SQLite

  assert( top+nByte <= (int)pPage->pBt->usableSize );
  *pIdx = top;
  return SQLITE_OK;
}

/*
** Return a section of the pPage->aData to the freelist.
** The first byte of the new free block is pPage->aData[iStart]
** and the size of the block is iSize bytes.
**
** Most of the effort here is involved in coalesing adjacent
** free blocks into a single big free block.
*/
static int freeSpace(MemPage *pPage, int iStart, int iSize){
  int iPtr;                  /* Address of pointer to next freeblock */
  int iFreeBlk;              /* Address of the next freeblock */
  int hdr;                   /* Page header size.  0 or 100 */
  int iLast;                 /* Largest possible freeblock offset */
  unsigned char *data = pPage->aData;   /* Page content */

  assert( pPage->pBt!=0 );
  assert( sqlite3PagerIswriteable(pPage->pDbPage) );
  assert( iStart>=pPage->hdrOffset+6+pPage->childPtrSize );
  assert( (iStart + iSize) <= (int)pPage->pBt->usableSize );
  assert( sqlite3_mutex_held(pPage->pBt->mutex) );
  assert( iSize>=4 );   /* Minimum cell size is 4 */

  if( pPage->pBt->btsFlags & BTS_SECURE_DELETE ){
    /* Overwrite deleted information with zeros when the secure_delete
    ** option is enabled */
    memset(&data[iStart], 0, iSize);
  }

  /* Add the space back into the linked list of freeblocks.  Note that
  ** even though the freeblock list was checked by btreeInitPage(),
  ** btreeInitPage() did not detect overlapping cells or
  ** freeblocks that overlapped cells.   Nor does it detect when the
  ** cell content area exceeds the value in the page header.  If these
  ** situations arise, then subsequent insert operations might corrupt
  ** the freelist.  So we do need to check for corruption while scanning
  ** the freelist.
  */
  hdr = pPage->hdrOffset;
  iPtr = hdr + 1;
  iLast = pPage->pBt->usableSize - 4;
  assert( iStart<=iLast );
  while( (iFreeBlk = get2byte(&data[iPtr]))<iStart && iFreeBlk>0 ){
    if( iFreeBlk<iPtr+4 ){
      return SQLITE_CORRUPT_BKPT;
    }
    iPtr = iFreeBlk;
  }
  if( iFreeBlk>iLast ){
    return SQLITE_CORRUPT_BKPT;
  }
  assert( iFreeBlk>iPtr || iFreeBlk==0 );
  put2byte(&data[iPtr], iStart);
  put2byte(&data[iStart], iFreeBlk);
  put2byte(&data[iStart+2], iSize);
  pPage->nFree = pPage->nFree + (u16)iSize;

  /* Coalesce adjacent free blocks */
  iPtr = hdr + 1;
  while( (iFreeBlk = get2byte(&data[iPtr]))>0 ){
    int iNextBlk;      /* Next freeblock after iFreeBlk */
    int szFreeBlk;     /* Size of iFreeBlk */
    assert( iFreeBlk>iPtr );
    assert( iFreeBlk <= (int)pPage->pBt->usableSize-4 );
    iNextBlk = get2byte(&data[iFreeBlk]);
    szFreeBlk = get2byte(&data[iFreeBlk+2]);
    if( iFreeBlk + szFreeBlk + 3 >= iNextBlk && iNextBlk>0 ){
      int nFrag;     /* Fragment bytes in between iFreeBlk and iNextBlk */
      int x;         /* Temp value */
      nFrag = iNextBlk - (iFreeBlk+szFreeBlk);
      if( (nFrag<0) || (nFrag>(int)data[hdr+7]) ){
        return SQLITE_CORRUPT_BKPT;
      }
      data[hdr+7] -= (u8)nFrag;
      x = get2byte(&data[iNextBlk]);
      put2byte(&data[iFreeBlk], x);
      x = iNextBlk + get2byte(&data[iNextBlk+2]) - iFreeBlk;
      put2byte(&data[iFreeBlk+2], x);
    }else{
      iPtr = iFreeBlk;
    }
  }

  /* If the cell content area begins with a freeblock, remove it. */
  if( data[hdr+1]==data[hdr+5] && data[hdr+2]==data[hdr+6] ){
    int top;
    iFreeBlk = get2byte(&data[hdr+1]);
    memcpy(&data[hdr+1], &data[iFreeBlk], 2);
    top = get2byte(&data[hdr+5]) + get2byte(&data[iFreeBlk+2]);
    put2byte(&data[hdr+5], top);
  }
  assert( sqlite3PagerIswriteable(pPage->pDbPage) );
  return SQLITE_OK;
}

/*