/*
** 2009 Oct 23
**
** The author disclaims copyright to this source code.  In place of
** a legal notice, here is a blessing:
**
**    May you do good and not evil.
**    May you find forgiveness for yourself and forgive others.
**    May you share freely, never taking more than you give.
**
******************************************************************************
*/

#if !defined(SQLITE_CORE) || defined(SQLITE_ENABLE_FTS3)

#include "fts3Int.h"
#include <string.h>
#include <assert.h>


/*
** Used as an fts3ExprIterate() context when loading phrase doclists to
** Fts3Expr.aDoclist[]/nDoclist.
*/
typedef struct LoadDoclistCtx LoadDoclistCtx;
struct LoadDoclistCtx {
  Fts3Cursor *pCsr;               /* FTS3 Cursor */
  int nPhrase;                    /* Number of phrases seen so far */
  int nToken;                     /* Number of tokens seen so far */
};

/*
** The following types are used as part of the implementation of the 
** fts3BestSnippet() routine.
*/
typedef struct SnippetIter SnippetIter;
typedef struct SnippetPhrase SnippetPhrase;
typedef struct SnippetFragment SnippetFragment;

struct SnippetIter {
  Fts3Cursor *pCsr;               /* Cursor snippet is being generated from */
  int iCol;                       /* Extract snippet from this column */
  int nSnippet;                   /* Requested snippet length (in tokens) */
  int nPhrase;                    /* Number of phrases in query */
  SnippetPhrase *aPhrase;         /* Array of size nPhrase */
  int iCurrent;                   /* First token of current snippet */
};

struct SnippetPhrase {
  int nToken;                     /* Number of tokens in phrase */
  char *pList;                    /* Pointer to start of phrase position list */
  int iHead;                      /* Next value in position list */
  char *pHead;                    /* Position list data following iHead */
  int iTail;                      /* Next value in trailing position list */
  char *pTail;                    /* Position list data following iTail */
};

struct SnippetFragment {
  int iCol;                       /* Column snippet is extracted from */
  int iPos;                       /* Index of first token in snippet */
  u64 covered;                    /* Mask of query phrases covered */
  u64 hlmask;                     /* Mask of snippet terms to highlight */
};

/*
** This type is used as an fts3ExprIterate() context object while 
** accumulating the data returned by the matchinfo() function.
*/
typedef struct MatchInfo MatchInfo;
struct MatchInfo {
  Fts3Cursor *pCursor;            /* FTS3 Cursor */
  int nCol;                       /* Number of columns in table */
  u32 *aMatchinfo;                /* Pre-allocated buffer */
};



/*
** The snippet() and offsets() functions both return text values. An instance
** of the following structure is used to accumulate those values while the
** functions are running. See fts3StringAppend() for details.
*/
typedef struct StrBuffer StrBuffer;
struct StrBuffer {
  char *z;                        /* Pointer to buffer containing string */
  int n;                          /* Length of z in bytes (excl. nul-term) */
  int nAlloc;                     /* Allocated size of buffer z in bytes */
};


/*
** This function is used to help iterate through a position-list. A position
** list is a list of unique integers, sorted from smallest to largest. Each
** element of the list is represented by an FTS3 varint that takes the value
** of the difference between the current element and the previous one plus
** two. For example, to store the position-list:
**
**     4 9 113
**
** the three varints:
**
**     6 7 106
**
** are encoded.
**
** When this function is called, *pp points to the start of an element of
** the list. *piPos contains the value of the previous entry in the list.
** After it returns, *piPos contains the value of the next element of the
** list and *pp is advanced to the following varint.
*/
static void fts3GetDeltaPosition(char **pp, int *piPos){
  int iVal;
  *pp += sqlite3Fts3GetVarint32(*pp, &iVal);
  *piPos += (iVal-2);
}

/*
** Helper function for fts3ExprIterate() (see below).
*/
static int fts3ExprIterate2(
  Fts3Expr *pExpr,                /* Expression to iterate phrases of */
  int *piPhrase,                  /* Pointer to phrase counter */
  int (*x)(Fts3Expr*,int,void*),  /* Callback function to invoke for phrases */
  void *pCtx                      /* Second argument to pass to callback */
){
  int rc;                         /* Return code */
  int eType = pExpr->eType;       /* Type of expression node pExpr */

  if( eType!=FTSQUERY_PHRASE ){
    assert( pExpr->pLeft && pExpr->pRight );
    rc = fts3ExprIterate2(pExpr->pLeft, piPhrase, x, pCtx);
    if( rc==SQLITE_OK && eType!=FTSQUERY_NOT ){
      rc = fts3ExprIterate2(pExpr->pRight, piPhrase, x, pCtx);
    }
  }else{
    rc = x(pExpr, *piPhrase, pCtx);
    (*piPhrase)++;
  }
  return rc;
}

/*
** Iterate through all phrase nodes in an FTS3 query, except those that
** are part of a sub-tree that is the right-hand-side of a NOT operator.
** For each phrase node found, the supplied callback function is invoked.
**
** If the callback function returns anything other than SQLITE_OK, 
** the iteration is abandoned and the error code returned immediately.
** Otherwise, SQLITE_OK is returned after a callback has been made for
** all eligible phrase nodes.
*/
static int fts3ExprIterate(
  Fts3Expr *pExpr,                /* Expression to iterate phrases of */
  int (*x)(Fts3Expr*,int,void*),  /* Callback function to invoke for phrases */
  void *pCtx                      /* Second argument to pass to callback */
){
  int iPhrase = 0;                /* Variable used as the phrase counter */
  return fts3ExprIterate2(pExpr, &iPhrase, x, pCtx);
}

/*
** The argument to this function is always a phrase node. Its doclist 
** (Fts3Expr.aDoclist[]) and the doclists associated with all phrase nodes
** to the left of this one in the query tree have already been loaded.
**
** If this phrase node is part of a series of phrase nodes joined by 
** NEAR operators (and is not the left-most of said series), then elements are
** removed from the phrases doclist consistent with the NEAR restriction. If
** required, elements may be removed from the doclists of phrases to the
** left of this one that are part of the same series of NEAR operator 
** connected phrases.
**
** If an OOM error occurs, SQLITE_NOMEM is returned. Otherwise, SQLITE_OK.
*/
static int fts3ExprNearTrim(Fts3Expr *pExpr){
  int rc = SQLITE_OK;
  Fts3Expr *pParent = pExpr->pParent;

  assert( pExpr->eType==FTSQUERY_PHRASE );
  while( rc==SQLITE_OK
   && pParent 
   && pParent->eType==FTSQUERY_NEAR 
   && pParent->pRight==pExpr 
  ){
    /* This expression (pExpr) is the right-hand-side of a NEAR operator. 
    ** Find the expression to the left of the same operator.
    */
    int nNear = pParent->nNear;
    Fts3Expr *pLeft = pParent->pLeft;

    if( pLeft->eType!=FTSQUERY_PHRASE ){
      assert( pLeft->eType==FTSQUERY_NEAR );
      assert( pLeft->pRight->eType==FTSQUERY_PHRASE );
      pLeft = pLeft->pRight;
    }

    rc = sqlite3Fts3ExprNearTrim(pLeft, pExpr, nNear);

    pExpr = pLeft;
    pParent = pExpr->pParent;
  }

  return rc;
}

/*
** This is an fts3ExprIterate() callback used while loading the doclists
** for each phrase into Fts3Expr.aDoclist[]/nDoclist. See also
** fts3ExprLoadDoclists().
*/
static int fts3ExprLoadDoclistsCb1(Fts3Expr *pExpr, int iPhrase, void *ctx){
  int rc = SQLITE_OK;
  LoadDoclistCtx *p = (LoadDoclistCtx *)ctx;

  UNUSED_PARAMETER(iPhrase);

  p->nPhrase++;
  p->nToken += pExpr->pPhrase->nToken;

  if( pExpr->isLoaded==0 ){
    rc = sqlite3Fts3ExprLoadDoclist(p->pCsr, pExpr);
    pExpr->isLoaded = 1;
    if( rc==SQLITE_OK ){
      rc = fts3ExprNearTrim(pExpr);
    }
  }

  return rc;
}

/*
** This is an fts3ExprIterate() callback used while loading the doclists
** for each phrase into Fts3Expr.aDoclist[]/nDoclist. See also
** fts3ExprLoadDoclists().
*/
static int fts3ExprLoadDoclistsCb2(Fts3Expr *pExpr, int iPhrase, void *ctx){
  UNUSED_PARAMETER(iPhrase);
  UNUSED_PARAMETER(ctx);
  if( pExpr->aDoclist ){
    pExpr->pCurrent = pExpr->aDoclist;
    pExpr->iCurrent = 0;
    pExpr->pCurrent += sqlite3Fts3GetVarint(pExpr->pCurrent, &pExpr->iCurrent);
  }
  return SQLITE_OK;
}

/*
** Load the doclists for each phrase in the query associated with FTS3 cursor
** pCsr. 
**
** If pnPhrase is not NULL, then *pnPhrase is set to the number of matchable 
** phrases in the expression (all phrases except those directly or 
** indirectly descended from the right-hand-side of a NOT operator). If 
** pnToken is not NULL, then it is set to the number of tokens in all
** matchable phrases of the expression.
*/
static int fts3ExprLoadDoclists(
  Fts3Cursor *pCsr,               /* Fts3 cursor for current query */
  int *pnPhrase,                  /* OUT: Number of phrases in query */
  int *pnToken                    /* OUT: Number of tokens in query */
){
  int rc;                         /* Return Code */
  LoadDoclistCtx sCtx = {0,0,0};  /* Context for fts3ExprIterate() */
  sCtx.pCsr = pCsr;
  rc = fts3ExprIterate(pCsr->pExpr, fts3ExprLoadDoclistsCb1, (void *)&sCtx);
  if( rc==SQLITE_OK ){
    (void)fts3ExprIterate(pCsr->pExpr, fts3ExprLoadDoclistsCb2, 0);
  }
  if( pnPhrase ) *pnPhrase = sCtx.nPhrase;
  if( pnToken ) *pnToken = sCtx.nToken;
  return rc;
}

/*
** Advance the position list iterator specified by the first two 
** arguments so that it points to the first element with a value greater
** than or equal to parameter iNext.
*/
static void fts3SnippetAdvance(char **ppIter, int *piIter, int iNext){
  char *pIter = *ppIter;
  if( pIter ){
    int iIter = *piIter;

    while( iIter<iNext ){
      if( 0==(*pIter & 0xFE) ){
        iIter = -1;
        pIter = 0;
        break;
      }
      fts3GetDeltaPosition(&pIter, &iIter);
    }

    *piIter = iIter;
    *ppIter = pIter;
  }
}

/*
** Advance the snippet iterator to the next candidate snippet.
*/
static int fts3SnippetNextCandidate(SnippetIter *pIter){
  int i;                          /* Loop counter */

  if( pIter->iCurrent<0 ){
    /* The SnippetIter object has just been initialized. The first snippet
    ** candidate always starts at offset 0 (even if this candidate has a
    ** score of 0.0).
    */
    pIter->iCurrent = 0;

    /* Advance the 'head' iterator of each phrase to the first offset that
    ** is greater than or equal to (iNext+nSnippet).
    */
    for(i=0; i<pIter->nPhrase; i++){
      SnippetPhrase *pPhrase = &pIter->aPhrase[i];
      fts3SnippetAdvance(&pPhrase->pHead, &pPhrase->iHead, pIter->nSnippet);
    }
  }else{
    int iStart;
    int iEnd = 0x7FFFFFFF;

    for(i=0; i<pIter->nPhrase; i++){
      SnippetPhrase *pPhrase = &pIter->aPhrase[i];
      if( pPhrase->pHead && pPhrase->iHead<iEnd ){
        iEnd = pPhrase->iHead;
      }
    }
    if( iEnd==0x7FFFFFFF ){
      return 1;
    }

    pIter->iCurrent = iStart = iEnd - pIter->nSnippet + 1;
    for(i=0; i<pIter->nPhrase; i++){
      SnippetPhrase *pPhrase = &pIter->aPhrase[i];
      fts3SnippetAdvance(&pPhrase->pHead, &pPhrase->iHead, iEnd+1);
      fts3SnippetAdvance(&pPhrase->pTail, &pPhrase->iTail, iStart);
    }
  }

  return 0;
}

/*
** Retrieve information about the current candidate snippet of snippet 
** iterator pIter.
*/
static void fts3SnippetDetails(
  SnippetIter *pIter,             /* Snippet iterator */
  u64 mCovered,                   /* Bitmask of phrases already covered */
  int *piToken,                   /* OUT: First token of proposed snippet */
  int *piScore,                   /* OUT: "Score" for this snippet */
  u64 *pmCover,                   /* OUT: Bitmask of phrases covered */
  u64 *pmHighlight                /* OUT: Bitmask of terms to highlight */
){
  int iStart = pIter->iCurrent;   /* First token of snippet */
  int iScore = 0;                 /* Score of this snippet */
  int i;                          /* Loop counter */
  u64 mCover = 0;                 /* Mask of phrases covered by this snippet */
  u64 mHighlight = 0;             /* Mask of tokens to highlight in snippet */

  for(i=0; i<pIter->nPhrase; i++){
    SnippetPhrase *pPhrase = &pIter->aPhrase[i];
    if( pPhrase->pTail ){
      char *pCsr = pPhrase->pTail;
      int iCsr = pPhrase->iTail;

      while( iCsr<(iStart+pIter->nSnippet) ){
        int j;
        u64 mPhrase = (u64)1 << i;
        u64 mPos = (u64)1 << (iCsr - iStart);
        assert( iCsr>=iStart );
        if( (mCover|mCovered)&mPhrase ){
          iScore++;
        }else{
          iScore += 1000;
        }
        mCover |= mPhrase;

        for(j=0; j<pPhrase->nToken; j++){
          mHighlight |= (mPos>>j);
        }

        if( 0==(*pCsr & 0x0FE) ) break;
        fts3GetDeltaPosition(&pCsr, &iCsr);
      }
    }
  }

  /* Set the output variables before returning. */
  *piToken = iStart;
  *piScore = iScore;
  *pmCover = mCover;
  *pmHighlight = mHighlight;
}

/*
** This function is an fts3ExprIterate() callback used by fts3BestSnippet().
** Each invocation populates an element of the SnippetIter.aPhrase[] array.
*/
static int fts3SnippetFindPositions(Fts3Expr *pExpr, int iPhrase, void *ctx){
  SnippetIter *p = (SnippetIter *)ctx;
  SnippetPhrase *pPhrase = &p->aPhrase[iPhrase];
  char *pCsr;

  pPhrase->nToken = pExpr->pPhrase->nToken;

  pCsr = sqlite3Fts3FindPositions(pExpr, p->pCsr->iPrevId, p->iCol);
  if( pCsr ){
    int iFirst = 0;
    pPhrase->pList = pCsr;
    fts3GetDeltaPosition(&pCsr, &iFirst);
    pPhrase->pHead = pCsr;
    pPhrase->pTail = pCsr;
    pPhrase->iHead = iFirst;
    pPhrase->iTail = iFirst;
  }else{
    assert( pPhrase->pList==0 && pPhrase->pHead==0 && pPhrase->pTail==0 );
  }

  return SQLITE_OK;
}

/*
** Select the fragment of text consisting of nFragment contiguous tokens 
** from column iCol that represent the "best" snippet. The best snippet
** is the snippet with the highest score, where scores are calculated
** by adding:
**
**   (a) +1 point for each occurence of a matchable phrase in the snippet.
**
**   (b) +1000 points for the first occurence of each matchable phrase in 
**       the snippet for which the corresponding mCovered bit is not set.
**
** The selected snippet parameters are stored in structure *pFragment before
** returning. The score of the selected snippet is stored in *piScore
** before returning.
*/
static int fts3BestSnippet(
  int nSnippet,                   /* Desired snippet length */
  Fts3Cursor *pCsr,               /* Cursor to create snippet for */
  int iCol,                       /* Index of column to create snippet from */
  u64 mCovered,                   /* Mask of phrases already covered */
  u64 *pmSeen,                    /* IN/OUT: Mask of phrases seen */
  SnippetFragment *pFragment,     /* OUT: Best snippet found */
  int *piScore                    /* OUT: Score of snippet pFragment */
){
  int rc;                         /* Return Code */
  int nList;                      /* Number of phrases in expression */
  SnippetIter sIter;              /* Iterates through snippet candidates */
  int nByte;                      /* Number of bytes of space to allocate */
  int iBestScore = -1;            /* Best snippet score found so far */
  int i;                          /* Loop counter */

  memset(&sIter, 0, sizeof(sIter));

  /* Iterate through the phrases in the expression to count them. The same
  ** callback makes sure the doclists are loaded for each phrase.
  */
  rc = fts3ExprLoadDoclists(pCsr, &nList, 0);
  if( rc!=SQLITE_OK ){
    return rc;
  }

  /* Now that it is known how many phrases there are, allocate and zero
  ** the required space using malloc().
  */
  nByte = sizeof(SnippetPhrase) * nList;
  sIter.aPhrase = (SnippetPhrase *)sqlite3_malloc(nByte);
  if( !sIter.aPhrase ){
    return SQLITE_NOMEM;
  }
  memset(sIter.aPhrase, 0, nByte);

  /* Initialize the contents of the SnippetIter object. Then iterate through
  ** the set of phrases in the expression to populate the aPhrase[] array.
  */
  sIter.pCsr = pCsr;
  sIter.iCol = iCol;
  sIter.nSnippet = nSnippet;
  sIter.nPhrase = nList;
  sIter.iCurrent = -1;
  (void)fts3ExprIterate(pCsr->pExpr, fts3SnippetFindPositions, (void *)&sIter);

  /* Set the *pmSeen output variable. */
  for(i=0; i<nList; i++){
    if( sIter.aPhrase[i].pHead ){
      *pmSeen |= (u64)1 << i;
    }
  }

  /* Loop through all candidate snippets. Store the best snippet in 
  ** *pFragment. Store its associated 'score' in iBestScore.
  */
  pFragment->iCol = iCol;
  while( !fts3SnippetNextCandidate(&sIter) ){
    int iPos;
    int iScore;
    u64 mCover;
    u64 mHighlight;
    fts3SnippetDetails(&sIter, mCovered, &iPos, &iScore, &mCover, &mHighlight);
    assert( iScore>=0 );
    if( iScore>iBestScore ){
      pFragment->iPos = iPos;
      pFragment->hlmask = mHighlight;
      pFragment->covered = mCover;
      iBestScore = iScore;
    }
  }

  sqlite3_free(sIter.aPhrase);
  *piScore = iBestScore;
  return SQLITE_OK;
}


/*
** Append a string to the string-buffer passed as the first argument.
**
** If nAppend is negative, then the length of the string zAppend is
** determined using strlen().
*/
static int fts3StringAppend(
  StrBuffer *pStr,                /* Buffer to append to */
  const char *zAppend,            /* Pointer to data to append to buffer */
  int nAppend                     /* Size of zAppend in bytes (or -1) */
){
  if( nAppend<0 ){
    nAppend = (int)strlen(zAppend);
  }

  /* If there is insufficient space allocated at StrBuffer.z, use realloc()
  ** to grow the buffer until so that it is big enough to accomadate the
  ** appended data.
  */
  if( pStr->n+nAppend+1>=pStr->nAlloc ){
    int nAlloc = pStr->nAlloc+nAppend+100;
    char *zNew = sqlite3_realloc(pStr->z, nAlloc);
    if( !zNew ){
      return SQLITE_NOMEM;
    }
    pStr->z = zNew;
    pStr->nAlloc = nAlloc;
  }

  /* Append the data to the string buffer. */
  memcpy(&pStr->z[pStr->n], zAppend, nAppend);
  pStr->n += nAppend;
  pStr->z[pStr->n] = '\0';

  return SQLITE_OK;
}

/*
** The fts3BestSnippet() function often selects snippets that end with a
** query term. That is, the final term of the snippet is always a term
** that requires highlighting. For example, if 'X' is a highlighted term
** and '.' is a non-highlighted term, BestSnippet() may select:
**
**     ........X.....X
**
** This function "shifts" the beginning of the snippet forward in the 
** document so that there are approximately the same number of 
** non-highlighted terms to the right of the final highlighted term as there
** are to the left of the first highlighted term. For example, to this:
**
**     ....X.....X....
**
** This is done as part of extracting the snippet text, not when selecting
** the snippet. Snippet selection is done based on doclists only, so there
** is no way for fts3BestSnippet() to know whether or not the document 
** actually contains terms that follow the final highlighted term. 
*/
static int fts3SnippetShift(
  Fts3Table *pTab,                /* FTS3 table snippet comes from */
  int nSnippet,                   /* Number of tokens desired for snippet */
  const char *zDoc,               /* Document text to extract snippet from */
  int nDoc,                       /* Size of buffer zDoc in bytes */
  int *piPos,                     /* IN/OUT: First token of snippet */
  u64 *pHlmask                    /* IN/OUT: Mask of tokens to highlight */
){
  u64 hlmask = *pHlmask;          /* Local copy of initial highlight-mask */

  if( hlmask ){
    int nLeft;                    /* Tokens to the left of first highlight */
    int nRight;                   /* Tokens to the right of last highlight */
    int nDesired;                 /* Ideal number of tokens to shift forward */

    for(nLeft=0; !(hlmask & ((u64)1 << nLeft)); nLeft++);
    for(nRight=0; !(hlmask & ((u64)1 << (nSnippet-1-nRight))); nRight++);
    nDesired = (nLeft-nRight)/2;

    /* Ideally, the start of the snippet should be pushed forward in the
    ** document nDesired tokens. This block checks if there are actually
    ** nDesired tokens to the right of the snippet. If so, *piPos and
    ** *pHlMask are updated to shift the snippet nDesired tokens to the
    ** right. Otherwise, the snippet is shifted by the number of tokens
    ** available.
    */
    if( nDesired>0 ){
      int nShift;                 /* Number of tokens to shift snippet by */
      int iCurrent = 0;           /* Token counter */
      int rc;                     /* Return Code */
      sqlite3_tokenizer_module *pMod;
      sqlite3_tokenizer_cursor *pC;
      pMod = (sqlite3_tokenizer_module *)pTab->pTokenizer->pModule;

      /* Open a cursor on zDoc/nDoc. Check if there are (nSnippet+nDesired)
      ** or more tokens in zDoc/nDoc.
      */
      rc = pMod->xOpen(pTab->pTokenizer, zDoc, nDoc, &pC);
      if( rc!=SQLITE_OK ){
        return rc;
      }
      pC->pTokenizer = pTab->pTokenizer;
      while( rc==SQLITE_OK && iCurrent<(nSnippet+nDesired) ){
        const char *ZDUMMY; int DUMMY1, DUMMY2, DUMMY3;
        rc = pMod->xNext(pC, &ZDUMMY, &DUMMY1, &DUMMY2, &DUMMY3, &iCurrent);
      }
      pMod->xClose(pC);
      if( rc!=SQLITE_OK && rc!=SQLITE_DONE ){ return rc; }

      nShift = (rc==SQLITE_DONE)+iCurrent-nSnippet;
      assert( nShift<=nDesired );
      if( nShift>0 ){
        *piPos += nShift;
        *pHlmask = hlmask >> nShift;
      }
    }
  }
  return SQLITE_OK;
}

/*
** Extract the snippet text for fragment pFragment from cursor pCsr and
** append it to string buffer pOut.
*/
static int fts3SnippetText(
  Fts3Cursor *pCsr,               /* FTS3 Cursor */
  SnippetFragment *pFragment,     /* Snippet to extract */
  int iFragment,                  /* Fragment number */
  int isLast,                     /* True for final fragment in snippet */
  int nSnippet,                   /* Number of tokens in extracted snippet */
  const char *zOpen,              /* String inserted before highlighted term */
  const char *zClose,             /* String inserted after highlighted term */
  const char *zEllipsis,          /* String inserted between snippets */
  StrBuffer *pOut                 /* Write output here */
){
  Fts3Table *pTab = (Fts3Table *)pCsr->base.pVtab;
  int rc;                         /* Return code */
  const char *zDoc;               /* Document text to extract snippet from */
  int nDoc;                       /* Size of zDoc in bytes */
  int iCurrent = 0;               /* Current token number of document */
  int iEnd = 0;                   /* Byte offset of end of current token */
  int isShiftDone = 0;            /* True after snippet is shifted */
  int iPos = pFragment->iPos;     /* First token of snippet */
  u64 hlmask = pFragment->hlmask; /* Highlight-mask for snippet */
  int iCol = pFragment->iCol+1;   /* Query column to extract text from */
  sqlite3_tokenizer_module *pMod; /* Tokenizer module methods object */
  sqlite3_tokenizer_cursor *pC;   /* Tokenizer cursor open on zDoc/nDoc */
  const char *ZDUMMY;             /* Dummy argument used with tokenizer */
  int DUMMY1;                     /* Dummy argument used with tokenizer */
  
  zDoc = (const char *)sqlite3_column_text(pCsr->pStmt, iCol);
  if( zDoc==0 ){
    if( sqlite3_column_type(pCsr->pStmt, iCol)!=SQLITE_NULL ){
      return SQLITE_NOMEM;
    }
    return SQLITE_OK;
  }
  nDoc = sqlite3_column_bytes(pCsr->pStmt, iCol);

  /* Open a token cursor on the document. */
  pMod = (sqlite3_tokenizer_module *)pTab->pTokenizer->pModule;
  rc = pMod->xOpen(pTab->pTokenizer, zDoc, nDoc, &pC);
  if( rc!=SQLITE_OK ){
    return rc;
  }
  pC->pTokenizer = pTab->pTokenizer;

  while( rc==SQLITE_OK ){
    int iBegin;                   /* Offset in zDoc of start of token */
    int iFin;                     /* Offset in zDoc of end of token */
    int isHighlight;              /* True for highlighted terms */

    rc = pMod->xNext(pC, &ZDUMMY, &DUMMY1, &iBegin, &iFin, &iCurrent);
    if( rc!=SQLITE_OK ){
      if( rc==SQLITE_DONE ){
        /* Special case - the last token of the snippet is also the last token
        ** of the column. Append any punctuation that occurred between the end
        ** of the previous token and the end of the document to the output. 
        ** Then break out of the loop. */
        rc = fts3StringAppend(pOut, &zDoc[iEnd], -1);
      }
      break;
    }
    if( iCurrent<iPos ){ continue; }

    if( !isShiftDone ){
      int n = nDoc - iBegin;
      rc = fts3SnippetShift(pTab, nSnippet, &zDoc[iBegin], n, &iPos, &hlmask);
      isShiftDone = 1;

      /* Now that the shift has been done, check if the initial "..." are
      ** required. They are required if (a) this is not the first fragment,
      ** or (b) this fragment does not begin at position 0 of its column. 
      */
      if( rc==SQLITE_OK && (iPos>0 || iFragment>0) ){
        rc = fts3StringAppend(pOut, zEllipsis, -1);
      }
      if( rc!=SQLITE_OK || iCurrent<iPos ) continue;
    }

    if( iCurrent>=(iPos+nSnippet) ){
      if( isLast ){
        rc = fts3StringAppend(pOut, zEllipsis, -1);
      }
      break;
    }

    /* Set isHighlight to true if this term should be highlighted. */
    isHighlight = (hlmask & ((u64)1 << (iCurrent-iPos)))!=0;

    if( iCurrent>iPos ) rc = fts3StringAppend(pOut, &zDoc[iEnd], iBegin-iEnd);
    if( rc==SQLITE_OK && isHighlight ) rc = fts3StringAppend(pOut, zOpen, -1);
    if( rc==SQLITE_OK ) rc = fts3StringAppend(pOut, &zDoc[iBegin], iFin-iBegin);
    if( rc==SQLITE_OK && isHighlight ) rc = fts3StringAppend(pOut, zClose, -1);

    iEnd = iFin;
  }

  pMod->xClose(pC);
  return rc;
}


/*
** This function is used to count the entries in a column-list (a 
** delta-encoded list of term offsets within a single column of a single 
** row). When this function is called, *ppCollist should point to the
** beginning of the first varint in the column-list (the varint that
** contains the position of the first matching term in the column data).
** Before returning, *ppCollist is set to point to the first byte after
** the last varint in the column-list (either the 0x00 signifying the end
** of the position-list, or the 0x01 that precedes the column number of
** the next column in the position-list).
**
** The number of elements in the column-list is returned.
*/
static int fts3ColumnlistCount(char **ppCollist){
  char *pEnd = *ppCollist;
  char c = 0;
  int nEntry = 0;

  /* A column-list is terminated by either a 0x01 or 0x00. */
  while( 0xFE & (*pEnd | c) ){
    c = *pEnd++ & 0x80;
    if( !c ) nEntry++;
  }

  *ppCollist = pEnd;
  return nEntry;
}

static void fts3LoadColumnlistCounts(char **pp, u32 *aOut, int isGlobal){
  char *pCsr = *pp;
  while( *pCsr ){
    int nHit;
    sqlite3_int64 iCol = 0;
    if( *pCsr==0x01 ){
      pCsr++;
      pCsr += sqlite3Fts3GetVarint(pCsr, &iCol);
    }
    nHit = fts3ColumnlistCount(&pCsr);
    assert( nHit>0 );
    if( isGlobal ){
      aOut[iCol*3+1]++;
    }
    aOut[iCol*3] += nHit;
  }
  pCsr++;
  *pp = pCsr;
}

/*
** fts3ExprIterate() callback used to collect the "global" matchinfo stats
** for a single query. The "global" stats are those elements of the matchinfo
** array that are constant for all rows returned by the current query.
*/
static int fts3ExprGlobalMatchinfoCb(
  Fts3Expr *pExpr,                /* Phrase expression node */
  int iPhrase,                    /* Phrase number (numbered from zero) */
  void *pCtx                      /* Pointer to MatchInfo structure */
){
  MatchInfo *p = (MatchInfo *)pCtx;
  Fts3Cursor *pCsr = p->pCursor;
  char *pIter;
  char *pEnd;
  char *pFree = 0;
  const int iStart = 2 + (iPhrase * p->nCol * 3) + 1;

  assert( pExpr->isLoaded );
  assert( pExpr->eType==FTSQUERY_PHRASE );

  if( pCsr->pDeferred ){
    Fts3Phrase *pPhrase = pExpr->pPhrase;
    int ii;
    for(ii=0; ii<pPhrase->nToken; ii++){
      if( pPhrase->aToken[ii].bFulltext ) break;
    }
    if( ii<pPhrase->nToken ){
      int nFree = 0;
      int rc = sqlite3Fts3ExprLoadFtDoclist(pCsr, pExpr, &pFree, &nFree);
      if( rc!=SQLITE_OK ) return rc;
      pIter = pFree;
      pEnd = &pFree[nFree];
    }else{
      int nDoc = p->aMatchinfo[2 + 3*p->nCol*p->aMatchinfo[0]];
      for(ii=0; ii<p->nCol; ii++){
        p->aMatchinfo[iStart + ii*3] = nDoc;
        p->aMatchinfo[iStart + ii*3 + 1] = nDoc;
      }
      return SQLITE_OK;
    }
  }else{
    pIter = pExpr->aDoclist;
    pEnd = &pExpr->aDoclist[pExpr->nDoclist];
  }

  /* Fill in the global hit count matrix row for this phrase. */
  while( pIter<pEnd ){
    while( *pIter++ & 0x80 );      /* Skip past docid. */
    fts3LoadColumnlistCounts(&pIter, &p->aMatchinfo[iStart], 1);
  }

  sqlite3_free(pFree);
  return SQLITE_OK;
}

/*
** fts3ExprIterate() callback used to collect the "local" matchinfo stats
** for a single query. The "local" stats are those elements of the matchinfo
** array that are different for each row returned by the query.
*/
static int fts3ExprLocalMatchinfoCb(
  Fts3Expr *pExpr,                /* Phrase expression node */
  int iPhrase,                    /* Phrase number */
  void *pCtx                      /* Pointer to MatchInfo structure */
){
  MatchInfo *p = (MatchInfo *)pCtx;

  if( pExpr->aDoclist ){
    char *pCsr;
    int iStart = 2 + (iPhrase * p->nCol * 3);
    int i;

    for(i=0; i<p->nCol; i++) p->aMatchinfo[iStart+i*3] = 0;

    pCsr = sqlite3Fts3FindPositions(pExpr, p->pCursor->iPrevId, -1);
    if( pCsr ){
      fts3LoadColumnlistCounts(&pCsr, &p->aMatchinfo[iStart], 0);
    }
  }

  return SQLITE_OK;
}

/*
** Populate pCsr->aMatchinfo[] with data for the current row. The 
** 'matchinfo' data is an array of 32-bit unsigned integers (C type u32).
*/
static int fts3GetMatchinfo(Fts3Cursor *pCsr){
  MatchInfo sInfo;
  Fts3Table *pTab = (Fts3Table *)pCsr->base.pVtab;
  int rc = SQLITE_OK;

  sInfo.pCursor = pCsr;
  sInfo.nCol = pTab->nColumn;

  if( pCsr->aMatchinfo==0 ){
    /* If Fts3Cursor.aMatchinfo[] is NULL, then this is the first time the
    ** matchinfo function has been called for this query. In this case 
    ** allocate the array used to accumulate the matchinfo data and
    ** initialize those elements that are constant for every row.
    */
    int nPhrase;                  /* Number of phrases */
    int nMatchinfo;               /* Number of u32 elements in match-info */

    /* Load doclists for each phrase in the query. */
    rc = fts3ExprLoadDoclists(pCsr, &nPhrase, 0);
    if( rc!=SQLITE_OK ){
      return rc;
    }
    nMatchinfo = 2 + 3*sInfo.nCol*nPhrase;
    if( pTab->bHasDocsize ){
      nMatchinfo += 1 + 2*pTab->nColumn;
    }

    sInfo.aMatchinfo = (u32 *)sqlite3_malloc(sizeof(u32)*nMatchinfo);
    if( !sInfo.aMatchinfo ){ 
      return SQLITE_NOMEM;
    }
    memset(sInfo.aMatchinfo, 0, sizeof(u32)*nMatchinfo);

    /* First element of match-info is the number of phrases in the query */
    sInfo.aMatchinfo[0] = nPhrase;
    sInfo.aMatchinfo[1] = sInfo.nCol;
    if( pTab->bHasDocsize ){
      int ofst = 2 + 3*sInfo.aMatchinfo[0]*sInfo.aMatchinfo[1];
      rc = sqlite3Fts3MatchinfoDocsizeGlobal(pCsr, &sInfo.aMatchinfo[ofst]);
    }
    (void)fts3ExprIterate(pCsr->pExpr, fts3ExprGlobalMatchinfoCb,(void*)&sInfo);
    pCsr->aMatchinfo = sInfo.aMatchinfo;
    pCsr->isMatchinfoNeeded = 1;
  }

  sInfo.aMatchinfo = pCsr->aMatchinfo;
  if( rc==SQLITE_OK && pCsr->isMatchinfoNeeded ){
    (void)fts3ExprIterate(pCsr->pExpr, fts3ExprLocalMatchinfoCb, (void*)&sInfo);
    if( pTab->bHasDocsize ){
      int ofst = 2 + 3*sInfo.aMatchinfo[0]*sInfo.aMatchinfo[1];
      rc = sqlite3Fts3MatchinfoDocsizeLocal(pCsr, &sInfo.aMatchinfo[ofst]);
    }
    pCsr->isMatchinfoNeeded = 0;
  }

  return SQLITE_OK;
}

/*
** Implementation of snippet() function.
*/
void sqlite3Fts3Snippet(
  sqlite3_context *pCtx,          /* SQLite function call context */
  Fts3Cursor *pCsr,               /* Cursor object */
  const char *zStart,             /* Snippet start text - "<b>" */
  const char *zEnd,               /* Snippet end text - "</b>" */
  const char *zEllipsis,          /* Snippet ellipsis text - "<b>...</b>" */
  int iCol,                       /* Extract snippet from this column */
  int nToken                      /* Approximate number of tokens in snippet */
){
  Fts3Table *pTab = (Fts3Table *)pCsr->base.pVtab;
  int rc = SQLITE_OK;
  int i;
  StrBuffer res = {0, 0, 0};

  /* The returned text includes up to four fragments of text extracted from
  ** the data in the current row. The first iteration of the for(...) loop
  ** below attempts to locate a single fragment of text nToken tokens in 
  ** size that contains at least one instance of all phrases in the query
  ** expression that appear in the current row. If such a fragment of text
  ** cannot be found, the second iteration of the loop attempts to locate
  ** a pair of fragments, and so on.
  */
  int nSnippet = 0;               /* Number of fragments in this snippet */
  SnippetFragment aSnippet[4];    /* Maximum of 4 fragments per snippet */
  int nFToken = -1;               /* Number of tokens in each fragment */

  if( !pCsr->pExpr ){
    sqlite3_result_text(pCtx, "", 0, SQLITE_STATIC);
    return;
  }

  for(nSnippet=1; 1; nSnippet++){

    int iSnip;                    /* Loop counter 0..nSnippet-1 */
    u64 mCovered = 0;             /* Bitmask of phrases covered by snippet */
    u64 mSeen = 0;                /* Bitmask of phrases seen by BestSnippet() */

    if( nToken>=0 ){
      nFToken = (nToken+nSnippet-1) / nSnippet;
    }else{
      nFToken = -1 * nToken;
    }

    for(iSnip=0; iSnip<nSnippet; iSnip++){
      int iBestScore = -1;        /* Best score of columns checked so far */
      int iRead;                  /* Used to iterate through columns */
      SnippetFragment *pFragment = &aSnippet[iSnip];

      memset(pFragment, 0, sizeof(*pFragment));

      /* Loop through all columns of the table being considered for snippets.
      ** If the iCol argument to this function was negative, this means all
      ** columns of the FTS3 table. Otherwise, only column iCol is considered.
      */
      for(iRead=0; iRead<pTab->nColumn; iRead++){
        SnippetFragment sF;
        int iS;
        if( iCol>=0 && iRead!=iCol ) continue;

        /* Find the best snippet of nFToken tokens in column iRead. */
        rc = fts3BestSnippet(nFToken, pCsr, iRead, mCovered, &mSeen, &sF, &iS);
        if( rc!=SQLITE_OK ){
          goto snippet_out;
        }
        if( iS>iBestScore ){
          *pFragment = sF;
          iBestScore = iS;
        }
      }

      mCovered |= pFragment->covered;
    }

    /* If all query phrases seen by fts3BestSnippet() are present in at least
    ** one of the nSnippet snippet fragments, break out of the loop.
    */
    assert( (mCovered&mSeen)==mCovered );
    if( mSeen==mCovered || nSnippet==SizeofArray(aSnippet) ) break;
  }

  assert( nFToken>0 );

  for(i=0; i<nSnippet && rc==SQLITE_OK; i++){
    rc = fts3SnippetText(pCsr, &aSnippet[i], 
        i, (i==nSnippet-1), nFToken, zStart, zEnd, zEllipsis, &res
    );
  }

 snippet_out:
  sqlite3Fts3SegmentsClose(pTab);
  if( rc!=SQLITE_OK ){
    sqlite3_result_error_code(pCtx, rc);
    sqlite3_free(res.z);
  }else{
    sqlite3_result_text(pCtx, res.z, -1, sqlite3_free);
  }
}


typedef struct TermOffset TermOffset;
typedef struct TermOffsetCtx TermOffsetCtx;

struct TermOffset {
  char *pList;                    /* Position-list */
  int iPos;                       /* Position just read from pList */
  int iOff;                       /* Offset of this term from read positions */
};

struct TermOffsetCtx {
  int iCol;                       /* Column of table to populate aTerm for */
  int iTerm;
  sqlite3_int64 iDocid;
  TermOffset *aTerm;
};

/*
** This function is an fts3ExprIterate() callback used by sqlite3Fts3Offsets().
*/
static int fts3ExprTermOffsetInit(Fts3Expr *pExpr, int iPhrase, void *ctx){
  TermOffsetCtx *p = (TermOffsetCtx *)ctx;
  int nTerm;                      /* Number of tokens in phrase */
  int iTerm;                      /* For looping through nTerm phrase terms */
  char *pList;                    /* Pointer to position list for phrase */
  int iPos = 0;                   /* First position in position-list */

  UNUSED_PARAMETER(iPhrase);
  pList = sqlite3Fts3FindPositions(pExpr, p->iDocid, p->iCol);
  nTerm = pExpr->pPhrase->nToken;
  if( pList ){
    fts3GetDeltaPosition(&pList, &iPos);
    assert( iPos>=0 );
  }

  for(iTerm=0; iTerm<nTerm; iTerm++){
    TermOffset *pT = &p->aTerm[p->iTerm++];
    pT->iOff = nTerm-iTerm-1;
    pT->pList = pList;
    pT->iPos = iPos;
  }

  return SQLITE_OK;
}

/*
** Implementation of offsets() function.
*/
void sqlite3Fts3Offsets(
  sqlite3_context *pCtx,          /* SQLite function call context */
  Fts3Cursor *pCsr                /* Cursor object */
){
  Fts3Table *pTab = (Fts3Table *)pCsr->base.pVtab;
  sqlite3_tokenizer_module const *pMod = pTab->pTokenizer->pModule;
  const char *ZDUMMY;             /* Dummy argument used with xNext() */
  int NDUMMY;                     /* Dummy argument used with xNext() */
  int rc;                         /* Return Code */
  int nToken;                     /* Number of tokens in query */
  int iCol;                       /* Column currently being processed */
  StrBuffer res = {0, 0, 0};      /* Result string */
  TermOffsetCtx sCtx;             /* Context for fts3ExprTermOffsetInit() */

  if( !pCsr->pExpr ){
    sqlite3_result_text(pCtx, "", 0, SQLITE_STATIC);
    return;
  }

  memset(&sCtx, 0, sizeof(sCtx));
  assert( pCsr->isRequireSeek==0 );

  /* Count the number of terms in the query */
  rc = fts3ExprLoadDoclists(pCsr, 0, &nToken);
  if( rc!=SQLITE_OK ) goto offsets_out;

  /* Allocate the array of TermOffset iterators. */
  sCtx.aTerm = (TermOffset *)sqlite3_malloc(sizeof(TermOffset)*nToken);
  if( 0==sCtx.aTerm ){
    rc = SQLITE_NOMEM;
    goto offsets_out;
  }
  sCtx.iDocid = pCsr->iPrevId;

  /* Loop through the table columns, appending offset information to 
  ** string-buffer res for each column.
  */
  for(iCol=0; iCol<pTab->nColumn; iCol++){
    sqlite3_tokenizer_cursor *pC; /* Tokenizer cursor */
    int iStart;
    int iEnd;
    int iCurrent;
    const char *zDoc;
    int nDoc;

    /* Initialize the contents of sCtx.aTerm[] for column iCol. There is 
    ** no way that this operation can fail, so the return code from
    ** fts3ExprIterate() can be discarded.
    */
    sCtx.iCol = iCol;
    sCtx.iTerm = 0;
    (void)fts3ExprIterate(pCsr->pExpr, fts3ExprTermOffsetInit, (void *)&sCtx);

    /* Retreive the text stored in column iCol. If an SQL NULL is stored 
    ** in column iCol, jump immediately to the next iteration of the loop.
    ** If an OOM occurs while retrieving the data (this can happen if SQLite
    ** needs to transform the data from utf-16 to utf-8), return SQLITE_NOMEM 
    ** to the caller. 
    */
    zDoc = (const char *)sqlite3_column_text(pCsr->pStmt, iCol+1);
    nDoc = sqlite3_column_bytes(pCsr->pStmt, iCol+1);
    if( zDoc==0 ){
      if( sqlite3_column_type(pCsr->pStmt, iCol+1)==SQLITE_NULL ){
        continue;
      }
      rc = SQLITE_NOMEM;
      goto offsets_out;
    }

    /* Initialize a tokenizer iterator to iterate through column iCol. */
    rc = pMod->xOpen(pTab->pTokenizer, zDoc, nDoc, &pC);
    if( rc!=SQLITE_OK ) goto offsets_out;
    pC->pTokenizer = pTab->pTokenizer;

    rc = pMod->xNext(pC, &ZDUMMY, &NDUMMY, &iStart, &iEnd, &iCurrent);
    while( rc==SQLITE_OK ){
      int i;                      /* Used to loop through terms */
      int iMinPos = 0x7FFFFFFF;   /* Position of next token */
      TermOffset *pTerm = 0;      /* TermOffset associated with next token */

      for(i=0; i<nToken; i++){
        TermOffset *pT = &sCtx.aTerm[i];
        if( pT->pList && (pT->iPos-pT->iOff)<iMinPos ){
          iMinPos = pT->iPos-pT->iOff;
          pTerm = pT;
        }
      }

      if( !pTerm ){
        /* All offsets for this column have been gathered. */
        break;
      }else{
        assert( iCurrent<=iMinPos );
        if( 0==(0xFE&*pTerm->pList) ){
          pTerm->pList = 0;
        }else{
          fts3GetDeltaPosition(&pTerm->pList, &pTerm->iPos);
        }
        while( rc==SQLITE_OK && iCurrent<iMinPos ){
          rc = pMod->xNext(pC, &ZDUMMY, &NDUMMY, &iStart, &iEnd, &iCurrent);
        }
        if( rc==SQLITE_OK ){
          char aBuffer[64];
          sqlite3_snprintf(sizeof(aBuffer), aBuffer, 
              "%d %d %d %d ", iCol, pTerm-sCtx.aTerm, iStart, iEnd-iStart
          );
          rc = fts3StringAppend(&res, aBuffer, -1);
        }else if( rc==SQLITE_DONE ){
          rc = SQLITE_CORRUPT;
        }
      }
    }
    if( rc==SQLITE_DONE ){
      rc = SQLITE_OK;
    }

    pMod->xClose(pC);
    if( rc!=SQLITE_OK ) goto offsets_out;
  }

 offsets_out:
  sqlite3_free(sCtx.aTerm);
  assert( rc!=SQLITE_DONE );
  sqlite3Fts3SegmentsClose(pTab);
  if( rc!=SQLITE_OK ){
    sqlite3_result_error_code(pCtx,  rc);
    sqlite3_free(res.z);
  }else{
    sqlite3_result_text(pCtx, res.z, res.n-1, sqlite3_free);
  }
  return;
}

/*
** Implementation of matchinfo() function.
*/
void sqlite3Fts3Matchinfo(sqlite3_context *pContext, Fts3Cursor *pCsr){
  int rc;
  if( !pCsr->pExpr ){
    sqlite3_result_blob(pContext, "", 0, SQLITE_STATIC);
    return;
  }
  rc = fts3GetMatchinfo(pCsr);
  sqlite3Fts3SegmentsClose((Fts3Table *)pCsr->base.pVtab );
  if( rc!=SQLITE_OK ){
    sqlite3_result_error_code(pContext, rc);
  }else{
    Fts3Table *pTab = (Fts3Table*)pCsr->base.pVtab;
    int n = sizeof(u32)*(2+pCsr->aMatchinfo[0]*pCsr->aMatchinfo[1]*3);
    if( pTab->bHasDocsize ){
      n += sizeof(u32)*(1 + 2*pTab->nColumn);
    }
    sqlite3_result_blob(pContext, pCsr->aMatchinfo, n, SQLITE_TRANSIENT);
  }
}

#endif