SQLite

  assert( pPage->pBt->db!=0 );
  assert( sqlite3_mutex_held(pPage->pBt->mutex) );
  assert( pPage->pgno==sqlite3PagerPagenumber(pPage->pDbPage) );
  assert( pPage == sqlite3PagerGetExtra(pPage->pDbPage) );
  assert( pPage->aData == sqlite3PagerGetData(pPage->pDbPage) );

  if( !pPage->isInit ){
    u16 pc;            /* Address of a freeblock within pPage->aData[] */
    u32 pc;            /* Address of a freeblock within pPage->aData[] */
    u8 hdr;            /* Offset to beginning of page header */
    u8 *data;          /* Equal to pPage->aData */
    BtShared *pBt;        /* The main btree structure */
    int usableSize;    /* Amount of usable space on each page */
    u16 cellOffset;    /* Offset from start of page to first cell pointer */
    int nFree;         /* Number of unused bytes on the page */
    int top;           /* First byte of the cell content area */

    /* Compute the total free space on the page
    ** EVIDENCE-OF: R-23588-34450 The two-byte integer at offset 1 gives the
    ** start of the first freeblock on the page, or is zero if there are no
    ** freeblocks. */
    pc = get2byte(&data[hdr+1]);
    nFree = data[hdr+7] + top;  /* Init nFree to non-freeblock free space */
    while( pc>0 ){
      u16 next, size;
      if( pc<iCellFirst || pc>iCellLast ){
    if( pc>0 ){
      u32 next, size;
      if( pc<iCellFirst ){
        /* EVIDENCE-OF: R-55530-52930 In a well-formed b-tree page, there will
        ** always be at least one cell before the first freeblock.
        **
        ** Or, the freeblock is off the end of the page
        */
        return SQLITE_CORRUPT_BKPT; 
      }
      while( 1 ){
        if( pc>iCellLast ){
          return SQLITE_CORRUPT_BKPT; /* Freeblock off the end of the page */
        }
      next = get2byte(&data[pc]);
      size = get2byte(&data[pc+2]);
      if( (next>0 && next<=pc+size+3) || pc+size>usableSize ){
        /* Free blocks must be in ascending order. And the last byte of
        next = get2byte(&data[pc]);
        size = get2byte(&data[pc+2]);
        nFree = nFree + size;
        if( next<=pc+size+3 ) break;
        pc = next;
      }
      if( next>0 ){
        return SQLITE_CORRUPT_BKPT;  /* Freeblock not in ascending order */
        ** the free-block must lie on the database page.  */
        return SQLITE_CORRUPT_BKPT; 
      }
      if( pc+size>usableSize ){
        return SQLITE_CORRUPT_BKPT;  /* Last freeblock extends past page end */
      }
      nFree = nFree + size;
      pc = next;
    }

    /* At this point, nFree contains the sum of the offset to the start
    ** of the cell-content area plus the number of free bytes within
    ** the cell-content area. If this is greater than the usable-size
    ** of the page, then the page must be corrupted. This check also
    ** serves to verify that the offset to the start of the cell-content