SQLite

** Boston, MA  02111-1307, USA.
**
** Author contact information:
**   drh@hwaci.com
**   http://www.hwaci.com/drh/
**
*************************************************************************
** $Id: btree.c,v 1.11 2001/06/08 00:21:52 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.
**
** The basic idea is that each page of the file contains N database
** entries and N+1 pointers to subpages.
**
**   ----------------------------------------------------------------
**   |  Ptr(0) | Key(0) | Ptr(1) | Key(1) | ... | Key(N) | Ptr(N+1) |
**   ----------------------------------------------------------------
**
** All of the keys on the page that Ptr(0) points to have values less
** than Key(0).  All of the keys on page Ptr(1) and its subpages have
** values greater than Key(0) and less than Key(1).  All of the keys
** on Ptr(N+1) and its subpages have values greater than Key(N).  And
** so forth.
**
** Finding a particular key requires reading O(log(M)) pages from the file
** where M is the number of entries in the tree.
**
** In this implementation, a single file can hold one or more separate 
** BTrees.  Each BTree is identified by the index of its root page.  The
** key and data for any entry are combined to form the "payload".  Up to
** MX_LOCAL_PAYLOAD bytes of payload can be carried directly on the
** database page.  If the payload is larger than MX_LOCAL_PAYLOAD bytes
** then surplus bytes are stored on overflow pages.  The payload for an
** entry and the preceding pointer are combined to form a "Cell".  Each 
** page has a smaller header which contains the Ptr(N+1) pointer.
**
** The first page of the file contains a magic string used to verify that
** the file really is a valid BTree database, a pointer to a list of unused
** pages in the file, and some meta information.  The root of the first
** BTree begins on page 2 of the file.  (Pages are numbered beginning with
** 1, not 0.)  Thus a minimum database contains 2 pages.
*/
#include "sqliteInt.h"
#include "pager.h"
#include "btree.h"
#include <assert.h>

   "** This file contains an SQLite 2.0 database **"
#define MAGIC_SIZE (sizeof(zMagicHeader))

/*
** The first page of the database file contains a magic header string
** to identify the file as an SQLite database file.  It also contains
** a pointer to the first free page of the file.  Page 2 contains the
** root of the principle BTree.  The file might contain other BTrees
** rooted on pages above 2.
**
** The first page also contains SQLITE_N_BTREE_META integers that
** can be used by higher-level routines.
**
** Remember that pages are numbered beginning with 1.  (See pager.c
** for additional information.)  Page 0 does not exist and a page
** number of 0 is used to mean "no such page".
*/
struct PageOne {
  char zMagic[MAGIC_SIZE]; /* String that identifies the file as a database */
  Pgno firstList;          /* First free page in a list of all free pages */
  int aMeta[SQLITE_N_BTREE_META];  /* User defined integers */
};

/*
** Each database page has a header that is an instance of this
** structure.
**

** PageHdr.firstFree is 0 if there is no free space on this page.
** Otherwise, PageHdr.firstFree is the index in MemPage.aDisk[] of a 
** FreeBlk structure that describes the first block of free space.  
** All free space is defined by a linked list of FreeBlk structures.
**
** Data is stored in a linked list of Cell structures.  PageHdr.firstCell
** is the index into MemPage.aDisk[] of the first cell on the page.  The
** Cells are kept in sorted order.
**
** A Cell contains all information about a database entry and a pointer
** to a child page that contains other entries less than itself.  In
** other words, the i-th Cell contains both Ptr(i) and Key(i).  The
** right-most pointer of the page is contained in PageHdr.rightChild.
*/
struct PageHdr {
  Pgno rightChild;  /* Child page that comes after all cells on this page */
  u16 firstCell;    /* Index in MemPage.aDisk[] of the first cell */
  u16 firstFree;    /* Index in MemPage.aDisk[] of the first free block */
};

  Pgno ovfl;                        /* The first overflow page */
};

/*
** Free space on a page is remembered using a linked list of the FreeBlk
** structures.  Space on a database page is allocated in increments of
** at least 4 bytes and is always aligned to a 4-byte boundry.  The
** linked list of FreeBlks is always kept in order by address.
*/
struct FreeBlk {
  u16 iSize;      /* Number of bytes in this block of free space */
  u16 iNext;      /* Index in MemPage.aDisk[] of the next free block */
};

/*
** Number of bytes on a single overflow page.
*/
#define OVERFLOW_SIZE (SQLITE_PAGE_SIZE-sizeof(Pgno))

/*
** When the key and data for a single entry in the BTree will not fit in
** the MX_LOACAL_PAYLOAD bytes of space available on the database page,
** then all extra bytes are written to a linked list of overflow pages.
** Each overflow page is an instance of the following structure.
**
** Unused pages in the database are also represented by instances of
** the OverflowPage structure.  The PageOne.freeList field is the
** page number of the first page in a linked list of unused database
** pages.
*/

*/
struct MemPage {
  char aDisk[SQLITE_PAGE_SIZE];  /* Page data stored on disk */
  int isInit;                    /* True if auxiliary data is initialized */
  MemPage *pParent;              /* The parent of this page.  NULL for root */
  int nFree;                     /* Number of free bytes in aDisk[] */
  int nCell;                     /* Number of entries on this page */
  Cell *apCell[MX_CELL+1];       /* All data entires in sorted order */
}

/*
** 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.
*/

** A cursor is a pointer to a particular entry in the BTree.
** The entry is identified by its MemPage and the index in
** MemPage.apCell[] of the entry.
*/
struct BtCursor {
  Btree *pBt;               /* The Btree to which this cursor belongs */
  BtCursor *pPrev, *pNext;  /* List of all cursors */
  Pgno pgnoRoot;            /* The root page of this tree */
  MemPage *pPage;           /* Page that contains the entry */
  u16 idx;                  /* Index of the entry in pPage->apCell[] */
  u8 bSkipNext;             /* sqliteBtreeNext() is no-op if true */
  u8 iMatch;                /* compare result from last sqliteBtreeMoveto() */
};

/*

  pFBlk->iSize = SQLITE_PAGE_SIZE - pc;
  pFBlk->iNext = 0;
  ((PageHdr*)pPage)->firstFree = pc;
  memset(&pFBlk[1], 0, SQLITE_PAGE_SIZE - pc - sizeof(FreeBlk));
}

/*
** Allocate nByte bytes of space on a page.  nByte must be a 
** multiple of 4.
**
** Return the index into pPage->aDisk[] of the first byte of
** the new allocation. Or return 0 if there is not enough free
** space on the page to satisfy the allocation request.
**
** If the page contains nBytes of free space but does not contain
** nBytes of contiguous free space, then this routine automatically
** calls defragementPage() to consolidate all free space before 
** allocating the new chunk.
*/
static int allocateSpace(MemPage *pPage, int nByte){
  FreeBlk *p;
  u16 *pIdx;
  int start;

  assert( nByte==ROUNDUP(nByte) );

  pPage->nFree -= nByte;
  return start;
}

/*
** Return a section of the MemPage.aDisk[] to the freelist.
** The first byte of the new free block is pPage->aDisk[start]
** and the size of the block is "size" bytes.
**
** Most of the effort here is involved in coalesing adjacent
** free blocks into a single big free block.
*/
static void freeSpace(MemPage *pPage, int start, int size){
  int end = start + size;
  u16 *pIdx, idx;

}

/*
** 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 the
** BTree (usually page 2) has no parent and so for that page, 
** pParent==NULL.
**
** Return SQLITE_OK on success.  If we see that the page does
** not contained a well-formed database page, then return 
** SQLITE_CORRUPT.  Note that a return of SQLITE_OK does not
** guarantee that the page is well-formed.  It only shows that
** we failed to detect any corruption.
*/

  while( idx!=0 ){
    if( idx>SQLITE_PAGE_SIZE-sizeof(FreeBlk) ) goto page_format_error;
    if( idx<sizeof(PageHdr) ) goto page_format_error;
    pFBlk = (FreeBlk*)&pPage->aDisk[idx];
    pPage->nFree += pFBlk->iSize;
    if( pFBlk->iNext <= idx ) goto page_format_error;
    idx = pFBlk->iNext;
  }
  if( pPage->nCell==0 && pPage->nFree==0 ){
    /* As a special case, an uninitialized root page appears to be
    ** an empty database */
    return SQLITE_OK;
  }
  if( pPage->nFree!=freeSpace ) goto page_format_error;
  return SQLITE_OK;

page_format_error:
  return SQLITE_CORRUPT;
}

/*
** Recompute the MemPage.apCell[], MemPage.nCell, and MemPage.nFree parameters
** for a cell after the MemPage.aDisk[] content has be changed significantly.
**
** The computation here is similar to initPage() except that in this case
** the MemPage.aDisk[] field has been set up internally (instead of 
** having been read from disk) so we do not need to do as much error
** checking.
*/
static void reinitPage(MemPage *pPage){

    pPage->nFree += pFBlk->iSize;
    idx = pFBlk->iNext;
  }
  return SQLITE_OK;
}

/*
** Set up a raw page so that it looks like a database page holding
** no entries.
*/
static void zeroPage(MemPage *pPage){
  PageHdr *pHdr;
  FreeBlk *pFBlk;
  memset(pPage, 0, SQLITE_PAGE_SIZE);
  pHdr = (PageHdr*)pPage;
  pHdr->firstCell = 0;

  return rc;

page1_init_failed:
  sqlitepager_unref(pBt->page1);
  pBt->page1 = 0;
  return rc;
}

/*
** Create a new database by initializing the first two pages.
*/
static int newDatabase(Btree *pBt){
  MemPage *pRoot;
  PageOne *pP1;
  if( sqlitepager_pagecount(pBt->pPager)>0 ) return SQLITE_OK;
  pP1 = pBt->page1;
  rc = sqlitepager_write(pBt->page1);
  if( rc ) return rc;
  rc = sqlitepager_get(pBt->pPager, 2, &pRoot);
  if( rc ) return rc;
  rc = sqlitepager_write(pRoot);
  if( rc ){
    sqlitepager_unref(pRoot);
    return rc;
  }
  strcpy(pP1->zMagic, zMagicHeader);
  zeroPage(pRoot);
  sqlitepager_unref(pRoot);
  return SQLITE_OK;
}

/*
** Attempt to start a new transaction.
**
** A transaction must be started before attempting any changes
** to the database.  None of the following routines will work
** unless a transaction is started first:
**
**      sqliteBtreeCreateTable()
**      sqliteBtreeClearTable()
**      sqliteBtreeDropTable()
**      sqliteBtreeInsert()
**      sqliteBtreeDelete()
**      sqliteBtreeUpdateMeta()
*/
int sqliteBtreeBeginTrans(Btree *pBt){
  int rc;
  PageOne *pP1;
  if( pBt->inTrans ) return SQLITE_ERROR;
  if( pBt->page1==0 ){
    rc = lockBtree(pBt);
    if( rc!=SQLITE_OK ) return rc;
  }
  rc = sqlitepager_write(pBt->page1);
  if( rc==SQLITE_OK ){
    pBt->inTrans = 1;
  }





  return newDatabase(pBt);
}

/*
** Remove the last reference to the database file.  This will
** remove the read lock.
*/
static void unlockBtree(Btree *pBt){

  if( pBt->pCursor!=0 ) return SQLITE_ERROR;
  rc = sqlitepager_rollback(pBt->pPager);
  unlockBtree(pBt);
  return rc;
}

/*
** Create a new cursor for the BTree whose root is on the page
** iTable.  The act of acquiring a cursor gets a read lock on 
** the database file.
*/
int sqliteBtreeCursor(Btree *pBt, int iTable, BtCursor **ppCur){
  int rc;
  BtCursor *pCur;
  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 = sqlitepager_get(pBt->pPager, pCur->pgnoRoot, &pCur->pPage);
  if( rc!=SQLITE_OK ){
    goto create_cursor_exception;
  }
  rc = initPage(pCur->pPage, pCur->pgnoRoot, 0);
  if( rc!=SQLITE_OK ){
    goto create_cursor_exception;
  }
  pCur->pPrev = 0;
  pCur->pNext = pBt->pCursor;
  if( pCur->pNext ){
    pCur->pNext->pPrev = pCur;

/*
** Move the cursor to the root page
*/
static int moveToRoot(BtCursor *pCur){
  MemPage *pNew;
  int rc;

  rc = sqlitepager_get(pCur->pBt->pPager, pCur->pgnoRoot, &pNew);
  if( rc ) return rc;
  sqlitepager_unref(pCur->pPage);
  pCur->pPage = pNew;
  pCur->idx = 0;
  return SQLITE_OK;
}

  int rc;
  rc = moveToRoot(pCur);
  if( rc ) return rc;
  for(;;){
    int lwr, upr;
    Pgno chldPg;
    MemPage *pPage = pCur->pPage;
    int c = -1;
    lwr = 0;
    upr = pPage->nCell-1;
    while( lwr<=upr ){

      pCur->idx = (lwr+upr)/2;
      rc = compareKey(pCur, pKey, nKey, &c);
      if( rc ) return rc;
      if( c==0 ){
        pCur->iMatch = c;
        if( pRes ) *pRes = 0;
        return SQLITE_OK;

** needs to do that.
*/
static int freePage(Btree *pBt, void *pPage, Pgno pgno){
  PageOne *pPage1 = pBt->page1;
  OverflowPage *pOvfl = (OverflowPage*)pPage;
  int rc;
  int needOvflUnref = 0;

  if( pgno==0 ){
    assert( pOvfl!=0 );
    pgno = sqlitepager_pagenumber(pOvfl);
  }
  rc = sqlitepager_write(pPage1);
  if( rc ){
    return rc;

    pSpace += n;
  }
  return SQLITE_OK;
}

/*
** 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){
  MemPage *pThis;

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

static void reparentChildPages(Pager *pPager, Page *pPage){
  int i;
  for(i=0; i<pPage->nCell; i++){
    reparentPage(pPager, pPage->apCell[i]->leftChild, pPage);
  }
  reparentPage(pPager, ((PageHdr*)pPage)->rightChild, pPage);
}

/*
** This routine redistributes Cells on pPage and up to two siblings
** of pPage so that all pages have about the same amount of free space.
** Usually one siblings on either side of pPage are used in the repack,
** though both siblings might come from one side if pPage is the first
** or last child of its parent.  If pPage has fewer than two siblings
** (something which can only happen if pPage is the root page or a 
** child of root) then all siblings participate in the repack.
**
** The number of siblings of pPage might be increased or decreased by
** one in order to keep all pages between 2/3 and completely 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.
**
** Note that when this routine is called, some of the Cells on pPage
** might not actually be stored in pPage->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->aDisk[].
*/
static int repack(Btree *pBt, MemPage *pPage){
  MemPage *pParent;            /* The parent of pPage */
  MemPage *apOld[3];           /* pPage and up to two siblings before repack */
  Pgno pgnoOld[3];             /* Page numbers for each page in apOld[] */
  MemPage *apNew[4];           /* pPage and up to 3 siblings after repack */
  int idxDiv[3];               /* Indices of divider cells in pParent */
  Cell *apDiv[3];              /* Divider cells in pParent */
  int nCell;                   /* Number of cells in apCell[] */
  int nOld;                    /* Number of pages in apOld[] */
  int nNew;                    /* Number of pages in apNew[] */
  int perPage;                 /* Approximate number of bytes per page */
  int nDiv;                    /* Number of cells in apDiv[] */
  Cell *apCell[MX_CELL*3+5];   /* All cells from pages being repacked */
  int unrefPage = 0;           /* If true, then unref pPage when done */

  /*
  ** Early out if no repacking is needed.
  */
  if( pPage->nFree>=0 && pPage->nFree<SQLITE_PAGE_SIZE/2 ){
    return SQLITE_OK;
  }

  /*
  ** Find the parent of the page to be repacked.
  */
  pParent = pPage->pParent;

  /*
  ** If there is no parent, it means the page is the root page.
  ** special rules apply.
  */
  if( pParent==0 ){
    Pgno pgnoChild;
    Page *pChild;
    if( pPage->nCell==0 ){
      if( ((PageHdr*)pPage)->rightChild ){
        /* The root page is under full.  Copy the one child page
        ** into the root page and return.  This reduces the depth
        ** of the BTree by one.
        */
        pgnoChild = ((PageHdr*)pPage->rightChild;
        rc = sqlitepager_get(pBt, pgnoChild, &pChild);
        if( rc ) return rc;
        memcpy(pPage, pChild, SQLITE_PAGE_SIZE);
        pPage->isInit = 0;
        initPage(pPage, sqlitepager_pagenumber(pPage), 0);
        reparentChildPages(pBt->pPager, pPage);
        freePage(pBt, pChild, pgnoChild);
        sqlitepager_unref(pChild);
      }
      return SQLITE_OK;
    }
    if( pPage->nFree>=0 ){
      /* It is OK for the root page to be less than half full.
      */
      return SQLITE_OK;
    }
    /* If we get to here, it means the root page is over full.
    ** When this happens, Create a new child page and copy the
    ** contents of the root into the child.  Then make the root
    ** page and 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);
    if( rc ) return rc;
    memcpy(pChild, pPage, SQLITE_PAGE_SIZE);
    for(i=0; i<pPage->nCell; i++){
      if( pPage->apCell[i]>(Cell*)pPage && pPage->apCell[i]<(Cell*)&pPage[1] ){
        int offset = (int)pPage->apCell[i] - (int)pPage;
        pChild->apCell[i] = (Cell*)((int)pChild + offset);
      }else{
        pChild->apCell[i] = pPage->apCell[i];
      }
    }
    pChild->isInit = 1;
    pChild->nCell = pPage->nCell;
    pChild->nFree = pPage->nFree;
    /* reparentChildPages(pBt->pPager, pChild); */
    zeroPage(pPage);
    ((PageHdr*)pPage)->rightChild = pgnoChild;
    pParent = pPage;
    pPage = pChild;
    unrefPage = 1;
  }

  /*
  ** Find the Cell in the parent page that refers to the page
  ** to be repacked.
  */
  idx = -1;
  pgno = sqlitepager_pagenumber(pPage);
  for(i=0; i<pParent->nCell; i++){
    if( pParent->apCell[i]->h.leftChild==pgno ){
      idx = i;
      break;
    }
  }
  if( idx<0 && ((PageHdr*)pPage)->rightChild==pgno ){
    idx = pPage->nCell;
  }
  if( idx<0 ){
    rc = SQLITE_CORRUPT;
    goto end_of_repack;
  }

  /*
  ** Get sibling pages and their dividers
  */
  if( idx==pPage->nCell ){
    idx -= 2;
  }else{
    idx--;
  }
  if( idx<0 ) idx = 0;
  nDiv = 0;
  nOld = 0;
  for(i=0; i<3; i++){
    if( i+idx<pParent->nCell ){
      idxDiv[i] = i+idx;
      apDiv[i] = pParent->apCell[i+idx];
      nDiv++;
      pgnoOld[i] = apDiv[i]->h.leftChild;
      rc = sqlitepager_get(pBt, pgnoOld[i], &apOld[i]);
      if( rc ) goto end_of_repack;
      nOld++;
    }
    if( i+idx==pParent->nCell ){
      pgnoOld[i] = pParent->rightChild;
      rc = sqlitepager_get(pBt, pgnoOld[i], &apOld[i]);
      if( rc ) goto end_of_repack;
      nOld++;
    }
  }

  /*
  ** Get all cells
  */
  nCell = 0;
  for(i=0; i<nOld; i++){
    MemPage *pOld = apOld[i];
    for(j=0; j<pOld->nCell; j++){
      apCell[nCell++] = pOld->apCell[j];
    }
    if( i<nOld-1 ){
      apCell[nCell++] = apDiv[i];
    }
  }

  /*
  ** Estimate the number of pages needed
  */
  totalSize = 0;
  for(i=0; i<nCell; i++){
    totalSize += cellSize(apCell[i]);
  }
  nNew = (totalSize + (SQLITE_PAGE_SIZE - sizeof(PageHdr) - 1)) /
            (SQLITE_PAGE_SIZE - sizeof(PageHdr));
  perPage = totalSize/nNew;
  

  /*
  ** Allocate new pages
  */
  for(i=0; i<nNew; i++){
    rc = allocatePage(pBt, &apNew[i], &pgnoNew[i]);
    if( rc ) goto end_of_repack;
    zeroPage(apNew[i]);
  }

  /*
  ** Copy data into the new pages
  */
  for(i=0; i<nNew; i++){
  }

  /*
  ** Reparent children of all cells
  */

  /*
  ** Release the old pages
  */
  for(i=0; i<nOld; i++){
    releasePage(pBt, apOld[i], 0);
  }

  /*
  ** Repack the parent page, if necessary
  */
  if( needToRepackParent ){
    return repack(pParent);
  }
  rc = SQLITE_OK;

end_of_repack:
  if( unrefPage ){
    sqlitepager_unref(pPage);
  }
  return rc;
}

/*
** Attempt to move N or more bytes out of the page that the cursor
** points to into the left sibling page.  (The left sibling page
** contains cells that are less than the cells on this page.)  The
** entry that the cursor is pointing to cannot be moved.  Return
** TRUE if successful and FALSE if not.

){
  Cell newCell;
  int rc;
  int loc;
  MemPage *pPage;
  Btree *pBt = pCur->pBt;

  if( !pCur->pBt->inTrans ){
    return SQLITE_ERROR;  /* Must start a transaction first */
  }
  rc = sqliteBtreeMoveTo(pCur, pKey, nKey, &loc);
  if( rc ) return rc;
  rc = sqlitepager_write(pCur->pPage);
  if( rc ) return rc;
  rc = fillInCell(pBt, &newCell, pKey, nKey, pData, nData);
  if( rc ) return rc;
  if( loc==0 ){

    /* The page being refilled is the root of the BTree and it has
    ** no entries of its own.  If there is a child page, then make the
    ** child become the new root.
    */
    MemPage *pChild;
    Pgno pgnoChild;
    assert( pPage->pParent==0 );
    assert( sqlitepager_pagenumber(pPage)==pCur->pgnoRoot );
    pgnoChild = ((PageHdr*)pPage)->rightChild;
    if( pgnoChild==0 ){
      return SQLITE_OK;
    }
    rc = sqlitepager_get(pPager, pgno, &pChild);
    if( rc ) return rc;
    memcpy(pPage, pChild, SQLITE_PAGE_SIZE);
    memset(&pPage->aDisk[SQLITE_PAGE_SIZE], 0, EXTRA_SIZE);
    freePage(pCur->pBt, pChild, pgnoChild);
    sqlitepager_unref(pChild);
    rc = initPage(pPage, pCur->pgnoRoot, 0);
    reparentChildPages(pPager, pPage);
    return SQLITE_OK;
  }

  /** merge with siblings **/

  /** borrow from siblings **/

** sqliteBtreeNext() after a delete and the cursor will be left
** pointing to the first entry after the deleted entry.
*/
int sqliteBtreeDelete(BtCursor *pCur){
  MemPage *pPage = pCur->pPage;
  Cell *pCell;
  int rc;

  if( !pCur->pBt->inTrans ){
    return SQLITE_ERROR;  /* Must start a transaction first */
  }
  if( pCur->idx >= pPage->nCell ){
    return SQLITE_ERROR;  /* The cursor is not pointing to anything */
  }
  rc = sqlitepager_write(pPage);
  if( rc ) return rc;
  pCell = pPage->apCell[pCur->idx];
  if( pPage->pHdr->rightChild ){

    pCur->bSkipNext = 1;
  }else{
    pCur->idx--;
  }
  rc = refillPage(pCur);
  return rc;
}

/*
** Create a new BTree in the same file.  Write into *piTable the index
** of the root page of the new table.
*/
int sqliteBtreeCreateTable(Btree *pBt, int *piTable){
  MemPage *pRoot;
  Pgno pgnoRoot;
  int rc;
  if( !pBt->inTrans ){
    return SQLITE_ERROR;  /* Must start a transaction first */
  }
  rc = allocatePage(pBt, &pRoot, &pgnoRoot);
  if( rc ) return rc;
  sqlitepager_write(pRoot);
  zeroPage(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){
  MemPage *pPage;
  int rc;
  int i;
  Cell *pCell;
  int idx;

  rc = sqlitepager_get(pBt->pPager, pgno, &pPage);
  if( rc ) return rc;
  idx = ((PageHdr*)pPage)->firstCell;
  while( idx>0 ){
    pCell = (Cell*)&pPage->aDisk[idx];
    idx = pCell->h.iNext;
    if( pCell->h.leftChild ){
      rc = clearDatabasePage(pBt, pCell->h.leftChild);
      if( rc ) return rc;
    }
    rc = clearCell(pCell);
    if( rc ) return rc;
  }
  return freePage(pBt, pPage, pgno);
}

/*
** Delete all information from a single table in the database.
*/
int sqliteBtreeClearTable(Btree *pBt, int iTable){
  int rc;
  if( !pBt->inTrans ){
    return SQLITE_ERROR;  /* Must start a transaction first */
  }
  rc = clearDatabasePage(pBt, (Pgno)iTable);
  if( rc ){
    sqliteBtreeRollback(pBt);
    return rc;
  }
}

/*
** Erase all information in a table and add the root of the table to
** the freelist.  Except, the root of the principle table (the one on
** page 2) is never added to the freelist.
*/
int sqliteBtreeDropTable(Btree *pBt, int iTable){
  int rc;
  MemPage *pPage;
  if( !pBt->inTrans ){
    return SQLITE_ERROR;  /* Must start a transaction first */
  }
  rc = sqlitepager_get(pBt->pPager, (Pgno)iTable, &pPage);
  if( rc==SQLITE_OK ){
    rc = sqliteBtreeClearTable(pBt, iTable);
  }
  if( rc==SQLITE_OK && iTable!=2 ){
    rc = freePage(pBt, pPage, (Pgno)iTable);
  }
  sqlitepager_unref(pPage);
  return rc;  
}

/*
** Read the meta-information out of a database file.
*/
int sqliteBtreeGetMeta(Btree *pBt, int *aMeta){
  PageOne *pP1;
  int rc;

  rc = sqlitepager_get(pBt->pPager, 1, &pP1);
  if( rc ) return rc;
  memcpy(aMeta, pP1->aMeta, sizeof(pP1->aMeta));
  sqlitepager_unref(pP1);
  return SQLITE_OK;
}

/*
** Write meta-information back into the database.
*/
int sqliteBtreeUpdateMeta(Btree *pBt, int *aMeta){
  PageOne *pP1;
  int rc;
  if( !pBt->inTrans ){
    return SQLITE_ERROR;  /* Must start a transaction first */
  }
  pP1 = pBt->page1;
  rc = sqlitepager_write(pP1);
  if( rc ) return rc;
  memcpy(pP1->aMeta, aMeta, sizeof(pP1->aMeta));
  return SQLITE_OK;
}

**   drh@hwaci.com
**   http://www.hwaci.com/drh/
**
*************************************************************************
** This header file defines the interface that the sqlite B-Tree file
** subsystem.
**
** @(#) $Id: btree.h,v 1.4 2001/06/08 00:21:53 drh Exp $
*/

typedef struct Btree Btree;
typedef struct BtCursor BtCursor;

int sqliteBtreeOpen(const char *zFilename, int mode, Btree **ppBtree);
int sqliteBtreeClose(Btree*);

int sqliteBtreeBeginTrans(Btree*);
int sqliteBtreeCommit(Btree*);
int sqliteBtreeRollback(Btree*);

int sqliteBtreeCreateTable(Btree*, int*);
int sqliteBtreeDropTable(Btree*, int);
int sqliteBtreeClearTable(Btree*, int);

int sqliteBtreeCursor(Btree*, int iTable, BtCursor **ppCur);
int sqliteBtreeMoveto(BtCursor*, void *pKey, int nKey, *pRes);
int sqliteBtreeDelete(BtCursor*);
int sqliteBtreeInsert(BtCursor*, void *pKey, int nKey, void *pData, int nData);
int sqliteBtreeNext(BtCursor*, int *pRes);
int sqliteBtreeKeySize(BtCursor*, int *pSize);
int sqliteBtreeKey(BtCursor*, int offset, int amt, char *zBuf);
int sqliteBtreeDataSize(BtCursor*, int *pSize);
int sqliteBtreeData(BtCursor*, int offset, int amt, char *zBuf);
int sqliteBtreeCloseCursor(BtCursor*);

#define SQLITE_N_BTREE_META 3
int sqliteBtreeGetMeta(Btree*, int*);
int sqliteBtreeUpdateMeta(Btree*, int*);