SQLite

  memset(&data[iCellFirst], 0, cbrk-iCellFirst);
  assert( sqlite3PagerIswriteable(pPage->pDbPage) );
  if( cbrk-iCellFirst!=pPage->nFree ){
    return SQLITE_CORRUPT_BKPT;
  }
  return SQLITE_OK;
}




















































/*
** Allocate nByte bytes of space from within the B-Tree page passed
** as the first argument. Write into *pIdx the index into pPage->aData[]
** of the first byte of allocated space. Return either SQLITE_OK or
** an error code (usually SQLITE_CORRUPT).
**

  ** array entry offset, and if the freelist is not empty, then search the
  ** freelist looking for a free slot big enough to satisfy the request.
  */
  testcase( gap+2==top );
  testcase( gap+1==top );
  testcase( gap==top );
  if( gap+2<=top && (data[hdr+1] || data[hdr+2]) ){
    int pc, addr;
    for(addr=hdr+1; (pc = get2byte(&data[addr]))>0; addr=pc){
      int size;            /* Size of the free slot */
      if( pc>usableSize-4 || pc<addr+4 ){
        return SQLITE_CORRUPT_BKPT;
      }
      size = get2byte(&data[pc+2]);
      if( size>=nByte ){
        int x = size - nByte;
        testcase( x==4 );
        testcase( x==3 );
        if( x<4 ){
          if( data[hdr+7]>=60 ) goto defragment_page;
          /* Remove the slot from the free-list. Update the number of
          ** fragmented bytes within the page. */
          memcpy(&data[addr], &data[pc], 2);
          data[hdr+7] += (u8)x;
        }else if( size+pc > usableSize ){
          return SQLITE_CORRUPT_BKPT;
        }else{
          /* The slot remains on the free-list. Reduce its size to account
          ** for the portion used by the new allocation. */
          put2byte(&data[pc+2], x);
        }
        *pIdx = pc + x;
        return SQLITE_OK;
      }
    }
  }

  /* The request could not be fulfilled using a freelist slot.  Check
  ** to see if defragmentation is necessary.
  */
  testcase( gap+2+nByte==top );
  if( gap+2+nByte>top ){
defragment_page:
    testcase( pPage->nCell==0 );
    rc = defragmentPage(pPage);
    if( rc ) return rc;
    top = get2byteNotZero(&data[hdr+5]);
    assert( gap+nByte<=top );
  }

      ptrmapPutOvflPtr(pPage, pCell, pRC);
    }
#endif
  }
}

/*
** Add a list of cells to a page.  The page should be initially empty.


** The cells are guaranteed to fit on the page.
*/
static void assemblePage(
  MemPage *pPage,   /* The page to be assembled */
  int nCell,        /* The number of cells to add to this page */
  u8 **apCell,      /* Pointers to cell bodies */
  u16 *aSize        /* Sizes of the cells */
){
  int i;            /* Loop counter */
  u8 *pCellptr;     /* Address of next cell pointer */
  int cellbody;     /* Address of next cell body */
  u8 * const data = pPage->aData;             /* Pointer to data for pPage */
  const int hdr = pPage->hdrOffset;           /* Offset of header on pPage */
  const int nUsable = pPage->pBt->usableSize; /* Usable size of page */

  assert( pPage->nOverflow==0 );
  assert( sqlite3_mutex_held(pPage->pBt->mutex) );
  assert( nCell>=0 && nCell<=(int)MX_CELL(pPage->pBt)
            && (int)MX_CELL(pPage->pBt)<=10921);
  assert( sqlite3PagerIswriteable(pPage->pDbPage) );

  /* Check that the page has just been zeroed by zeroPage() */
  assert( pPage->nCell==0 );
  assert( get2byteNotZero(&data[hdr+5])==nUsable );

  pCellptr = pPage->aCellIdx;
  cellbody = nUsable;
  for(i=0; i<nCell; i++){
    u16 sz = aSize[i];
    cellbody -= sz;
    put2byte(pCellptr, cellbody);
    pCellptr += 2;
    memcpy(&data[cellbody], apCell[i], sz);
  }
  put2byte(&data[hdr+3], nCell);
  put2byte(&data[hdr+5], cellbody);
  pPage->nFree -= (nCell*2 + nUsable - cellbody);
  pPage->nCell = (u16)nCell;
}


static void rebuildPage(
  MemPage *pPg,                   /* Edit this page */
  int nCell,                      /* Final number of cells on page */
  u8 **apCell,                    /* Array of cells */
  u16 *szCell                     /* Array of cell sizes */
){
  const int hdr = pPg->hdrOffset;          /* Offset of header on pPg */
  u8 * const aData = pPg->aData;           /* Pointer to data for pPg */
  const int usableSize = pPg->pBt->usableSize;
  u8 * const pEnd = &aData[usableSize];
  int i;
  u8 *pCellptr = pPg->aCellIdx;
  u8 *pTmp = sqlite3PagerTempSpace(pPg->pBt->pPager);
  u8 *pData;

  i = get2byte(&aData[hdr+5]);
  memcpy(&pTmp[i], &aData[i], usableSize - i);
  pData = &aData[usableSize];


  for(i=0; i<nCell; i++){
    u8 *pCell = apCell[i];
    if( pCell>aData && pCell<pEnd ){
      pCell = &pTmp[pCell - aData];
    }
    pData -= szCell[i];
    memcpy(pData, pCell, szCell[i]);

  put2byte(&aData[hdr+1], 0);
  put2byte(&aData[hdr+3], pPg->nCell);
  put2byte(&aData[hdr+5], pData - aData);
  aData[hdr+7] = 0x00;
}

static u8 *pageFindSlot(MemPage *pPg, int nByte){

  const int hdr = pPg->hdrOffset;
  u8 * const aData = pPg->aData;
  int iAddr;
  int pc;
  int usableSize = pPg->pBt->usableSize;

  for(iAddr=hdr+1; (pc = get2byte(&aData[iAddr]))>0; iAddr=pc){
    int size;            /* Size of the free slot */
    if( pc>usableSize-4 || pc<iAddr+4 ) return 0;



    size = get2byte(&aData[pc+2]);
    if( size>=nByte ){
      int x = size - nByte;
      testcase( x==4 );

      testcase( x==3 );
      if( x<4 ){
        if( aData[hdr+7]>=60 ) return 0;
        /* Remove the slot from the free-list. Update the number of
         ** fragmented bytes within the page. */
        memcpy(&aData[iAddr], &aData[pc], 2);
        aData[hdr+7] += (u8)x;
      }else if( size+pc > usableSize ){
        return 0;
      }else{
        /* The slot remains on the free-list. Reduce its size to account



         ** for the portion used by the new allocation. */
        put2byte(&aData[pc+2], x);
      }
      return &aData[pc + x];
    }
  }

  return 0;
}

static int pageInsertArray(
  MemPage *pPg,
  u8 *pBegin,
  u8 **ppData, 
  u8 *pCellptr, 
  int nCell,
  u8 **apCell,                    /* Array of cells */
  u16 *szCell                     /* Array of cell sizes */
){
  int i;
  u8 *aData = pPg->aData;
  u8 *pData = *ppData;


  for(i=0; i<nCell; i++){
    int sz = szCell[i];
    u8 *pSlot;
    if( (pSlot = pageFindSlot(pPg, sz))==0 ){
      pData -= sz;
      if( pData<pBegin ) return 1;
      pSlot = pData;
    }
    memcpy(pSlot, apCell[i], sz);
    put2byte(pCellptr, (pSlot - aData));
    pCellptr += 2;
  }
  *ppData = pData;
  return 0;
}










static int pageFreeArray(
  MemPage *pPg,                   /* Page to edit */
  int nCell,                      /* Cells to delete */
  u8 **apCell,                    /* Array of cells */
  u16 *szCell                     /* Array of cell sizes */
){
  u8 * const aData = pPg->aData;

      }
      nRet++;
    }
  }
  if( pFree ) freeSpace(pPg, pFree - aData, szFree);
  return nRet;
}


/*
** The pPg->nFree field is invalid when this function returns. It is the
** responsibility of the caller to set it correctly.
*/
static void editPage(
  MemPage *pPg,                   /* Edit this page */

    if( pCell>=aData && pCell<&aData[pPg->pBt->usableSize] ){
      pCell = &pTmp[pCell - aData];
    }
    assert( 0==memcmp(pCell, &aData[iOff], szCell[i+iNew]) );
  }
#endif

#if 0
printf("EDIT\n");
#endif

  return;
 editpage_fail:
#if 0
  printf("REBUILD\n");
#endif
  /* Unable to edit this page. Rebuild it from scratch instead. */
  rebuildPage(pPg, nNew, &apCell[iNew], &szCell[iNew]);
}

/*
** The following parameters determine how many adjacent pages get involved
** in a balancing operation.  NN is the number of neighbors on either side

    u8 *pCell = pPage->apOvfl[0];
    u16 szCell = cellSizePtr(pPage, pCell);
    u8 *pStop;

    assert( sqlite3PagerIswriteable(pNew->pDbPage) );
    assert( pPage->aData[0]==(PTF_INTKEY|PTF_LEAFDATA|PTF_LEAF) );
    zeroPage(pNew, PTF_INTKEY|PTF_LEAFDATA|PTF_LEAF);
    assemblePage(pNew, 1, &pCell, &szCell);


    /* If this is an auto-vacuum database, update the pointer map
    ** with entries for the new page, and any pointer from the 
    ** cell on the page to an overflow page. If either of these
    ** operations fails, the return code is set, but the contents
    ** of the parent page are still manipulated by thh code below.
    ** That is Ok, at this point the parent page is guaranteed to

  int cntOld[NB+2];            /* Old index in aCell[] after i-th page */
  int szNew[NB+2];             /* Combined size of cells place on i-th page */
  u8 **apCell = 0;             /* All cells begin balanced */
  u16 *szCell;                 /* Local size of all cells in apCell[] */
  u8 *aSpace1;                 /* Space for copies of dividers cells */
  Pgno pgno;                   /* Temp var to store a page number in */

  int aShiftLeft[NB+2];
  int aShiftRight[NB+2];
  u8 abDone[NB+2];
  Pgno aPgno[NB+2];
  u16 aPgFlags[NB+2];

  memset(abDone, 0, sizeof(abDone));
  pBt = pParent->pBt;
  assert( sqlite3_mutex_held(pBt->mutex) );

    nNew>=5 ? apNew[4]->pgno : 0, nNew>=5 ? szNew[4] : 0,
    nNew>=5 ? cntNew[4] - cntNew[3] - !leafData : 0
  ));

  assert( sqlite3PagerIswriteable(pParent->pDbPage) );
  put4byte(pRight, apNew[nNew-1]->pgno);

  j = 0;
  for(i=0; i<nNew; i++){
    /* At this point, "j" is the apCell[] index of the first cell currently
    ** stored on page apNew[i]. Or, if apNew[i] was not one of the original 
    ** sibling pages, "j" should be set to nCell. Variable iFirst is set
    ** to the apCell[] index of the first cell that will appear on the
    ** page following this balancing operation.  */
    int iFirst = (i==0 ? 0 : cntNew[i-1] + !leafData);     /* new first cell */

#if 0
    MemPage *pNew = apNew[i];
    int iCell;
    int nCta = 0;
    int nFree;

    printf("REBUILD %d: %d@%d -> %d@%d", apNew[i]->pgno,
          pNew->nCell+pNew->nOverflow, j,
          cntNew[i] - iFirst, iFirst
    );
    for(iCell=iFirst; iCell<cntNew[i]; iCell++){
      if( apCell[iCell]<pNew->aData 
       || apCell[iCell]>=&pNew->aData[pBt->usableSize] 
      ){
        nCta += szCell[iCell];
      }
    }
    nFree = get2byte(&pNew->aData[pNew->hdrOffset+5]);
    nFree -= (pNew->cellOffset + (cntNew[i] - iFirst) * 2);
    printf(" cta=%d free=%d\n", nCta, nFree);
    if( i==(nNew-1) ){
      printf("-----\n");
      fflush(stdout);
    }
#endif

    assert( i<nOld || j==nCell );
    aShiftLeft[i] = j - iFirst;
    j += apNew[i]->nCell + apNew[i]->nOverflow;
    aShiftRight[i] = cntNew[i] - j;
    assert( i!=nOld-1 || j==nCell );
    if( j<nCell ) j += !leafData;
  }

  /* If the sibling pages are not leaves, ensure that the right-child pointer
  ** of the right-most new sibling page is set to the value that was 
  ** originally in the same field of the right-most old sibling page. */
  if( (pageFlags & PTF_LEAF)==0 && nOld!=nNew ){
    MemPage *pOld = (nNew>nOld ? apNew : apOld)[nOld-1];
    memcpy(&apNew[nNew-1]->aData[8], &pOld->aData[8], 4);
  }

    assert( sqlite3PagerIswriteable(pParent->pDbPage) );
  }

  /* Now update the actual sibling pages. The order in which they are updated
  ** is important, as this code needs to avoid disrupting any page from which
  ** cells may still to be read. In practice, this means:
  **
  **   1) If the aShiftLeft[] entry is less than 0, it is not safe to 
  **      update the page until the page to the left of the current page
  **      (apNew[i-1]) has already been updated.
  **
  **   2) If the aShiftRight[] entry is less than 0, it is not safe to 
  **      update the page until the page to the right of the current page
  **      (apNew[i+1]) has already been updated.
  **
  ** If neither of the above apply, the page is safe to update.
  */
  assert( aShiftRight[nNew-1]>=0 && aShiftLeft[0]==0 );
  for(i=0; i<nNew*2; i++){
    int iPg = (i>=nNew ? i-nNew : nNew-1-i);
    if( abDone[iPg]==0 
     && (aShiftLeft[iPg]>=0 || abDone[iPg-1])
     && (aShiftRight[iPg]>=0 || abDone[iPg+1])
    ){
      int iNew;
      int iOld;
      int nNewCell;

      if( iPg==0 ){
        iNew = iOld = 0;