** 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.
**
*************************************************************************
** $Id: btree.c,v 1.106 2004/04/29 14:42:46 drh Exp $
**
** This file implements a external (disk-based) database using BTrees.
** For a detailed discussion of BTrees, refer to
**
**     Donald E. Knuth, THE ART OF COMPUTER PROGRAMMING, Volume 3:
**     "Sorting And Searching", pages 473-480. Addison-Wesley
**     Publishing Company, Reading, Massachusetts.
................................................................................
**      *     zero or more pages numbers of leaves
*/
#include "sqliteInt.h"
#include "pager.h"
#include "btree.h"
#include <assert.h>






























/* Forward declarations */
typedef struct MemPage MemPage;

/*
** This is a magic string that appears at the beginning of every
** SQLite database in order to identify the file as a real database.
**                                  0123456789 123456 */
static const char zMagicHeader[] = "SQLite version 3";

/*
** Page type flags

*/
#define PTF_LEAF      0x01
#define PTF_ZERODATA  0x02
#define PTF_INTKEY    0x04


/*
** As each page of the file is loaded into memory, an instance of the following
** structure is appended and initialized to zero.  This structure stores
** information about the page that is decoded from the raw file page.
** The extra two entries in apCell[] are an allowance for this situation.
**
** The pParent field points back to the parent page.  This allows us to
** walk up the BTree from any leaf to the root.  Care must be taken to
** unref() the parent page pointer when this page is no longer referenced.
** The pageDestructor() routine handles that chore.
*/
struct MemPage {
................................................................................
  u8 zeroData;                   /* True if zero data flag is set */
  u8 hdrOffset;                  /* 100 for page 1.  0 otherwise */
  Pgno pgno;                     /* Page number for this page */
  MemPage *pParent;              /* The parent of this page.  NULL for root */
  int idxParent;                 /* Index in pParent->aCell[] of this node */
  int nFree;                     /* Number of free bytes on the page */
  int nCell;                     /* Number of entries on this page */

  unsigned char **aCell;         /* Pointer to start of each cell */
};

/*
** The in-memory image of a disk page has the auxiliary information appended
** to the end.  EXTRA_SIZE is the number of bytes of space needed to hold
** that extra information.
................................................................................
  int size;
  unsigned char *data;
#ifndef NDEBUG
  int cnt = 0;
#endif

  data = pPage->aData;
  assert( sqlitepager_iswriteable(data) );
  assert( pPage->pBt );
  if( nByte<4 ) nByte = 4;
  if( pPage->nFree<nByte || pPage->isOverfull ) return 0;
  hdr = pPage->hdrOffset;
  if( data[hdr+5]>=252 ){
    defragmentPage(pPage);
  }
................................................................................
  int addr, pbegin, pend;
#ifndef NDEBUG
  int tsize = 0;          /* Total size of all freeblocks */
#endif
  unsigned char *data = pPage->aData;

  assert( pPage->pBt!=0 );
  assert( sqlitepager_iswriteable(data) );
  assert( start>=pPage->hdrOffset+6+(pPage->leaf?0:4) );
  assert( end<=pPage->pBt->pageSize );
  if( size<4 ) size = 4;

  /* Add the space back into the linked list of freeblocks */
  addr = pPage->hdrOffset + 1;
  while( (pbegin = get2byte(&data[addr]))<start && pbegin>0 ){
................................................................................

#if 0
/*
** The following is the default comparison function for (non-integer)
** keys in the btrees.  This function returns negative, zero, or
** positive if the first key is less than, equal to, or greater than
** the second.




















**
*/
static int keyComp(
  void *userData,
  int nKey1, const unsigned char *aKey1, 
  int nKey2, const unsigned char *aKey2,
){
................................................................................
  }
  return 0;

bad_key:
  return 1;
}
#endif
















/*
** Initialize the auxiliary information for a disk block.
**
** The pParent parameter must be a pointer to the MemPage which
** is the parent of the page being initialized.  The root of a
** BTree has no parent and so for that page, pParent==NULL.
................................................................................
  MemPage *pPage,        /* The page to be initialized */
  MemPage *pParent       /* The parent.  Might be NULL */
){
  int c, pc, i, hdr;
  int sumCell = 0;       /* Total size of all cells */

  assert( pPage->pBt!=0 );


  assert( pPage->aData == &((unsigned char*)pPage)[pPage->pBt->pageSize] );

  if( pPage->pParent ){
    assert( pPage->pParent==pParent );
    return SQLITE_OK;
  }
  if( pParent ){
    pPage->pParent = pParent;

    sqlitepager_ref(pParent->aData);
  }
  if( pPage->isInit ) return SQLITE_OK;
  pPage->nCell = 0;
  assert( sqlitepager_pagenumber(pPage->aData)==pPage->pgno );
  pPage->hdrOffset = hdr = pgnoThis==1 ? 100 : 0;
  c = pPage->aData[pPage->hdrOffset];
  pPage->intKey = (c & PTF_INTKEY)!=0;
  pPage->zeroData = (c & PTF_ZERODATA)!=0;
  pPage->leaf = (c & PTF_INTKEY)!=0;

  /* Initialize the cell count and cell pointers */
  pc = get2byte(&data[hdr+3]);
  while( pc>0 ){
    if( pc>=pBt->pageSize ) return SQLITE_CORRUPT;
    if( pPage->nCell>pBt->pageSize ) return SQLITE_CORRUPT;
    pPage->nCell++;
    pc = get2byte(&data[pc]);
  }
  pPage->aCell = sqlite_malloc( sizeof(pPage->aCell[0])*pPage->nCell );
  if( pPage->aCell==0 ){
    return SQLITE_NOMEM;
  }
  pc = get2byte(&data[hdr+3]);
  for(i=0; pc>0; i++){
    pPage->aCell[i] = &data[pc];
    pc = get2byte(&data[pc]);
    sumCell += cellSize(pPage, &data[pc]);
................................................................................
*/
static void zeroPage(MemPage *pPage, int flags){
  unsigned char *data = pPage->aData;
  Btree *pBt = pPage->pBt;
  int hdr = pPage->hdrOffset;
  int first;

  assert( sqlitepager_iswriteable(data) );
  memset(&data[hdr], 0, pBt->pageSize - hdr);
  data[hdr] = flags;
  first = hdr + 6 + 4*((flags&0x01)!=0);
  put2byte(&data[hdr+1], first);
  put2byte(&data[first+2], pBt->pageSize - first);
  sqliteFree(pPage->aCell);
  pPage->aCell = 0;
................................................................................
  return SQLITE_OK;
}

/*
** Release a MemPage.  This should be called once for each prior
** call to getPage.
*/
static int releasePage(MemPage *pPage){
  if( pPage ){
    assert( pPage->aData );
    assert( pPage->pBt );
    assert( &pPage->aData[pPage->pBt->pageSize]==(unsigned char*)pPage );
    sqlitepager_unref(pPage->aData);
  }
}
................................................................................
    *ppBtree = 0;
    return rc;
  }
  sqlitepager_set_destructor(pBt->pPager, pageDestructor);
  pBt->pCursor = 0;
  pBt->page1 = 0;
  pBt->readOnly = sqlitepager_isreadonly(pBt->pPager);
  pBt->pageSize = SQLITE_PAGE_SIZE;
  pBt->maxLocal = (SQLITE_PAGE_SIZE-10)/4-12;
  *ppBtree = pBt;
  return SQLITE_OK;
}

/*
** Close an open database and invalidate all cursors.
*/
................................................................................
  int rc;
  BtCursor *pCur, *pRing;

  if( pBt->readOnly && wrFlag ){
    *ppCur = 0;
    return SQLITE_READONLY;
  }
  if( pBt->page1==0 ){
    rc = lockBtree(pBt);
    if( rc!=SQLITE_OK ){
      *ppCur = 0;
      return rc;
    }
  }
  pCur = sqliteMalloc( sizeof(*pCur) );
  if( pCur==0 ){
    rc = SQLITE_NOMEM;
    goto create_cursor_exception;
  }
  pCur->pgnoRoot = (Pgno)iTable;
  rc = getPage(pBt, pCur->pgnoRoot, (void**)&pCur->pPage);
  if( rc!=SQLITE_OK ){
    goto create_cursor_exception;
  }
  rc = initPage(pCur->pPage, 0);
  if( rc!=SQLITE_OK ){
    goto create_cursor_exception;
  }
................................................................................
}

/*
** Set *pSize to the size of the buffer needed to hold the value of
** the key for the current entry.  If the cursor is not pointing
** to a valid entry, *pSize is set to 0. 
**
** For a table with the intKey flag set, this routine returns the key
** itself, not the number of bytes in the key.
*/
int sqlite3BtreeKeySize(BtCursor *pCur, u64 *pSize){
  MemPage *pPage;

  pPage = pCur->pPage;
  assert( pPage!=0 );
................................................................................
** If pCur is not pointing to a valid entry return 0.
**
** The pointer returned is ephemeral.  The key may move
** or be destroyed on the next call to any Btree routine.
**
** This routine is used to do quick key comparisons in the
** common case where the entire key fits in the payload area
** of a cell and does not overflow onto secondary pages.






*/
void *sqlite3BtreeKeyFetch(BtCursor *pCur){
  unsigned char *aPayload;
  MemPage *pPage;
  Btree *pBt;
  u64 nData, nKey;

................................................................................
  pCur->pPage = pRoot;
  pCur->idx = 0;
  if( pRoot->nCell==0 && !pRoot->leaf ){
    Pgno subpage;
    assert( pRoot->pgno==1 );
    subpage = get4byte(&pRoot->aData[pRoot->hdrOffset+6]);
    assert( subpage>0 );
    rc = movetoChild(pCur, subpage);
  }
  return rc;
}

/*
** Move the cursor down to the left-most leaf entry beneath the
** entry to which it is currently pointing.
................................................................................
    if( rc ) return rc;
    sqlitepager_unref(pOvfl->aData);
  }
  return SQLITE_OK;
}

/*
** Compute the number of bytes required by a cell header.  If the *pHeader
** argument is not NULL, fill it in with the bytes of the header.
*/
static int makeCellHeader(
  MemPage *pPage,          /* The page that will contain the cell */
  u64 nKey,                /* Size of key, or the key value if intKey */
  int nData,               /* Size of data.  Ignored for zerodata */
  unsigned char *pHeader   /* Write header bytes here */
){
................................................................................
/*
** Fill in the payload section of a cell into the space provided.  If
** the payload will not completely fit in the cell, allocate additional
** overflow pages and fill them in.
*/
static int fillInCell(
  MemPage *pPage,                /* The page that contains the cell */
  unsigned char *pCell,          /* Pointer to start of payload area */
  int nHeader,                   /* Number of bytes in the cell header */
  const void *pKey, u64 nKey,    /* The key */
  const void *pData,int nData    /* The data */

){
  int nPayload;
  const void *pSrc;
  int nSrc, nSrc2;
  int spaceLeft;
  MemPage *pOvfl = 0;
  unsigned char *pPrior;
  unsigned char *pPayload;
  Btree *pBt = pPage->pBt;
  Pgno pgnoOvfl = 0;



  nPayload = nData;
  if( pPage->intKey ){
    pSrc = pData;
    nSrc = nData;
    nSrc2 = 0;
  }else{
    nPayload += nKey;
    pSrc = pKey;
    nSrc = nKey;
  }





  spaceLeft = pBt->maxLocal;
  pPayload = &pCell[nHeader];
  pPrior = &pPayload[pBt->maxLocal];

  while( nPayload>0 ){
    if( spaceLeft==0 ){
      rc =  allocatePage(pBt, &pOvfl, &pgnoOvfl, pgnoOvfl);
................................................................................
}

/*
** Change the MemPage.pParent pointer on the page whose number is
** given in the second argument so that MemPage.pParent holds the
** pointer in the third argument.
*/
static void reparentPage(Pager *pPager, Pgno pgno, MemPage *pNewParent,int idx){
  MemPage *pThis;


  if( pgno==0 ) return;
  assert( pPager!=0 );
  pThis = sqlitepager_lookup(pPager, pgno);

  if( pThis && pThis->isInit ){
    if( pThis->pParent!=pNewParent ){
      if( pThis->pParent ) sqlitepager_unref(pThis->pParent);
      pThis->pParent = pNewParent;
      if( pNewParent ) sqlitepager_ref(pNewParent);
    }
    pThis->idxParent = idx;
    sqlitepager_unref(pThis);
  }
}

/*
** Reparent all children of the given page to be the given page.


** In other words, for every child of pPage, invoke reparentPage()
** to make sure that each child knows that pPage is its parent.
**
** This routine gets called after you memcpy() one page into
** another.
*/
static void reparentChildPages(Btree *pBt, MemPage *pPage){
  int i;
  Pager *pPager = pBt->pPager;



  for(i=0; i<pPage->nCell; i++){
    reparentPage(pPager, SWAB32(pBt, pPage->apCell[i]->h.leftChild), pPage, i);

  }
  reparentPage(pPager, SWAB32(pBt, pPage->u.hdr.rightChild), pPage, i);

  pPage->idxShift = 0;
}

/*
** Remove the i-th cell from pPage.  This routine effects pPage only.
** The cell content is not freed or deallocated.  It is assumed that
** the cell content has been copied someplace else.  This routine just
................................................................................
** "sz" must be the number of bytes in the cell.
**
** Do not bother maintaining the integrity of the linked list of Cells.
** Only the pPage->apCell[] array is important.  The relinkCellList() 
** routine will be called soon after this routine in order to rebuild 
** the linked list.
*/
static void dropCell(Btree *pBt, MemPage *pPage, int idx, int sz){
  int j;
  assert( idx>=0 && idx<pPage->nCell );
  assert( sz==cellSize(pBt, pPage->apCell[idx]) );
  assert( sqlitepager_iswriteable(pPage) );
  freeSpace(pBt, pPage, Addr(pPage->apCell[idx]) - Addr(pPage), sz);


  for(j=idx; j<pPage->nCell-1; j++){
    pPage->apCell[j] = pPage->apCell[j+1];
  }
  pPage->nCell--;
  pPage->idxShift = 1;
}

/*
** Insert a new cell on pPage at cell index "i".  pCell points to the
................................................................................
** and set pPage->isOverfull.  
**
** Do not bother maintaining the integrity of the linked list of Cells.
** Only the pPage->apCell[] array is important.  The relinkCellList() 
** routine will be called soon after this routine in order to rebuild 
** the linked list.
*/
static void insertCell(Btree *pBt, MemPage *pPage, int i, Cell *pCell, int sz){
  int idx, j;
  assert( i>=0 && i<=pPage->nCell );
  assert( sz==cellSize(pBt, pCell) );
  assert( sqlitepager_iswriteable(pPage) );
  idx = allocateSpace(pBt, pPage, sz);

  for(j=pPage->nCell; j>i; j--){
    pPage->apCell[j] = pPage->apCell[j-1];
  }
  pPage->nCell++;
  if( idx<=0 ){
    pPage->isOverfull = 1;
    pPage->apCell[i] = pCell;
  }else{
    memcpy(&pPage->u.aDisk[idx], pCell, sz);
    pPage->apCell[i] = (Cell*)&pPage->u.aDisk[idx];
  }
  pPage->idxShift = 1;
}

/*
** Rebuild the linked list of cells on a page so that the cells
** occur in the order specified by the pPage->apCell[] array.  
** Invoke this routine once to repair damage after one or more
** invocations of either insertCell() or dropCell().
*/
static void relinkCellList(Btree *pBt, MemPage *pPage){
  int i;
  u16 *pIdx;
  assert( sqlitepager_iswriteable(pPage) );
  pIdx = &pPage->u.hdr.firstCell;

  for(i=0; i<pPage->nCell; i++){
    int idx = Addr(pPage->apCell[i]) - Addr(pPage);

    assert( idx>0 && idx<SQLITE_USABLE_SIZE );
    *pIdx = SWAB16(pBt, idx);
    pIdx = &pPage->apCell[i]->h.iNext;
  }
  *pIdx = 0;

}

/*
** Make a copy of the contents of pFrom into pTo.  The pFrom->apCell[]
** pointers that point into pFrom->u.aDisk[] must be adjusted to point
** into pTo->u.aDisk[] instead.  But some pFrom->apCell[] entries might
** not point to pFrom->u.aDisk[].  Those are unchanged.
*/
static void copyPage(MemPage *pTo, MemPage *pFrom){
  uptr from, to;
  int i;
  memcpy(pTo->u.aDisk, pFrom->u.aDisk, SQLITE_USABLE_SIZE);






  pTo->pParent = 0;
  pTo->isInit = 1;

  pTo->nCell = pFrom->nCell;
  pTo->nFree = pFrom->nFree;


  pTo->isOverfull = pFrom->isOverfull;
  to = Addr(pTo);
  from = Addr(pFrom);
  for(i=0; i<pTo->nCell; i++){
    uptr x = Addr(pFrom->apCell[i]);
    if( x>from && x<from+SQLITE_USABLE_SIZE ){
      *((uptr*)&pTo->apCell[i]) = x + to - from;
    }else{
      pTo->apCell[i] = pFrom->apCell[i];
    }
  }
}

/*
** The following parameters determine how many adjacent pages get involved
** in a balancing operation.  NN is the number of neighbors on either side
................................................................................
** The number of siblings of pPage might be increased or decreased by
** one in an effort to keep pages between 66% and 100% full. The root page
** is special and is allowed to be less than 66% full. If pPage is 
** the root page, then the depth of the tree might be increased
** or decreased by one, as necessary, to keep the root page from being
** overfull or empty.
**
** This routine calls relinkCellList() on its input page regardless of
** whether or not it does any real balancing.  Client routines will typically
** invoke insertCell() or dropCell() before calling this routine, so we
** need to call relinkCellList() to clean up the mess that those other
** routines left behind.
**
** pCur is left pointing to the same cell as when this routine was called
** even if that cell gets moved to a different page.  pCur may be NULL.
** Set the pCur parameter to NULL if you do not care about keeping track
** of a cell as that will save this routine the work of keeping track of it.
**
** Note that when this routine is called, some of the Cells on pPage
** might not actually be stored in pPage->u.aDisk[].  This can happen
** if the page is overfull.  Part of the job of this routine is to
** make sure all Cells for pPage once again fit in pPage->u.aDisk[].
**
** In the course of balancing the siblings of pPage, the parent of pPage
** might become overfull or underfull.  If that happens, then this routine
** is called recursively on the parent.
**
** If this routine fails for any reason, it might leave the database
** in a corrupted state.  So if this routine fails, the database should
** be rolled back.
*/
static int balance(Btree *pBt, MemPage *pPage, BtCursor *pCur){
  MemPage *pParent;            /* The parent of pPage */

  int nCell;                   /* Number of cells in apCell[] */
  int nOld;                    /* Number of pages in apOld[] */
  int nNew;                    /* Number of pages in apNew[] */
  int nDiv;                    /* Number of cells in apDiv[] */
  int i, j, k;                 /* Loop counters */
  int idx;                     /* Index of pPage in pParent->apCell[] */
  int nxDiv;                   /* Next divider slot in pParent->apCell[] */
  int rc;                      /* The return code */
  int iCur;                    /* apCell[iCur] is the cell of the cursor */
  MemPage *pOldCurPage;        /* The cursor originally points to this page */
  int subtotal;                /* Subtotal of bytes in cells on one page */
  MemPage *extraUnref = 0;     /* A page that needs to be unref-ed */
  MemPage *apOld[NB];          /* pPage and up to two siblings */
  Pgno pgnoOld[NB];            /* Page numbers for each page in apOld[] */

  MemPage *apNew[NB+1];        /* pPage and up to NB siblings after balancing */
  Pgno pgnoNew[NB+1];          /* Page numbers for each page in apNew[] */
  int idxDiv[NB];              /* Indices of divider cells in pParent */
  Cell *apDiv[NB];             /* Divider cells in pParent */
  Cell aTemp[NB];              /* Temporary holding area for apDiv[] */
  int cntNew[NB+1];            /* Index in apCell[] of cell after i-th page */
  int szNew[NB+1];             /* Combined size of cells place on i-th page */
  MemPage aOld[NB];            /* Temporary copies of pPage and its siblings */
  Cell *apCell[(MX_CELL+2)*NB]; /* All cells from pages being balanced */
  int szCell[(MX_CELL+2)*NB];  /* Local size of all cells */


  /* 
  ** Return without doing any work if pPage is neither overfull nor
  ** underfull.
  */
  assert( sqlitepager_iswriteable(pPage) );

  if( !pPage->isOverfull && pPage->nFree<SQLITE_USABLE_SIZE/2 
        && pPage->nCell>=2){
    relinkCellList(pBt, pPage);
    return SQLITE_OK;
  }

  /*
  ** Find the parent of the page to be balanceed.
  ** If there is no parent, it means this page is the root page and
  ** special rules apply.
  */
  pParent = pPage->pParent;
  if( pParent==0 ){
    Pgno pgnoChild;
    MemPage *pChild;
    assert( pPage->isInit );
    if( pPage->nCell==0 ){
................................................................................
        relinkCellList(pPage);
      }else{
        /* The root page is empty but has one child.  Transfer the
        ** information from that one child into the root page if it 
        ** will fit.  This reduces the depth of the BTree by one.
        **
        ** If the root page is page 1, it has less space available than

        ** the child, so it might not be able to hold all of the information

        ** in the child.  If this is the case, then do not do the transfer.
        ** Leave page 1 empty except for the right-pointer to the child page.
        ** The child page becomes the virtual root of the tree.
        */
        pgnoChild = get4byte(pPage->aData[pPage->hdrOffset+6]);
        assert( pgnoChild>0 && pgnoChild<=sqlit3pager_pagecount(pBt->pPager) );
        rc = getPage(pBt, pgnoChild, &pChild);
        if( rc ) return rc;
        if( pPage->pgno==1 ){
          rc = initPage(pChild);
          if( rc ) return rc;
          if( pChild->nFree>=100 ){







          }

        }else{




          memcpy(pPage, pChild, SQLITE_USABLE_SIZE);
          pPage->isInit = 0;
          rc = initPage(pBt, pPage, sqlitepager_pagenumber(pPage), 0);


          assert( rc==SQLITE_OK );
          reparentChildPages(pBt, pPage);
        if( pCur && pCur->pPage==pChild ){
          sqlitepager_unref(pChild);
          pCur->pPage = pPage;
          sqlitepager_ref(pPage);
        }
        freePage(pBt, pChild, pgnoChild);

        sqlitepager_unref(pChild);
      }else{
        relinkCellList(pBt, pPage);
      }
      return SQLITE_OK;
    }
    if( !pPage->isOverfull ){
      /* It is OK for the root page to be less than half full.
      */
      relinkCellList(pBt, pPage);
................................................................................
    ** If we get to here, it means the root page is overfull.
    ** When this happens, Create a new child page and copy the
    ** contents of the root into the child.  Then make the root
    ** page an empty page with rightChild pointing to the new
    ** child.  Then fall thru to the code below which will cause
    ** the overfull child page to be split.
    */
    rc = sqlitepager_write(pPage);
    if( rc ) return rc;
    rc = allocatePage(pBt, &pChild, &pgnoChild, sqlitepager_pagenumber(pPage));
    if( rc ) return rc;
    assert( sqlitepager_iswriteable(pChild) );
    copyPage(pChild, pPage);
    pChild->pParent = pPage;
    pChild->idxParent = 0;
    sqlitepager_ref(pPage);
    pChild->isOverfull = 1;
    if( pCur && pCur->pPage==pPage ){
      sqlitepager_unref(pPage);
      pCur->pPage = pChild;
    }else{
      extraUnref = pChild;
    }
    zeroPage(pBt, pPage);
    pPage->u.hdr.rightChild = SWAB32(pBt, pgnoChild);
    pParent = pPage;
    pPage = pChild;
  }
  rc = sqlitepager_write(pParent);
  if( rc ) return rc;
  assert( pParent->isInit );
  
  /*
  ** Find the Cell in the parent page whose h.leftChild points back
  ** to pPage.  The "idx" variable is the index of that cell.  If pPage
  ** is the rightmost child of pParent then set idx to pParent->nCell 
  */
  if( pParent->idxShift ){
    Pgno pgno, swabPgno;

    pgno = sqlitepager_pagenumber(pPage);
    swabPgno = SWAB32(pBt, pgno);
    for(idx=0; idx<pParent->nCell; idx++){
      if( pParent->apCell[idx]->h.leftChild==swabPgno ){
        break;
      }
    }
    assert( idx<pParent->nCell || pParent->u.hdr.rightChild==swabPgno );

  }else{
    idx = pPage->idxParent;
  }

  /*
  ** Initialize variables so that it will be safe to jump
  ** directly to balance_cleanup at any moment.
  */
  nOld = nNew = 0;
  sqlitepager_ref(pParent);

  /*
  ** Find sibling pages to pPage and the Cells in pParent that divide
  ** the siblings.  An attempt is made to find NN siblings on either
  ** side of pPage.  More siblings are taken from one side, however, if
  ** pPage there are fewer than NN siblings on the other side.  If pParent
  ** has NB or fewer children then all children of pParent are taken.
  */
  nxDiv = idx - NN;
  if( nxDiv + NB > pParent->nCell ){
................................................................................
  if( nxDiv<0 ){
    nxDiv = 0;
  }
  nDiv = 0;
  for(i=0, k=nxDiv; i<NB; i++, k++){
    if( k<pParent->nCell ){
      idxDiv[i] = k;
      apDiv[i] = pParent->apCell[k];
      nDiv++;
      pgnoOld[i] = SWAB32(pBt, apDiv[i]->h.leftChild);

    }else if( k==pParent->nCell ){
      pgnoOld[i] = SWAB32(pBt, pParent->u.hdr.rightChild);

    }else{
      break;
    }
    rc = sqlitepager_get(pBt->pPager, pgnoOld[i], (void**)&apOld[i]);
    if( rc ) goto balance_cleanup;
    rc = initPage(pBt, apOld[i], pgnoOld[i], pParent);
    if( rc ) goto balance_cleanup;
    apOld[i]->idxParent = k;
    nOld++;
  }

  /*
  ** Set iCur to be the index in apCell[] of the cell that the cursor
  ** is pointing to.  We will need this later on in order to keep the
  ** cursor pointing at the same cell.  If pCur points to a page that
  ** has no involvement with this rebalancing, then set iCur to a large
  ** number so that the iCur==j tests always fail in the main cell
  ** distribution loop below.
  */
  if( pCur ){
    iCur = 0;
    for(i=0; i<nOld; i++){
      if( pCur->pPage==apOld[i] ){
        iCur += pCur->idx;
        break;
      }
      iCur += apOld[i]->nCell;
      if( i<nOld-1 && pCur->pPage==pParent && pCur->idx==idxDiv[i] ){
        break;
      }
      iCur++;
    }
    pOldCurPage = pCur->pPage;
  }

  /*
  ** Make copies of the content of pPage and its siblings into aOld[].
  ** The rest of this function will use data from the copies rather
  ** that the original pages since the original pages will be in the
  ** process of being overwritten.
  */
  for(i=0; i<nOld; i++){



    copyPage(&aOld[i], apOld[i]);
  }

  /*
  ** Load pointers to all cells on sibling pages and the divider cells
  ** into the local apCell[] array.  Make copies of the divider cells
  ** into aTemp[] and remove the the divider Cells from pParent.







  */
  nCell = 0;

  for(i=0; i<nOld; i++){
    MemPage *pOld = &aOld[i];
    for(j=0; j<pOld->nCell; j++){
      apCell[nCell] = pOld->apCell[j];
      szCell[nCell] = cellSize(pBt, apCell[nCell]);
      nCell++;
    }
    if( i<nOld-1 ){
      szCell[nCell] = cellSize(pBt, apDiv[i]);
      memcpy(&aTemp[i], apDiv[i], szCell[nCell]);
      apCell[nCell] = &aTemp[i];
      dropCell(pBt, pParent, nxDiv, szCell[nCell]);
      assert( SWAB32(pBt, apCell[nCell]->h.leftChild)==pgnoOld[i] );
      apCell[nCell]->h.leftChild = pOld->u.hdr.rightChild;







      nCell++;
    }
  }

  /*
  ** Figure out the number of pages needed to hold all nCell cells.
  ** Store this number in "k".  Also compute szNew[] which is the total
  ** size of all cells on the i-th page and cntNew[] which is the index
  ** in apCell[] of the cell that divides path i from path i+1.  
  ** cntNew[k] should equal nCell.
  **
  ** This little patch of code is critical for keeping the tree
  ** balanced. 
  */

  for(subtotal=k=i=0; i<nCell; i++){
    subtotal += szCell[i];
    if( subtotal > USABLE_SPACE ){
      szNew[k] = subtotal - szCell[i];
      cntNew[k] = i;
      subtotal = 0;
      k++;
    }
  }
  szNew[k] = subtotal;
  cntNew[k] = nCell;
  k++;
  for(i=k-1; i>0; i--){
    while( szNew[i]<USABLE_SPACE/2 ){
      cntNew[i-1]--;
      assert( cntNew[i-1]>0 );
      szNew[i] += szCell[cntNew[i-1]];
      szNew[i-1] -= szCell[cntNew[i-1]-1];
    }
  }
  assert( cntNew[0]>0 );

  /*
  ** Allocate k new pages.  Reuse old pages where possible.
  */


  for(i=0; i<k; i++){
    if( i<nOld ){
      apNew[i] = apOld[i];
      pgnoNew[i] = pgnoOld[i];
      apOld[i] = 0;
      sqlitepager_write(apNew[i]);
    }else{
      rc = allocatePage(pBt, &apNew[i], &pgnoNew[i], pgnoNew[i-1]);
      if( rc ) goto balance_cleanup;
    }
    nNew++;
    zeroPage(pBt, apNew[i]);
    apNew[i]->isInit = 1;
  }

  /* Free any old pages that were not reused as new pages.
  */
  while( i<nOld ){
    rc = freePage(pBt, apOld[i], pgnoOld[i]);
    if( rc ) goto balance_cleanup;
    sqlitepager_unref(apOld[i]);
    apOld[i] = 0;
    i++;
  }

  /*
  ** Put the new pages in accending order.  This helps to
  ** keep entries in the disk file in order so that a scan
................................................................................
  /*
  ** Evenly distribute the data in apCell[] across the new pages.
  ** Insert divider cells into pParent as necessary.
  */
  j = 0;
  for(i=0; i<nNew; i++){
    MemPage *pNew = apNew[i];


    while( j<cntNew[i] ){
      assert( pNew->nFree>=szCell[j] );
      if( pCur && iCur==j ){ pCur->pPage = pNew; pCur->idx = pNew->nCell; }
      insertCell(pBt, pNew, pNew->nCell, apCell[j], szCell[j]);
      j++;
    }
    assert( pNew->nCell>0 );
    assert( !pNew->isOverfull );
    relinkCellList(pBt, pNew);
    if( i<nNew-1 && j<nCell ){
      pNew->u.hdr.rightChild = apCell[j]->h.leftChild;
      apCell[j]->h.leftChild = SWAB32(pBt, pgnoNew[i]);
      if( pCur && iCur==j ){ pCur->pPage = pParent; pCur->idx = nxDiv; }
      insertCell(pBt, pParent, nxDiv, apCell[j], szCell[j]);




      j++;
      nxDiv++;
    }
  }
  assert( j==nCell );
  apNew[nNew-1]->u.hdr.rightChild = aOld[nOld-1].u.hdr.rightChild;
  if( nxDiv==pParent->nCell ){
    pParent->u.hdr.rightChild = SWAB32(pBt, pgnoNew[nNew-1]);
  }else{
    pParent->apCell[nxDiv]->h.leftChild = SWAB32(pBt, pgnoNew[nNew-1]);
  }
  if( pCur ){
    if( j<=iCur && pCur->pPage==pParent && pCur->idx>idxDiv[nOld-1] ){
      assert( pCur->pPage==pOldCurPage );
      pCur->idx += nNew - nOld;
    }else{
      assert( pOldCurPage!=0 );
      sqlitepager_ref(pCur->pPage);
      sqlitepager_unref(pOldCurPage);
    }
  }

  /*
  ** Reparent children of all cells.
  */
  for(i=0; i<nNew; i++){
    reparentChildPages(pBt, apNew[i]);
  }
  reparentChildPages(pBt, pParent);

  /*
  ** balance the parent page.
  */
  rc = balance(pBt, pParent, pCur);

  /*
  ** Cleanup before returning.
  */
balance_cleanup:
  if( extraUnref ){
    sqlitepager_unref(extraUnref);
  }
  for(i=0; i<nOld; i++){
    if( apOld[i]!=0 && apOld[i]!=&aOld[i] ) sqlitepager_unref(apOld[i]);
  }
  for(i=0; i<nNew; i++){
    sqlitepager_unref(apNew[i]);
  }
  if( pCur && pCur->pPage==0 ){
    pCur->pPage = pParent;
    pCur->idx = 0;
  }else{
    sqlitepager_unref(pParent);
  }
  return rc;
}

/*
** This routine checks all cursors that point to the same table
** as pCur points to.  If any of those cursors were opened with
** wrFlag==0 then this routine returns SQLITE_LOCKED.  If all
................................................................................
  return SQLITE_OK;
}

/*
** Insert a new record into the BTree.  The key is given by (pKey,nKey)
** and the data is given by (pData,nData).  The cursor is used only to
** define what database the record should be inserted into.  The cursor
** is left pointing at the new record.



*/
int sqlite3BtreeInsert(
  BtCursor *pCur,                /* Insert data into the table of this cursor */
  const void *pKey, int nKey,    /* The key of the new record */
  const void *pData, int nData   /* The data of the new record */
){
  Cell newCell;
  int rc;
  int loc;
  int szNew;
  MemPage *pPage;
  Btree *pBt = pCur->pBt;


  if( pCur->pPage==0 ){
    return SQLITE_ABORT;  /* A rollback destroyed this cursor */
  }
  if( !pBt->inTrans || nKey+nData==0 ){
    /* Must start a transaction before doing an insert */
    return pBt->readOnly ? SQLITE_READONLY : SQLITE_ERROR;
................................................................................
  }
  if( checkReadLocks(pCur) ){
    return SQLITE_LOCKED; /* The table pCur points to has a read lock */
  }
  rc = sqlite3BtreeMoveto(pCur, pKey, nKey, &loc);
  if( rc ) return rc;
  pPage = pCur->pPage;

  assert( pPage->isInit );
  rc = sqlitepager_write(pPage);
  if( rc ) return rc;
  rc = fillInCell(pBt, &newCell, pKey, nKey, pData, nData);
  if( rc ) return rc;
  szNew = cellSize(pBt, &newCell);
  if( loc==0 ){
    newCell.h.leftChild = pPage->apCell[pCur->idx]->h.leftChild;






    rc = clearCell(pBt, pPage->apCell[pCur->idx]);
    if( rc ) return rc;
    dropCell(pBt, pPage, pCur->idx, cellSize(pBt, pPage->apCell[pCur->idx]));

  }else if( loc<0 && pPage->nCell>0 ){
    assert( pPage->u.hdr.rightChild==0 );  /* Must be a leaf page */

    pCur->idx++;
  }else{
    assert( pPage->u.hdr.rightChild==0 );  /* Must be a leaf page */

  }
  insertCell(pBt, pPage, pCur->idx, &newCell, szNew);
  rc = balance(pCur->pBt, pPage, pCur);
  /* sqliteBtreePageDump(pCur->pBt, pCur->pgnoRoot, 1); */
  /* fflush(stdout); */

  pCur->eSkip = SKIP_INVALID;
  return rc;
}

/*
** Delete the entry that the cursor is pointing to.
**
** The cursor is left pointing at either the next or the previous
** entry.  If the cursor is left pointing to the next entry, then 
** the pCur->eSkip flag is set to SKIP_NEXT which forces the next call to 
** sqliteBtreeNext() to be a no-op.  That way, you can always call
** sqliteBtreeNext() after a delete and the cursor will be left
** pointing to the first entry after the deleted entry.  Similarly,
** pCur->eSkip is set to SKIP_PREV is the cursor is left pointing to
** the entry prior to the deleted entry so that a subsequent call to
** sqliteBtreePrevious() will always leave the cursor pointing at the
** entry immediately before the one that was deleted.
*/
int sqlite3BtreeDelete(BtCursor *pCur){
  MemPage *pPage = pCur->pPage;
  Cell *pCell;
  int rc;
  Pgno pgnoChild;
  Btree *pBt = pCur->pBt;

  assert( pPage->isInit );
  if( pCur->pPage==0 ){
    return SQLITE_ABORT;  /* A rollback destroyed this cursor */
................................................................................
    return SQLITE_PERM;   /* Did not open this cursor for writing */
  }
  if( checkReadLocks(pCur) ){
    return SQLITE_LOCKED; /* The table pCur points to has a read lock */
  }
  rc = sqlitepager_write(pPage);
  if( rc ) return rc;
  pCell = pPage->apCell[pCur->idx];
  pgnoChild = SWAB32(pBt, pCell->h.leftChild);


  clearCell(pBt, pCell);
  if( pgnoChild ){

    /*
    ** The entry we are about to delete is not a leaf so if we do not
    ** do something we will leave a hole on an internal page.
    ** We have to fill the hole by moving in a cell from a leaf.  The
    ** next Cell after the one to be deleted is guaranteed to exist and
    ** to be a leaf so we can use it.
    */
    BtCursor leafCur;
    Cell *pNext;
    int szNext;
    int notUsed;
    getTempCursor(pCur, &leafCur);
    rc = sqlite3BtreeNext(&leafCur, &notUsed);
    if( rc!=SQLITE_OK ){
      if( rc!=SQLITE_NOMEM ) rc = SQLITE_CORRUPT;
      return rc;
    }
    rc = sqlitepager_write(leafCur.pPage);
    if( rc ) return rc;
    dropCell(pBt, pPage, pCur->idx, cellSize(pBt, pCell));
    pNext = leafCur.pPage->apCell[leafCur.idx];
    szNext = cellSize(pBt, pNext);
    pNext->h.leftChild = SWAB32(pBt, pgnoChild);
    insertCell(pBt, pPage, pCur->idx, pNext, szNext);
    rc = balance(pBt, pPage, pCur);
    if( rc ) return rc;
    pCur->eSkip = SKIP_NEXT;
    dropCell(pBt, leafCur.pPage, leafCur.idx, szNext);
    rc = balance(pBt, leafCur.pPage, pCur);
    releaseTempCursor(&leafCur);
  }else{
    dropCell(pBt, pPage, pCur->idx, cellSize(pBt, pCell));
    if( pCur->idx>=pPage->nCell ){
      pCur->idx = pPage->nCell-1;
      if( pCur->idx<0 ){ 
        pCur->idx = 0;
        pCur->eSkip = SKIP_NEXT;
      }else{
        pCur->eSkip = SKIP_PREV;
      }
    }else{
      pCur->eSkip = SKIP_NEXT;
    }
    rc = balance(pBt, pPage, pCur);
  }

  return rc;
}

/*
** Create a new BTree table.  Write into *piTable the page
** number for the root page of the new table.
**
................................................................................
    return pBt->readOnly ? SQLITE_READONLY : SQLITE_ERROR;
  }
  if( pBt->readOnly ){
    return SQLITE_READONLY;
  }
  rc = allocatePage(pBt, &pRoot, &pgnoRoot, 0);
  if( rc ) return rc;
  assert( sqlitepager_iswriteable(pRoot) );
  zeroPage(pBt, pRoot);
  sqlitepager_unref(pRoot);
  *piTable = (int)pgnoRoot;
  return SQLITE_OK;
}

/*
** Erase the given database page and all its children.  Return
** the page to the freelist.
*/
static int clearDatabasePage(Btree *pBt, Pgno pgno, int freePageFlag){





  MemPage *pPage;
  int rc;
  Cell *pCell;
  int idx;

  rc = sqlitepager_get(pBt->pPager, pgno, (void**)&pPage);

  if( rc ) return rc;
  rc = sqlitepager_write(pPage);
  if( rc ) return rc;
  rc = initPage(pBt, pPage, pgno, 0);
  if( rc ) return rc;
  idx = SWAB16(pBt, pPage->u.hdr.firstCell);
  while( idx>0 ){
    pCell = (Cell*)&pPage->u.aDisk[idx];
    idx = SWAB16(pBt, pCell->h.iNext);
    if( pCell->h.leftChild ){



      rc = clearDatabasePage(pBt, SWAB32(pBt, pCell->h.leftChild), 1);
      if( rc ) return rc;
    }
    rc = clearCell(pBt, pCell);
    if( rc ) return rc;
  }
  if( pPage->u.hdr.rightChild ){

    rc = clearDatabasePage(pBt, SWAB32(pBt, pPage->u.hdr.rightChild), 1);
    if( rc ) return rc;
  }
  if( freePageFlag ){
    rc = freePage(pBt, pPage, pgno);
  }else{
    zeroPage(pBt, pPage);
  }
  sqlitepager_unref(pPage);
  return rc;
}

/*
** Delete all information from a single table in the database.
*/
int sqlite3BtreeClearTable(Btree *pBt, int iTable){
................................................................................
    return pBt->readOnly ? SQLITE_READONLY : SQLITE_ERROR;
  }
  for(pCur=pBt->pCursor; pCur; pCur=pCur->pNext){
    if( pCur->pgnoRoot==(Pgno)iTable ){
      return SQLITE_LOCKED;  /* Cannot drop a table that has a cursor */
    }
  }
  rc = sqlitepager_get(pBt->pPager, (Pgno)iTable, (void**)&pPage);
  if( rc ) return rc;
  rc = sqlite3BtreeClearTable(pBt, iTable);
  if( rc ) return rc;
  if( iTable>2 ){
    rc = freePage(pBt, pPage, iTable);
  }else{
    zeroPage(pBt, pPage);
  }
  sqlitepager_unref(pPage);
  return rc;  
}

#if 0 /* UNTESTED */
/*
** Copy all cell data from one database file into another.
** pages back the freelist.
*/
static int copyCell(Btree *pBtFrom, BTree *pBtTo, Cell *pCell){
  Pager *pFromPager = pBtFrom->pPager;
  OverflowPage *pOvfl;
  Pgno ovfl, nextOvfl;
  Pgno *pPrev;
  int rc = SQLITE_OK;
  MemPage *pNew, *pPrevPg;
  Pgno new;

  if( NKEY(pBtTo, pCell->h) + NDATA(pBtTo, pCell->h) <= MX_LOCAL_PAYLOAD ){
    return SQLITE_OK;
  }
  pPrev = &pCell->ovfl;
  pPrevPg = 0;
  ovfl = SWAB32(pBtTo, pCell->ovfl);
  while( ovfl && rc==SQLITE_OK ){
    rc = sqlitepager_get(pFromPager, ovfl, (void**)&pOvfl);
    if( rc ) return rc;
    nextOvfl = SWAB32(pBtFrom, pOvfl->iNext);
    rc = allocatePage(pBtTo, &pNew, &new, 0);
    if( rc==SQLITE_OK ){
      rc = sqlitepager_write(pNew);
      if( rc==SQLITE_OK ){
        memcpy(pNew, pOvfl, SQLITE_USABLE_SIZE);
        *pPrev = SWAB32(pBtTo, new);
        if( pPrevPg ){
          sqlitepager_unref(pPrevPg);
        }
        pPrev = &pOvfl->iNext;
        pPrevPg = pNew;
      }
    }
    sqlitepager_unref(pOvfl);
    ovfl = nextOvfl;
  }
  if( pPrevPg ){
    sqlitepager_unref(pPrevPg);
  }
  return rc;
}
#endif


#if 0 /* UNTESTED */
/*
** Copy a page of data from one database over to another.
*/
static int copyDatabasePage(
  Btree *pBtFrom,
  Pgno pgnoFrom,
  Btree *pBtTo,
  Pgno *pTo
){
  MemPage *pPageFrom, *pPage;
  Pgno to;
  int rc;
  Cell *pCell;
  int idx;

  rc = sqlitepager_get(pBtFrom->pPager, pgno, (void**)&pPageFrom);
  if( rc ) return rc;
  rc = allocatePage(pBt, &pPage, pTo, 0);
  if( rc==SQLITE_OK ){
    rc = sqlitepager_write(pPage);
  }
  if( rc==SQLITE_OK ){
    memcpy(pPage, pPageFrom, SQLITE_USABLE_SIZE);
    idx = SWAB16(pBt, pPage->u.hdr.firstCell);
    while( idx>0 ){
      pCell = (Cell*)&pPage->u.aDisk[idx];
      idx = SWAB16(pBt, pCell->h.iNext);
      if( pCell->h.leftChild ){
        Pgno newChld;
        rc = copyDatabasePage(pBtFrom, SWAB32(pBtFrom, pCell->h.leftChild),
                              pBtTo, &newChld);
        if( rc ) return rc;
        pCell->h.leftChild = SWAB32(pBtFrom, newChld);
      }
      rc = copyCell(pBtFrom, pBtTo, pCell);
      if( rc ) return rc;
    }
    if( pPage->u.hdr.rightChild ){
      Pgno newChld;
      rc = copyDatabasePage(pBtFrom, SWAB32(pBtFrom, pPage->u.hdr.rightChild), 
                            pBtTo, &newChld);
      if( rc ) return rc;
      pPage->u.hdr.rightChild = SWAB32(pBtTo, newChild);
    }
  }
  sqlitepager_unref(pPage);
  return rc;
}
#endif

/*
** Read the meta-information out of a database file.
*/
int sqlite3BtreeGetMeta(Btree *pBt, int idx, u32 *pMeta){
  int rc;
  int i;



  rc = sqlitepager_get(pBt->pPager, 1, (void**)&pP1);
  if( rc ) return rc;
  aMeta[0] = SWAB32(pBt, pP1->nFree);
  for(i=0; i<sizeof(pP1->aMeta)/sizeof(pP1->aMeta[0]); i++){
    aMeta[i+1] = SWAB32(pBt, pP1->aMeta[i]);
  }

  sqlitepager_unref(pP1);
  return SQLITE_OK;
}

/*
** Write meta-information back into the database.
*/
int sqlite3BtreeUpdateMeta(Btree *pBt, int idx, u32 iMeta){
  PageOne *pP1;
  int rc, i;

  if( !pBt->inTrans ){
    return pBt->readOnly ? SQLITE_READONLY : SQLITE_ERROR;
  }
  pP1 = pBt->page1;

  rc = sqlitepager_write(pP1);
  if( rc ) return rc;   
  for(i=0; i<sizeof(pP1->aMeta)/sizeof(pP1->aMeta[0]); i++){
    pP1->aMeta[i] = SWAB32(pBt, aMeta[i+1]);
  }
  return SQLITE_OK;
}

/******************************************************************************
** The complete implementation of the BTree subsystem is above this line.
** All the code the follows is for testing and troubleshooting the BTree
** subsystem.  None of the code that follows is used during normal operation.
................................................................................
#ifdef SQLITE_TEST
static int fileBtreePageDump(Btree *pBt, int pgno, int recursive){
  int rc;
  MemPage *pPage;
  int i, j;
  int nFree;
  u16 idx;

  char range[20];
  unsigned char payload[20];
  rc = sqlitepager_get(pBt->pPager, (Pgno)pgno, (void**)&pPage);

  if( rc ){
    return rc;
  }
  if( recursive ) printf("PAGE %d:\n", pgno);

  i = 0;
  idx = SWAB16(pBt, pPage->u.hdr.firstCell);


  while( idx>0 && idx<=SQLITE_USABLE_SIZE-MIN_CELL_SIZE ){
    Cell *pCell = (Cell*)&pPage->u.aDisk[idx];




    int sz = cellSize(pBt, pCell);
    sprintf(range,"%d..%d", idx, idx+sz-1);






    sz = NKEY(pBt, pCell->h) + NDATA(pBt, pCell->h);
    if( sz>sizeof(payload)-1 ) sz = sizeof(payload)-1;
    memcpy(payload, pCell->aPayload, sz);
    for(j=0; j<sz; j++){
      if( payload[j]<0x20 || payload[j]>0x7f ) payload[j] = '.';
    }
    payload[sz] = 0;
    printf(
      "cell %2d: i=%-10s chld=%-4d nk=%-4d nd=%-4d payload=%s\n",
      i, range, (int)pCell->h.leftChild, 
      NKEY(pBt, pCell->h), NDATA(pBt, pCell->h),
      payload
    );
    if( pPage->isInit && pPage->apCell[i]!=pCell ){
      printf("**** apCell[%d] does not match on prior entry ****\n", i);
    }
    i++;
    idx = SWAB16(pBt, pCell->h.iNext);

  }
  if( idx!=0 ){
    printf("ERROR: next cell index out of range: %d\n", idx);
  }
  printf("right_child: %d\n", SWAB32(pBt, pPage->u.hdr.rightChild));
  nFree = 0;
  i = 0;
  idx = SWAB16(pBt, pPage->u.hdr.firstFree);
  while( idx>0 && idx<SQLITE_USABLE_SIZE ){
    FreeBlk *p = (FreeBlk*)&pPage->u.aDisk[idx];
    sprintf(range,"%d..%d", idx, idx+p->iSize-1);
    nFree += SWAB16(pBt, p->iSize);
    printf("freeblock %2d: i=%-10s size=%-4d total=%d\n",
       i, range, SWAB16(pBt, p->iSize), nFree);
    idx = SWAB16(pBt, p->iNext);


    i++;
  }
  if( idx!=0 ){
    printf("ERROR: next freeblock index out of range: %d\n", idx);
  }
  if( recursive && pPage->u.hdr.rightChild!=0 ){
    idx = SWAB16(pBt, pPage->u.hdr.firstCell);
    while( idx>0 && idx<SQLITE_USABLE_SIZE-MIN_CELL_SIZE ){
      Cell *pCell = (Cell*)&pPage->u.aDisk[idx];
      fileBtreePageDump(pBt, SWAB32(pBt, pCell->h.leftChild), 1);
      idx = SWAB16(pBt, pCell->h.iNext);
    }
    fileBtreePageDump(pBt, SWAB32(pBt, pPage->u.hdr.rightChild), 1);
  }
  sqlitepager_unref(pPage);
  return SQLITE_OK;
}
#endif

#ifdef SQLITE_TEST
................................................................................
**
** This routine is used for testing and debugging only.
*/
static int fileBtreeCursorDump(BtCursor *pCur, int *aResult){
  int cnt, idx;
  MemPage *pPage = pCur->pPage;
  Btree *pBt = pCur->pBt;

  aResult[0] = sqlitepager_pagenumber(pPage);
  aResult[1] = pCur->idx;
  aResult[2] = pPage->nCell;
  if( pCur->idx>=0 && pCur->idx<pPage->nCell ){
    aResult[3] = cellSize(pBt, pPage->apCell[pCur->idx]);
    aResult[6] = SWAB32(pBt, pPage->apCell[pCur->idx]->h.leftChild);
  }else{
    aResult[3] = 0;
    aResult[6] = 0;
  }
  aResult[4] = pPage->nFree;
  cnt = 0;
  idx = SWAB16(pBt, pPage->u.hdr.firstFree);
  while( idx>0 && idx<SQLITE_USABLE_SIZE ){
    cnt++;
    idx = SWAB16(pBt, ((FreeBlk*)&pPage->u.aDisk[idx])->iNext);
  }
  aResult[5] = cnt;
  aResult[7] = SWAB32(pBt, pPage->u.hdr.rightChild);
  return SQLITE_OK;
}
#endif

/*
** Return the pager associated with a BTree.  This routine is used for
** testing and debugging only.
................................................................................
  int iPage,            /* Page number for first page in the list */
  int N,                /* Expected number of pages in the list */
  char *zContext        /* Context for error messages */
){
  int i;
  char zMsg[100];
  while( N-- > 0 ){
    OverflowPage *pOvfl;
    if( iPage<1 ){
      sprintf(zMsg, "%d pages missing from overflow list", N+1);
      checkAppendMsg(pCheck, zContext, zMsg);
      break;
    }
    if( checkRef(pCheck, iPage, zContext) ) break;
    if( sqlitepager_get(pCheck->pPager, (Pgno)iPage, (void**)&pOvfl) ){
      sprintf(zMsg, "failed to get page %d", iPage);
      checkAppendMsg(pCheck, zContext, zMsg);
      break;
    }
    if( isFreeList ){
      FreelistInfo *pInfo = (FreelistInfo*)pOvfl->aPayload;
      int n = SWAB32(pCheck->pBt, pInfo->nFree);
      for(i=0; i<n; i++){
        checkRef(pCheck, SWAB32(pCheck->pBt, pInfo->aFree[i]), zContext);

      }
      N -= n;
    }
    iPage = SWAB32(pCheck->pBt, pOvfl->iNext);
    sqlitepager_unref(pOvfl);
  }
}

/*
** Return negative if zKey1<zKey2.
** Return zero if zKey1==zKey2.
................................................................................

  /* Check that the page exists
  */
  cur.pBt = pBt = pCheck->pBt;
  if( iPage==0 ) return 0;
  if( checkRef(pCheck, iPage, zParentContext) ) return 0;
  sprintf(zContext, "On tree page %d: ", iPage);
  if( (rc = sqlitepager_get(pCheck->pPager, (Pgno)iPage, (void**)&pPage))!=0 ){
    sprintf(zMsg, "unable to get the page. error code=%d", rc);
    checkAppendMsg(pCheck, zContext, zMsg);
    return 0;
  }
  if( (rc = initPage(pBt, pPage, (Pgno)iPage, pParent))!=0 ){
    sprintf(zMsg, "initPage() returns error code %d", rc);
    checkAppendMsg(pCheck, zContext, zMsg);
    sqlitepager_unref(pPage);
    return 0;
  }



  /* Check out all the cells.
  */
  depth = 0;
  if( zLowerBound ){
    zKey1 = sqliteMalloc( nLower+1 );
    memcpy(zKey1, zLowerBound, nLower);
................................................................................
    }else if( hit[i]>1 ){
      sprintf(zMsg, "Multiple uses for byte %d of page %d", i, iPage);
      checkAppendMsg(pCheck, zMsg, 0);
      break;
    }
  }

  /* Check that free space is kept to a minimum
  */
#if 0
  if( pParent && pParent->nCell>2 && pPage->nFree>3*SQLITE_USABLE_SIZE/4 ){
    sprintf(zMsg, "free space (%d) greater than max (%d)", pPage->nFree,
       SQLITE_USABLE_SIZE/3);
    checkAppendMsg(pCheck, zContext, zMsg);
  }
#endif

  sqlitepager_unref(pPage);
  return depth;
}

/*
** This routine does a complete check of the given BTree file.  aRoot[] is
** an array of pages numbers were each page number is the root page of
** a table.  nRoot is the number of entries in aRoot.

** 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.
**
*************************************************************************
** $Id: btree.c,v 1.107 2004/05/02 21:12:19 drh Exp $
**
** This file implements a external (disk-based) database using BTrees.
** For a detailed discussion of BTrees, refer to
**
**     Donald E. Knuth, THE ART OF COMPUTER PROGRAMMING, Volume 3:
**     "Sorting And Searching", pages 473-480. Addison-Wesley
**     Publishing Company, Reading, Massachusetts.
................................................................................
**      *     zero or more pages numbers of leaves
*/
#include "sqliteInt.h"
#include "pager.h"
#include "btree.h"
#include <assert.h>


/* Maximum page size.  The upper bound on this value is 65536 (a limit
** imposed by the 2-byte offset at the beginning of each cell.)  The
** maximum page size determines the amount of stack space allocated
** by many of the routines in this module.  On embedded architectures
** or any machine where memory and especially stack memory is limited,
** one may wish to chose a smaller value for the maximum page size.
*/
#ifndef MX_PAGE_SIZE
# define MX_PAGE_SIZE 1024
#endif

/* Individual entries or "cells" are limited in size so that at least
** this many cells will fit on one page.  Changing this value will result
** in an incompatible database.
*/
#define MN_CELLS_PER_PAGE 4

/* The following value is the maximum cell size assuming a maximum page
** size give above.
*/
#define MX_CELL_SIZE  ((MX_PAGE_SIZE-10)/MN_CELLS_PER_PAGE)

/* The maximum number of cells on a single page of the database.  This
** assumes a minimum cell size of 3 bytes.  Such small cells will be
** exceedingly rare, but they are possible.
*/
#define MX_CELL ((MX_PAGE_SIZE-10)/3)

/* Forward declarations */
typedef struct MemPage MemPage;

/*
** This is a magic string that appears at the beginning of every
** SQLite database in order to identify the file as a real database.
**                                  0123456789 123456 */
static const char zMagicHeader[] = "SQLite version 3";

/*
** Page type flags.  An ORed combination of these flags appear as the
** first byte of every BTree page.
*/
#define PTF_LEAF      0x01
#define PTF_ZERODATA  0x02
#define PTF_INTKEY    0x04
/* Idea for the future:  PTF_LEAFDATA */

/*
** As each page of the file is loaded into memory, an instance of the following
** structure is appended and initialized to zero.  This structure stores
** information about the page that is decoded from the raw file page.

**
** The pParent field points back to the parent page.  This allows us to
** walk up the BTree from any leaf to the root.  Care must be taken to
** unref() the parent page pointer when this page is no longer referenced.
** The pageDestructor() routine handles that chore.
*/
struct MemPage {
................................................................................
  u8 zeroData;                   /* True if zero data flag is set */
  u8 hdrOffset;                  /* 100 for page 1.  0 otherwise */
  Pgno pgno;                     /* Page number for this page */
  MemPage *pParent;              /* The parent of this page.  NULL for root */
  int idxParent;                 /* Index in pParent->aCell[] of this node */
  int nFree;                     /* Number of free bytes on the page */
  int nCell;                     /* Number of entries on this page */
  int nCellAlloc;                /* Number of slots allocated in aCell[] */
  unsigned char **aCell;         /* Pointer to start of each cell */
};

/*
** The in-memory image of a disk page has the auxiliary information appended
** to the end.  EXTRA_SIZE is the number of bytes of space needed to hold
** that extra information.
................................................................................
  int size;
  unsigned char *data;
#ifndef NDEBUG
  int cnt = 0;
#endif

  data = pPage->aData;
  assert( sqlitepager_iswriteable(data->aData) );
  assert( pPage->pBt );
  if( nByte<4 ) nByte = 4;
  if( pPage->nFree<nByte || pPage->isOverfull ) return 0;
  hdr = pPage->hdrOffset;
  if( data[hdr+5]>=252 ){
    defragmentPage(pPage);
  }
................................................................................
  int addr, pbegin, pend;
#ifndef NDEBUG
  int tsize = 0;          /* Total size of all freeblocks */
#endif
  unsigned char *data = pPage->aData;

  assert( pPage->pBt!=0 );
  assert( sqlitepager_iswriteable(data->aData) );
  assert( start>=pPage->hdrOffset+6+(pPage->leaf?0:4) );
  assert( end<=pPage->pBt->pageSize );
  if( size<4 ) size = 4;

  /* Add the space back into the linked list of freeblocks */
  addr = pPage->hdrOffset + 1;
  while( (pbegin = get2byte(&data[addr]))<start && pbegin>0 ){
................................................................................

#if 0
/*
** The following is the default comparison function for (non-integer)
** keys in the btrees.  This function returns negative, zero, or
** positive if the first key is less than, equal to, or greater than
** the second.
**
** The key consists of multiple fields.  Each field begins with a variable
** length integer which determines the field type and the number of bytes
** of key data to follow for that field.
**
**   initial varint     bytes to follow    type
**   --------------     ---------------    ---------------
**      0                     0            NULL
**      1                     1            signed integer
**      2                     2            signed integer
**      3                     4            signed integer
**      4                     8            signed integer
**      5                     8            IEEE float
**     6..12                               reserved for expansion
**    N>=12 and even       (N-12)/2        BLOB
**    N>=13 and odd        (N-13)/2        text
**
** For a particular database, text is always either UTF-8, UTF-16BE, or
** UTF-16LE.  Which of these three formats to use is determined by one
** of the meta values in the file header.
**
*/
static int keyComp(
  void *userData,
  int nKey1, const unsigned char *aKey1, 
  int nKey2, const unsigned char *aKey2,
){
................................................................................
  }
  return 0;

bad_key:
  return 1;
}
#endif

/*
** Resize the aCell[] array of the given page so that it is able to
** hold at least nNewSz entries.
**
** Return SQLITE_OK or SQLITE_NOMEM.
*/
static int resizeCellArray(MemPage *pPage, int nNewSz){
  if( pPage->nCellAlloc<nNewSize ){
    pPage->aCell = sqliteRealloc(pPage->aCell, nNewSz*sizeof(pPage->aCell[0]) );
    if( sqlite_malloc_failed ) return SQLITE_NOMEM;
    pPage->nCellAlloc = nNewSize;
  }
  return SQLITE_OK;
}

/*
** Initialize the auxiliary information for a disk block.
**
** The pParent parameter must be a pointer to the MemPage which
** is the parent of the page being initialized.  The root of a
** BTree has no parent and so for that page, pParent==NULL.
................................................................................
  MemPage *pPage,        /* The page to be initialized */
  MemPage *pParent       /* The parent.  Might be NULL */
){
  int c, pc, i, hdr;
  int sumCell = 0;       /* Total size of all cells */

  assert( pPage->pBt!=0 );
  assert( pParent==0 || pParent->pBt==pPage->pBt );
  assert( pPage->pgno==sqlitepager_pagenumber(pPage->aData) );
  assert( pPage->aData == &((unsigned char*)pPage)[pPage->pBt->pageSize] );
  assert( pPage->isInit==0 || pPage->pParent==pParent );
  if( pPage->isInit ) return SQLITE_OK;
  assert( pPage->pParent==0 );



  pPage->pParent = pParent;
  if( pParent ){
    sqlitepager_ref(pParent->aData);
  }

  pPage->nCell = pPage->nCellAlloc = 0;

  pPage->hdrOffset = hdr = pPage->pgno==1 ? 100 : 0;
  c = pPage->aData[hdr];
  pPage->intKey = (c & PTF_INTKEY)!=0;
  pPage->zeroData = (c & PTF_ZERODATA)!=0;
  pPage->leaf = (c & PTF_LEAF)!=0;

  /* Initialize the cell count and cell pointers */
  pc = get2byte(&data[hdr+3]);
  while( pc>0 ){
    if( pc>=pBt->pageSize ) return SQLITE_CORRUPT;
    if( pPage->nCell>pBt->pageSize ) return SQLITE_CORRUPT;
    pPage->nCell++;
    pc = get2byte(&data[pc]);
  }
  if( resizeCellArray(pPage, pPage->nCell) ){

    return SQLITE_NOMEM;
  }
  pc = get2byte(&data[hdr+3]);
  for(i=0; pc>0; i++){
    pPage->aCell[i] = &data[pc];
    pc = get2byte(&data[pc]);
    sumCell += cellSize(pPage, &data[pc]);
................................................................................
*/
static void zeroPage(MemPage *pPage, int flags){
  unsigned char *data = pPage->aData;
  Btree *pBt = pPage->pBt;
  int hdr = pPage->hdrOffset;
  int first;

  assert( sqlitepager_iswriteable(data->aData) );
  memset(&data[hdr], 0, pBt->pageSize - hdr);
  data[hdr] = flags;
  first = hdr + 6 + 4*((flags&0x01)!=0);
  put2byte(&data[hdr+1], first);
  put2byte(&data[first+2], pBt->pageSize - first);
  sqliteFree(pPage->aCell);
  pPage->aCell = 0;
................................................................................
  return SQLITE_OK;
}

/*
** Release a MemPage.  This should be called once for each prior
** call to getPage.
*/
static void releasePage(MemPage *pPage){
  if( pPage ){
    assert( pPage->aData );
    assert( pPage->pBt );
    assert( &pPage->aData[pPage->pBt->pageSize]==(unsigned char*)pPage );
    sqlitepager_unref(pPage->aData);
  }
}
................................................................................
    *ppBtree = 0;
    return rc;
  }
  sqlitepager_set_destructor(pBt->pPager, pageDestructor);
  pBt->pCursor = 0;
  pBt->page1 = 0;
  pBt->readOnly = sqlitepager_isreadonly(pBt->pPager);
  pBt->pageSize = SQLITE_PAGE_SIZE;  /* FIX ME - read from header */
  pBt->maxLocal = (pBt->pageSize-10)/4-12;
  *ppBtree = pBt;
  return SQLITE_OK;
}

/*
** Close an open database and invalidate all cursors.
*/
................................................................................
  int rc;
  BtCursor *pCur, *pRing;

  if( pBt->readOnly && wrFlag ){
    *ppCur = 0;
    return SQLITE_READONLY;
  }
  if( pBt->pPage1==0 ){
    rc = lockBtree(pBt);
    if( rc!=SQLITE_OK ){
      *ppCur = 0;
      return rc;
    }
  }
  pCur = sqliteMalloc( sizeof(*pCur) );
  if( pCur==0 ){
    rc = SQLITE_NOMEM;
    goto create_cursor_exception;
  }
  pCur->pgnoRoot = (Pgno)iTable;
  rc = getPage(pBt, pCur->pgnoRoot, &pCur->pPage);
  if( rc!=SQLITE_OK ){
    goto create_cursor_exception;
  }
  rc = initPage(pCur->pPage, 0);
  if( rc!=SQLITE_OK ){
    goto create_cursor_exception;
  }
................................................................................
}

/*
** Set *pSize to the size of the buffer needed to hold the value of
** the key for the current entry.  If the cursor is not pointing
** to a valid entry, *pSize is set to 0. 
**
** For a table with the INTKEY flag set, this routine returns the key
** itself, not the number of bytes in the key.
*/
int sqlite3BtreeKeySize(BtCursor *pCur, u64 *pSize){
  MemPage *pPage;

  pPage = pCur->pPage;
  assert( pPage!=0 );
................................................................................
** If pCur is not pointing to a valid entry return 0.
**
** The pointer returned is ephemeral.  The key may move
** or be destroyed on the next call to any Btree routine.
**
** This routine is used to do quick key comparisons in the
** common case where the entire key fits in the payload area
** of a cell and does not overflow onto secondary pages.  If
** this routine returns 0 for a valid cursor, the caller will
** need to allocate a buffer big enough to hold the whole key
** then use sqlite3BtreeKey() to copy the key value into the
** buffer.  That is substantially slower.  But fortunately,
** most keys are small enough to fit in the payload area so
** the slower method is rarely needed.
*/
void *sqlite3BtreeKeyFetch(BtCursor *pCur){
  unsigned char *aPayload;
  MemPage *pPage;
  Btree *pBt;
  u64 nData, nKey;

................................................................................
  pCur->pPage = pRoot;
  pCur->idx = 0;
  if( pRoot->nCell==0 && !pRoot->leaf ){
    Pgno subpage;
    assert( pRoot->pgno==1 );
    subpage = get4byte(&pRoot->aData[pRoot->hdrOffset+6]);
    assert( subpage>0 );
    rc = moveToChild(pCur, subpage);
  }
  return rc;
}

/*
** Move the cursor down to the left-most leaf entry beneath the
** entry to which it is currently pointing.
................................................................................
    if( rc ) return rc;
    sqlitepager_unref(pOvfl->aData);
  }
  return SQLITE_OK;
}

/*
** Compute the number of bytes required by a cell header.  Fill in
** the nData and nKey values of the header that pHeader points to.
*/
static int makeCellHeader(
  MemPage *pPage,          /* The page that will contain the cell */
  u64 nKey,                /* Size of key, or the key value if intKey */
  int nData,               /* Size of data.  Ignored for zerodata */
  unsigned char *pHeader   /* Write header bytes here */
){
................................................................................
/*
** Fill in the payload section of a cell into the space provided.  If
** the payload will not completely fit in the cell, allocate additional
** overflow pages and fill them in.
*/
static int fillInCell(
  MemPage *pPage,                /* The page that contains the cell */
  unsigned char *pCell,          /* Complete text of the cell */

  const void *pKey, u64 nKey,    /* The key */
  const void *pData,int nData,   /* The data */
  int *pnSize                    /* Write cell size here */
){
  int nPayload;
  const void *pSrc;
  int nSrc, nSrc2;
  int spaceLeft;
  MemPage *pOvfl = 0;
  unsigned char *pPrior;
  unsigned char *pPayload;
  Btree *pBt = pPage->pBt;
  Pgno pgnoOvfl = 0;
  int nHeader;

  nHeader = makeCellHeader(pPage, pCell, nKey, nData);
  nPayload = nData;
  if( pPage->intKey ){
    pSrc = pData;
    nSrc = nData;
    nSrc2 = 0;
  }else{
    nPayload += nKey;
    pSrc = pKey;
    nSrc = nKey;
  }
  if( nPayload>pBt->maxLocal ){
    *pnSize = nHeader + pBt->maxLocal + 4;
  }else{
    *pnSize = nHeader + nPayload;
  }
  spaceLeft = pBt->maxLocal;
  pPayload = &pCell[nHeader];
  pPrior = &pPayload[pBt->maxLocal];

  while( nPayload>0 ){
    if( spaceLeft==0 ){
      rc =  allocatePage(pBt, &pOvfl, &pgnoOvfl, pgnoOvfl);
................................................................................
}

/*
** Change the MemPage.pParent pointer on the page whose number is
** given in the second argument so that MemPage.pParent holds the
** pointer in the third argument.
*/
static void reparentPage(Btree *pBt, Pgno pgno, MemPage *pNewParent, int idx){
  MemPage *pThis;
  unsigned char *aData;

  if( pgno==0 ) return;
  assert( pBt->pPager!=0 );
  aData = sqlitepager_lookup(pBt->pPager, pgno);
  pThis = (MemPage)&aData[pBt->pageSize];
  if( pThis && pThis->isInit ){
    if( pThis->pParent!=pNewParent ){
      if( pThis->pParent ) sqlitepager_unref(pThis->pParent->aData);
      pThis->pParent = pNewParent;
      if( pNewParent ) sqlitepager_ref(pNewParent->aData);
    }
    pThis->idxParent = idx;
    sqlitepager_unref(aData);
  }
}

/*
** Change the pParent pointer of all children of pPage to point back
** to pPage.
**
** In other words, for every child of pPage, invoke reparentPage()
** to make sure that each child knows that pPage is its parent.
**
** This routine gets called after you memcpy() one page into
** another.
*/
static void reparentChildPages(MemPage *pPage){
  int i;
  Btree *pBt;

  if( pPage->left ) return;
  pBt = pPage->pBt;
  for(i=0; i<pPage->nCell; i++){

    reparentPage(pBt, get4byte(&pPage->aCell[i][2]), pPage, i);
  }

  reparentPage(pBt, get4byte(&pPage->aData[pPage->hdrOffset+6]), pPage, i);
  pPage->idxShift = 0;
}

/*
** Remove the i-th cell from pPage.  This routine effects pPage only.
** The cell content is not freed or deallocated.  It is assumed that
** the cell content has been copied someplace else.  This routine just
................................................................................
** "sz" must be the number of bytes in the cell.
**
** Do not bother maintaining the integrity of the linked list of Cells.
** Only the pPage->apCell[] array is important.  The relinkCellList() 
** routine will be called soon after this routine in order to rebuild 
** the linked list.
*/
static void dropCell(MemPage *pPage, int idx, int sz){
  int j;
  assert( idx>=0 && idx<pPage->nCell );
  assert( sz==cellSize(pPage, pPage->aCell[idx]) );
  assert( sqlitepager_iswriteable(pPage->aData) );
  assert( pPage->aCell[idx]>=pPage->aData );
  assert( pPage->aCell[idx]<&pPage->aData[pPage->pBt->pageSize-sz] );
  freeSpace(pPage, idx, sz);
  for(j=idx; j<pPage->nCell-1; j++){
    pPage->aCell[j] = pPage->aCell[j+1];
  }
  pPage->nCell--;
  pPage->idxShift = 1;
}

/*
** Insert a new cell on pPage at cell index "i".  pCell points to the
................................................................................
** and set pPage->isOverfull.  
**
** Do not bother maintaining the integrity of the linked list of Cells.
** Only the pPage->apCell[] array is important.  The relinkCellList() 
** routine will be called soon after this routine in order to rebuild 
** the linked list.
*/
static void insertCell(MemPage *pPage, int i, unsigned char *pCell, int sz){
  int idx, j;
  assert( i>=0 && i<=pPage->nCell );
  assert( sz==cellSize(pBt, pCell) );
  assert( sqlitepager_iswriteable(pPage->aData) );
  idx = allocateSpace(pBt, pPage, sz);
  resizeCellArray(pPage, pPage->nCell+1);
  for(j=pPage->nCell; j>i; j--){
    pPage->aCell[j] = pPage->aCell[j-1];
  }
  pPage->nCell++;
  if( idx<=0 ){
    pPage->isOverfull = 1;
    pPage->aCell[i] = pCell;
  }else{
    memcpy(&pPage->aData[idx], pCell, sz);
    pPage->aCell[i] = &pPage->aData[idx];
  }
  pPage->idxShift = 1;
}

/*
** Rebuild the linked list of cells on a page so that the cells
** occur in the order specified by the pPage->aCell[] array.  
** Invoke this routine once to repair damage after one or more
** invocations of either insertCell() or dropCell().
*/
static void relinkCellList(MemPage *pPage){
  int i, idxFrom;

  assert( sqlitepager_iswriteable(pPage->aData) );

  idxFrom = pPage->hdrOffset+3;
  for(i=0; i<pPage->nCell; i++){
    int idx = Addr(pPage->aCell[i]) - Addr(pPage);
    assert( idx>pPage->hdrOffset && idx<pPage->pBt->pageSize );
    put2byte(&pPage->aData[idxFrom], idx);
    idxFrom = idx;

  }

  put2byte(&pPage->aData[idxFrom], 0);
}

/*
** Make a copy of the contents of pFrom into pTo.  The pFrom->aCell[]
** pointers that point into pFrom->aData[] must be adjusted to point
** into pTo->aData[] instead.  But some pFrom->aCell[] entries might
** not point to pFrom->aData[].  Those are unchanged.
*/
static void copyPage(MemPage *pTo, MemPage *pFrom){
  uptr from, to;
  int i;
  int pageSize;
  int ofst;

  assert( pTo->hdrOffset==0 );
  ofst = pFrom->hdrOffset;
  pageSize = pTo->pBt->pageSize;
  memcpy(pTo->aData, &pFrom->aData[ofst], pageSize - ofst);
  pTo->pParent = 0;
  pTo->isInit = 1;
  resizeCellArray(pTo, pFrom->nCell);
  pTo->nCell = pFrom->nCell;
  pTo->nFree = pFrom->nFree + ofst;
  assert( pTo->aData[5]<155 );
  pTo->aData[5] += ofst;
  pTo->isOverfull = pFrom->isOverfull;
  to = Addr(pTo->aData);
  from = Addr(pFrom->aData);
  for(i=0; i<pTo->nCell; i++){
    uptr x = Addr(pFrom->aCell[i]);
    if( x>from && x<from+pageSize ){
      *((uptr*)&pTo->aCell[i]) = x + to - from;
    }else{
      pTo->aCell[i] = pFrom->aCell[i];
    }
  }
}

/*
** The following parameters determine how many adjacent pages get involved
** in a balancing operation.  NN is the number of neighbors on either side
................................................................................
** The number of siblings of pPage might be increased or decreased by
** one in an effort to keep pages between 66% and 100% full. The root page
** is special and is allowed to be less than 66% full. If pPage is 
** the root page, then the depth of the tree might be increased
** or decreased by one, as necessary, to keep the root page from being
** overfull or empty.
**
** This routine alwyas calls relinkCellList() on its input page regardless of
** whether or not it does any real balancing.  Client routines will typically
** invoke insertCell() or dropCell() before calling this routine, so we
** need to call relinkCellList() to clean up the mess that those other
** routines left behind.
**





** Note that when this routine is called, some of the Cells on pPage
** might not actually be stored in pPage->aData[].  This can happen
** if the page is overfull.  Part of the job of this routine is to
** make sure all Cells for pPage once again fit in pPage->aData[].
**
** In the course of balancing the siblings of pPage, the parent of pPage
** might become overfull or underfull.  If that happens, then this routine
** is called recursively on the parent.
**
** If this routine fails for any reason, it might leave the database
** in a corrupted state.  So if this routine fails, the database should
** be rolled back.
*/
static int balance(MemPage *pPage){
  MemPage *pParent;            /* The parent of pPage */
  Btree *pBt;                  /* The whole database */
  int nCell;                   /* Number of cells in apCell[] */
  int nOld;                    /* Number of pages in apOld[] */
  int nNew;                    /* Number of pages in apNew[] */
  int nDiv;                    /* Number of cells in apDiv[] */
  int i, j, k;                 /* Loop counters */
  int idx;                     /* Index of pPage in pParent->apCell[] */
  int nxDiv;                   /* Next divider slot in pParent->apCell[] */
  int rc;                      /* The return code */
  int iCur;                    /* apCell[iCur] is the cell of the cursor */
  MemPage *pOldCurPage;        /* The cursor originally points to this page */
  int subtotal;                /* Subtotal of bytes in cells on one page */

  MemPage *apOld[NB];          /* pPage and up to two siblings */
  Pgno pgnoOld[NB];            /* Page numbers for each page in apOld[] */
  MemPage *apCopy[NB];         /* Private copies of apOld[] pages */
  MemPage *apNew[NB+1];        /* pPage and up to NB siblings after balancing */
  Pgno pgnoNew[NB+1];          /* Page numbers for each page in apNew[] */
  int idxDiv[NB];              /* Indices of divider cells in pParent */
  u8 *apDiv[NB];               /* Divider cells in pParent */
  u8 aTemp[NB][MX_CELL_SIZE];  /* Temporary holding area for apDiv[] */
  int cntNew[NB+1];            /* Index in apCell[] of cell after i-th page */
  int szNew[NB+1];             /* Combined size of cells place on i-th page */

  u8 *apCell[(MX_CELL+2)*NB];  /* All cells from pages being balanced */
  int szCell[(MX_CELL+2)*NB];  /* Local size of all cells */
  u8 aCopy[NB][MX_PAGE_SIZE+sizeof(MemPage)];  /* Space for apCopy[] */

  /* 
  ** Return without doing any work if pPage is neither overfull nor
  ** underfull.
  */
  assert( sqlitepager_iswriteable(pPage->aData) );
  pBt = pPage->pBt;
  if( !pPage->isOverfull && pPage->nFree<pBt->pageSize/2 && pPage->nCell>=2){

    relinkCellList(pPage);
    return SQLITE_OK;
  }

  /*
  ** Find the parent of the page to be balanced.  If there is no parent,
  ** it means this page is the root page and special rules apply.

  */
  pParent = pPage->pParent;
  if( pParent==0 ){
    Pgno pgnoChild;
    MemPage *pChild;
    assert( pPage->isInit );
    if( pPage->nCell==0 ){
................................................................................
        relinkCellList(pPage);
      }else{
        /* The root page is empty but has one child.  Transfer the
        ** information from that one child into the root page if it 
        ** will fit.  This reduces the depth of the BTree by one.
        **
        ** If the root page is page 1, it has less space available than
        ** its child (due to the 100 byte header that occurs at the beginning
        ** of the database fle), so it might not be able to hold all of the 
        ** information currently contained in the child.  If this is the 
        ** case, then do not do the transfer.  Leave page 1 empty except
        ** for the right-pointer to the child page.  The child page becomes
        ** the virtual root of the tree.
        */
        pgnoChild = get4byte(pPage->aData[pPage->hdrOffset+6]);
        assert( pgnoChild>0 && pgnoChild<=sqlit3pager_pagecount(pBt->pPager) );
        rc = getPage(pBt, pgnoChild, &pChild);
        if( rc ) return rc;
        if( pPage->pgno==1 ){
          rc = initPage(pChild, pPage);
          if( rc ) return rc;
          if( pChild->nFree>=100 ){
            /* The child information will fit on the root page, so do the
            ** copy */
            zeroPage(pPage, pChild->aData[0]);
            resizeCellArray(pPage, pChild->nCell);
            for(i=0; i<pChild->nCell; i++){
              insertCell(pPage, i, pChild->aCell[i], 
                        cellSize(pChild, pChild->aCell[i]));
            }
            freePage(pChild);
          }else{
            /* The child has more information that will fit on the root.
            ** The tree is already balanced.  Do nothing. */
          }
        }else{
          memcpy(pPage, pChild, pBt->pageSize);
          pPage->isInit = 0;

          pPage->pParent = 0;
          rc = initPage(pPage, 0);
          assert( rc==SQLITE_OK );


          freePage(pChild);


        }

        reparentChildPages(pPage);
        releasePage(pChild);
      }else{
        relinkCellList(pPage);
      }
      return SQLITE_OK;
    }
    if( !pPage->isOverfull ){
      /* It is OK for the root page to be less than half full.
      */
      relinkCellList(pBt, pPage);
................................................................................
    ** If we get to here, it means the root page is overfull.
    ** When this happens, Create a new child page and copy the
    ** contents of the root into the child.  Then make the root
    ** page an empty page with rightChild pointing to the new
    ** child.  Then fall thru to the code below which will cause
    ** the overfull child page to be split.
    */


    rc = allocatePage(pBt, &pChild, &pgnoChild, pPage->pgno);
    if( rc ) return rc;
    assert( sqlitepager_iswriteable(pChild->aData) );
    copyPage(pChild, pPage);
    pChild->pParent = pPage;
    pChild->idxParent = 0;
    sqlitepager_ref(pPage->aData);
    pChild->isOverfull = 1;
    zeroPage(pPage, pPage->aData[pPage->hdrOffset] & ~PTF_LEAF);
    put4byte(&pPage->aData[pPage->hdrOffset+6], pChild->pgno);






    pParent = pPage;
    pPage = pChild;
  }
  rc = sqlitepager_write(pParent->aData);
  if( rc ) return rc;
  assert( pParent->isInit );
  
  /*
  ** Find the cell in the parent page whose left child points back
  ** to pPage.  The "idx" variable is the index of that cell.  If pPage
  ** is the rightmost child of pParent then set idx to pParent->nCell 
  */
  if( pParent->idxShift ){
    Pgno pgno, swabPgno;
    pgno = pPage->pgno;
    assert( pgno==sqlitepager_pagenumber(pPage->aData) );

    for(idx=0; idx<pParent->nCell; idx++){
      if( get4byte(pParent->aCell[idx][2])==pgno ){
        break;
      }
    }
    assert( idx<pParent->nCell
             || get4byte(&pParent->aData[pParent->hdrOffset+6])==pgno );
  }else{
    idx = pPage->idxParent;
  }

  /*
  ** Initialize variables so that it will be safe to jump
  ** directly to balance_cleanup at any moment.
  */
  nOld = nNew = 0;
  sqlitepager_ref(pParent->aData);

  /*
  ** Find sibling pages to pPage and the cells in pParent that divide
  ** the siblings.  An attempt is made to find NN siblings on either
  ** side of pPage.  More siblings are taken from one side, however, if
  ** pPage there are fewer than NN siblings on the other side.  If pParent
  ** has NB or fewer children then all children of pParent are taken.
  */
  nxDiv = idx - NN;
  if( nxDiv + NB > pParent->nCell ){
................................................................................
  if( nxDiv<0 ){
    nxDiv = 0;
  }
  nDiv = 0;
  for(i=0, k=nxDiv; i<NB; i++, k++){
    if( k<pParent->nCell ){
      idxDiv[i] = k;
      apDiv[i] = pParent->aCell[k];
      nDiv++;
      assert( !pParent->left );
      pgnoOld[i] = get4byte(&apDev[i][2]);
    }else if( k==pParent->nCell ){

      pgnoOld[i] = get4byte(&pParent->aData[pParent->hdrOffset+6]);
    }else{
      break;
    }
    rc = getPage(pBt, pgnoOld[i], &apOld[i]);
    if( rc ) goto balance_cleanup;
    rc = initPage(apOld[i], pParent);
    if( rc ) goto balance_cleanup;
    apOld[i]->idxParent = k;
    nOld++;
  }

























  /*
  ** Make copies of the content of pPage and its siblings into aOld[].
  ** The rest of this function will use data from the copies rather
  ** that the original pages since the original pages will be in the
  ** process of being overwritten.
  */
  for(i=0; i<nOld; i++){
    apCopy[i] = (MemPage*)&aCopy[i+1][-sizeof(MemPage)];
    memset(apCopy[i], 0, sizeof(MemPage));
    apCopy[i]->aData = &((u8*)apCopy)[-pBt->pageSize];
    copyPage(apCopy[i], apOld[i]);
  }

  /*
  ** Load pointers to all cells on sibling pages and the divider cells
  ** into the local apCell[] array.  Make copies of the divider cells
  ** into aTemp[] and remove the the divider Cells from pParent.
  **
  ** If the siblings are on leaf pages, then the child pointers of the
  ** divider cells are stripped from the cells before they are copied
  ** into aTemp[].  In this wall, all cells in apCell[] are without
  ** child pointers.  If siblings are not leaves, then all cell in
  ** apCell[] include child pointers.  Either way, all cells in apCell[]
  ** are alike.
  */
  nCell = 0;
  leafCorrection = pPage->leaf*4;
  for(i=0; i<nOld; i++){
    MemPage *pOld = apCopy[i];
    for(j=0; j<pOld->nCell; j++){
      apCell[nCell] = pOld->aCell[j];
      szCell[nCell] = cellSize(pOld, apCell[nCell]);
      nCell++;
    }
    if( i<nOld-1 ){
      szCell[nCell] = cellSize(pParent, apDiv[i]) - leafCorrection;
      memcpy(aTemp[i], apDiv[i], szCell[nCell] + leafCorrection);
      apCell[nCell] = &aTemp[i][leafCorrection];
      dropCell(pParent, nxDiv, szCell[nCell]);
      assert( get4byte(&apCell[nCell][2])==pgnoOld[i] );
      if( !pOld->leaf ){
        assert( leafCorrection==0 );
        /* The right pointer of the child page pOld becomes the left
        ** pointer of the divider cell */
        memcpy(&apCell[nCell][2], &pOld->aData[pOld->hdrOffset+6], 4);
      }else{
        assert( leafCorrection==4 );
      }
      nCell++;
    }
  }

  /*
  ** Figure out the number of pages needed to hold all nCell cells.
  ** Store this number in "k".  Also compute szNew[] which is the total
  ** size of all cells on the i-th page and cntNew[] which is the index
  ** in apCell[] of the cell that divides page i from page i+1.  
  ** cntNew[k] should equal nCell.
  **
  ** This little patch of code is critical for keeping the tree
  ** balanced. 
  */
  usableSpace = pBt->pageSize - 10 + leafCorrection;
  for(subtotal=k=i=0; i<nCell; i++){
    subtotal += szCell[i];
    if( subtotal > usableSpace ){
      szNew[k] = subtotal - szCell[i];
      cntNew[k] = i;
      subtotal = 0;
      k++;
    }
  }
  szNew[k] = subtotal;
  cntNew[k] = nCell;
  k++;
  for(i=k-1; i>0; i--){
    while( szNew[i]<usableSpace/2 ){
      cntNew[i-1]--;
      assert( cntNew[i-1]>0 );
      szNew[i] += szCell[cntNew[i-1]];
      szNew[i-1] -= szCell[cntNew[i-1]-1];
    }
  }
  assert( cntNew[0]>0 );

  /*
  ** Allocate k new pages.  Reuse old pages where possible.
  */
  assert( pPage->pgno>1 );
  pageFlags = pPage->aData[0];
  for(i=0; i<k; i++){
    if( i<nOld ){
      apNew[i] = apOld[i];
      pgnoNew[i] = pgnoOld[i];
      apOld[i] = 0;
      sqlitepager_write(apNew[i]);
    }else{
      rc = allocatePage(pBt, &apNew[i], &pgnoNew[i], pgnoNew[i-1]);
      if( rc ) goto balance_cleanup;
    }
    nNew++;
    zeroPage(apNew[i], pageFlags);
    apNew[i]->isInit = 1;
  }

  /* Free any old pages that were not reused as new pages.
  */
  while( i<nOld ){
    rc = freePage(apOld[i]);
    if( rc ) goto balance_cleanup;
    sqlitepager_unref(apOld[i]->aData);
    apOld[i] = 0;
    i++;
  }

  /*
  ** Put the new pages in accending order.  This helps to
  ** keep entries in the disk file in order so that a scan
................................................................................
  /*
  ** Evenly distribute the data in apCell[] across the new pages.
  ** Insert divider cells into pParent as necessary.
  */
  j = 0;
  for(i=0; i<nNew; i++){
    MemPage *pNew = apNew[i];
    assert( pNew->pgno==pgnoNew[i] );
    resizeCellArray(pNew, cntNew[i] - j);
    while( j<cntNew[i] ){
      assert( pNew->nFree>=szCell[j] );

      insertCell(pBt, pNew, pNew->nCell, apCell[j], szCell[j]);
      j++;
    }
    assert( pNew->nCell>0 );
    assert( !pNew->isOverfull );
    relinkCellList(pNew);
    if( i<nNew-1 && j<nCell ){
      u8 *pCell = apCell[j];
      if( !pNew->leaf ){
        memcpy(&pNew->aData[6], &apCell[j][2], 4);
      }else{
        pCell -= 4;
      }
      insertCell(pParent, nxDiv, pCell, szCell[j]+leafCorrection);
      put4byte(&pParent->aCell[nxDiv][2], pNew->pgno);
      j++;
      nxDiv++;
    }
  }
  assert( j==nCell );
  if( (pageFlags & PTF_LEAF)==0 ){
    memcpy(&apNew[nNew-1]->aData[6], &apCopy[nOld-1]->aData[6], 4);
  }
  if( nxDiv==pParent->nCell ){
    /* Right-most sibling is the right-most child of pParent */
    put4byte(&pParent->aData[pParent->hdrOffset+6], pgnoNew[nNew-1]);
  }else{
    /* Right-most sibling is the left child of the first entry in pParent
    ** past the right-most divider entry */
    put4byte(&pParent->apCell[nxDiv][2], pgnoNew[nNew-1]);





  }

  /*
  ** Reparent children of all cells.
  */
  for(i=0; i<nNew; i++){
    reparentChildPages(apNew[i]);
  }
  reparentChildPages(pParent);

  /*
  ** balance the parent page.
  */
  rc = balance(pParent);

  /*
  ** Cleanup before returning.
  */
balance_cleanup:



  for(i=0; i<nOld; i++){
    if( apOld[i]!=0 ) sqlitepager_unref(apOld[i]->aData);
  }
  for(i=0; i<nNew; i++){
    sqlitepager_unref(apNew[i]->aData);
  }




  sqlitepager_unref(pParent->aData);

  return rc;
}

/*
** This routine checks all cursors that point to the same table
** as pCur points to.  If any of those cursors were opened with
** wrFlag==0 then this routine returns SQLITE_LOCKED.  If all
................................................................................
  return SQLITE_OK;
}

/*
** Insert a new record into the BTree.  The key is given by (pKey,nKey)
** and the data is given by (pData,nData).  The cursor is used only to
** define what database the record should be inserted into.  The cursor
** is left pointing at a random location.
**
** For an INTKEY table, only the nKey value of the key is used.  pKey is
** ignored.  For a ZERODATA table, the pData and nData are both ignored.
*/
int sqlite3BtreeInsert(
  BtCursor *pCur,                /* Insert data into the table of this cursor */
  const void *pKey, u64 nKey,    /* The key of the new record */
  const void *pData, int nData   /* The data of the new record */
){

  int rc;
  int loc;
  int szNew;
  MemPage *pPage;
  Btree *pBt = pCur->pBt;
  unsigned char newCell[MX_CELL_SIZE], *oldCell;

  if( pCur->pPage==0 ){
    return SQLITE_ABORT;  /* A rollback destroyed this cursor */
  }
  if( !pBt->inTrans || nKey+nData==0 ){
    /* Must start a transaction before doing an insert */
    return pBt->readOnly ? SQLITE_READONLY : SQLITE_ERROR;
................................................................................
  }
  if( checkReadLocks(pCur) ){
    return SQLITE_LOCKED; /* The table pCur points to has a read lock */
  }
  rc = sqlite3BtreeMoveto(pCur, pKey, nKey, &loc);
  if( rc ) return rc;
  pPage = pCur->pPage;
  assert( nData==0 || pPage->zeroData!=0 );
  assert( pPage->isInit );
  rc = sqlitepager_write(pPage);
  if( rc ) return rc;
  rc = fillInCell(pPage, &newCell, pKey, nKey, pData, nData, &szNew);
  if( rc ) return rc;
  assert( szNew==cellSize(pPage, newCell) );
  if( loc==0 ){
    int szOld
    assert( pCur->idx>=0 && pCur->idx<pPage->nPage );
    oldCell = pPage->aCell[pCur->idx];
    if( !pPage->leaf ){
      memcpy(&newCell[2], &oldCell[2], 4);
    }
    szOld = cellSize(pPage, oldCell);
    rc = clearCell(pPage, oldCell);
    if( rc ) return rc;

    dropCell(pPage, pCur->idx, szOld);
  }else if( loc<0 && pPage->nCell>0 ){

    assert( pPage->leaf );
    pCur->idx++;
  }else{

    assert( pPage->leaf );
  }
  insertCell(pPage, pCur->idx, &newCell, szNew);
  rc = balance(pPage);
  /* sqliteBtreePageDump(pCur->pBt, pCur->pgnoRoot, 1); */
  /* fflush(stdout); */
  moveToRoot(pCur);
  pCur->eSkip = SKIP_INVALID;
  return rc;
}

/*
** Delete the entry that the cursor is pointing to.  The cursor

** is left pointing at a random location.









*/
int sqlite3BtreeDelete(BtCursor *pCur){
  MemPage *pPage = pCur->pPage;
  unsigned char *pCell;
  int rc;
  Pgno pgnoChild;
  Btree *pBt = pCur->pBt;

  assert( pPage->isInit );
  if( pCur->pPage==0 ){
    return SQLITE_ABORT;  /* A rollback destroyed this cursor */
................................................................................
    return SQLITE_PERM;   /* Did not open this cursor for writing */
  }
  if( checkReadLocks(pCur) ){
    return SQLITE_LOCKED; /* The table pCur points to has a read lock */
  }
  rc = sqlitepager_write(pPage);
  if( rc ) return rc;
  pCell = pPage->aCell[pCur->idx];
  if( !pPage->leaf ){
    pgnoChild = get4byte(&pCell[2]);
  }
  clearCell(pPage, pCell);

  if( !pPage->leaf ){
    /*
    ** The entry we are about to delete is not a leaf so if we do not
    ** do something we will leave a hole on an internal page.
    ** We have to fill the hole by moving in a cell from a leaf.  The
    ** next Cell after the one to be deleted is guaranteed to exist and
    ** to be a leaf so we can use it.
    */
    BtCursor leafCur;
    unsigned char *pNext;
    int szNext;
    int notUsed;
    getTempCursor(pCur, &leafCur);
    rc = sqlite3BtreeNext(&leafCur, &notUsed);
    if( rc!=SQLITE_OK ){
      if( rc!=SQLITE_NOMEM ) rc = SQLITE_CORRUPT;
      return rc;
    }
    rc = sqlitepager_write(leafCur.pPage);
    if( rc ) return rc;
    dropCell(pPage, pCur->idx, cellSize(pPage, pCell));
    pNext = leafCur.pPage->aCell[leafCur.idx];
    szNext = cellSize(leafCur.pPage, pNext);
    insertCell(pPage, pCur->idx, &pNext[-4], szNext+4);
    put4byte(&pNext[-2], pgnoChild);
    rc = balance(pPage);
    if( rc ) return rc;

    dropCell(leafCur.pPage, leafCur.idx, szNext);
    rc = balance(leafCur.pPage);
    releaseTempCursor(&leafCur);
  }else{
    dropCell(pPage, pCur->idx, cellSize(pPage, pCell));











    rc = balance(pPage);
  }
  moveToRoot(pCur);
  return rc;
}

/*
** Create a new BTree table.  Write into *piTable the page
** number for the root page of the new table.
**
................................................................................
    return pBt->readOnly ? SQLITE_READONLY : SQLITE_ERROR;
  }
  if( pBt->readOnly ){
    return SQLITE_READONLY;
  }
  rc = allocatePage(pBt, &pRoot, &pgnoRoot, 0);
  if( rc ) return rc;
  assert( sqlitepager_iswriteable(pRoot->aData) );
  zeroPage(pBt, pRoot);
  sqlitepager_unref(pRoot);
  *piTable = (int)pgnoRoot;
  return SQLITE_OK;
}

/*
** Erase the given database page and all its children.  Return
** the page to the freelist.
*/
static int clearDatabasePage(
  Btree *pBt,           /* The BTree that contains the table */
  Pgno pgno,            /* Page number to clear */
  MemPage *pParent,     /* Parent page.  NULL for the root */
  int freePageFlag      /* Deallocate page if true */
){
  MemPage *pPage;
  int rc;
  unsigned char *pCell;
  int i;


  rc = getPage(pBt, pgno, &pPage);
  if( rc ) return rc;
  rc = sqlitepager_write(pPage->aData);
  if( rc ) return rc;
  rc = initPage(pPage, pParent);
  if( rc ) return rc;





  for(i=0; i<pPage->nCell; i++){
    pCell = pPage->aCell[i];
    if( !pPage->leaf ){
      rc = clearDatabasePage(pBt, get4byte(&pCell[2]), 1);
      if( rc ) return rc;
    }
    rc = clearCell(pPage, pCell);
    if( rc ) return rc;
  }

  if( !pPage->left ){
    rc = clearDatabasePage(pBt, get4byte(&pPage->aData[6]), 1);
    if( rc ) return rc;
  }
  if( freePageFlag ){
    rc = freePage(pPage);
  }else{
    zeroPage(pPage, pPage->aData[0]);
  }
  releasePage(pPage);
  return rc;
}

/*
** Delete all information from a single table in the database.
*/
int sqlite3BtreeClearTable(Btree *pBt, int iTable){
................................................................................
    return pBt->readOnly ? SQLITE_READONLY : SQLITE_ERROR;
  }
  for(pCur=pBt->pCursor; pCur; pCur=pCur->pNext){
    if( pCur->pgnoRoot==(Pgno)iTable ){
      return SQLITE_LOCKED;  /* Cannot drop a table that has a cursor */
    }
  }
  rc = getPage(pBt, (Pgno)iTable, pPage);
  if( rc ) return rc;
  rc = sqlite3BtreeClearTable(pBt, iTable);
  if( rc ) return rc;
  if( iTable>1 ){
    rc = freePage(pBt, pPage, iTable);
  }else{
    zeroPage(pBt, pPage);
  }
  releasePage(pPage);
  return rc;  
}


















































/*


















































** Read the meta-information out of a database file.
*/
int sqlite3BtreeGetMeta(Btree *pBt, int idx, u32 *pMeta){
  int rc;
  int i;
  unsigned char *pP1;

  assert( idx>=0 && idx<15 );
  rc = sqlitepager_get(pBt->pPager, 1, (void**)&pP1);
  if( rc ) return rc;




  *pMeta = get4byte(&pP1[40 + idx*4]);
  sqlitepager_unref(pP1);
  return SQLITE_OK;
}

/*
** Write meta-information back into the database.
*/
int sqlite3BtreeUpdateMeta(Btree *pBt, int idx, u32 iMeta){
  unsigned char *pP1;
  int rc, i;
  assert( idx>=0 && idx<15 );
  if( !pBt->inTrans ){
    return pBt->readOnly ? SQLITE_READONLY : SQLITE_ERROR;
  }
  rc = sqlitepager_get(pBt->pPager, 1, (void**)&pP1);
  if( rc ) return rc;
  rc = sqlitepager_write(pP1);
  if( rc ) return rc;

  put4byte(&pP1[40 + idx*4], iMeta);

  return SQLITE_OK;
}

/******************************************************************************
** The complete implementation of the BTree subsystem is above this line.
** All the code the follows is for testing and troubleshooting the BTree
** subsystem.  None of the code that follows is used during normal operation.
................................................................................
#ifdef SQLITE_TEST
static int fileBtreePageDump(Btree *pBt, int pgno, int recursive){
  int rc;
  MemPage *pPage;
  int i, j;
  int nFree;
  u16 idx;
  int hdrOffset;
  char range[20];
  unsigned char payload[20];

  rc = getPage(pBt, (Pgno)pgno, &pPage);
  if( rc ){
    return rc;
  }
  printf("PAGE %d:  flags=0x%02x  frag=%d\n", pgno, pPage->aData[0],
    pPage->aData[5]);
  i = 0;

  hdrOffset = pgno==1 ? 100 : 0;
  idx = get2byte(&pPage->aData[hdrOffset+3]);
  while( idx>0 && idx<=pBt->pageSize ){

    u64 nData, nKey;
    int nHeader;
    Pgno child;
    unsigned char *pCell = &pPage->aData[idx];
    int sz = cellSize(pPage, pCell);
    sprintf(range,"%d..%d", idx, idx+sz-1);
    parseCellHeader(pPage, pCell, &nData, &nKey, &nHeader);
    if( pPage->leaf ){
      child = 0;
    }else{
      child = get4byte(&pCell[2]);
    }
    sz = NKEY(pBt, pCell->h) + NDATA(pBt, pCell->h);
    if( sz>sizeof(payload)-1 ) sz = sizeof(payload)-1;
    memcpy(payload, pCell->aPayload, sz);
    for(j=0; j<sz; j++){
      if( payload[j]<0x20 || payload[j]>0x7f ) payload[j] = '.';
    }
    payload[sz] = 0;
    printf(
      "cell %2d: i=%-10s chld=%-4d nk=%-4d nd=%-4d payload=%s\n",
      i, range, child, (int)nKey, (int)nData, payload


    );
    if( pPage->isInit && pPage->aCell[i]!=pCell ){
      printf("**** aCell[%d] does not match on prior entry ****\n", i);
    }
    i++;

    idx = get2byte(pCell);
  }
  if( idx!=0 ){
    printf("ERROR: next cell index out of range: %d\n", idx);
  }
  if( !pPage->leaf ){
    printf("right_child: %d\n", get4byte(&pPage->aData[6]));
  }
  nFree = 0;
  i = 0;
  idx = get2byte(&pPage->aData[hdrOffset+1]);
  while( idx>0 && idx<SQLITE_USABLE_SIZE ){
    int sz = get2byte(&pPage->aData[idx+2]);
    sprintf(range,"%d..%d", idx, idx+sz-1);
    nFree += sz;
    printf("freeblock %2d: i=%-10s size=%-4d total=%d\n",
       i, range, sz, nFree);
    idx = get2byte(&pPage->aData[idx]);
    i++;
  }
  if( idx!=0 ){
    printf("ERROR: next freeblock index out of range: %d\n", idx);
  }
  if( recursive && !pPage->left ){
    idx = get2byte(&pPage->aData[hdrOffset+3]);
    while( idx>0 && idx<SQLITE_USABLE_SIZE-MIN_CELL_SIZE ){
      unsigned char *pCell = &pPage->aData[idx];
      fileBtreePageDump(pBt, get4byte(&pPage->aData[idx+2]), 1);
      idx = get2byte(&pPage->aData[idx]);
    }
    fileBtreePageDump(pBt, get4byte(&pPage->aData[hdrOffset+6]), 1);
  }
  sqlitepager_unref(pPage);
  return SQLITE_OK;
}
#endif

#ifdef SQLITE_TEST
................................................................................
**
** This routine is used for testing and debugging only.
*/
static int fileBtreeCursorDump(BtCursor *pCur, int *aResult){
  int cnt, idx;
  MemPage *pPage = pCur->pPage;
  Btree *pBt = pCur->pBt;
  assert( pPage->isInit );
  aResult[0] = sqlitepager_pagenumber(pPage);
  aResult[1] = pCur->idx;
  aResult[2] = pPage->nCell;
  if( pCur->idx>=0 && pCur->idx<pPage->nCell ){
    aResult[3] = cellSize(pPage, pPage->aCell[pCur->idx]);
    aResult[6] = pPage->leaf ? 0 : get4byte(&pPage->aCell[pCur->idx][2]);
  }else{
    aResult[3] = 0;
    aResult[6] = 0;
  }
  aResult[4] = pPage->nFree;
  cnt = 0;
  idx = get2byte(&pPage->aData[pPage->hdrOffset+1]);
  while( idx>0 && idx<SQLITE_USABLE_SIZE ){
    cnt++;
    idx = get2byte(&pPage->aData[idx]);
  }
  aResult[5] = cnt;
  aResult[7] = pPage->leaf ? 0 : get4byte(&pPage->aData[pPage->hdrOffset+6]);
  return SQLITE_OK;
}
#endif

/*
** Return the pager associated with a BTree.  This routine is used for
** testing and debugging only.
................................................................................
  int iPage,            /* Page number for first page in the list */
  int N,                /* Expected number of pages in the list */
  char *zContext        /* Context for error messages */
){
  int i;
  char zMsg[100];
  while( N-- > 0 ){
    unsigned char *pOvfl;
    if( iPage<1 ){
      sprintf(zMsg, "%d pages missing from overflow list", N+1);
      checkAppendMsg(pCheck, zContext, zMsg);
      break;
    }
    if( checkRef(pCheck, iPage, zContext) ) break;
    if( sqlitepager_get(pCheck->pPager, (Pgno)iPage, (void**)&pOvfl) ){
      sprintf(zMsg, "failed to get page %d", iPage);
      checkAppendMsg(pCheck, zContext, zMsg);
      break;
    }
    if( isFreeList ){
      int n = get4byte(&pOvfl[4]);

      for(i=0; i<n; i++){

        checkRef(pCheck, get4byte(&pOvfl[8+i*4]), zContext);
      }
      N -= n;
    }
    iPage = get4byte(pOvfl);
    sqlitepager_unref(pOvfl);
  }
}

/*
** Return negative if zKey1<zKey2.
** Return zero if zKey1==zKey2.
................................................................................

  /* Check that the page exists
  */
  cur.pBt = pBt = pCheck->pBt;
  if( iPage==0 ) return 0;
  if( checkRef(pCheck, iPage, zParentContext) ) return 0;
  sprintf(zContext, "On tree page %d: ", iPage);
  if( (rc = getPage(pBt, (Pgno)iPage, &pPage))!=0 ){
    sprintf(zMsg, "unable to get the page. error code=%d", rc);
    checkAppendMsg(pCheck, zContext, zMsg);
    return 0;
  }
  if( (rc = initPage(pPage, pParent))!=0 ){
    sprintf(zMsg, "initPage() returns error code %d", rc);
    checkAppendMsg(pCheck, zContext, zMsg);
    sqlitepager_unref(pPage);
    return 0;
  }

#if 0

  /* Check out all the cells.
  */
  depth = 0;
  if( zLowerBound ){
    zKey1 = sqliteMalloc( nLower+1 );
    memcpy(zKey1, zLowerBound, nLower);
................................................................................
    }else if( hit[i]>1 ){
      sprintf(zMsg, "Multiple uses for byte %d of page %d", i, iPage);
      checkAppendMsg(pCheck, zMsg, 0);
      break;
    }
  }









#endif

  releasePage(pPage);
  return depth;
}

/*
** This routine does a complete check of the given BTree file.  aRoot[] is
** an array of pages numbers were each page number is the root page of
** a table.  nRoot is the number of entries in aRoot.