SQLite

**    ixNx[3] = Number of cells in Child-1 and Child-2 + both divider cells
**    ixNx[4] = Total number of cells.
**
** For a table-btree, the concept is similar, except only apEnd[0]..apEnd[2]
** are used and they point to the leaf pages only, and the ixNx value are:
**
**    ixNx[0] = Number of cells in Child-1.
**    ixNx[1] = Number of cells in Child-1 and Child-2.
**    ixNx[2] = Total number of cells.
**
** Sometimes when deleting, a child page can have zero cells.  In those
** cases, ixNx[] entries with higher indexes, and the corresponding apEnd[]
** entries, shift down.  The end result is that each ixNx[] entry should
** be larger than the previous
*/
typedef struct CellArray CellArray;
struct CellArray {
  int nCell;              /* Number of cells in apCell[] */
  MemPage *pRef;          /* Reference page */
  u8 **apCell;            /* All cells begin balanced */
  u16 *szCell;            /* Local size of all cells in apCell[] */

  ** 
  */
  usableSpace = pBt->usableSize - 12 + leafCorrection;
  for(i=k=0; i<nOld; i++, k++){
    MemPage *p = apOld[i];
    b.apEnd[k] = p->aDataEnd;
    b.ixNx[k] = cntOld[i];
    if( k && b.ixNx[k]==b.ixNx[k-1] ){
      k--;  /* Omit b.ixNx[] entry for child pages with no cells */
    }
    if( !leafData ){
      k++;
      b.apEnd[k] = pParent->aDataEnd;
      b.ixNx[k] = cntOld[i]+1;
    }
    assert( p->nFree>=0 );
    szNew[i] = usableSpace - p->nFree;

  ** associated with the right-child of each sibling may also need to be 
  ** updated. This happens below, after the sibling pages have been 
  ** populated, not here.
  */
  if( ISAUTOVACUUM ){
    MemPage *pOld;
    MemPage *pNew = pOld = apNew[0];

    int cntOldNext = pNew->nCell + pNew->nOverflow;

    int iNew = 0;
    int iOld = 0;

    for(i=0; i<b.nCell; i++){
      u8 *pCell = b.apCell[i];
      while( i==cntOldNext ){
        iOld++;
        assert( iOld<nNew || iOld<nOld );
        pOld = iOld<nNew ? apNew[iOld] : apOld[iOld];
        cntOldNext += pOld->nCell + pOld->nOverflow + !leafData;

      }
      if( i==cntNew[iNew] ){
        pNew = apNew[++iNew];
        if( !leafData ) continue;
      }

      /* Cell pCell is destined for new sibling page pNew. Originally, it
      ** was either part of sibling page iOld (possibly an overflow cell), 
      ** or else the divider cell to the left of sibling page iOld. So,
      ** if sibling page iOld had the same page number as pNew, and if
      ** pCell really was a part of sibling page iOld (not a divider or
      ** overflow cell), we can skip updating the pointer map entries.  */
      if( iOld>=nNew
       || pNew->pgno!=aPgno[iOld]
       || !SQLITE_WITHIN(pCell,pOld->aData,pOld->aDataEnd)
      ){
        if( !leafCorrection ){
          ptrmapPut(pBt, get4byte(pCell), PTRMAP_BTREE, pNew->pgno, &rc);
        }
        if( cachedCellSize(&b,i)>pNew->minLocal ){
          ptrmapPutOvflPtr(pNew, pOld, pCell, &rc);
        }