SQLite

** structure. Unless SQLITE_PCACHE_SEPARATE_HEADER is defined, a buffer of
** PgHdr1.pCache->szPage bytes is allocated directly before this structure 
** in memory.
*/
struct PgHdr1 {
  sqlite3_pcache_page page;
  unsigned int iKey;             /* Key value (page number) */
  u8 isPinned;                   /* Page in use, not on the LRU list */
  PgHdr1 *pNext;                 /* Next in hash table chain */
  PCache1 *pCache;               /* Cache that currently owns this page */
  PgHdr1 *pLruNext;              /* Next in LRU list of unpinned pages */
  PgHdr1 *pLruPrev;              /* Previous in LRU list of unpinned pages */
};

/*

/*
** 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.


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

  assert( pPage!=0 );
  assert( pPage->isPinned==0 );
  pCache = pPage->pCache;
  pGroup = pCache->pGroup;
  assert( pPage->pLruNext || pPage==pGroup->pLruTail );
  assert( pPage->pLruPrev || pPage==pGroup->pLruHead );
  assert( sqlite3_mutex_held(pGroup->mutex) );

  if( pPage->pLruPrev ){
    pPage->pLruPrev->pLruNext = pPage->pLruNext;
  }else{
    pGroup->pLruHead = pPage->pLruNext;
  }
  if( pPage->pLruNext ){
    pPage->pLruNext->pLruPrev = pPage->pLruPrev;
  }else{




    pGroup->pLruTail = pPage->pLruPrev;
  }
  pPage->pLruNext = 0;
  pPage->pLruPrev = 0;
  pPage->isPinned = 1;
  pCache->nRecyclable--;

}


/*
** Remove the page supplied as an argument from the hash table 
** (PCache1.apHash structure) that it is currently stored in.
**

** to recycle pages to reduce the number allocated to nMaxPage.
*/
static void pcache1EnforceMaxPage(PGroup *pGroup){
  assert( sqlite3_mutex_held(pGroup->mutex) );
  while( pGroup->nCurrentPage>pGroup->nMaxPage && pGroup->pLruTail ){
    PgHdr1 *p = pGroup->pLruTail;
    assert( p->pCache->pGroup==pGroup );
    assert( p->isPinned==0 );
    pcache1PinPage(p);
    pcache1RemoveFromHash(p);
    pcache1FreePage(p);
  }
}

/*

  for(h=0; h<pCache->nHash; h++){
    PgHdr1 **pp = &pCache->apHash[h]; 
    PgHdr1 *pPage;
    while( (pPage = *pp)!=0 ){
      if( pPage->iKey>=iLimit ){
        pCache->nPage--;
        *pp = pPage->pNext;
        if( !pPage->isPinned ) pcache1PinPage(pPage);
        pcache1FreePage(pPage);
      }else{
        pp = &pPage->pNext;
        TESTONLY( nPage++; )
      }
    }
  }

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

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

  if( pCache->bPurgeable && pGroup->pLruTail && (
         (pCache->nPage+1>=pCache->nMax)
      || pGroup->nCurrentPage>=pGroup->nMaxPage
      || pcache1UnderMemoryPressure(pCache)
  )){
    PCache1 *pOther;
    pPage = pGroup->pLruTail;
    assert( pPage->isPinned==0 );
    pcache1RemoveFromHash(pPage);
    pcache1PinPage(pPage);
    pOther = pPage->pCache;

    /* We want to verify that szPage and szExtra are the same for pOther
    ** and pCache.  Assert that we can verify this by comparing sums. */
    assert( (pCache->szPage & (pCache->szPage-1))==0 && pCache->szPage>=512 );

    unsigned int h = iKey % pCache->nHash;
    pCache->nPage++;
    pPage->iKey = iKey;
    pPage->pNext = pCache->apHash[h];
    pPage->pCache = pCache;
    pPage->pLruPrev = 0;
    pPage->pLruNext = 0;
    pPage->isPinned = 1;
    *(void **)pPage->page.pExtra = 0;
    pCache->apHash[h] = pPage;
  }

fetch_out:
  if( pPage && iKey>pCache->iMaxKey ){
    pCache->iMaxKey = iKey;

  pcache1EnterMutex(pGroup);

  /* It is an error to call this function if the page is already 
  ** part of the PGroup LRU list.
  */
  assert( pPage->pLruPrev==0 && pPage->pLruNext==0 );
  assert( pGroup->pLruHead!=pPage && pGroup->pLruTail!=pPage );
  assert( pPage->isPinned==1 );

  if( reuseUnlikely || pGroup->nCurrentPage>pGroup->nMaxPage ){
    pcache1RemoveFromHash(pPage);
    pcache1FreePage(pPage);
  }else{
    /* Add the page to the PGroup LRU list. */
    if( pGroup->pLruHead ){
      pGroup->pLruHead->pLruPrev = pPage;
      pPage->pLruNext = pGroup->pLruHead;
      pGroup->pLruHead = pPage;
    }else{
      pGroup->pLruTail = pPage;
      pGroup->pLruHead = pPage;
    }
    pCache->nRecyclable++;
    pPage->isPinned = 0;
  }

  pcache1LeaveMutex(pCache->pGroup);
}

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

    PgHdr1 *p;
    pcache1EnterMutex(&pcache1.grp);
    while( (nReq<0 || nFree<nReq) && ((p=pcache1.grp.pLruTail)!=0) ){
      nFree += pcache1MemSize(p->page.pBuf);
#ifdef SQLITE_PCACHE_SEPARATE_HEADER
      nFree += sqlite3MemSize(p);
#endif
      assert( p->isPinned==0 );
      pcache1PinPage(p);
      pcache1RemoveFromHash(p);
      pcache1FreePage(p);
    }
    pcache1LeaveMutex(&pcache1.grp);
  }
  return nFree;

  int *pnMax,          /* OUT: Global maximum cache size */
  int *pnMin,          /* OUT: Sum of PCache1.nMin for purgeable caches */
  int *pnRecyclable    /* OUT: Total number of pages available for recycling */
){
  PgHdr1 *p;
  int nRecyclable = 0;
  for(p=pcache1.grp.pLruHead; p; p=p->pLruNext){
    assert( p->isPinned==0 );
    nRecyclable++;
  }
  *pnCurrent = pcache1.grp.nCurrentPage;
  *pnMax = (int)pcache1.grp.nMaxPage;
  *pnMin = (int)pcache1.grp.nMinPage;
  *pnRecyclable = nRecyclable;
}
#endif