SQLite

**
** Use the separate sqlite3BtreeCursorRestore() routine to restore a cursor
** back to where it ought to be if this routine returns true.
*/
int sqlite3BtreeCursorHasMoved(BtCursor *pCur){
  return pCur->eState!=CURSOR_VALID;
}

/*
** Return a pointer to a fake BtCursor object that will always answer
** false to the sqlite3BtreeCursorHasMoved() routine above.  The fake
** cursor returned must not be used with any other Btree interface.
*/
BtCursor *sqlite3BtreeFakeValidCursor(void){
  static u8 fakeCursor = CURSOR_VALID;
  assert( offsetof(BtCursor, eState)==0 );
  return (BtCursor*)&fakeCursor;
}

/*
** This routine restores a cursor back to its original position after it
** has been moved by some outside activity (such as a btree rebalance or
** a row having been deleted out from under the cursor).  
**
** On success, the *pDifferentRow parameter is false if the cursor is left

int sqlite3BtreeCursor(
  Btree*,                              /* BTree containing table to open */
  int iTable,                          /* Index of root page */
  int wrFlag,                          /* 1 for writing.  0 for read-only */
  struct KeyInfo*,                     /* First argument to compare function */
  BtCursor *pCursor                    /* Space to write cursor structure */
);
BtCursor *sqlite3BtreeFakeValidCursor(void);
int sqlite3BtreeCursorSize(void);
void sqlite3BtreeCursorZero(BtCursor*);
void sqlite3BtreeCursorHintFlags(BtCursor*, unsigned);
#ifdef SQLITE_ENABLE_CURSOR_HINTS
void sqlite3BtreeCursorHint(BtCursor*, int, ...);
#endif

**
** skipNext meaning:
**    eState==SKIPNEXT && skipNext>0:  Next sqlite3BtreeNext() is no-op.
**    eState==SKIPNEXT && skipNext<0:  Next sqlite3BtreePrevious() is no-op.
**    eState==FAULT:                   Cursor fault with skipNext as error code.
*/
struct BtCursor {
  u8 eState;                /* One of the CURSOR_XXX constants (see below) */
  u8 curFlags;              /* zero or more BTCF_* flags defined below */
  u8 curPagerFlags;         /* Flags to send to sqlite3PagerGet() */
  u8 hints;                 /* As configured by CursorSetHints() */
  int nOvflAlloc;           /* Allocated size of aOverflow[] array */
  Btree *pBtree;            /* The Btree to which this cursor belongs */
  BtShared *pBt;            /* The BtShared this cursor points to */
  BtCursor *pNext;          /* Forms a linked list of all cursors */
  Pgno *aOverflow;          /* Cache of overflow page locations */
  CellInfo info;            /* A parse of the cell we are pointing at */
  i64 nKey;                 /* Size of pKey, or last integer key */
  void *pKey;               /* Saved key that was cursor last known position */
  Pgno pgnoRoot;            /* The root page of this tree */

  int skipNext;    /* Prev() is noop if negative. Next() is noop if positive.
                   ** Error code if eState==CURSOR_FAULT */




  /* All fields above are zeroed when the cursor is allocated.  See
  ** sqlite3BtreeCursorZero().  Fields that follow must be manually
  ** initialized. */
  i8 iPage;                 /* Index of current page in apPage */
  u8 curIntKey;             /* Value of apPage[0]->intKey */
  u16 ix;                   /* Current index for apPage[iPage] */
  u16 aiIdx[BTCURSOR_MAX_DEPTH-1];     /* Current index in apPage[i] */

  assert( pC->eCurType!=CURTYPE_VTAB );
  assert( pC->eCurType!=CURTYPE_PSEUDO || pC->nullRow );
  assert( pC->eCurType!=CURTYPE_SORTER );

  if( pC->cacheStatus!=p->cacheCtr ){                /*OPTIMIZATION-IF-FALSE*/
    if( pC->nullRow ){
      if( pC->eCurType==CURTYPE_PSEUDO ){
        /* For the special case of as pseudo-cursor, the seekResult field
        ** identifies the register that holds the record */
        assert( pC->seekResult>0 );
        pReg = &aMem[pC->seekResult];
        assert( pReg->flags & MEM_Blob );
        assert( memIsValid(pReg) );
        pC->payloadSize = pC->szRow = pReg->n;
        pC->aRow = (u8*)pReg->z;
      }else{
        sqlite3VdbeMemSetNull(pDest);
        goto op_column_out;

  VdbeCursor *pCx;

  assert( pOp->p1>=0 );
  assert( pOp->p3>=0 );
  pCx = allocateCursor(p, pOp->p1, pOp->p3, -1, CURTYPE_PSEUDO);
  if( pCx==0 ) goto no_mem;
  pCx->nullRow = 1;
  pCx->seekResult = pOp->p2;
  pCx->isTable = 1;
  /* Give this pseudo-cursor a fake BtCursor pointer so that pCx
  ** can be safely passed to sqlite3VdbeCursorMoveto().  This avoids a test
  ** for pCx->eCurType==CURTYPE_BTREE inside of sqlite3VdbeCursorMoveto()
  ** which is a performance optimization */
  pCx->uc.pCursor = sqlite3BtreeFakeValidCursor();
  assert( pOp->p5==0 );
  break;
}

/* Opcode: Close P1 * * * *
**
** Close a cursor previously opened as P1.  If P1 is not

  /* Cached OP_Column parse information is only valid if cacheStatus matches
  ** Vdbe.cacheCtr.  Vdbe.cacheCtr will never take on the value of
  ** CACHE_STALE (0) and so setting cacheStatus=CACHE_STALE guarantees that
  ** the cache is out of date. */
  u32 cacheStatus;        /* Cache is valid if this matches Vdbe.cacheCtr */
  int seekResult;         /* Result of previous sqlite3BtreeMoveto() or 0
                          ** if there have been no prior seeks on the cursor. */
  /* seekResult does not distinguish between "no seeks have ever occurred
  ** on this cursor" and "the most recent seek was an exact match".
  ** For CURTYPE_PSEUDO, seekResult is the register holding the record */

  /* When a new VdbeCursor is allocated, only the fields above are zeroed.
  ** The fields that follow are uninitialized, and must be individually
  ** initialized prior to first use. */
  VdbeCursor *pAltCursor; /* Associated index cursor from which to read */
  union {
    BtCursor *pCursor;          /* CURTYPE_BTREE or _PSEUDO.  Btree cursor */
    sqlite3_vtab_cursor *pVCur; /* CURTYPE_VTAB.              Vtab cursor */

    VdbeSorter *pSorter;        /* CURTYPE_SORTER.            Sorter object */
  } uc;
  KeyInfo *pKeyInfo;      /* Info about index keys needed by index cursors */
  u32 iHdrOffset;         /* Offset to next unparsed byte of the header */
  Pgno pgnoRoot;          /* Root page of the open btree cursor */
  i16 nField;             /* Number of fields in the header */
  u16 nHdrParsed;         /* Number of header fields parsed so far */
  i64 movetoTarget;       /* Argument to the deferred sqlite3BtreeMoveto() */

** a NULL row.
**
** If the cursor is already pointing to the correct row and that row has
** not been deleted out from under the cursor, then this routine is a no-op.
*/
int sqlite3VdbeCursorMoveto(VdbeCursor **pp, int *piCol){
  VdbeCursor *p = *pp;
  assert( p->eCurType==CURTYPE_BTREE || p->eCurType==CURTYPE_PSEUDO );
  if( p->deferredMoveto ){
    int iMap;
    if( p->aAltMap && (iMap = p->aAltMap[1+*piCol])>0 ){
      *pp = p->pAltCursor;
      *piCol = iMap - 1;
      return SQLITE_OK;
    }
    return handleDeferredMoveto(p);
  }
  if( sqlite3BtreeCursorHasMoved(p->uc.pCursor) ){
    return handleMovedCursor(p);

  }
  return SQLITE_OK;
}

/*
** The following functions:
**