		** LRU list, then this function is a no-op.
		**
		** The PGroup mutex must be held when this function is called.
		**
		** If pPage is NULL then this routine is a no-op.
		*/

		private static void pcache1PinPage(PgHdr1 pPage)
		{
			PCache1 pCache;
			PGroup pGroup;

			if (pPage == null)
				return;
			pCache = pPage.pCache;
			pGroup = pCache.pGroup;
			Debug.Assert(sqlite3_mutex_held(pGroup.mutex));
			if (pPage.pLruNext != null || pPage == pGroup.pLruTail)
			{
				if (pPage.pLruPrev != null)
				{
					pPage.pLruPrev.pLruNext = pPage.pLruNext;
				}
				if (pPage.pLruNext != null)
				{
					pPage.pLruNext.pLruPrev = pPage.pLruPrev;
				}
				if (pGroup.pLruHead == pPage)
				{
					pGroup.pLruHead = pPage.pLruNext;
				}
				if (pGroup.pLruTail == pPage)
				{
					pGroup.pLruTail = pPage.pLruPrev;
				}
				pPage.pLruNext = null;
				pPage.pLruPrev = null;
				pPage.pCache.nRecyclable--;
			}
		}

		/*
		** Remove the page supplied as an argument from the hash table
		** (PCache1.apHash structure) that it is currently stored in.
		**
		** The PGroup mutex must be held when this function is called.
		*/

		private static void pcache1RemoveFromHash(PgHdr1 pPage)
		{
			int h;
			PCache1 pCache = pPage.pCache;
			PgHdr1 pp;
			PgHdr1 pPrev = null;

			Debug.Assert(sqlite3_mutex_held(pCache.pGroup.mutex));
			h = (int)(pPage.iKey % pCache.nHash);
			for (pp = pCache.apHash[h]; pp != pPage; pPrev = pp, pp = pp.pNext)
				;
			if (pPrev == null)
				pCache.apHash[h] = pp.pNext;
			else
				pPrev.pNext = pp.pNext; // pCache.apHash[h] = pp.pNext;
			pCache.nPage--;
		}

		/*
		** If there are currently more than nMaxPage pages allocated, try
		** to recycle pages to reduce the number allocated to nMaxPage.
		*/

		private static void pcache1EnforceMaxPage(PGroup pGroup)
		{
			Debug.Assert(sqlite3_mutex_held(pGroup.mutex));
			while (pGroup.nCurrentPage > pGroup.nMaxPage && pGroup.pLruTail != null)
			{
				PgHdr1 p = pGroup.pLruTail;
				Debug.Assert(p.pCache.pGroup == pGroup);
				pcache1PinPage(p);
				pcache1RemoveFromHash(p);
				pcache1FreePage(ref p);
			}
		}

		/*
		** Discard all pages from cache pCache with a page number (key value)
		** greater than or equal to iLimit. Any pinned pages that meet this
		** criteria are unpinned before they are discarded.
		**
		** The PCache mutex must be held when this function is called.
		*/

		private static void pcache1TruncateUnsafe(
		  PCache1 pCache,             /* The cache to truncate */
		  uint iLimit          /* Drop pages with this pgno or larger */
		)
		{
#if !NDEBUG || SQLITE_COVERAGE_TEST //TESTONLY( uint nPage = 0; )  /* To assert pCache.nPage is correct */
			uint nPage = 0;
#endif
			uint h;
			Debug.Assert(sqlite3_mutex_held(pCache.pGroup.mutex));
			for (h = 0; h < pCache.nHash; h++)
			{
				PgHdr1 pPrev = null;
				PgHdr1 pp = pCache.apHash[h];
				PgHdr1 pPage;
				while ((pPage = pp) != null)
				{
					if (pPage.iKey >= iLimit)
					{
						pCache.nPage--;
						pp = pPage.pNext;
						pcache1PinPage(pPage);
						if (pCache.apHash[h] == pPage)
							pCache.apHash[h] = pPage.pNext;
						else
							pPrev.pNext = pp;
						pcache1FreePage(ref pPage);
					}
					else
					{
						pp = pPage.pNext;
#if !NDEBUG || SQLITE_COVERAGE_TEST //TESTONLY( nPage++; )
						nPage++;
#endif
					}
					pPrev = pPage;
				}
			}
#if !NDEBUG || SQLITE_COVERAGE_TEST
			Debug.Assert(pCache.nPage == nPage);
#endif
		}

		/******************************************************************************/
		/******** sqlite3_pcache Methods **********************************************/

		/*
		** Implementation of the sqlite3_pcache.xInit method.
		*/

		private static int pcache1Init<T>(T NotUsed)
		{
			UNUSED_PARAMETER(NotUsed);
			Debug.Assert(pcache1.isInit == false);
			pcache1 = new PCacheGlobal();//memset(&pcache1, 0, sizeof(pcache1));
			if (sqlite3GlobalConfig.bCoreMutex)
			{
				pcache1.grp.mutex = sqlite3_mutex_alloc(SQLITE_MUTEX_STATIC_LRU);
				pcache1.mutex = sqlite3_mutex_alloc(SQLITE_MUTEX_STATIC_PMEM);
			}
			pcache1.grp.mxPinned = 10;
			pcache1.isInit = true;
			return SQLITE_OK;
		}

		/*
		** Implementation of the sqlite3_pcache.xShutdown method.
		** Note that the static mutex allocated in xInit does
		** not need to be freed.
		*/

		private static void pcache1Shutdown<T>(T NotUsed)
		{
			UNUSED_PARAMETER(NotUsed);
			Debug.Assert(pcache1.isInit);
			pcache1 = new PCacheGlobal();//;memset( &pcache1, 0, sizeof( pcache1 ) );
		}

		/*
		** Implementation of the sqlite3_pcache.xCreate method.
		**
		** Allocate a new cache.
		*/

		private static sqlite3_pcache pcache1Create(int szPage, bool bPurgeable)
		{
			PCache1 pCache;      /* The newly created page cache */
			PGroup pGroup;       /* The group the new page cache will belong to */
			int sz;               /* Bytes of memory required to allocate the new cache */

			/*
			** The seperateCache variable is true if each PCache has its own private
			** PGroup.  In other words, separateCache is true for mode (1) where no
			** mutexing is required.
			**
			**   *  Always use a unified cache (mode-2) if ENABLE_MEMORY_MANAGEMENT
			**
			**   *  Always use a unified cache in single-threaded applications
			**
			**   *  Otherwise (if multi-threaded and ENABLE_MEMORY_MANAGEMENT is off)
			**      use separate caches (mode-1)
			*/
#if (SQLITE_ENABLE_MEMORY_MANAGEMENT) || !SQLITE_THREADSAF
			const int separateCache = 0;
#else
  int separateCache = sqlite3GlobalConfig.bCoreMutex>0;
#endif

			//sz = sizeof( PCache1 ) + sizeof( PGroup ) * separateCache;
			pCache = new PCache1();//(PCache1)sqlite3_malloc( sz );
			//if ( pCache != null )
			//{
			//memset( pCache, 0, sz );
			if (separateCache == 0)
			{
				pGroup = pcache1.grp;
			}
			////else
			////{
			////pGroup = new PGroup();//(PGroup)pCache[1];
			////pGroup.mxPinned = 10;
			////}
			pGroup = pcache1.grp;
			pCache.pGroup = pGroup;
			pCache.szPage = szPage;
			pCache.bPurgeable = bPurgeable;//( bPurgeable ? 1 : 0 );
			if (bPurgeable)
			{
				pCache.nMin = 10;
				pcache1EnterMutex(pGroup);
				pGroup.nMinPage += (int)pCache.nMin;
				pGroup.mxPinned = pGroup.nMaxPage + 10 - pGroup.nMinPage;
				pcache1LeaveMutex(pGroup);
			}
			//}
			return (sqlite3_pcache)pCache;
		}

		/*
		** Implementation of the sqlite3_pcache.xCachesize method.
		**
		** Configure the cache_size limit for a cache.
		*/

		private static void pcache1Cachesize(sqlite3_pcache p, int nMax)
		{
			PCache1 pCache = (PCache1)p;
			if (pCache.bPurgeable)
			{
				PGroup pGroup = pCache.pGroup;
				pcache1EnterMutex(pGroup);
				pGroup.nMaxPage += nMax - pCache.nMax;
				pGroup.mxPinned = pGroup.nMaxPage + 10 - pGroup.nMinPage;
				pCache.nMax = nMax;
				pCache.n90pct = pCache.nMax * 9 / 10;
				pcache1EnforceMaxPage(pGroup);
				pcache1LeaveMutex(pGroup);
			}
		}

		/*
		** Implementation of the sqlite3_pcache.xPagecount method.
		*/

		private static int pcache1Pagecount(sqlite3_pcache p)
		{
			int n;
			PCache1 pCache = (PCache1)p;
			pcache1EnterMutex(pCache.pGroup);
			n = (int)pCache.nPage;
			pcache1LeaveMutex(pCache.pGroup);
			return n;
		}

		/*
		** Implementation of the sqlite3_pcache.xFetch method.
		**
		** Fetch a page by key value.
		**
		** Whether or not a new page may be allocated by this function depends on
		** the value of the createFlag argument.  0 means do not allocate a new
		** page.  1 means allocate a new page if space is easily available.  2
		** means to try really hard to allocate a new page.
		**
		** For a non-purgeable cache (a cache used as the storage for an in-memory
		** database) there is really no difference between createFlag 1 and 2.  So
		** the calling function (pcache.c) will never have a createFlag of 1 on
		** a non-purgable cache.
		**
		** There are three different approaches to obtaining space for a page,
		** depending on the value of parameter createFlag (which may be 0, 1 or 2).
		**
		**   1. Regardless of the value of createFlag, the cache is searched for a
		**      copy of the requested page. If one is found, it is returned.
		**
		**   2. If createFlag==0 and the page is not already in the cache, NULL is
		**      returned.
		**
		**   3. If createFlag is 1, and the page is not already in the cache, then
		**      return NULL (do not allocate a new page) if any of the following
		**      conditions are true:
		**
		**       (a) the number of pages pinned by the cache is greater than
		**           PCache1.nMax, or
		**
		**       (b) the number of pages pinned by the cache is greater than
		**           the sum of nMax for all purgeable caches, less the sum of
		**           nMin for all other purgeable caches, or
		**
		**   4. If none of the first three conditions apply and the cache is marked
		**      as purgeable, and if one of the following is true:
		**
		**       (a) The number of pages allocated for the cache is already
		**           PCache1.nMax, or
		**
		**       (b) The number of pages allocated for all purgeable caches is
		**           already equal to or greater than the sum of nMax for all
		**           purgeable caches,
		**
		**       (c) The system is under memory pressure and wants to avoid
		**           unnecessary pages cache entry allocations
		**
		**      then attempt to recycle a page from the LRU list. If it is the right
		**      size, return the recycled buffer. Otherwise, free the buffer and
		**      proceed to step 5.
		**
		**   5. Otherwise, allocate and return a new page buffer.
		*/

		private static PgHdr pcache1Fetch(sqlite3_pcache p, Pgno iKey, int createFlag)
		{
			int nPinned;
			PCache1 pCache = (PCache1)p;
			PGroup pGroup;
			PgHdr1 pPage = null;

			Debug.Assert(pCache.bPurgeable || createFlag != 1);
			Debug.Assert(pCache.bPurgeable || pCache.nMin == 0);
			Debug.Assert(pCache.bPurgeable == false || pCache.nMin == 10);
			Debug.Assert(pCache.nMin == 0 || pCache.bPurgeable);
			pcache1EnterMutex(pGroup = pCache.pGroup);

			/* Step 1: Search the hash table for an existing entry. */
			if (pCache.nHash > 0)
			{
				int h = (int)(iKey % pCache.nHash);
				for (pPage = pCache.apHash[h]; pPage != null && pPage.iKey != iKey; pPage = pPage.pNext)
					;
			}

			/* Step 2: Abort if no existing page is found and createFlag is 0 */
			if (pPage != null || createFlag == 0)
			{
				pcache1PinPage(pPage);
				goto fetch_out;
			}

			/* The pGroup local variable will normally be initialized by the
			** pcache1EnterMutex() macro above.  But if SQLITE_MUTEX_OMIT is defined,
			** then pcache1EnterMutex() is a no-op, so we have to initialize the
			** local variable here.  Delaying the initialization of pGroup is an
			** optimization:  The common case is to exit the module before reaching
			** this point.
			*/
#if  SQLITE_MUTEX_OMIT
      pGroup = pCache.pGroup;
#endif

			/* Step 3: Abort if createFlag is 1 but the cache is nearly full */
			nPinned = pCache.nPage - pCache.nRecyclable;
			Debug.Assert(nPinned >= 0);
			Debug.Assert(pGroup.mxPinned == pGroup.nMaxPage + 10 - pGroup.nMinPage);
			Debug.Assert(pCache.n90pct == pCache.nMax * 9 / 10);
			if (createFlag == 1 && (
				  nPinned >= pGroup.mxPinned
			   || nPinned >= (int)pCache.n90pct
			   || pcache1UnderMemoryPressure(pCache)
			))
			{
				goto fetch_out;
			}

			if (pCache.nPage >= pCache.nHash && pcache1ResizeHash(pCache) != 0)
			{
				goto fetch_out;
			}

			/* Step 4. Try to recycle a page. */
			if (pCache.bPurgeable && pGroup.pLruTail != null && (
				   (pCache.nPage + 1 >= pCache.nMax)
				|| pGroup.nCurrentPage >= pGroup.nMaxPage
				|| pcache1UnderMemoryPressure(pCache)
			))
			{
				PCache1 pOtherCache;
				pPage = pGroup.pLruTail;
				pcache1RemoveFromHash(pPage);
				pcache1PinPage(pPage);
				if ((pOtherCache = pPage.pCache).szPage != pCache.szPage)
				{
					pcache1FreePage(ref pPage);
					pPage = null;
				}
				else
				{
					pGroup.nCurrentPage -=
							 (pOtherCache.bPurgeable ? 1 : 0) - (pCache.bPurgeable ? 1 : 0);
				}
			}

			/* Step 5. If a usable page buffer has still not been found,
			** attempt to allocate a new one.
			*/
			if (null == pPage)
			{
				if (createFlag == 1)
					sqlite3BeginBenignMalloc();
				pcache1LeaveMutex(pGroup);
				pPage = pcache1AllocPage(pCache);
				pcache1EnterMutex(pGroup);
				if (createFlag == 1)
					sqlite3EndBenignMalloc();
			}

			if (pPage != null)
			{
				int h = (int)(iKey % pCache.nHash);
				pCache.nPage++;
				pPage.iKey = iKey;
				pPage.pNext = pCache.apHash[h];
				pPage.pCache = pCache;
				pPage.pLruPrev = null;
				pPage.pLruNext = null;
				PGHDR1_TO_PAGE(pPage).Clear();// *(void **)(PGHDR1_TO_PAGE(pPage)) = 0;
				pPage.pPgHdr.pPgHdr1 = pPage;
				pCache.apHash[h] = pPage;
			}

		fetch_out:
			if (pPage != null && iKey > pCache.iMaxKey)
			{
				pCache.iMaxKey = iKey;
			}
			pcache1LeaveMutex(pGroup);
			return (pPage != null ? PGHDR1_TO_PAGE(pPage) : null);
		}

		/*
		** Implementation of the sqlite3_pcache.xUnpin method.
		**
		** Mark a page as unpinned (eligible for asynchronous recycling).
		*/

		private static void pcache1Unpin(sqlite3_pcache p, PgHdr pPg, bool reuseUnlikely)
		{
			PCache1 pCache = (PCache1)p;
			PgHdr1 pPage = PAGE_TO_PGHDR1(pCache, pPg);
			PGroup pGroup = pCache.pGroup;

			Debug.Assert(pPage.pCache == pCache);
			pcache1EnterMutex(pGroup);

			/* It is an error to call this function if the page is already
			** part of the PGroup LRU list.
			*/
			Debug.Assert(pPage.pLruPrev == null && pPage.pLruNext == null);
			Debug.Assert(pGroup.pLruHead != pPage && pGroup.pLruTail != pPage);

			if (reuseUnlikely || pGroup.nCurrentPage > pGroup.nMaxPage)
			{
				pcache1RemoveFromHash(pPage);
				pcache1FreePage(ref pPage);
			}
			else
			{
				/* Add the page to the PGroup LRU list. */
				if (pGroup.pLruHead != null)
				{
					pGroup.pLruHead.pLruPrev = pPage;
					pPage.pLruNext = pGroup.pLruHead;
					pGroup.pLruHead = pPage;
				}
				else
				{
					pGroup.pLruTail = pPage;
					pGroup.pLruHead = pPage;
				}
				pCache.nRecyclable++;
			}

			pcache1LeaveMutex(pCache.pGroup);
		}

		/*
		** Implementation of the sqlite3_pcache.xRekey method.
		*/

		private static void pcache1Rekey(
		  sqlite3_pcache p,
		  PgHdr pPg,
		  Pgno iOld,
		  Pgno iNew
		)
		{
			PCache1 pCache = (PCache1)p;
			PgHdr1 pPage = PAGE_TO_PGHDR1(pCache, pPg);
			PgHdr1 pp;
			int h;
			Debug.Assert(pPage.iKey == iOld);
			Debug.Assert(pPage.pCache == pCache);

			pcache1EnterMutex(pCache.pGroup);

			h = (int)(iOld % pCache.nHash);
			pp = pCache.apHash[h];
			while ((pp) != pPage)
			{
				pp = (pp).pNext;
			}
			if (pp == pCache.apHash[h])
				pCache.apHash[h] = pp.pNext;
			else
				pp.pNext = pPage.pNext;

			h = (int)(iNew % pCache.nHash);
			pPage.iKey = iNew;
			pPage.pNext = pCache.apHash[h];
			pCache.apHash[h] = pPage;
			if (iNew > pCache.iMaxKey)
			{
				pCache.iMaxKey = iNew;
			}

			pcache1LeaveMutex(pCache.pGroup);
		}

		/*
		** Implementation of the sqlite3_pcache.xTruncate method.
		**
		** Discard all unpinned pages in the cache with a page number equal to
		** or greater than parameter iLimit. Any pinned pages with a page number
		** equal to or greater than iLimit are implicitly unpinned.
		*/

		private static void pcache1Truncate(sqlite3_pcache p, Pgno iLimit)
		{
			PCache1 pCache = (PCache1)p;
			pcache1EnterMutex(pCache.pGroup);
			if (iLimit <= pCache.iMaxKey)
			{
				pcache1TruncateUnsafe(pCache, iLimit);
				pCache.iMaxKey = iLimit - 1;
			}
			pcache1LeaveMutex(pCache.pGroup);
		}

		/*
		** Implementation of the sqlite3_pcache.xDestroy method.
		**
		** Destroy a cache allocated using pcache1Create().
		*/

		private static void pcache1Destroy(ref sqlite3_pcache p)
		{
			PCache1 pCache = (PCache1)p;
			PGroup pGroup = pCache.pGroup;
			Debug.Assert(pCache.bPurgeable || (pCache.nMax == 0 && pCache.nMin == 0));
			pcache1EnterMutex(pGroup);
			pcache1TruncateUnsafe(pCache, 0);
			pGroup.nMaxPage -= pCache.nMax;
			pGroup.nMinPage -= pCache.nMin;
			pGroup.mxPinned = pGroup.nMaxPage + 10 - pGroup.nMinPage;
			pcache1EnforceMaxPage(pGroup);
			pcache1LeaveMutex(pGroup);
			//sqlite3_free(  pCache.apHash );
			//sqlite3_free( pCache );
			p = null;
		}

		/*
		** This function is called during initialization (sqlite3_initialize()) to
		** install the default pluggable cache module, assuming the user has not
		** already provided an alternative.
		*/

		private static void sqlite3PCacheSetDefault()
		{
			sqlite3_pcache_methods defaultMethods = new sqlite3_pcache_methods(
		  0,                       /* pArg */
			(dxPC_Init)pcache1Init,           /* xInit */
			(dxPC_Shutdown)pcache1Shutdown,   /* xShutdown */
			(dxPC_Create)pcache1Create,       /* xCreate */
			(dxPC_Cachesize)pcache1Cachesize, /* xCachesize */
			(dxPC_Pagecount)pcache1Pagecount, /* xPagecount */
			(dxPC_Fetch)pcache1Fetch,         /* xFetch */
			(dxPC_Unpin)pcache1Unpin,         /* xUnpin */
			(dxPC_Rekey)pcache1Rekey,         /* xRekey */
			(dxPC_Truncate)pcache1Truncate,   /* xTruncate */
			(dxPC_Destroy)pcache1Destroy      /* xDestroy */
		);
			sqlite3_config(SQLITE_CONFIG_PCACHE, defaultMethods);
		}

#if SQLITE_ENABLE_MEMORY_MANAGEMENT
/*
** This function is called to free superfluous dynamically allocated memory
** held by the pager system. Memory in use by any SQLite pager allocated
** by the current thread may be sqlite3_free()ed.
**
** nReq is the number of bytes of memory required. Once this much has
** been released, the function returns. The return value is the total number
** of bytes of memory released.
*/
int sqlite3PcacheReleaseMemory(int nReq){
  int nFree = 0;
  Debug.Assert( sqlite3_mutex_notheld(pcache1.grp.mutex) );
  Debug.Assert( sqlite3_mutex_notheld(pcache1.mutex) );
  if( pcache1.pStart==0 ){
    PgHdr1 p;
    pcache1EnterMutex(&pcache1.grp);
    while( (nReq<0 || nFree<nReq) && ((p=pcache1.grp.pLruTail)!=0) ){
      nFree += pcache1MemSize(PGHDR1_TO_PAGE(p));
      PCache1pinPage(p);
      pcache1RemoveFromHash(p);
      pcache1FreePage(p);
    }
    pcache1LeaveMutex(&pcache1.grp);
  }
  return nFree;
}
#endif //* SQLITE_ENABLE_MEMORY_MANAGEMENT */

#if SQLITE_TEST
    /*
** This function is used by test procedures to inspect the internal state
** of the global cache.
*/
    static void sqlite3PcacheStats(
      out int pnCurrent,      /* OUT: Total number of pages cached */
      out int pnMax,          /* OUT: Global maximum cache size */
      out int pnMin,          /* OUT: Sum of PCache1.nMin for purgeable caches */
      out int pnRecyclable    /* OUT: Total number of pages available for recycling */
    )
    {
      PgHdr1 p;
      int nRecyclable = 0;
      for ( p = pcache1.grp.pLruHead; p != null; p = p.pLruNext )
      {
        nRecyclable++;
      }
      pnCurrent = pcache1.grp.nCurrentPage;
      pnMax = pcache1.grp.nMaxPage;
      pnMin = pcache1.grp.nMinPage;
      pnRecyclable = nRecyclable;
    }
#endif
	}
}