** Interface to code in fts5_index.c. fts5_index.c contains contains code
** to access the data stored in the %_data table.
*/

typedef struct Fts5Index Fts5Index;
typedef struct Fts5IndexIter Fts5IndexIter;







/*
** Values used as part of the flags argument passed to IndexQuery().
*/
#define FTS5INDEX_QUERY_PREFIX     0x0001   /* Prefix query */
#define FTS5INDEX_QUERY_DESC       0x0002   /* Docs in descending rowid order */
#define FTS5INDEX_QUERY_TEST_NOIDX 0x0004   /* Do not use prefix index */
#define FTS5INDEX_QUERY_SCAN       0x0008   /* Scan query (fts5vocab) */
................................................................................

/*
** This interface is used by the fts5vocab module.
*/
const char *sqlite3Fts5IterTerm(Fts5IndexIter*, int*);
int sqlite3Fts5IterNextScan(Fts5IndexIter*);




/*
** Insert or remove data to or from the index. Each time a document is 
** added to or removed from the index, this function is called one or more
** times.
**
** For an insert, it must be called once for each token in the new document.
................................................................................

int sqlite3Fts5IndexReinit(Fts5Index *p);
int sqlite3Fts5IndexOptimize(Fts5Index *p);
int sqlite3Fts5IndexMerge(Fts5Index *p, int nMerge);

int sqlite3Fts5IndexLoadConfig(Fts5Index *p);

int sqlite3Fts5IterCollist(Fts5IndexIter*, const u8 **, int*);

/*
** End of interface to code in fts5_index.c.
**************************************************************************/

/**************************************************************************
** Interface to code in fts5_varint.c. 
*/

** Interface to code in fts5_index.c. fts5_index.c contains contains code
** to access the data stored in the %_data table.
*/

typedef struct Fts5Index Fts5Index;
typedef struct Fts5IndexIter Fts5IndexIter;

struct Fts5IndexIter {
  i64 iRowid;
  const u8 *pData;
  int nData;
};

/*
** Values used as part of the flags argument passed to IndexQuery().
*/
#define FTS5INDEX_QUERY_PREFIX     0x0001   /* Prefix query */
#define FTS5INDEX_QUERY_DESC       0x0002   /* Docs in descending rowid order */
#define FTS5INDEX_QUERY_TEST_NOIDX 0x0004   /* Do not use prefix index */
#define FTS5INDEX_QUERY_SCAN       0x0008   /* Scan query (fts5vocab) */
................................................................................

/*
** This interface is used by the fts5vocab module.
*/
const char *sqlite3Fts5IterTerm(Fts5IndexIter*, int*);
int sqlite3Fts5IterNextScan(Fts5IndexIter*);

int sqlite3Fts5IterCollist(Fts5IndexIter*, const u8 **, int*);


/*
** Insert or remove data to or from the index. Each time a document is 
** added to or removed from the index, this function is called one or more
** times.
**
** For an insert, it must be called once for each token in the new document.
................................................................................

int sqlite3Fts5IndexReinit(Fts5Index *p);
int sqlite3Fts5IndexOptimize(Fts5Index *p);
int sqlite3Fts5IndexMerge(Fts5Index *p, int nMerge);

int sqlite3Fts5IndexLoadConfig(Fts5Index *p);



/*
** End of interface to code in fts5_index.c.
**************************************************************************/

/**************************************************************************
** Interface to code in fts5_varint.c. 
*/

  assert( pTerm->pSynonym );
  for(p=pTerm; p; p=p->pSynonym){
    Fts5IndexIter *pIter = p->pIter;
    if( sqlite3Fts5IterEof(pIter)==0 && sqlite3Fts5IterRowid(pIter)==iRowid ){
      const u8 *a;
      int n;

      if( bCollist ){
        rc = sqlite3Fts5IterCollist(pIter, &a, &n);
      }else{
        i64 dummy;

        rc = sqlite3Fts5IterPoslist(pIter, pColset, &a, &n, &dummy);



      }

      if( rc!=SQLITE_OK ) goto synonym_poslist_out;
      if( n==0 ) continue;
      if( nIter==nAlloc ){
        int nByte = sizeof(Fts5PoslistReader) * nAlloc * 2;
        Fts5PoslistReader *aNew = (Fts5PoslistReader*)sqlite3_malloc(nByte);
................................................................................
    int bFlag = 0;
    const u8 *a = 0;
    if( pTerm->pSynonym ){
      rc = fts5ExprSynonymList(
          pTerm, 0, pColset, pNode->iRowid, &bFlag, (u8**)&a, &n
      );
    }else{


      rc = sqlite3Fts5IterPoslist(pTerm->pIter, pColset, &a, &n, &dummy);



    }
    if( rc!=SQLITE_OK ) goto ismatch_out;
    sqlite3Fts5PoslistReaderInit(a, n, &aIter[i]);
    aIter[i].bFlag = (u8)bFlag;
    if( aIter[i].bEof ) goto ismatch_out;
  }

................................................................................
  if( pExpr->pConfig->eDetail!=FTS5_DETAIL_FULL ){
    Fts5ExprTerm *pTerm;
    Fts5ExprPhrase *pPhrase = pNear->apPhrase[0];
    pPhrase->poslist.n = 0;
    for(pTerm=&pPhrase->aTerm[0]; pTerm; pTerm=pTerm->pSynonym){
      Fts5IndexIter *pIter = pTerm->pIter;
      if( sqlite3Fts5IterEof(pIter)==0 ){

        int n;
        i64 iRowid;
        rc = sqlite3Fts5IterPoslist(pIter, pNear->pColset, 0, &n, &iRowid);
        if( rc!=SQLITE_OK ){
          *pRc = rc;
          return 0;
        }else if( iRowid==pNode->iRowid && n>0 ){
          pPhrase->poslist.n = 1;
        }




      }
    }
    return pPhrase->poslist.n;
  }else{
    int i;

    /* Check that each phrase in the nearset matches the current row.
................................................................................
    for(i=0; rc==SQLITE_OK && i<pNear->nPhrase; i++){
      Fts5ExprPhrase *pPhrase = pNear->apPhrase[i];
      if( pPhrase->nTerm>1 || pPhrase->aTerm[0].pSynonym || pNear->pColset ){
        int bMatch = 0;
        rc = fts5ExprPhraseIsMatch(pNode, pNear->pColset, pPhrase, &bMatch);
        if( bMatch==0 ) break;
      }else{



        rc = sqlite3Fts5IterPoslistBuffer(
            pPhrase->aTerm[0].pIter, &pPhrase->poslist
        );

      }
    }

    *pRc = rc;
    if( i==pNear->nPhrase && (i==1 || fts5ExprNearIsMatch(pRc, pNear)) ){
      return 1;
    }
................................................................................
  Fts5ExprNode *pNode             /* The "NEAR" node (FTS5_TERM) */
){
  /* As this "NEAR" object is actually a single phrase that consists 
  ** of a single term only, grab pointers into the poslist managed by the
  ** fts5_index.c iterator object. This is much faster than synthesizing 
  ** a new poslist the way we have to for more complicated phrase or NEAR
  ** expressions.  */
  Fts5ExprNearset *pNear = pNode->pNear;
  Fts5ExprPhrase *pPhrase = pNear->apPhrase[0];
  Fts5IndexIter *pIter = pPhrase->aTerm[0].pIter;
  Fts5Colset *pColset = pNear->pColset;
  int rc;

  assert( pNode->eType==FTS5_TERM );
  assert( pNear->nPhrase==1 && pPhrase->nTerm==1 );
  assert( pPhrase->aTerm[0].pSynonym==0 );

  rc = sqlite3Fts5IterPoslist(pIter, pColset, 




      (const u8**)&pPhrase->poslist.p, (int*)&pPhrase->poslist.n, &pNode->iRowid
  );
  pNode->bNomatch = (pPhrase->poslist.n==0);
  return rc;
}

/*
** All individual term iterators in pNear are guaranteed to be valid when
** this function is called. This function checks if all term iterators
** point to the same rowid, and if not, advances them until they do.
** If an EOF is reached before this happens, *pbEof is set to true before
................................................................................
  int *pnCollist
){
  Fts5ExprPhrase *pPhrase = pExpr->apExprPhrase[iPhrase];
  Fts5ExprNode *pNode = pPhrase->pNode;
  int rc = SQLITE_OK;

  assert( iPhrase>=0 && iPhrase<pExpr->nPhrase );


  if( pNode->bEof==0 
   && pNode->iRowid==pExpr->pRoot->iRowid 
   && pPhrase->poslist.n>0
  ){
    Fts5ExprTerm *pTerm = &pPhrase->aTerm[0];
    if( pTerm->pSynonym ){
      int bDel = 0;
................................................................................
        sqlite3Fts5BufferSet(&rc, &pPhrase->poslist, *pnCollist, a);
        *ppCollist = pPhrase->poslist.p;
        sqlite3_free(a);
      }else{
        *ppCollist = a;
      }
    }else{
      sqlite3Fts5IterCollist(pPhrase->aTerm[0].pIter, ppCollist, pnCollist);

    }
  }else{
    *ppCollist = 0;
    *pnCollist = 0;
  }

  return rc;
}

  assert( pTerm->pSynonym );
  for(p=pTerm; p; p=p->pSynonym){
    Fts5IndexIter *pIter = p->pIter;
    if( sqlite3Fts5IterEof(pIter)==0 && sqlite3Fts5IterRowid(pIter)==iRowid ){
      const u8 *a;
      int n;

      if( 0 && bCollist ){
        rc = sqlite3Fts5IterCollist(pIter, &a, &n);
      }else{
        i64 dummy;
#if 0
        rc = sqlite3Fts5IterPoslist(pIter, pColset, &a, &n, &dummy);
#endif
        a = pIter->pData;
        n = pIter->nData;
      }

      if( rc!=SQLITE_OK ) goto synonym_poslist_out;
      if( n==0 ) continue;
      if( nIter==nAlloc ){
        int nByte = sizeof(Fts5PoslistReader) * nAlloc * 2;
        Fts5PoslistReader *aNew = (Fts5PoslistReader*)sqlite3_malloc(nByte);
................................................................................
    int bFlag = 0;
    const u8 *a = 0;
    if( pTerm->pSynonym ){
      rc = fts5ExprSynonymList(
          pTerm, 0, pColset, pNode->iRowid, &bFlag, (u8**)&a, &n
      );
    }else{
      Fts5IndexIter *pIter = pTerm->pIter;
#if 0
      rc = sqlite3Fts5IterPoslist(pTerm->pIter, pColset, &a, &n, &dummy);
#endif
      a = pIter->pData;
      n = pIter->nData;
    }
    if( rc!=SQLITE_OK ) goto ismatch_out;
    sqlite3Fts5PoslistReaderInit(a, n, &aIter[i]);
    aIter[i].bFlag = (u8)bFlag;
    if( aIter[i].bEof ) goto ismatch_out;
  }

................................................................................
  if( pExpr->pConfig->eDetail!=FTS5_DETAIL_FULL ){
    Fts5ExprTerm *pTerm;
    Fts5ExprPhrase *pPhrase = pNear->apPhrase[0];
    pPhrase->poslist.n = 0;
    for(pTerm=&pPhrase->aTerm[0]; pTerm; pTerm=pTerm->pSynonym){
      Fts5IndexIter *pIter = pTerm->pIter;
      if( sqlite3Fts5IterEof(pIter)==0 ){
#if 0
        int n;
        i64 iRowid;
        rc = sqlite3Fts5IterPoslist(pIter, pNear->pColset, 0, &n, &iRowid);
        if( rc!=SQLITE_OK ){
          *pRc = rc;
          return 0;
        }else if( iRowid==pNode->iRowid && n>0 ){
          pPhrase->poslist.n = 1;
        }
#endif
        if( pIter->iRowid==pNode->iRowid && pIter->nData>0 ){
          pPhrase->poslist.n = 1;
        }
      }
    }
    return pPhrase->poslist.n;
  }else{
    int i;

    /* Check that each phrase in the nearset matches the current row.
................................................................................
    for(i=0; rc==SQLITE_OK && i<pNear->nPhrase; i++){
      Fts5ExprPhrase *pPhrase = pNear->apPhrase[i];
      if( pPhrase->nTerm>1 || pPhrase->aTerm[0].pSynonym || pNear->pColset ){
        int bMatch = 0;
        rc = fts5ExprPhraseIsMatch(pNode, pNear->pColset, pPhrase, &bMatch);
        if( bMatch==0 ) break;
      }else{
        Fts5IndexIter *pIter = pPhrase->aTerm[0].pIter;
        fts5BufferSet(&rc, &pPhrase->poslist, pIter->nData, pIter->pData);
#if 0
        rc = sqlite3Fts5IterPoslistBuffer(
            pPhrase->aTerm[0].pIter, &pPhrase->poslist
        );
#endif
      }
    }

    *pRc = rc;
    if( i==pNear->nPhrase && (i==1 || fts5ExprNearIsMatch(pRc, pNear)) ){
      return 1;
    }
................................................................................
  Fts5ExprNode *pNode             /* The "NEAR" node (FTS5_TERM) */
){
  /* As this "NEAR" object is actually a single phrase that consists 
  ** of a single term only, grab pointers into the poslist managed by the
  ** fts5_index.c iterator object. This is much faster than synthesizing 
  ** a new poslist the way we have to for more complicated phrase or NEAR
  ** expressions.  */

  Fts5ExprPhrase *pPhrase = pNode->pNear->apPhrase[0];
  Fts5IndexIter *pIter = pPhrase->aTerm[0].pIter;



  assert( pNode->eType==FTS5_TERM );
  assert( pNode->pNear->nPhrase==1 && pPhrase->nTerm==1 );
  assert( pPhrase->aTerm[0].pSynonym==0 );


  pPhrase->poslist.n = pIter->nData;
  if( pExpr->pConfig->eDetail==FTS5_DETAIL_FULL ){
    pPhrase->poslist.p = (u8*)pIter->pData;
  }
  pNode->iRowid = pIter->iRowid;

  pNode->bNomatch = (pPhrase->poslist.n==0);
  return SQLITE_OK;
}

/*
** All individual term iterators in pNear are guaranteed to be valid when
** this function is called. This function checks if all term iterators
** point to the same rowid, and if not, advances them until they do.
** If an EOF is reached before this happens, *pbEof is set to true before
................................................................................
  int *pnCollist
){
  Fts5ExprPhrase *pPhrase = pExpr->apExprPhrase[iPhrase];
  Fts5ExprNode *pNode = pPhrase->pNode;
  int rc = SQLITE_OK;

  assert( iPhrase>=0 && iPhrase<pExpr->nPhrase );
  assert( pExpr->pConfig->eDetail==FTS5_DETAIL_COLUMNS );

  if( pNode->bEof==0 
   && pNode->iRowid==pExpr->pRoot->iRowid 
   && pPhrase->poslist.n>0
  ){
    Fts5ExprTerm *pTerm = &pPhrase->aTerm[0];
    if( pTerm->pSynonym ){
      int bDel = 0;
................................................................................
        sqlite3Fts5BufferSet(&rc, &pPhrase->poslist, *pnCollist, a);
        *ppCollist = pPhrase->poslist.p;
        sqlite3_free(a);
      }else{
        *ppCollist = a;
      }
    }else{
      *ppCollist = pPhrase->aTerm[0].pIter->pData;
      *pnCollist = pPhrase->aTerm[0].pIter->nData;
    }
  }else{
    *ppCollist = 0;
    *pnCollist = 0;
  }

  return rc;
}

#define FTS5_DATA_ZERO_PADDING 8
#define FTS5_DATA_PADDING 20

typedef struct Fts5Data Fts5Data;
typedef struct Fts5DlidxIter Fts5DlidxIter;
typedef struct Fts5DlidxLvl Fts5DlidxLvl;
typedef struct Fts5DlidxWriter Fts5DlidxWriter;

typedef struct Fts5PageWriter Fts5PageWriter;
typedef struct Fts5SegIter Fts5SegIter;
typedef struct Fts5DoclistIter Fts5DoclistIter;
typedef struct Fts5SegWriter Fts5SegWriter;
typedef struct Fts5Structure Fts5Structure;
typedef struct Fts5StructureLevel Fts5StructureLevel;
typedef struct Fts5StructureSegment Fts5StructureSegment;
................................................................................
** aFirst[1] contains the index in aSeg[] of the iterator that points to
** the smallest key overall. aFirst[0] is unused. 
**
** poslist:
**   Used by sqlite3Fts5IterPoslist() when the poslist needs to be buffered.
**   There is no way to tell if this is populated or not.
*/
struct Fts5IndexIter {


  Fts5Index *pIndex;              /* Index that owns this iterator */
  Fts5Structure *pStruct;         /* Database structure for this iterator */
  Fts5Buffer poslist;             /* Buffer containing current poslist */





  int nSeg;                       /* Size of aSeg[] array */
  int bRev;                       /* True to iterate in reverse order */
  u8 bSkipEmpty;                  /* True to skip deleted entries */
  u8 bEof;                        /* True at EOF */
  u8 bFiltered;                   /* True if column-filter already applied */

................................................................................
}

/*
** Return true if the iterator passed as the second argument currently
** points to a delete marker. A delete marker is an entry with a 0 byte
** position-list.
*/
static int fts5MultiIterIsEmpty(Fts5Index *p, Fts5IndexIter *pIter){
  Fts5SegIter *pSeg = &pIter->aSeg[pIter->aFirst[1].iFirst];
  return (p->rc==SQLITE_OK && pSeg->pLeaf && pSeg->nPos==0);
}

/*
** Advance iterator pIter to the next entry.
**
................................................................................
/*
** This function is used as part of the big assert() procedure implemented by
** fts5AssertMultiIterSetup(). It ensures that the result currently stored
** in *pRes is the correct result of comparing the current positions of the
** two iterators.
*/
static void fts5AssertComparisonResult(
  Fts5IndexIter *pIter, 
  Fts5SegIter *p1,
  Fts5SegIter *p2,
  Fts5CResult *pRes
){
  int i1 = p1 - pIter->aSeg;
  int i2 = p2 - pIter->aSeg;

................................................................................

/*
** This function is a no-op unless SQLITE_DEBUG is defined when this module
** is compiled. In that case, this function is essentially an assert() 
** statement used to verify that the contents of the pIter->aFirst[] array
** are correct.
*/
static void fts5AssertMultiIterSetup(Fts5Index *p, Fts5IndexIter *pIter){
  if( p->rc==SQLITE_OK ){
    Fts5SegIter *pFirst = &pIter->aSeg[ pIter->aFirst[1].iFirst ];
    int i;

    assert( (pFirst->pLeaf==0)==pIter->bEof );

    /* Check that pIter->iSwitchRowid is set correctly. */
................................................................................
** Do the comparison necessary to populate pIter->aFirst[iOut].
**
** If the returned value is non-zero, then it is the index of an entry
** in the pIter->aSeg[] array that is (a) not at EOF, and (b) pointing
** to a key that is a duplicate of another, higher priority, 
** segment-iterator in the pSeg->aSeg[] array.
*/
static int fts5MultiIterDoCompare(Fts5IndexIter *pIter, int iOut){
  int i1;                         /* Index of left-hand Fts5SegIter */
  int i2;                         /* Index of right-hand Fts5SegIter */
  int iRes;
  Fts5SegIter *p1;                /* Left-hand Fts5SegIter */
  Fts5SegIter *p2;                /* Right-hand Fts5SegIter */
  Fts5CResult *pRes = &pIter->aFirst[iOut];

................................................................................
  }while( p->rc==SQLITE_OK );
}


/*
** Free the iterator object passed as the second argument.
*/
static void fts5MultiIterFree(Fts5Index *p, Fts5IndexIter *pIter){
  if( pIter ){
    int i;
    for(i=0; i<pIter->nSeg; i++){
      fts5SegIterClear(&pIter->aSeg[i]);
    }
    fts5StructureRelease(pIter->pStruct);
    fts5BufferFree(&pIter->poslist);
    sqlite3_free(pIter);
  }
}

static void fts5MultiIterAdvanced(
  Fts5Index *p,                   /* FTS5 backend to iterate within */
  Fts5IndexIter *pIter,           /* Iterator to update aFirst[] array for */
  int iChanged,                   /* Index of sub-iterator just advanced */
  int iMinset                     /* Minimum entry in aFirst[] to set */
){
  int i;
  for(i=(pIter->nSeg+iChanged)/2; i>=iMinset && p->rc==SQLITE_OK; i=i/2){
    int iEq;
    if( (iEq = fts5MultiIterDoCompare(pIter, i)) ){
................................................................................
**
** If non-zero is returned, the caller should call fts5MultiIterAdvanced()
** on the iterator instead. That function does the same as this one, except
** that it deals with more complicated cases as well.
*/ 
static int fts5MultiIterAdvanceRowid(
  Fts5Index *p,                   /* FTS5 backend to iterate within */
  Fts5IndexIter *pIter,           /* Iterator to update aFirst[] array for */
  int iChanged                    /* Index of sub-iterator just advanced */

){
  Fts5SegIter *pNew = &pIter->aSeg[iChanged];

  if( pNew->iRowid==pIter->iSwitchRowid
   || (pNew->iRowid<pIter->iSwitchRowid)==pIter->bRev
  ){
    int i;
................................................................................
      pRes->iFirst = (u16)(pNew - pIter->aSeg);
      if( i==1 ) break;

      pOther = &pIter->aSeg[ pIter->aFirst[i ^ 0x0001].iFirst ];
    }
  }


  return 0;
}

/*
** Set the pIter->bEof variable based on the state of the sub-iterators.
*/
static void fts5MultiIterSetEof(Fts5IndexIter *pIter){
  Fts5SegIter *pSeg = &pIter->aSeg[ pIter->aFirst[1].iFirst ];
  pIter->bEof = pSeg->pLeaf==0;
  pIter->iSwitchRowid = pSeg->iRowid;
}

/*
** Move the iterator to the next entry. 
................................................................................
**
** If an error occurs, an error code is left in Fts5Index.rc. It is not 
** considered an error if the iterator reaches EOF, or if it is already at 
** EOF when this function is called.
*/
static void fts5MultiIterNext(
  Fts5Index *p, 
  Fts5IndexIter *pIter,
  int bFrom,                      /* True if argument iFrom is valid */
  i64 iFrom                       /* Advance at least as far as this */
){
  if( p->rc==SQLITE_OK ){
    int bUseFrom = bFrom;
    do {

      int iFirst = pIter->aFirst[1].iFirst;
      int bNewTerm = 0;
      Fts5SegIter *pSeg = &pIter->aSeg[iFirst];
      assert( p->rc==SQLITE_OK );
      if( bUseFrom && pSeg->pDlidx ){
        fts5SegIterNextFrom(p, pSeg, iFrom);
      }else{
        pSeg->xNext(p, pSeg, &bNewTerm);
      }

      if( pSeg->pLeaf==0 || bNewTerm 
       || fts5MultiIterAdvanceRowid(p, pIter, iFirst)
      ){
        fts5MultiIterAdvanced(p, pIter, iFirst, 1);
        fts5MultiIterSetEof(pIter);


      }

      fts5AssertMultiIterSetup(p, pIter);





      bUseFrom = 0;
    }while( pIter->bSkipEmpty && fts5MultiIterIsEmpty(p, pIter) );
  }
}

static void fts5MultiIterNext2(
  Fts5Index *p, 
  Fts5IndexIter *pIter,
  int *pbNewTerm                  /* OUT: True if *might* be new term */
){
  assert( pIter->bSkipEmpty );
  if( p->rc==SQLITE_OK ){
    do {
      int iFirst = pIter->aFirst[1].iFirst;
      Fts5SegIter *pSeg = &pIter->aSeg[iFirst];
      int bNewTerm = 0;

      assert( p->rc==SQLITE_OK );
      pSeg->xNext(p, pSeg, &bNewTerm);
      if( pSeg->pLeaf==0 || bNewTerm 
       || fts5MultiIterAdvanceRowid(p, pIter, iFirst)
      ){
        fts5MultiIterAdvanced(p, pIter, iFirst, 1);
        fts5MultiIterSetEof(pIter);
        *pbNewTerm = 1;
      }else{
        *pbNewTerm = 0;
      }
      fts5AssertMultiIterSetup(p, pIter);

    }while( fts5MultiIterIsEmpty(p, pIter) );
  }
}




static Fts5IndexIter *fts5MultiIterAlloc(
  Fts5Index *p,                   /* FTS5 backend to iterate within */
  int nSeg
){
  Fts5IndexIter *pNew;
  int nSlot;                      /* Power of two >= nSeg */

  for(nSlot=2; nSlot<nSeg; nSlot=nSlot*2);
  pNew = fts5IdxMalloc(p, 
      sizeof(Fts5IndexIter) +             /* pNew */
      sizeof(Fts5SegIter) * (nSlot-1) +   /* pNew->aSeg[] */
      sizeof(Fts5CResult) * nSlot         /* pNew->aFirst[] */
  );
  if( pNew ){
    pNew->nSeg = nSlot;
    pNew->aFirst = (Fts5CResult*)&pNew->aSeg[nSlot];
    pNew->pIndex = p;

  }
  return pNew;
}

/*
** Allocate a new Fts5IndexIter object.
**
** The new object will be used to iterate through data in structure pStruct.
** If iLevel is -ve, then all data in all segments is merged. Or, if iLevel
** is zero or greater, data from the first nSegment segments on level iLevel
** is merged.
**
** The iterator initially points to the first term/rowid entry in the 
................................................................................
  Fts5Index *p,                   /* FTS5 backend to iterate within */
  Fts5Structure *pStruct,         /* Structure of specific index */
  int bSkipEmpty,                 /* True to ignore delete-keys */
  int flags,                      /* FTS5INDEX_QUERY_XXX flags */
  const u8 *pTerm, int nTerm,     /* Term to seek to (or NULL/0) */
  int iLevel,                     /* Level to iterate (-1 for all) */
  int nSegment,                   /* Number of segments to merge (iLevel>=0) */
  Fts5IndexIter **ppOut           /* New object */
){
  int nSeg = 0;                   /* Number of segment-iters in use */
  int iIter = 0;                  /* */
  int iSeg;                       /* Used to iterate through segments */
  Fts5Buffer buf = {0,0,0};       /* Buffer used by fts5SegIterSeekInit() */
  Fts5StructureLevel *pLvl;
  Fts5IndexIter *pNew;

  assert( (pTerm==0 && nTerm==0) || iLevel<0 );

  /* Allocate space for the new multi-seg-iterator. */
  if( p->rc==SQLITE_OK ){
    if( iLevel<0 ){
      assert( pStruct->nSegment==fts5StructureCountSegments(pStruct) );
................................................................................
    fts5MultiIterFree(p, pNew);
    *ppOut = 0;
  }
  fts5BufferFree(&buf);
}

/*
** Create an Fts5IndexIter that iterates through the doclist provided
** as the second argument.
*/
static void fts5MultiIterNew2(
  Fts5Index *p,                   /* FTS5 backend to iterate within */
  Fts5Data *pData,                /* Doclist to iterate through */
  int bDesc,                      /* True for descending rowid order */
  Fts5IndexIter **ppOut           /* New object */
){
  Fts5IndexIter *pNew;
  pNew = fts5MultiIterAlloc(p, 2);
  if( pNew ){
    Fts5SegIter *pIter = &pNew->aSeg[1];

    pNew->bFiltered = 1;
    pIter->flags = FTS5_SEGITER_ONETERM;
    if( pData->szLeaf>0 ){
................................................................................
  fts5DataRelease(pData);
}

/*
** Return true if the iterator is at EOF or if an error has occurred. 
** False otherwise.
*/
static int fts5MultiIterEof(Fts5Index *p, Fts5IndexIter *pIter){
  assert( p->rc 
      || (pIter->aSeg[ pIter->aFirst[1].iFirst ].pLeaf==0)==pIter->bEof 
  );
  return (p->rc || pIter->bEof);
}

/*
** Return the rowid of the entry that the iterator currently points
** to. If the iterator points to EOF when this function is called the
** results are undefined.
*/
static i64 fts5MultiIterRowid(Fts5IndexIter *pIter){
  assert( pIter->aSeg[ pIter->aFirst[1].iFirst ].pLeaf );
  return pIter->aSeg[ pIter->aFirst[1].iFirst ].iRowid;
}

/*
** Move the iterator to the next entry at or following iMatch.
*/
static void fts5MultiIterNextFrom(
  Fts5Index *p, 
  Fts5IndexIter *pIter, 
  i64 iMatch
){
  while( 1 ){
    i64 iRowid;
    fts5MultiIterNext(p, pIter, 1, iMatch);
    if( fts5MultiIterEof(p, pIter) ) break;
    iRowid = fts5MultiIterRowid(pIter);
................................................................................
  }
}

/*
** Return a pointer to a buffer containing the term associated with the 
** entry that the iterator currently points to.
*/
static const u8 *fts5MultiIterTerm(Fts5IndexIter *pIter, int *pn){
  Fts5SegIter *p = &pIter->aSeg[ pIter->aFirst[1].iFirst ];
  *pn = p->term.n;
  return p->term.p;
}

static void fts5ChunkIterate(
  Fts5Index *p,                   /* Index object */
................................................................................
}

/*
** Iterator pIter was used to iterate through the input segments of on an
** incremental merge operation. This function is called if the incremental
** merge step has finished but the input has not been completely exhausted.
*/
static void fts5TrimSegments(Fts5Index *p, Fts5IndexIter *pIter){
  int i;
  Fts5Buffer buf;
  memset(&buf, 0, sizeof(Fts5Buffer));
  for(i=0; i<pIter->nSeg; i++){
    Fts5SegIter *pSeg = &pIter->aSeg[i];
    if( pSeg->pSeg==0 ){
      /* no-op */
................................................................................
  Fts5Structure **ppStruct,       /* IN/OUT: Stucture of index */
  int iLvl,                       /* Level to read input from */
  int *pnRem                      /* Write up to this many output leaves */
){
  Fts5Structure *pStruct = *ppStruct;
  Fts5StructureLevel *pLvl = &pStruct->aLevel[iLvl];
  Fts5StructureLevel *pLvlOut;
  Fts5IndexIter *pIter = 0;       /* Iterator to read input data */
  int nRem = pnRem ? *pnRem : 0;  /* Output leaf pages left to write */
  int nInput;                     /* Number of input segments */
  Fts5SegWriter writer;           /* Writer object */
  Fts5StructureSegment *pSeg;     /* Output segment */
  Fts5Buffer term;
  int bOldest;                    /* True if the output segment is the oldest */
  int eDetail = p->pConfig->eDetail;
................................................................................
  }
  return p - (*pa);
}

static int fts5AppendRowid(
  Fts5Index *p,
  i64 iDelta,
  Fts5IndexIter *pMulti,
  Fts5Colset *pColset,
  Fts5Buffer *pBuf
){
  fts5BufferAppendVarint(&p->rc, pBuf, iDelta);
  return 0;
}

................................................................................
** even iDelta).
**
** If an error occurs, an error code is left in p->rc. 
*/
static int fts5AppendPoslist(
  Fts5Index *p,
  i64 iDelta,
  Fts5IndexIter *pMulti,
  Fts5Colset *pColset,
  Fts5Buffer *pBuf
){
  if( p->rc==SQLITE_OK ){
    Fts5SegIter *pSeg = &pMulti->aSeg[ pMulti->aFirst[1].iFirst ];
    assert( fts5MultiIterEof(p, pMulti)==0 );
    assert( pSeg->nPos>0 );
................................................................................

static void fts5SetupPrefixIter(
  Fts5Index *p,                   /* Index to read from */
  int bDesc,                      /* True for "ORDER BY rowid DESC" */
  const u8 *pToken,               /* Buffer containing prefix to match */
  int nToken,                     /* Size of buffer pToken in bytes */
  Fts5Colset *pColset,            /* Restrict matches to these columns */
  Fts5IndexIter **ppIter          /* OUT: New iterator */
){
  Fts5Structure *pStruct;
  Fts5Buffer *aBuf;
  const int nBuf = 32;

  void (*xMerge)(Fts5Index*, Fts5Buffer*, Fts5Buffer*);
  int (*xAppend)(Fts5Index*, i64, Fts5IndexIter*, Fts5Colset*, Fts5Buffer*);
  if( p->pConfig->eDetail==FTS5_DETAIL_NONE ){
    xMerge = fts5MergeRowidLists;
    xAppend = fts5AppendRowid;
  }else{
    xMerge = fts5MergePrefixLists;
    xAppend = fts5AppendPoslist;
  }
................................................................................
  aBuf = (Fts5Buffer*)fts5IdxMalloc(p, sizeof(Fts5Buffer)*nBuf);
  pStruct = fts5StructureRead(p);

  if( aBuf && pStruct ){
    const int flags = FTS5INDEX_QUERY_SCAN;
    int i;
    i64 iLastRowid = 0;
    Fts5IndexIter *p1 = 0;     /* Iterator used to gather data from index */
    Fts5Data *pData;
    Fts5Buffer doclist;
    int bNewTerm = 1;

    memset(&doclist, 0, sizeof(doclist));
    for(fts5MultiIterNew(p, pStruct, 1, flags, pToken, nToken, -1, 0, &p1);
        fts5MultiIterEof(p, p1)==0;
................................................................................
      );
    }
  }

  return rc;
}





















/*
































































































































** Open a new iterator to iterate though all rowid that match the 
** specified token or token prefix.
*/
int sqlite3Fts5IndexQuery(
  Fts5Index *p,                   /* FTS index to query */
  const char *pToken, int nToken, /* Token (or prefix) to query for */
  int flags,                      /* Mask of FTS5INDEX_QUERY_X flags */
  Fts5Colset *pColset,            /* Match these columns only */
  Fts5IndexIter **ppIter          /* OUT: New iterator object */
){
  Fts5Config *pConfig = p->pConfig;
  Fts5IndexIter *pRet = 0;
  int iIdx = 0;
  Fts5Buffer buf = {0, 0, 0};

  /* If the QUERY_SCAN flag is set, all other flags must be clear. */
  assert( (flags & FTS5INDEX_QUERY_SCAN)==0 || flags==FTS5INDEX_QUERY_SCAN );

  if( sqlite3Fts5BufferSize(&p->rc, &buf, nToken+1)==0 ){

    memcpy(&buf.p[1], pToken, nToken);

#ifdef SQLITE_DEBUG





    /* If the QUERY_TEST_NOIDX flag was specified, then this must be a
    ** prefix-query. Instead of using a prefix-index (if one exists), 
    ** evaluate the prefix query using the main FTS index. This is used
    ** for internal sanity checking by the integrity-check in debug 
    ** mode only.  */

    if( pConfig->bPrefixIndex==0 || (flags & FTS5INDEX_QUERY_TEST_NOIDX) ){
      assert( flags & FTS5INDEX_QUERY_PREFIX );
      iIdx = 1+pConfig->nPrefix;
    }else
#endif
    if( flags & FTS5INDEX_QUERY_PREFIX ){
      int nChar = fts5IndexCharlen(pToken, nToken);
      for(iIdx=1; iIdx<=pConfig->nPrefix; iIdx++){
        if( pConfig->aPrefix[iIdx-1]==nChar ) break;
      }
    }

    if( iIdx<=pConfig->nPrefix ){

      Fts5Structure *pStruct = fts5StructureRead(p);
      buf.p[0] = (u8)(FTS5_MAIN_PREFIX + iIdx);
      if( pStruct ){
        fts5MultiIterNew(p, pStruct, 1, flags, buf.p, nToken+1, -1, 0, &pRet);
        fts5StructureRelease(pStruct);
      }
    }else{

      int bDesc = (flags & FTS5INDEX_QUERY_DESC)!=0;
      buf.p[0] = FTS5_MAIN_PREFIX;
      fts5SetupPrefixIter(p, bDesc, buf.p, nToken+1, pColset, &pRet);
    }







    if( p->rc ){
      sqlite3Fts5IterClose(pRet);
      pRet = 0;
      fts5CloseReader(p);
    }

    *ppIter = pRet;
    sqlite3Fts5BufferFree(&buf);
  }
  return fts5IndexReturn(p);
}

/*
** Return true if the iterator passed as the only argument is at EOF.
*/
int sqlite3Fts5IterEof(Fts5IndexIter *pIter){
  assert( pIter->pIndex->rc==SQLITE_OK );
  return pIter->bEof;
}

/*
** Move to the next matching rowid. 
*/
int sqlite3Fts5IterNext(Fts5IndexIter *pIter){

  assert( pIter->pIndex->rc==SQLITE_OK );
  fts5MultiIterNext(pIter->pIndex, pIter, 0, 0);
  return fts5IndexReturn(pIter->pIndex);
}

/*
** Move to the next matching term/rowid. Used by the fts5vocab module.
*/
int sqlite3Fts5IterNextScan(Fts5IndexIter *pIter){

  Fts5Index *p = pIter->pIndex;

  assert( pIter->pIndex->rc==SQLITE_OK );

  fts5MultiIterNext(p, pIter, 0, 0);
  if( p->rc==SQLITE_OK ){
    Fts5SegIter *pSeg = &pIter->aSeg[ pIter->aFirst[1].iFirst ];
................................................................................
}

/*
** Move to the next matching rowid that occurs at or after iMatch. The
** definition of "at or after" depends on whether this iterator iterates
** in ascending or descending rowid order.
*/
int sqlite3Fts5IterNextFrom(Fts5IndexIter *pIter, i64 iMatch){

  fts5MultiIterNextFrom(pIter->pIndex, pIter, iMatch);
  return fts5IndexReturn(pIter->pIndex);
}

/*
** Return the current rowid.
*/
i64 sqlite3Fts5IterRowid(Fts5IndexIter *pIter){
  return fts5MultiIterRowid(pIter);
}

/*
** Return the current term.
*/
const char *sqlite3Fts5IterTerm(Fts5IndexIter *pIter, int *pn){
  int n;
  const char *z = (const char*)fts5MultiIterTerm(pIter, &n);
  *pn = n-1;
  return &z[1];
}


static int fts5IndexExtractColset (
  Fts5Colset *pColset,            /* Colset to filter on */
  const u8 *pPos, int nPos,       /* Position list */
  Fts5Buffer *pBuf                /* Output buffer */
){
  int rc = SQLITE_OK;
  int i;

  fts5BufferZero(pBuf);
  for(i=0; i<pColset->nCol; i++){
    const u8 *pSub = pPos;
    int nSub = fts5IndexExtractCol(&pSub, nPos, pColset->aiCol[i]);
    if( nSub ){
      fts5BufferAppendBlob(&rc, pBuf, nSub, pSub);
    }
  }
  return rc;
}


/*
** Return a pointer to a buffer containing a copy of the position list for
** the current entry. Output variable *pn is set to the size of the buffer 
** in bytes before returning.
**
** The returned position list does not include the "number of bytes" varint
** field that starts the position list on disk.
*/
int sqlite3Fts5IterPoslist(
  Fts5IndexIter *pIter, 
  Fts5Colset *pColset,            /* Column filter (or NULL) */
  const u8 **pp,                  /* OUT: Pointer to position-list data */
  int *pn,                        /* OUT: Size of position-list in bytes */
  i64 *piRowid                    /* OUT: Current rowid */
){

  Fts5SegIter *pSeg = &pIter->aSeg[ pIter->aFirst[1].iFirst ];
  int eDetail = pIter->pIndex->pConfig->eDetail;

  assert( pIter->pIndex->rc==SQLITE_OK );
  *piRowid = pSeg->iRowid;
  if( eDetail==FTS5_DETAIL_NONE ){
    *pn = pSeg->nPos;
................................................................................
    }
    *pn = pIter->poslist.n;
  }
  return fts5IndexReturn(pIter->pIndex);
}

int sqlite3Fts5IterCollist(
  Fts5IndexIter *pIter, 
  const u8 **pp,                  /* OUT: Pointer to position-list data */
  int *pn                         /* OUT: Size of position-list in bytes */
){

  assert( pIter->pIndex->pConfig->eDetail==FTS5_DETAIL_COLUMNS );
  *pp = pIter->poslist.p;
  *pn = pIter->poslist.n;
  return SQLITE_OK;
}

/*
** This function is similar to sqlite3Fts5IterPoslist(), except that it
** copies the position list into the buffer supplied as the second 
** argument.
*/
int sqlite3Fts5IterPoslistBuffer(Fts5IndexIter *pIter, Fts5Buffer *pBuf){

  Fts5Index *p = pIter->pIndex;
  Fts5SegIter *pSeg = &pIter->aSeg[ pIter->aFirst[1].iFirst ];
  assert( p->rc==SQLITE_OK );
  fts5BufferZero(pBuf);
  fts5SegiterPoslist(p, pSeg, 0, pBuf);
  return fts5IndexReturn(p);
}

/*
** Close an iterator opened by an earlier call to sqlite3Fts5IndexQuery().
*/
void sqlite3Fts5IterClose(Fts5IndexIter *pIter){
  if( pIter ){

    Fts5Index *pIndex = pIter->pIndex;
    fts5MultiIterFree(pIter->pIndex, pIter);
    fts5CloseReader(pIndex);
  }
}

/*
................................................................................
  const char *z,                  /* Index key to query for */
  int n,                          /* Size of index key in bytes */
  int flags,                      /* Flags for Fts5IndexQuery */
  u64 *pCksum                     /* IN/OUT: Checksum value */
){
  int eDetail = p->pConfig->eDetail;
  u64 cksum = *pCksum;
  Fts5IndexIter *pIdxIter = 0;
  Fts5Buffer buf = {0, 0, 0};
  int rc = sqlite3Fts5IndexQuery(p, z, n, flags, 0, &pIdxIter);

  while( rc==SQLITE_OK && 0==sqlite3Fts5IterEof(pIdxIter) ){
    i64 rowid = sqlite3Fts5IterRowid(pIdxIter);

    if( eDetail==FTS5_DETAIL_NONE ){
      cksum ^= sqlite3Fts5IndexEntryCksum(rowid, 0, 0, iIdx, z, n);
    }else{
      rc = sqlite3Fts5IterPoslistBuffer(pIdxIter, &buf);
      if( rc==SQLITE_OK ){
        Fts5PoslistReader sReader;
        for(sqlite3Fts5PoslistReaderInit(buf.p, buf.n, &sReader);
            sReader.bEof==0;
            sqlite3Fts5PoslistReaderNext(&sReader)
        ){
          int iCol = FTS5_POS2COLUMN(sReader.iPos);
          int iOff = FTS5_POS2OFFSET(sReader.iPos);
          cksum ^= sqlite3Fts5IndexEntryCksum(rowid, iCol, iOff, iIdx, z, n);
        }
      }
    }
    if( rc==SQLITE_OK ){
      rc = sqlite3Fts5IterNext(pIdxIter);
    }
  }
  sqlite3Fts5IterClose(pIdxIter);
  fts5BufferFree(&buf);

  *pCksum = cksum;
  return rc;
}


/*
................................................................................
** error, or some other SQLite error code if another error (e.g. OOM)
** occurs.
*/
int sqlite3Fts5IndexIntegrityCheck(Fts5Index *p, u64 cksum){
  int eDetail = p->pConfig->eDetail;
  u64 cksum2 = 0;                 /* Checksum based on contents of indexes */
  Fts5Buffer poslist = {0,0,0};   /* Buffer used to hold a poslist */
  Fts5IndexIter *pIter;           /* Used to iterate through entire index */
  Fts5Structure *pStruct;         /* Index structure */

#ifdef SQLITE_DEBUG
  /* Used by extra internal tests only run if NDEBUG is not defined */
  u64 cksum3 = 0;                 /* Checksum based on contents of indexes */
  Fts5Buffer term = {0,0,0};      /* Buffer used to hold most recent term */
#endif

#define FTS5_DATA_ZERO_PADDING 8
#define FTS5_DATA_PADDING 20

typedef struct Fts5Data Fts5Data;
typedef struct Fts5DlidxIter Fts5DlidxIter;
typedef struct Fts5DlidxLvl Fts5DlidxLvl;
typedef struct Fts5DlidxWriter Fts5DlidxWriter;
typedef struct Fts5Iter Fts5Iter;
typedef struct Fts5PageWriter Fts5PageWriter;
typedef struct Fts5SegIter Fts5SegIter;
typedef struct Fts5DoclistIter Fts5DoclistIter;
typedef struct Fts5SegWriter Fts5SegWriter;
typedef struct Fts5Structure Fts5Structure;
typedef struct Fts5StructureLevel Fts5StructureLevel;
typedef struct Fts5StructureSegment Fts5StructureSegment;
................................................................................
** aFirst[1] contains the index in aSeg[] of the iterator that points to
** the smallest key overall. aFirst[0] is unused. 
**
** poslist:
**   Used by sqlite3Fts5IterPoslist() when the poslist needs to be buffered.
**   There is no way to tell if this is populated or not.
*/
struct Fts5Iter {
  Fts5IndexIter base;             /* Base class containing output vars */

  Fts5Index *pIndex;              /* Index that owns this iterator */
  Fts5Structure *pStruct;         /* Database structure for this iterator */
  Fts5Buffer poslist;             /* Buffer containing current poslist */
  Fts5Colset *pColset;            /* Restrict matches to these columns */

  /* Invoked to set output variables. */
  void (*xSetOutputs)(Fts5Iter*, Fts5SegIter*);

  int nSeg;                       /* Size of aSeg[] array */
  int bRev;                       /* True to iterate in reverse order */
  u8 bSkipEmpty;                  /* True to skip deleted entries */
  u8 bEof;                        /* True at EOF */
  u8 bFiltered;                   /* True if column-filter already applied */

................................................................................
}

/*
** Return true if the iterator passed as the second argument currently
** points to a delete marker. A delete marker is an entry with a 0 byte
** position-list.
*/
static int fts5MultiIterIsEmpty(Fts5Index *p, Fts5Iter *pIter){
  Fts5SegIter *pSeg = &pIter->aSeg[pIter->aFirst[1].iFirst];
  return (p->rc==SQLITE_OK && pSeg->pLeaf && pSeg->nPos==0);
}

/*
** Advance iterator pIter to the next entry.
**
................................................................................
/*
** This function is used as part of the big assert() procedure implemented by
** fts5AssertMultiIterSetup(). It ensures that the result currently stored
** in *pRes is the correct result of comparing the current positions of the
** two iterators.
*/
static void fts5AssertComparisonResult(
  Fts5Iter *pIter, 
  Fts5SegIter *p1,
  Fts5SegIter *p2,
  Fts5CResult *pRes
){
  int i1 = p1 - pIter->aSeg;
  int i2 = p2 - pIter->aSeg;

................................................................................

/*
** This function is a no-op unless SQLITE_DEBUG is defined when this module
** is compiled. In that case, this function is essentially an assert() 
** statement used to verify that the contents of the pIter->aFirst[] array
** are correct.
*/
static void fts5AssertMultiIterSetup(Fts5Index *p, Fts5Iter *pIter){
  if( p->rc==SQLITE_OK ){
    Fts5SegIter *pFirst = &pIter->aSeg[ pIter->aFirst[1].iFirst ];
    int i;

    assert( (pFirst->pLeaf==0)==pIter->bEof );

    /* Check that pIter->iSwitchRowid is set correctly. */
................................................................................
** Do the comparison necessary to populate pIter->aFirst[iOut].
**
** If the returned value is non-zero, then it is the index of an entry
** in the pIter->aSeg[] array that is (a) not at EOF, and (b) pointing
** to a key that is a duplicate of another, higher priority, 
** segment-iterator in the pSeg->aSeg[] array.
*/
static int fts5MultiIterDoCompare(Fts5Iter *pIter, int iOut){
  int i1;                         /* Index of left-hand Fts5SegIter */
  int i2;                         /* Index of right-hand Fts5SegIter */
  int iRes;
  Fts5SegIter *p1;                /* Left-hand Fts5SegIter */
  Fts5SegIter *p2;                /* Right-hand Fts5SegIter */
  Fts5CResult *pRes = &pIter->aFirst[iOut];

................................................................................
  }while( p->rc==SQLITE_OK );
}


/*
** Free the iterator object passed as the second argument.
*/
static void fts5MultiIterFree(Fts5Index *p, Fts5Iter *pIter){
  if( pIter ){
    int i;
    for(i=0; i<pIter->nSeg; i++){
      fts5SegIterClear(&pIter->aSeg[i]);
    }
    fts5StructureRelease(pIter->pStruct);
    fts5BufferFree(&pIter->poslist);
    sqlite3_free(pIter);
  }
}

static void fts5MultiIterAdvanced(
  Fts5Index *p,                   /* FTS5 backend to iterate within */
  Fts5Iter *pIter,                /* Iterator to update aFirst[] array for */
  int iChanged,                   /* Index of sub-iterator just advanced */
  int iMinset                     /* Minimum entry in aFirst[] to set */
){
  int i;
  for(i=(pIter->nSeg+iChanged)/2; i>=iMinset && p->rc==SQLITE_OK; i=i/2){
    int iEq;
    if( (iEq = fts5MultiIterDoCompare(pIter, i)) ){
................................................................................
**
** If non-zero is returned, the caller should call fts5MultiIterAdvanced()
** on the iterator instead. That function does the same as this one, except
** that it deals with more complicated cases as well.
*/ 
static int fts5MultiIterAdvanceRowid(
  Fts5Index *p,                   /* FTS5 backend to iterate within */
  Fts5Iter *pIter,                /* Iterator to update aFirst[] array for */
  int iChanged,                   /* Index of sub-iterator just advanced */
  Fts5SegIter **ppFirst
){
  Fts5SegIter *pNew = &pIter->aSeg[iChanged];

  if( pNew->iRowid==pIter->iSwitchRowid
   || (pNew->iRowid<pIter->iSwitchRowid)==pIter->bRev
  ){
    int i;
................................................................................
      pRes->iFirst = (u16)(pNew - pIter->aSeg);
      if( i==1 ) break;

      pOther = &pIter->aSeg[ pIter->aFirst[i ^ 0x0001].iFirst ];
    }
  }

  *ppFirst = pNew;
  return 0;
}

/*
** Set the pIter->bEof variable based on the state of the sub-iterators.
*/
static void fts5MultiIterSetEof(Fts5Iter *pIter){
  Fts5SegIter *pSeg = &pIter->aSeg[ pIter->aFirst[1].iFirst ];
  pIter->bEof = pSeg->pLeaf==0;
  pIter->iSwitchRowid = pSeg->iRowid;
}

/*
** Move the iterator to the next entry. 
................................................................................
**
** If an error occurs, an error code is left in Fts5Index.rc. It is not 
** considered an error if the iterator reaches EOF, or if it is already at 
** EOF when this function is called.
*/
static void fts5MultiIterNext(
  Fts5Index *p, 
  Fts5Iter *pIter,
  int bFrom,                      /* True if argument iFrom is valid */
  i64 iFrom                       /* Advance at least as far as this */
){

  int bUseFrom = bFrom;

  while( p->rc==SQLITE_OK ){
    int iFirst = pIter->aFirst[1].iFirst;
    int bNewTerm = 0;
    Fts5SegIter *pSeg = &pIter->aSeg[iFirst];
    assert( p->rc==SQLITE_OK );
    if( bUseFrom && pSeg->pDlidx ){
      fts5SegIterNextFrom(p, pSeg, iFrom);
    }else{
      pSeg->xNext(p, pSeg, &bNewTerm);
    }

    if( pSeg->pLeaf==0 || bNewTerm 
     || fts5MultiIterAdvanceRowid(p, pIter, iFirst, &pSeg)
    ){
      fts5MultiIterAdvanced(p, pIter, iFirst, 1);
      fts5MultiIterSetEof(pIter);
      pSeg = &pIter->aSeg[pIter->aFirst[1].iFirst];
      if( pSeg->pLeaf==0 ) return;
    }

    fts5AssertMultiIterSetup(p, pIter);
    assert( pSeg==&pIter->aSeg[pIter->aFirst[1].iFirst] && pSeg->pLeaf );
    if( pIter->bSkipEmpty==0 || pSeg->nPos ){
      pIter->xSetOutputs(pIter, pSeg);
      return;
    }
    bUseFrom = 0;

  }
}

static void fts5MultiIterNext2(
  Fts5Index *p, 
  Fts5Iter *pIter,
  int *pbNewTerm                  /* OUT: True if *might* be new term */
){
  assert( pIter->bSkipEmpty );
  if( p->rc==SQLITE_OK ){
    do {
      int iFirst = pIter->aFirst[1].iFirst;
      Fts5SegIter *pSeg = &pIter->aSeg[iFirst];
      int bNewTerm = 0;

      assert( p->rc==SQLITE_OK );
      pSeg->xNext(p, pSeg, &bNewTerm);
      if( pSeg->pLeaf==0 || bNewTerm 
       || fts5MultiIterAdvanceRowid(p, pIter, iFirst, &pSeg)
      ){
        fts5MultiIterAdvanced(p, pIter, iFirst, 1);
        fts5MultiIterSetEof(pIter);
        *pbNewTerm = 1;
      }else{
        *pbNewTerm = 0;
      }
      fts5AssertMultiIterSetup(p, pIter);

    }while( fts5MultiIterIsEmpty(p, pIter) );
  }
}

static void fts5IterSetOutputs_Noop(Fts5Iter *pIter, Fts5SegIter *pSeg){
}

static Fts5Iter *fts5MultiIterAlloc(
  Fts5Index *p,                   /* FTS5 backend to iterate within */
  int nSeg
){
  Fts5Iter *pNew;
  int nSlot;                      /* Power of two >= nSeg */

  for(nSlot=2; nSlot<nSeg; nSlot=nSlot*2);
  pNew = fts5IdxMalloc(p, 
      sizeof(Fts5Iter) +                  /* pNew */
      sizeof(Fts5SegIter) * (nSlot-1) +   /* pNew->aSeg[] */
      sizeof(Fts5CResult) * nSlot         /* pNew->aFirst[] */
  );
  if( pNew ){
    pNew->nSeg = nSlot;
    pNew->aFirst = (Fts5CResult*)&pNew->aSeg[nSlot];
    pNew->pIndex = p;
    pNew->xSetOutputs = fts5IterSetOutputs_Noop;
  }
  return pNew;
}

/*
** Allocate a new Fts5Iter object.
**
** The new object will be used to iterate through data in structure pStruct.
** If iLevel is -ve, then all data in all segments is merged. Or, if iLevel
** is zero or greater, data from the first nSegment segments on level iLevel
** is merged.
**
** The iterator initially points to the first term/rowid entry in the 
................................................................................
  Fts5Index *p,                   /* FTS5 backend to iterate within */
  Fts5Structure *pStruct,         /* Structure of specific index */
  int bSkipEmpty,                 /* True to ignore delete-keys */
  int flags,                      /* FTS5INDEX_QUERY_XXX flags */
  const u8 *pTerm, int nTerm,     /* Term to seek to (or NULL/0) */
  int iLevel,                     /* Level to iterate (-1 for all) */
  int nSegment,                   /* Number of segments to merge (iLevel>=0) */
  Fts5Iter **ppOut                /* New object */
){
  int nSeg = 0;                   /* Number of segment-iters in use */
  int iIter = 0;                  /* */
  int iSeg;                       /* Used to iterate through segments */
  Fts5Buffer buf = {0,0,0};       /* Buffer used by fts5SegIterSeekInit() */
  Fts5StructureLevel *pLvl;
  Fts5Iter *pNew;

  assert( (pTerm==0 && nTerm==0) || iLevel<0 );

  /* Allocate space for the new multi-seg-iterator. */
  if( p->rc==SQLITE_OK ){
    if( iLevel<0 ){
      assert( pStruct->nSegment==fts5StructureCountSegments(pStruct) );
................................................................................
    fts5MultiIterFree(p, pNew);
    *ppOut = 0;
  }
  fts5BufferFree(&buf);
}

/*
** Create an Fts5Iter that iterates through the doclist provided
** as the second argument.
*/
static void fts5MultiIterNew2(
  Fts5Index *p,                   /* FTS5 backend to iterate within */
  Fts5Data *pData,                /* Doclist to iterate through */
  int bDesc,                      /* True for descending rowid order */
  Fts5Iter **ppOut                /* New object */
){
  Fts5Iter *pNew;
  pNew = fts5MultiIterAlloc(p, 2);
  if( pNew ){
    Fts5SegIter *pIter = &pNew->aSeg[1];

    pNew->bFiltered = 1;
    pIter->flags = FTS5_SEGITER_ONETERM;
    if( pData->szLeaf>0 ){
................................................................................
  fts5DataRelease(pData);
}

/*
** Return true if the iterator is at EOF or if an error has occurred. 
** False otherwise.
*/
static int fts5MultiIterEof(Fts5Index *p, Fts5Iter *pIter){
  assert( p->rc 
      || (pIter->aSeg[ pIter->aFirst[1].iFirst ].pLeaf==0)==pIter->bEof 
  );
  return (p->rc || pIter->bEof);
}

/*
** Return the rowid of the entry that the iterator currently points
** to. If the iterator points to EOF when this function is called the
** results are undefined.
*/
static i64 fts5MultiIterRowid(Fts5Iter *pIter){
  assert( pIter->aSeg[ pIter->aFirst[1].iFirst ].pLeaf );
  return pIter->aSeg[ pIter->aFirst[1].iFirst ].iRowid;
}

/*
** Move the iterator to the next entry at or following iMatch.
*/
static void fts5MultiIterNextFrom(
  Fts5Index *p, 
  Fts5Iter *pIter, 
  i64 iMatch
){
  while( 1 ){
    i64 iRowid;
    fts5MultiIterNext(p, pIter, 1, iMatch);
    if( fts5MultiIterEof(p, pIter) ) break;
    iRowid = fts5MultiIterRowid(pIter);
................................................................................
  }
}

/*
** Return a pointer to a buffer containing the term associated with the 
** entry that the iterator currently points to.
*/
static const u8 *fts5MultiIterTerm(Fts5Iter *pIter, int *pn){
  Fts5SegIter *p = &pIter->aSeg[ pIter->aFirst[1].iFirst ];
  *pn = p->term.n;
  return p->term.p;
}

static void fts5ChunkIterate(
  Fts5Index *p,                   /* Index object */
................................................................................
}

/*
** Iterator pIter was used to iterate through the input segments of on an
** incremental merge operation. This function is called if the incremental
** merge step has finished but the input has not been completely exhausted.
*/
static void fts5TrimSegments(Fts5Index *p, Fts5Iter *pIter){
  int i;
  Fts5Buffer buf;
  memset(&buf, 0, sizeof(Fts5Buffer));
  for(i=0; i<pIter->nSeg; i++){
    Fts5SegIter *pSeg = &pIter->aSeg[i];
    if( pSeg->pSeg==0 ){
      /* no-op */
................................................................................
  Fts5Structure **ppStruct,       /* IN/OUT: Stucture of index */
  int iLvl,                       /* Level to read input from */
  int *pnRem                      /* Write up to this many output leaves */
){
  Fts5Structure *pStruct = *ppStruct;
  Fts5StructureLevel *pLvl = &pStruct->aLevel[iLvl];
  Fts5StructureLevel *pLvlOut;
  Fts5Iter *pIter = 0;       /* Iterator to read input data */
  int nRem = pnRem ? *pnRem : 0;  /* Output leaf pages left to write */
  int nInput;                     /* Number of input segments */
  Fts5SegWriter writer;           /* Writer object */
  Fts5StructureSegment *pSeg;     /* Output segment */
  Fts5Buffer term;
  int bOldest;                    /* True if the output segment is the oldest */
  int eDetail = p->pConfig->eDetail;
................................................................................
  }
  return p - (*pa);
}

static int fts5AppendRowid(
  Fts5Index *p,
  i64 iDelta,
  Fts5Iter *pMulti,
  Fts5Colset *pColset,
  Fts5Buffer *pBuf
){
  fts5BufferAppendVarint(&p->rc, pBuf, iDelta);
  return 0;
}

................................................................................
** even iDelta).
**
** If an error occurs, an error code is left in p->rc. 
*/
static int fts5AppendPoslist(
  Fts5Index *p,
  i64 iDelta,
  Fts5Iter *pMulti,
  Fts5Colset *pColset,
  Fts5Buffer *pBuf
){
  if( p->rc==SQLITE_OK ){
    Fts5SegIter *pSeg = &pMulti->aSeg[ pMulti->aFirst[1].iFirst ];
    assert( fts5MultiIterEof(p, pMulti)==0 );
    assert( pSeg->nPos>0 );
................................................................................

static void fts5SetupPrefixIter(
  Fts5Index *p,                   /* Index to read from */
  int bDesc,                      /* True for "ORDER BY rowid DESC" */
  const u8 *pToken,               /* Buffer containing prefix to match */
  int nToken,                     /* Size of buffer pToken in bytes */
  Fts5Colset *pColset,            /* Restrict matches to these columns */
  Fts5Iter **ppIter          /* OUT: New iterator */
){
  Fts5Structure *pStruct;
  Fts5Buffer *aBuf;
  const int nBuf = 32;

  void (*xMerge)(Fts5Index*, Fts5Buffer*, Fts5Buffer*);
  int (*xAppend)(Fts5Index*, i64, Fts5Iter*, Fts5Colset*, Fts5Buffer*);
  if( p->pConfig->eDetail==FTS5_DETAIL_NONE ){
    xMerge = fts5MergeRowidLists;
    xAppend = fts5AppendRowid;
  }else{
    xMerge = fts5MergePrefixLists;
    xAppend = fts5AppendPoslist;
  }
................................................................................
  aBuf = (Fts5Buffer*)fts5IdxMalloc(p, sizeof(Fts5Buffer)*nBuf);
  pStruct = fts5StructureRead(p);

  if( aBuf && pStruct ){
    const int flags = FTS5INDEX_QUERY_SCAN;
    int i;
    i64 iLastRowid = 0;
    Fts5Iter *p1 = 0;     /* Iterator used to gather data from index */
    Fts5Data *pData;
    Fts5Buffer doclist;
    int bNewTerm = 1;

    memset(&doclist, 0, sizeof(doclist));
    for(fts5MultiIterNew(p, pStruct, 1, flags, pToken, nToken, -1, 0, &p1);
        fts5MultiIterEof(p, p1)==0;
................................................................................
      );
    }
  }

  return rc;
}


static int fts5IndexExtractColset (
  Fts5Colset *pColset,            /* Colset to filter on */
  const u8 *pPos, int nPos,       /* Position list */
  Fts5Buffer *pBuf                /* Output buffer */
){
  int rc = SQLITE_OK;
  int i;

  fts5BufferZero(pBuf);
  for(i=0; i<pColset->nCol; i++){
    const u8 *pSub = pPos;
    int nSub = fts5IndexExtractCol(&pSub, nPos, pColset->aiCol[i]);
    if( nSub ){
      fts5BufferAppendBlob(&rc, pBuf, nSub, pSub);
    }
  }
  return rc;
}

/*
** xSetOutputs callback used by detail=none tables.
*/
static void fts5IterSetOutputs_None(Fts5Iter *pIter, Fts5SegIter *pSeg){
  assert( pIter->pIndex->pConfig->eDetail==FTS5_DETAIL_NONE );
  pIter->base.iRowid = pSeg->iRowid;
  pIter->base.nData = pSeg->nPos;
}

/*
** xSetOutputs callback used by detail=full and detail=col tables when no
** column filters are specified.
*/
static void fts5IterSetOutputs_Nocolset(Fts5Iter *pIter, Fts5SegIter *pSeg){
  pIter->base.iRowid = pSeg->iRowid;
  pIter->base.nData = pSeg->nPos;

  assert( pIter->pIndex->pConfig->eDetail!=FTS5_DETAIL_NONE );
  assert( pIter->pColset==0 || pIter->bFiltered );

  if( pSeg->iLeafOffset+pSeg->nPos<=pSeg->pLeaf->szLeaf ){
    /* All data is stored on the current page. Populate the output 
    ** variables to point into the body of the page object. */
    pIter->base.pData = &pSeg->pLeaf->p[pSeg->iLeafOffset];
  }else{
    /* The data is distributed over two or more pages. Copy it into the
    ** Fts5Iter.poslist buffer and then set the output pointer to point
    ** to this buffer.  */
    fts5BufferZero(&pIter->poslist);
    fts5SegiterPoslist(pIter->pIndex, pSeg, 0, &pIter->poslist);
    pIter->base.pData = pIter->poslist.p;
  }
}

/*
** xSetOutputs callback used by detail=col when there is a column filter.
*/
static void fts5IterSetOutputs_Col(Fts5Iter *pIter, Fts5SegIter *pSeg){
  Fts5Colset *pColset = pIter->pColset;
  pIter->base.iRowid = pSeg->iRowid;

  assert( pIter->pIndex->pConfig->eDetail==FTS5_DETAIL_COLUMNS );
  assert( pColset );

  if( pSeg->iLeafOffset+pSeg->nPos<=pSeg->pLeaf->szLeaf && 0 ){
    /* All data is stored on the current page. Populate the output 
    ** variables to point into the body of the page object. */
    Fts5PoslistWriter writer = {0};
    const u8 *a = &pSeg->pLeaf->p[pSeg->iLeafOffset];
    int n = pSeg->nPos;
    int iCol = 0;
    i64 iPos = 0;
    int iOff = 0;

    fts5BufferZero(&pIter->poslist);
    while( 0==sqlite3Fts5PoslistNext64(a, n, &iOff, &iPos) ){
      if( iPos==pColset->aiCol[iCol] ){
        sqlite3Fts5PoslistWriterAppend(&pIter->poslist, &writer, iPos);
        if( ++iCol>=pColset->nCol ) break;
      }
    }

  }else{
    /* The data is distributed over two or more pages. Copy it into the
    ** Fts5Iter.poslist buffer and then set the output pointer to point
    ** to this buffer.  */
    fts5BufferZero(&pIter->poslist);
    fts5SegiterPoslist(pIter->pIndex, pSeg, pColset, &pIter->poslist);
  }

  pIter->base.pData = pIter->poslist.p;
  pIter->base.nData = pIter->poslist.n;
}

/*
** xSetOutputs callback used by detail=full when there is a column filter.
*/
static void fts5IterSetOutputs_Full(Fts5Iter *pIter, Fts5SegIter *pSeg){
  Fts5Colset *pColset = pIter->pColset;
  pIter->base.iRowid = pSeg->iRowid;

  assert( pIter->pIndex->pConfig->eDetail==FTS5_DETAIL_FULL );
  assert( pColset );

  if( pSeg->iLeafOffset+pSeg->nPos<=pSeg->pLeaf->szLeaf ){
    /* All data is stored on the current page. Populate the output 
    ** variables to point into the body of the page object. */
    const u8 *a = &pSeg->pLeaf->p[pSeg->iLeafOffset];
    if( pColset->nCol==1 ){
      pIter->base.nData = fts5IndexExtractCol(&a, pSeg->nPos,pColset->aiCol[0]);
      pIter->base.pData = a;
    }else{
      fts5BufferZero(&pIter->poslist);
      fts5IndexExtractColset(pColset, a, pSeg->nPos, &pIter->poslist);
      pIter->base.pData = pIter->poslist.p;
      pIter->base.nData = pIter->poslist.n;
    }
  }else{
    /* The data is distributed over two or more pages. Copy it into the
    ** Fts5Iter.poslist buffer and then set the output pointer to point
    ** to this buffer.  */
    fts5BufferZero(&pIter->poslist);
    fts5SegiterPoslist(pIter->pIndex, pSeg, pColset, &pIter->poslist);
    pIter->base.pData = pIter->poslist.p;
    pIter->base.nData = pIter->poslist.n;
  }
}

static void fts5IterSetOutputCb(Fts5Iter *pIter){
  int eDetail = pIter->pIndex->pConfig->eDetail;
  if( eDetail==FTS5_DETAIL_NONE ){
    pIter->xSetOutputs = fts5IterSetOutputs_None;
  }

  else if( pIter->pColset==0 || pIter->bFiltered ){
    pIter->xSetOutputs = fts5IterSetOutputs_Nocolset;
  }

  else if( eDetail==FTS5_DETAIL_FULL ){
    pIter->xSetOutputs = fts5IterSetOutputs_Full;
  }

  else{
    assert( eDetail==FTS5_DETAIL_COLUMNS );
    pIter->xSetOutputs = fts5IterSetOutputs_Col;
  }
}

/*
** Open a new iterator to iterate though all rowid that match the 
** specified token or token prefix.
*/
int sqlite3Fts5IndexQuery(
  Fts5Index *p,                   /* FTS index to query */
  const char *pToken, int nToken, /* Token (or prefix) to query for */
  int flags,                      /* Mask of FTS5INDEX_QUERY_X flags */
  Fts5Colset *pColset,            /* Match these columns only */
  Fts5IndexIter **ppIter          /* OUT: New iterator object */
){
  Fts5Config *pConfig = p->pConfig;
  Fts5Iter *pRet = 0;

  Fts5Buffer buf = {0, 0, 0};

  /* If the QUERY_SCAN flag is set, all other flags must be clear. */
  assert( (flags & FTS5INDEX_QUERY_SCAN)==0 || flags==FTS5INDEX_QUERY_SCAN );

  if( sqlite3Fts5BufferSize(&p->rc, &buf, nToken+1)==0 ){
    int iIdx = 0;                 /* Index to search */
    memcpy(&buf.p[1], pToken, nToken);


    /* Figure out which index to search and set iIdx accordingly. If this
    ** is a prefix query for which there is no prefix index, set iIdx to
    ** greater than pConfig->nPrefix to indicate that the query will be
    ** satisfied by scanning multiple terms in the main index.
    **
    ** If the QUERY_TEST_NOIDX flag was specified, then this must be a
    ** prefix-query. Instead of using a prefix-index (if one exists), 
    ** evaluate the prefix query using the main FTS index. This is used
    ** for internal sanity checking by the integrity-check in debug 
    ** mode only.  */
#ifdef SQLITE_DEBUG
    if( pConfig->bPrefixIndex==0 || (flags & FTS5INDEX_QUERY_TEST_NOIDX) ){
      assert( flags & FTS5INDEX_QUERY_PREFIX );
      iIdx = 1+pConfig->nPrefix;
    }else
#endif
    if( flags & FTS5INDEX_QUERY_PREFIX ){
      int nChar = fts5IndexCharlen(pToken, nToken);
      for(iIdx=1; iIdx<=pConfig->nPrefix; iIdx++){
        if( pConfig->aPrefix[iIdx-1]==nChar ) break;
      }
    }

    if( iIdx<=pConfig->nPrefix ){
      /* Straight index lookup */
      Fts5Structure *pStruct = fts5StructureRead(p);
      buf.p[0] = (u8)(FTS5_MAIN_PREFIX + iIdx);
      if( pStruct ){
        fts5MultiIterNew(p, pStruct, 1, flags, buf.p, nToken+1, -1, 0, &pRet);
        fts5StructureRelease(pStruct);
      }
    }else{
      /* Scan multiple terms in the main index */
      int bDesc = (flags & FTS5INDEX_QUERY_DESC)!=0;
      buf.p[0] = FTS5_MAIN_PREFIX;
      fts5SetupPrefixIter(p, bDesc, buf.p, nToken+1, pColset, &pRet);
    }

    if( p->rc==SQLITE_OK ){
      Fts5SegIter *pSeg = &pRet->aSeg[pRet->aFirst[1].iFirst];
      pRet->pColset = pColset;
      fts5IterSetOutputCb(pRet);
      if( pSeg->pLeaf ) pRet->xSetOutputs(pRet, pSeg);
    }
    if( p->rc ){
      sqlite3Fts5IterClose(&pRet->base);
      pRet = 0;
      fts5CloseReader(p);
    }

    *ppIter = &pRet->base;
    sqlite3Fts5BufferFree(&buf);
  }
  return fts5IndexReturn(p);
}

/*
** Return true if the iterator passed as the only argument is at EOF.
*/
int sqlite3Fts5IterEof(Fts5IndexIter *pIter){
  assert( ((Fts5Iter*)pIter)->pIndex->rc==SQLITE_OK );
  return ((Fts5Iter*)pIter)->bEof;
}

/*
** Move to the next matching rowid. 
*/
int sqlite3Fts5IterNext(Fts5IndexIter *pIndexIter){
  Fts5Iter *pIter = (Fts5Iter*)pIndexIter;
  assert( pIter->pIndex->rc==SQLITE_OK );
  fts5MultiIterNext(pIter->pIndex, pIter, 0, 0);
  return fts5IndexReturn(pIter->pIndex);
}

/*
** Move to the next matching term/rowid. Used by the fts5vocab module.
*/
int sqlite3Fts5IterNextScan(Fts5IndexIter *pIndexIter){
  Fts5Iter *pIter = (Fts5Iter*)pIndexIter;
  Fts5Index *p = pIter->pIndex;

  assert( pIter->pIndex->rc==SQLITE_OK );

  fts5MultiIterNext(p, pIter, 0, 0);
  if( p->rc==SQLITE_OK ){
    Fts5SegIter *pSeg = &pIter->aSeg[ pIter->aFirst[1].iFirst ];
................................................................................
}

/*
** Move to the next matching rowid that occurs at or after iMatch. The
** definition of "at or after" depends on whether this iterator iterates
** in ascending or descending rowid order.
*/
int sqlite3Fts5IterNextFrom(Fts5IndexIter *pIndexIter, i64 iMatch){
  Fts5Iter *pIter = (Fts5Iter*)pIndexIter;
  fts5MultiIterNextFrom(pIter->pIndex, pIter, iMatch);
  return fts5IndexReturn(pIter->pIndex);
}

/*
** Return the current rowid.
*/
i64 sqlite3Fts5IterRowid(Fts5IndexIter *pIndexIter){
  return fts5MultiIterRowid((Fts5Iter*)pIndexIter);
}

/*
** Return the current term.
*/
const char *sqlite3Fts5IterTerm(Fts5IndexIter *pIndexIter, int *pn){
  int n;
  const char *z = (const char*)fts5MultiIterTerm((Fts5Iter*)pIndexIter, &n);
  *pn = n-1;
  return &z[1];
}






















/*
** Return a pointer to a buffer containing a copy of the position list for
** the current entry. Output variable *pn is set to the size of the buffer 
** in bytes before returning.
**
** The returned position list does not include the "number of bytes" varint
** field that starts the position list on disk.
*/
int sqlite3Fts5IterPoslist(
  Fts5IndexIter *pIndexIter, 
  Fts5Colset *pColset,            /* Column filter (or NULL) */
  const u8 **pp,                  /* OUT: Pointer to position-list data */
  int *pn,                        /* OUT: Size of position-list in bytes */
  i64 *piRowid                    /* OUT: Current rowid */
){
  Fts5Iter *pIter = (Fts5Iter*)pIndexIter;
  Fts5SegIter *pSeg = &pIter->aSeg[ pIter->aFirst[1].iFirst ];
  int eDetail = pIter->pIndex->pConfig->eDetail;

  assert( pIter->pIndex->rc==SQLITE_OK );
  *piRowid = pSeg->iRowid;
  if( eDetail==FTS5_DETAIL_NONE ){
    *pn = pSeg->nPos;
................................................................................
    }
    *pn = pIter->poslist.n;
  }
  return fts5IndexReturn(pIter->pIndex);
}

int sqlite3Fts5IterCollist(
  Fts5IndexIter *pIndexIter, 
  const u8 **pp,                  /* OUT: Pointer to position-list data */
  int *pn                         /* OUT: Size of position-list in bytes */
){
  Fts5Iter *pIter = (Fts5Iter*)pIndexIter;
  assert( pIter->pIndex->pConfig->eDetail==FTS5_DETAIL_COLUMNS );
  *pp = pIter->poslist.p;
  *pn = pIter->poslist.n;
  return SQLITE_OK;
}

/*
** This function is similar to sqlite3Fts5IterPoslist(), except that it
** copies the position list into the buffer supplied as the second 
** argument.
*/
int sqlite3Fts5IterPoslistBuffer(Fts5IndexIter *pIndexIter, Fts5Buffer *pBuf){
  Fts5Iter *pIter = (Fts5Iter*)pIndexIter;
  Fts5Index *p = pIter->pIndex;
  Fts5SegIter *pSeg = &pIter->aSeg[ pIter->aFirst[1].iFirst ];
  assert( p->rc==SQLITE_OK );
  fts5BufferZero(pBuf);
  fts5SegiterPoslist(p, pSeg, 0, pBuf);
  return fts5IndexReturn(p);
}

/*
** Close an iterator opened by an earlier call to sqlite3Fts5IndexQuery().
*/
void sqlite3Fts5IterClose(Fts5IndexIter *pIndexIter){
  if( pIndexIter ){
    Fts5Iter *pIter = (Fts5Iter*)pIndexIter;
    Fts5Index *pIndex = pIter->pIndex;
    fts5MultiIterFree(pIter->pIndex, pIter);
    fts5CloseReader(pIndex);
  }
}

/*
................................................................................
  const char *z,                  /* Index key to query for */
  int n,                          /* Size of index key in bytes */
  int flags,                      /* Flags for Fts5IndexQuery */
  u64 *pCksum                     /* IN/OUT: Checksum value */
){
  int eDetail = p->pConfig->eDetail;
  u64 cksum = *pCksum;
  Fts5IndexIter *pIter = 0;

  int rc = sqlite3Fts5IndexQuery(p, z, n, flags, 0, &pIter);

  while( rc==SQLITE_OK && 0==sqlite3Fts5IterEof(pIter) ){
    i64 rowid = sqlite3Fts5IterRowid(pIter);

    if( eDetail==FTS5_DETAIL_NONE ){
      cksum ^= sqlite3Fts5IndexEntryCksum(rowid, 0, 0, iIdx, z, n);
    }else{


      Fts5PoslistReader sReader;
      for(sqlite3Fts5PoslistReaderInit(pIter->pData, pIter->nData, &sReader);
          sReader.bEof==0;
          sqlite3Fts5PoslistReaderNext(&sReader)
      ){
        int iCol = FTS5_POS2COLUMN(sReader.iPos);
        int iOff = FTS5_POS2OFFSET(sReader.iPos);
        cksum ^= sqlite3Fts5IndexEntryCksum(rowid, iCol, iOff, iIdx, z, n);

      }
    }
    if( rc==SQLITE_OK ){
      rc = sqlite3Fts5IterNext(pIter);
    }
  }
  sqlite3Fts5IterClose(pIter);


  *pCksum = cksum;
  return rc;
}


/*
................................................................................
** error, or some other SQLite error code if another error (e.g. OOM)
** occurs.
*/
int sqlite3Fts5IndexIntegrityCheck(Fts5Index *p, u64 cksum){
  int eDetail = p->pConfig->eDetail;
  u64 cksum2 = 0;                 /* Checksum based on contents of indexes */
  Fts5Buffer poslist = {0,0,0};   /* Buffer used to hold a poslist */
  Fts5Iter *pIter;                /* Used to iterate through entire index */
  Fts5Structure *pStruct;         /* Index structure */

#ifdef SQLITE_DEBUG
  /* Used by extra internal tests only run if NDEBUG is not defined */
  u64 cksum3 = 0;                 /* Checksum based on contents of indexes */
  Fts5Buffer term = {0,0,0};      /* Buffer used to hold most recent term */
#endif

      assert( pTab->eType==FTS5_VOCAB_COL || pTab->eType==FTS5_VOCAB_ROW );
      while( rc==SQLITE_OK ){
        i64 dummy;
        const u8 *pPos; int nPos;   /* Position list */
        i64 iPos = 0;               /* 64-bit position read from poslist */
        int iOff = 0;               /* Current offset within position list */



        switch( pCsr->pConfig->eDetail ){
          case FTS5_DETAIL_FULL:
            rc = sqlite3Fts5IterPoslist(pCsr->pIter, 0, &pPos, &nPos, &dummy);
            if( rc==SQLITE_OK ){


              if( pTab->eType==FTS5_VOCAB_ROW ){
                while( 0==sqlite3Fts5PoslistNext64(pPos, nPos, &iOff, &iPos) ){
                  pCsr->aCnt[0]++;
                }
                pCsr->aDoc[0]++;
              }else{
                int iCol = -1;
                while( 0==sqlite3Fts5PoslistNext64(pPos, nPos, &iOff, &iPos) ){
                  int ii = FTS5_POS2COLUMN(iPos);
                  pCsr->aCnt[ii]++;
                  if( iCol!=ii ){
                    if( ii>=nCol ){
                      rc = FTS5_CORRUPT;
                      break;
                    }
                    pCsr->aDoc[ii]++;
                    iCol = ii;
                  }
                }
              }
            }
            break;

          case FTS5_DETAIL_COLUMNS:
            if( pTab->eType==FTS5_VOCAB_ROW ){
              pCsr->aDoc[0]++;
            }else{
              Fts5Buffer buf = {0, 0, 0};
              rc = sqlite3Fts5IterPoslistBuffer(pCsr->pIter, &buf);
              if( rc==SQLITE_OK ){
                while( 0==sqlite3Fts5PoslistNext64(buf.p, buf.n, &iOff,&iPos) ){
                  assert_nc( iPos>=0 && iPos<nCol );
                  if( iPos>=nCol ){
                    rc = FTS5_CORRUPT;
                    break;
                  }
                  pCsr->aDoc[iPos]++;
                }
              }
              sqlite3Fts5BufferFree(&buf);
            }
            break;

          default: 
            assert( pCsr->pConfig->eDetail==FTS5_DETAIL_NONE );
            pCsr->aDoc[0]++;
            break;

      assert( pTab->eType==FTS5_VOCAB_COL || pTab->eType==FTS5_VOCAB_ROW );
      while( rc==SQLITE_OK ){
        i64 dummy;
        const u8 *pPos; int nPos;   /* Position list */
        i64 iPos = 0;               /* 64-bit position read from poslist */
        int iOff = 0;               /* Current offset within position list */

        pPos = pCsr->pIter->pData;
        nPos = pCsr->pIter->nData;
        switch( pCsr->pConfig->eDetail ){
          case FTS5_DETAIL_FULL:


            pPos = pCsr->pIter->pData;
            nPos = pCsr->pIter->nData;
            if( pTab->eType==FTS5_VOCAB_ROW ){
              while( 0==sqlite3Fts5PoslistNext64(pPos, nPos, &iOff, &iPos) ){
                pCsr->aCnt[0]++;
              }
              pCsr->aDoc[0]++;
            }else{
              int iCol = -1;
              while( 0==sqlite3Fts5PoslistNext64(pPos, nPos, &iOff, &iPos) ){
                int ii = FTS5_POS2COLUMN(iPos);
                pCsr->aCnt[ii]++;
                if( iCol!=ii ){
                  if( ii>=nCol ){
                    rc = FTS5_CORRUPT;
                    break;
                  }
                  pCsr->aDoc[ii]++;
                  iCol = ii;

                }
              }
            }
            break;

          case FTS5_DETAIL_COLUMNS:
            if( pTab->eType==FTS5_VOCAB_ROW ){
              pCsr->aDoc[0]++;
            }else{



              while( 0==sqlite3Fts5PoslistNext64(pPos, nPos, &iOff,&iPos) ){
                assert_nc( iPos>=0 && iPos<nCol );
                if( iPos>=nCol ){
                  rc = FTS5_CORRUPT;
                  break;
                }
                pCsr->aDoc[iPos]++;
              }


            }
            break;

          default: 
            assert( pCsr->pConfig->eDetail==FTS5_DETAIL_NONE );
            pCsr->aDoc[0]++;
            break;

    }

    set res [fts5_query_data $expr xx]
    do_execsql_test 1.$tn2.$tn.[llength $res].asc {
      SELECT rowid, fts5_test_poslist(xx), fts5_test_collist(xx) 
      FROM xx WHERE xx match $expr
    } $res


    set res [fts5_query_data $expr xx DESC]
    do_execsql_test 1.$tn2.$tn.[llength $res].desc {
      SELECT rowid, fts5_test_poslist(xx), fts5_test_collist(xx) 
      FROM xx WHERE xx match $expr ORDER BY 1 DESC
    } $res
  }

    }

    set res [fts5_query_data $expr xx]
    do_execsql_test 1.$tn2.$tn.[llength $res].asc {
      SELECT rowid, fts5_test_poslist(xx), fts5_test_collist(xx) 
      FROM xx WHERE xx match $expr
    } $res


    set res [fts5_query_data $expr xx DESC]
    do_execsql_test 1.$tn2.$tn.[llength $res].desc {
      SELECT rowid, fts5_test_poslist(xx), fts5_test_collist(xx) 
      FROM xx WHERE xx match $expr ORDER BY 1 DESC
    } $res
  }