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SQLite training in Houston TX on 2019-11-05 (details)
Part of the 2019 Tcl Conference

Artifact ae896d78e45cbbf7d1f37f4aaa4cb7e135c3ddb3:


/*
** 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.
**
******************************************************************************
**
** This file is part of the SQLite FTS3 extension module. Specifically,
** this file contains code to insert, update and delete rows from FTS3
** tables. It also contains code to merge FTS3 b-tree segments. Some
** of the sub-routines used to merge segments are also used by the query 
** code in fts3.c.
*/

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

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

/*
** When full-text index nodes are loaded from disk, the buffer that they
** are loaded into has the following number of bytes of padding at the end 
** of it. i.e. if a full-text index node is 900 bytes in size, then a buffer
** of 920 bytes is allocated for it.
**
** This means that if we have a pointer into a buffer containing node data,
** it is always safe to read up to two varints from it without risking an
** overread, even if the node data is corrupted.
*/
#define FTS3_NODE_PADDING (FTS3_VARINT_MAX*2)

typedef struct PendingList PendingList;
typedef struct SegmentNode SegmentNode;
typedef struct SegmentWriter SegmentWriter;

/*
** Data structure used while accumulating terms in the pending-terms hash
** table. The hash table entry maps from term (a string) to a malloc'd
** instance of this structure.
*/
struct PendingList {
  int nData;
  char *aData;
  int nSpace;
  sqlite3_int64 iLastDocid;
  sqlite3_int64 iLastCol;
  sqlite3_int64 iLastPos;
};


/*
** Each cursor has a (possibly empty) linked list of the following objects.
*/
struct Fts3DeferredToken {
  Fts3PhraseToken *pToken;        /* Pointer to corresponding expr token */
  int iCol;                       /* Column token must occur in */
  Fts3DeferredToken *pNext;       /* Next in list of deferred tokens */
  PendingList *pList;             /* Doclist is assembled here */
};

/*
** An instance of this structure is used to iterate through the terms on
** a contiguous set of segment b-tree leaf nodes. Although the details of
** this structure are only manipulated by code in this file, opaque handles
** of type Fts3SegReader* are also used by code in fts3.c to iterate through
** terms when querying the full-text index. See functions:
**
**   sqlite3Fts3SegReaderNew()
**   sqlite3Fts3SegReaderFree()
**   sqlite3Fts3SegReaderCost()
**   sqlite3Fts3SegReaderIterate()
**
** Methods used to manipulate Fts3SegReader structures:
**
**   fts3SegReaderNext()
**   fts3SegReaderFirstDocid()
**   fts3SegReaderNextDocid()
*/
struct Fts3SegReader {
  int iIdx;                       /* Index within level, or 0x7FFFFFFF for PT */

  sqlite3_int64 iStartBlock;      /* Rowid of first leaf block to traverse */
  sqlite3_int64 iLeafEndBlock;    /* Rowid of final leaf block to traverse */
  sqlite3_int64 iEndBlock;        /* Rowid of final block in segment (or 0) */
  sqlite3_int64 iCurrentBlock;    /* Current leaf block (or 0) */

  char *aNode;                    /* Pointer to node data (or NULL) */
  int nNode;                      /* Size of buffer at aNode (or 0) */
  Fts3HashElem **ppNextElem;

  /* Variables set by fts3SegReaderNext(). These may be read directly
  ** by the caller. They are valid from the time SegmentReaderNew() returns
  ** until SegmentReaderNext() returns something other than SQLITE_OK
  ** (i.e. SQLITE_DONE).
  */
  int nTerm;                      /* Number of bytes in current term */
  char *zTerm;                    /* Pointer to current term */
  int nTermAlloc;                 /* Allocated size of zTerm buffer */
  char *aDoclist;                 /* Pointer to doclist of current entry */
  int nDoclist;                   /* Size of doclist in current entry */

  /* The following variables are used to iterate through the current doclist */
  char *pOffsetList;
  sqlite3_int64 iDocid;
};

#define fts3SegReaderIsPending(p) ((p)->ppNextElem!=0)
#define fts3SegReaderIsRootOnly(p) ((p)->aNode==(char *)&(p)[1])

/*
** An instance of this structure is used to create a segment b-tree in the
** database. The internal details of this type are only accessed by the
** following functions:
**
**   fts3SegWriterAdd()
**   fts3SegWriterFlush()
**   fts3SegWriterFree()
*/
struct SegmentWriter {
  SegmentNode *pTree;             /* Pointer to interior tree structure */
  sqlite3_int64 iFirst;           /* First slot in %_segments written */
  sqlite3_int64 iFree;            /* Next free slot in %_segments */
  char *zTerm;                    /* Pointer to previous term buffer */
  int nTerm;                      /* Number of bytes in zTerm */
  int nMalloc;                    /* Size of malloc'd buffer at zMalloc */
  char *zMalloc;                  /* Malloc'd space (possibly) used for zTerm */
  int nSize;                      /* Size of allocation at aData */
  int nData;                      /* Bytes of data in aData */
  char *aData;                    /* Pointer to block from malloc() */
};

/*
** Type SegmentNode is used by the following three functions to create
** the interior part of the segment b+-tree structures (everything except
** the leaf nodes). These functions and type are only ever used by code
** within the fts3SegWriterXXX() family of functions described above.
**
**   fts3NodeAddTerm()
**   fts3NodeWrite()
**   fts3NodeFree()
*/
struct SegmentNode {
  SegmentNode *pParent;           /* Parent node (or NULL for root node) */
  SegmentNode *pRight;            /* Pointer to right-sibling */
  SegmentNode *pLeftmost;         /* Pointer to left-most node of this depth */
  int nEntry;                     /* Number of terms written to node so far */
  char *zTerm;                    /* Pointer to previous term buffer */
  int nTerm;                      /* Number of bytes in zTerm */
  int nMalloc;                    /* Size of malloc'd buffer at zMalloc */
  char *zMalloc;                  /* Malloc'd space (possibly) used for zTerm */
  int nData;                      /* Bytes of valid data so far */
  char *aData;                    /* Node data */
};

/*
** Valid values for the second argument to fts3SqlStmt().
*/
#define SQL_DELETE_CONTENT             0
#define SQL_IS_EMPTY                   1
#define SQL_DELETE_ALL_CONTENT         2 
#define SQL_DELETE_ALL_SEGMENTS        3
#define SQL_DELETE_ALL_SEGDIR          4
#define SQL_DELETE_ALL_DOCSIZE         5
#define SQL_DELETE_ALL_STAT            6
#define SQL_SELECT_CONTENT_BY_ROWID    7
#define SQL_NEXT_SEGMENT_INDEX         8
#define SQL_INSERT_SEGMENTS            9
#define SQL_NEXT_SEGMENTS_ID          10
#define SQL_INSERT_SEGDIR             11
#define SQL_SELECT_LEVEL              12
#define SQL_SELECT_ALL_LEVEL          13
#define SQL_SELECT_LEVEL_COUNT        14
#define SQL_SELECT_SEGDIR_COUNT_MAX   15
#define SQL_DELETE_SEGDIR_BY_LEVEL    16
#define SQL_DELETE_SEGMENTS_RANGE     17
#define SQL_CONTENT_INSERT            18
#define SQL_DELETE_DOCSIZE            19
#define SQL_REPLACE_DOCSIZE           20
#define SQL_SELECT_DOCSIZE            21
#define SQL_SELECT_DOCTOTAL           22
#define SQL_REPLACE_DOCTOTAL          23

/*
** This function is used to obtain an SQLite prepared statement handle
** for the statement identified by the second argument. If successful,
** *pp is set to the requested statement handle and SQLITE_OK returned.
** Otherwise, an SQLite error code is returned and *pp is set to 0.
**
** If argument apVal is not NULL, then it must point to an array with
** at least as many entries as the requested statement has bound 
** parameters. The values are bound to the statements parameters before
** returning.
*/
static int fts3SqlStmt(
  Fts3Table *p,                   /* Virtual table handle */
  int eStmt,                      /* One of the SQL_XXX constants above */
  sqlite3_stmt **pp,              /* OUT: Statement handle */
  sqlite3_value **apVal           /* Values to bind to statement */
){
  const char *azSql[] = {
/* 0  */  "DELETE FROM %Q.'%q_content' WHERE rowid = ?",
/* 1  */  "SELECT NOT EXISTS(SELECT docid FROM %Q.'%q_content' WHERE rowid!=?)",
/* 2  */  "DELETE FROM %Q.'%q_content'",
/* 3  */  "DELETE FROM %Q.'%q_segments'",
/* 4  */  "DELETE FROM %Q.'%q_segdir'",
/* 5  */  "DELETE FROM %Q.'%q_docsize'",
/* 6  */  "DELETE FROM %Q.'%q_stat'",
/* 7  */  "SELECT %s FROM %Q.'%q_content' AS x WHERE rowid=?",
/* 8  */  "SELECT (SELECT max(idx) FROM %Q.'%q_segdir' WHERE level = ?) + 1",
/* 9  */  "INSERT INTO %Q.'%q_segments'(blockid, block) VALUES(?, ?)",
/* 10 */  "SELECT coalesce((SELECT max(blockid) FROM %Q.'%q_segments') + 1, 1)",
/* 11 */  "INSERT INTO %Q.'%q_segdir' VALUES(?,?,?,?,?,?)",

          /* Return segments in order from oldest to newest.*/ 
/* 12 */  "SELECT idx, start_block, leaves_end_block, end_block, root "
            "FROM %Q.'%q_segdir' WHERE level = ? ORDER BY idx ASC",
/* 13 */  "SELECT idx, start_block, leaves_end_block, end_block, root "
            "FROM %Q.'%q_segdir' ORDER BY level DESC, idx ASC",

/* 14 */  "SELECT count(*) FROM %Q.'%q_segdir' WHERE level = ?",
/* 15 */  "SELECT count(*), max(level) FROM %Q.'%q_segdir'",

/* 16 */  "DELETE FROM %Q.'%q_segdir' WHERE level = ?",
/* 17 */  "DELETE FROM %Q.'%q_segments' WHERE blockid BETWEEN ? AND ?",
/* 18 */  "INSERT INTO %Q.'%q_content' VALUES(%s)",
/* 19 */  "DELETE FROM %Q.'%q_docsize' WHERE docid = ?",
/* 20 */  "REPLACE INTO %Q.'%q_docsize' VALUES(?,?)",
/* 21 */  "SELECT size FROM %Q.'%q_docsize' WHERE docid=?",
/* 22 */  "SELECT value FROM %Q.'%q_stat' WHERE id=0",
/* 23 */  "REPLACE INTO %Q.'%q_stat' VALUES(0,?)",
  };
  int rc = SQLITE_OK;
  sqlite3_stmt *pStmt;

  assert( SizeofArray(azSql)==SizeofArray(p->aStmt) );
  assert( eStmt<SizeofArray(azSql) && eStmt>=0 );
  
  pStmt = p->aStmt[eStmt];
  if( !pStmt ){
    char *zSql;
    if( eStmt==SQL_CONTENT_INSERT ){
      zSql = sqlite3_mprintf(azSql[eStmt], p->zDb, p->zName, p->zWriteExprlist);
    }else if( eStmt==SQL_SELECT_CONTENT_BY_ROWID ){
      zSql = sqlite3_mprintf(azSql[eStmt], p->zReadExprlist, p->zDb, p->zName);
    }else{
      zSql = sqlite3_mprintf(azSql[eStmt], p->zDb, p->zName);
    }
    if( !zSql ){
      rc = SQLITE_NOMEM;
    }else{
      rc = sqlite3_prepare_v2(p->db, zSql, -1, &pStmt, NULL);
      sqlite3_free(zSql);
      assert( rc==SQLITE_OK || pStmt==0 );
      p->aStmt[eStmt] = pStmt;
    }
  }
  if( apVal ){
    int i;
    int nParam = sqlite3_bind_parameter_count(pStmt);
    for(i=0; rc==SQLITE_OK && i<nParam; i++){
      rc = sqlite3_bind_value(pStmt, i+1, apVal[i]);
    }
  }
  *pp = pStmt;
  return rc;
}

static int fts3SelectDocsize(
  Fts3Table *pTab,                /* FTS3 table handle */
  int eStmt,                      /* Either SQL_SELECT_DOCSIZE or DOCTOTAL */
  sqlite3_int64 iDocid,           /* Docid to bind for SQL_SELECT_DOCSIZE */
  sqlite3_stmt **ppStmt           /* OUT: Statement handle */
){
  sqlite3_stmt *pStmt = 0;        /* Statement requested from fts3SqlStmt() */
  int rc;                         /* Return code */

  assert( eStmt==SQL_SELECT_DOCSIZE || eStmt==SQL_SELECT_DOCTOTAL );

  rc = fts3SqlStmt(pTab, eStmt, &pStmt, 0);
  if( rc==SQLITE_OK ){
    if( eStmt==SQL_SELECT_DOCSIZE ){
      sqlite3_bind_int64(pStmt, 1, iDocid);
    }
    rc = sqlite3_step(pStmt);
    if( rc!=SQLITE_ROW ){
      rc = sqlite3_reset(pStmt);
      if( rc==SQLITE_OK ) rc = SQLITE_CORRUPT;
      pStmt = 0;
    }else{
      rc = SQLITE_OK;
    }
  }

  *ppStmt = pStmt;
  return rc;
}

int sqlite3Fts3SelectDoctotal(
  Fts3Table *pTab,                /* Fts3 table handle */
  sqlite3_stmt **ppStmt           /* OUT: Statement handle */
){
  return fts3SelectDocsize(pTab, SQL_SELECT_DOCTOTAL, 0, ppStmt);
}

int sqlite3Fts3SelectDocsize(
  Fts3Table *pTab,                /* Fts3 table handle */
  sqlite3_int64 iDocid,           /* Docid to read size data for */
  sqlite3_stmt **ppStmt           /* OUT: Statement handle */
){
  return fts3SelectDocsize(pTab, SQL_SELECT_DOCSIZE, iDocid, ppStmt);
}

/*
** Similar to fts3SqlStmt(). Except, after binding the parameters in
** array apVal[] to the SQL statement identified by eStmt, the statement
** is executed.
**
** Returns SQLITE_OK if the statement is successfully executed, or an
** SQLite error code otherwise.
*/
static void fts3SqlExec(
  int *pRC,                /* Result code */
  Fts3Table *p,            /* The FTS3 table */
  int eStmt,               /* Index of statement to evaluate */
  sqlite3_value **apVal    /* Parameters to bind */
){
  sqlite3_stmt *pStmt;
  int rc;
  if( *pRC ) return;
  rc = fts3SqlStmt(p, eStmt, &pStmt, apVal); 
  if( rc==SQLITE_OK ){
    sqlite3_step(pStmt);
    rc = sqlite3_reset(pStmt);
  }
  *pRC = rc;
}


/*
** This function ensures that the caller has obtained a shared-cache
** table-lock on the %_content table. This is required before reading
** data from the fts3 table. If this lock is not acquired first, then
** the caller may end up holding read-locks on the %_segments and %_segdir
** tables, but no read-lock on the %_content table. If this happens 
** a second connection will be able to write to the fts3 table, but
** attempting to commit those writes might return SQLITE_LOCKED or
** SQLITE_LOCKED_SHAREDCACHE (because the commit attempts to obtain 
** write-locks on the %_segments and %_segdir ** tables). 
**
** We try to avoid this because if FTS3 returns any error when committing
** a transaction, the whole transaction will be rolled back. And this is
** not what users expect when they get SQLITE_LOCKED_SHAREDCACHE. It can
** still happen if the user reads data directly from the %_segments or
** %_segdir tables instead of going through FTS3 though.
*/
int sqlite3Fts3ReadLock(Fts3Table *p){
  int rc;                         /* Return code */
  sqlite3_stmt *pStmt;            /* Statement used to obtain lock */

  rc = fts3SqlStmt(p, SQL_SELECT_CONTENT_BY_ROWID, &pStmt, 0);
  if( rc==SQLITE_OK ){
    sqlite3_bind_null(pStmt, 1);
    sqlite3_step(pStmt);
    rc = sqlite3_reset(pStmt);
  }
  return rc;
}

/*
** Set *ppStmt to a statement handle that may be used to iterate through
** all rows in the %_segdir table, from oldest to newest. If successful,
** return SQLITE_OK. If an error occurs while preparing the statement, 
** return an SQLite error code.
**
** There is only ever one instance of this SQL statement compiled for
** each FTS3 table.
**
** The statement returns the following columns from the %_segdir table:
**
**   0: idx
**   1: start_block
**   2: leaves_end_block
**   3: end_block
**   4: root
*/
int sqlite3Fts3AllSegdirs(Fts3Table *p, int iLevel, sqlite3_stmt **ppStmt){
  int rc;
  sqlite3_stmt *pStmt = 0;
  if( iLevel<0 ){
    rc = fts3SqlStmt(p, SQL_SELECT_ALL_LEVEL, &pStmt, 0);
  }else{
    rc = fts3SqlStmt(p, SQL_SELECT_LEVEL, &pStmt, 0);
    if( rc==SQLITE_OK ) sqlite3_bind_int(pStmt, 1, iLevel);
  }
  *ppStmt = pStmt;
  return rc;
}


/*
** Append a single varint to a PendingList buffer. SQLITE_OK is returned
** if successful, or an SQLite error code otherwise.
**
** This function also serves to allocate the PendingList structure itself.
** For example, to create a new PendingList structure containing two
** varints:
**
**   PendingList *p = 0;
**   fts3PendingListAppendVarint(&p, 1);
**   fts3PendingListAppendVarint(&p, 2);
*/
static int fts3PendingListAppendVarint(
  PendingList **pp,               /* IN/OUT: Pointer to PendingList struct */
  sqlite3_int64 i                 /* Value to append to data */
){
  PendingList *p = *pp;

  /* Allocate or grow the PendingList as required. */
  if( !p ){
    p = sqlite3_malloc(sizeof(*p) + 100);
    if( !p ){
      return SQLITE_NOMEM;
    }
    p->nSpace = 100;
    p->aData = (char *)&p[1];
    p->nData = 0;
  }
  else if( p->nData+FTS3_VARINT_MAX+1>p->nSpace ){
    int nNew = p->nSpace * 2;
    p = sqlite3_realloc(p, sizeof(*p) + nNew);
    if( !p ){
      sqlite3_free(*pp);
      *pp = 0;
      return SQLITE_NOMEM;
    }
    p->nSpace = nNew;
    p->aData = (char *)&p[1];
  }

  /* Append the new serialized varint to the end of the list. */
  p->nData += sqlite3Fts3PutVarint(&p->aData[p->nData], i);
  p->aData[p->nData] = '\0';
  *pp = p;
  return SQLITE_OK;
}

/*
** Add a docid/column/position entry to a PendingList structure. Non-zero
** is returned if the structure is sqlite3_realloced as part of adding
** the entry. Otherwise, zero.
**
** If an OOM error occurs, *pRc is set to SQLITE_NOMEM before returning.
** Zero is always returned in this case. Otherwise, if no OOM error occurs,
** it is set to SQLITE_OK.
*/
static int fts3PendingListAppend(
  PendingList **pp,               /* IN/OUT: PendingList structure */
  sqlite3_int64 iDocid,           /* Docid for entry to add */
  sqlite3_int64 iCol,             /* Column for entry to add */
  sqlite3_int64 iPos,             /* Position of term for entry to add */
  int *pRc                        /* OUT: Return code */
){
  PendingList *p = *pp;
  int rc = SQLITE_OK;

  assert( !p || p->iLastDocid<=iDocid );

  if( !p || p->iLastDocid!=iDocid ){
    sqlite3_int64 iDelta = iDocid - (p ? p->iLastDocid : 0);
    if( p ){
      assert( p->nData<p->nSpace );
      assert( p->aData[p->nData]==0 );
      p->nData++;
    }
    if( SQLITE_OK!=(rc = fts3PendingListAppendVarint(&p, iDelta)) ){
      goto pendinglistappend_out;
    }
    p->iLastCol = -1;
    p->iLastPos = 0;
    p->iLastDocid = iDocid;
  }
  if( iCol>0 && p->iLastCol!=iCol ){
    if( SQLITE_OK!=(rc = fts3PendingListAppendVarint(&p, 1))
     || SQLITE_OK!=(rc = fts3PendingListAppendVarint(&p, iCol))
    ){
      goto pendinglistappend_out;
    }
    p->iLastCol = iCol;
    p->iLastPos = 0;
  }
  if( iCol>=0 ){
    assert( iPos>p->iLastPos || (iPos==0 && p->iLastPos==0) );
    rc = fts3PendingListAppendVarint(&p, 2+iPos-p->iLastPos);
    if( rc==SQLITE_OK ){
      p->iLastPos = iPos;
    }
  }

 pendinglistappend_out:
  *pRc = rc;
  if( p!=*pp ){
    *pp = p;
    return 1;
  }
  return 0;
}

/*
** Tokenize the nul-terminated string zText and add all tokens to the
** pending-terms hash-table. The docid used is that currently stored in
** p->iPrevDocid, and the column is specified by argument iCol.
**
** If successful, SQLITE_OK is returned. Otherwise, an SQLite error code.
*/
static int fts3PendingTermsAdd(
  Fts3Table *p,                   /* Table into which text will be inserted */
  const char *zText,              /* Text of document to be inserted */
  int iCol,                       /* Column into which text is being inserted */
  u32 *pnWord                     /* OUT: Number of tokens inserted */
){
  int rc;
  int iStart;
  int iEnd;
  int iPos;
  int nWord = 0;

  char const *zToken;
  int nToken;

  sqlite3_tokenizer *pTokenizer = p->pTokenizer;
  sqlite3_tokenizer_module const *pModule = pTokenizer->pModule;
  sqlite3_tokenizer_cursor *pCsr;
  int (*xNext)(sqlite3_tokenizer_cursor *pCursor,
      const char**,int*,int*,int*,int*);

  assert( pTokenizer && pModule );

  rc = pModule->xOpen(pTokenizer, zText, -1, &pCsr);
  if( rc!=SQLITE_OK ){
    return rc;
  }
  pCsr->pTokenizer = pTokenizer;

  xNext = pModule->xNext;
  while( SQLITE_OK==rc
      && SQLITE_OK==(rc = xNext(pCsr, &zToken, &nToken, &iStart, &iEnd, &iPos))
  ){
    PendingList *pList;
 
    if( iPos>=nWord ) nWord = iPos+1;

    /* Positions cannot be negative; we use -1 as a terminator internally.
    ** Tokens must have a non-zero length.
    */
    if( iPos<0 || !zToken || nToken<=0 ){
      rc = SQLITE_ERROR;
      break;
    }

    pList = (PendingList *)fts3HashFind(&p->pendingTerms, zToken, nToken);
    if( pList ){
      p->nPendingData -= (pList->nData + nToken + sizeof(Fts3HashElem));
    }
    if( fts3PendingListAppend(&pList, p->iPrevDocid, iCol, iPos, &rc) ){
      if( pList==fts3HashInsert(&p->pendingTerms, zToken, nToken, pList) ){
        /* Malloc failed while inserting the new entry. This can only 
        ** happen if there was no previous entry for this token.
        */
        assert( 0==fts3HashFind(&p->pendingTerms, zToken, nToken) );
        sqlite3_free(pList);
        rc = SQLITE_NOMEM;
      }
    }
    if( rc==SQLITE_OK ){
      p->nPendingData += (pList->nData + nToken + sizeof(Fts3HashElem));
    }
  }

  pModule->xClose(pCsr);
  *pnWord = nWord;
  return (rc==SQLITE_DONE ? SQLITE_OK : rc);
}

/* 
** Calling this function indicates that subsequent calls to 
** fts3PendingTermsAdd() are to add term/position-list pairs for the
** contents of the document with docid iDocid.
*/
static int fts3PendingTermsDocid(Fts3Table *p, sqlite_int64 iDocid){
  /* TODO(shess) Explore whether partially flushing the buffer on
  ** forced-flush would provide better performance.  I suspect that if
  ** we ordered the doclists by size and flushed the largest until the
  ** buffer was half empty, that would let the less frequent terms
  ** generate longer doclists.
  */
  if( iDocid<=p->iPrevDocid || p->nPendingData>p->nMaxPendingData ){
    int rc = sqlite3Fts3PendingTermsFlush(p);
    if( rc!=SQLITE_OK ) return rc;
  }
  p->iPrevDocid = iDocid;
  return SQLITE_OK;
}

/*
** Discard the contents of the pending-terms hash table. 
*/
void sqlite3Fts3PendingTermsClear(Fts3Table *p){
  Fts3HashElem *pElem;
  for(pElem=fts3HashFirst(&p->pendingTerms); pElem; pElem=fts3HashNext(pElem)){
    sqlite3_free(fts3HashData(pElem));
  }
  fts3HashClear(&p->pendingTerms);
  p->nPendingData = 0;
}

/*
** This function is called by the xUpdate() method as part of an INSERT
** operation. It adds entries for each term in the new record to the
** pendingTerms hash table.
**
** Argument apVal is the same as the similarly named argument passed to
** fts3InsertData(). Parameter iDocid is the docid of the new row.
*/
static int fts3InsertTerms(Fts3Table *p, sqlite3_value **apVal, u32 *aSz){
  int i;                          /* Iterator variable */
  for(i=2; i<p->nColumn+2; i++){
    const char *zText = (const char *)sqlite3_value_text(apVal[i]);
    if( zText ){
      int rc = fts3PendingTermsAdd(p, zText, i-2, &aSz[i-2]);
      if( rc!=SQLITE_OK ){
        return rc;
      }
    }
    aSz[p->nColumn] += sqlite3_value_bytes(apVal[i]);
  }
  return SQLITE_OK;
}

/*
** This function is called by the xUpdate() method for an INSERT operation.
** The apVal parameter is passed a copy of the apVal argument passed by
** SQLite to the xUpdate() method. i.e:
**
**   apVal[0]                Not used for INSERT.
**   apVal[1]                rowid
**   apVal[2]                Left-most user-defined column
**   ...
**   apVal[p->nColumn+1]     Right-most user-defined column
**   apVal[p->nColumn+2]     Hidden column with same name as table
**   apVal[p->nColumn+3]     Hidden "docid" column (alias for rowid)
*/
static int fts3InsertData(
  Fts3Table *p,                   /* Full-text table */
  sqlite3_value **apVal,          /* Array of values to insert */
  sqlite3_int64 *piDocid          /* OUT: Docid for row just inserted */
){
  int rc;                         /* Return code */
  sqlite3_stmt *pContentInsert;   /* INSERT INTO %_content VALUES(...) */

  /* Locate the statement handle used to insert data into the %_content
  ** table. The SQL for this statement is:
  **
  **   INSERT INTO %_content VALUES(?, ?, ?, ...)
  **
  ** The statement features N '?' variables, where N is the number of user
  ** defined columns in the FTS3 table, plus one for the docid field.
  */
  rc = fts3SqlStmt(p, SQL_CONTENT_INSERT, &pContentInsert, &apVal[1]);
  if( rc!=SQLITE_OK ){
    return rc;
  }

  /* There is a quirk here. The users INSERT statement may have specified
  ** a value for the "rowid" field, for the "docid" field, or for both.
  ** Which is a problem, since "rowid" and "docid" are aliases for the
  ** same value. For example:
  **
  **   INSERT INTO fts3tbl(rowid, docid) VALUES(1, 2);
  **
  ** In FTS3, this is an error. It is an error to specify non-NULL values
  ** for both docid and some other rowid alias.
  */
  if( SQLITE_NULL!=sqlite3_value_type(apVal[3+p->nColumn]) ){
    if( SQLITE_NULL==sqlite3_value_type(apVal[0])
     && SQLITE_NULL!=sqlite3_value_type(apVal[1])
    ){
      /* A rowid/docid conflict. */
      return SQLITE_ERROR;
    }
    rc = sqlite3_bind_value(pContentInsert, 1, apVal[3+p->nColumn]);
    if( rc!=SQLITE_OK ) return rc;
  }

  /* Execute the statement to insert the record. Set *piDocid to the 
  ** new docid value. 
  */
  sqlite3_step(pContentInsert);
  rc = sqlite3_reset(pContentInsert);

  *piDocid = sqlite3_last_insert_rowid(p->db);
  return rc;
}



/*
** Remove all data from the FTS3 table. Clear the hash table containing
** pending terms.
*/
static int fts3DeleteAll(Fts3Table *p){
  int rc = SQLITE_OK;             /* Return code */

  /* Discard the contents of the pending-terms hash table. */
  sqlite3Fts3PendingTermsClear(p);

  /* Delete everything from the %_content, %_segments and %_segdir tables. */
  fts3SqlExec(&rc, p, SQL_DELETE_ALL_CONTENT, 0);
  fts3SqlExec(&rc, p, SQL_DELETE_ALL_SEGMENTS, 0);
  fts3SqlExec(&rc, p, SQL_DELETE_ALL_SEGDIR, 0);
  if( p->bHasDocsize ){
    fts3SqlExec(&rc, p, SQL_DELETE_ALL_DOCSIZE, 0);
  }
  if( p->bHasStat ){
    fts3SqlExec(&rc, p, SQL_DELETE_ALL_STAT, 0);
  }
  return rc;
}

/*
** The first element in the apVal[] array is assumed to contain the docid
** (an integer) of a row about to be deleted. Remove all terms from the
** full-text index.
*/
static void fts3DeleteTerms( 
  int *pRC,               /* Result code */
  Fts3Table *p,           /* The FTS table to delete from */
  sqlite3_value **apVal,  /* apVal[] contains the docid to be deleted */
  u32 *aSz                /* Sizes of deleted document written here */
){
  int rc;
  sqlite3_stmt *pSelect;

  if( *pRC ) return;
  rc = fts3SqlStmt(p, SQL_SELECT_CONTENT_BY_ROWID, &pSelect, apVal);
  if( rc==SQLITE_OK ){
    if( SQLITE_ROW==sqlite3_step(pSelect) ){
      int i;
      for(i=1; i<=p->nColumn; i++){
        const char *zText = (const char *)sqlite3_column_text(pSelect, i);
        rc = fts3PendingTermsAdd(p, zText, -1, &aSz[i-1]);
        if( rc!=SQLITE_OK ){
          sqlite3_reset(pSelect);
          *pRC = rc;
          return;
        }
        aSz[p->nColumn] += sqlite3_column_bytes(pSelect, i);
      }
    }
    rc = sqlite3_reset(pSelect);
  }else{
    sqlite3_reset(pSelect);
  }
  *pRC = rc;
}

/*
** Forward declaration to account for the circular dependency between
** functions fts3SegmentMerge() and fts3AllocateSegdirIdx().
*/
static int fts3SegmentMerge(Fts3Table *, int);

/* 
** This function allocates a new level iLevel index in the segdir table.
** Usually, indexes are allocated within a level sequentially starting
** with 0, so the allocated index is one greater than the value returned
** by:
**
**   SELECT max(idx) FROM %_segdir WHERE level = :iLevel
**
** However, if there are already FTS3_MERGE_COUNT indexes at the requested
** level, they are merged into a single level (iLevel+1) segment and the 
** allocated index is 0.
**
** If successful, *piIdx is set to the allocated index slot and SQLITE_OK
** returned. Otherwise, an SQLite error code is returned.
*/
static int fts3AllocateSegdirIdx(Fts3Table *p, int iLevel, int *piIdx){
  int rc;                         /* Return Code */
  sqlite3_stmt *pNextIdx;         /* Query for next idx at level iLevel */
  int iNext = 0;                  /* Result of query pNextIdx */

  /* Set variable iNext to the next available segdir index at level iLevel. */
  rc = fts3SqlStmt(p, SQL_NEXT_SEGMENT_INDEX, &pNextIdx, 0);
  if( rc==SQLITE_OK ){
    sqlite3_bind_int(pNextIdx, 1, iLevel);
    if( SQLITE_ROW==sqlite3_step(pNextIdx) ){
      iNext = sqlite3_column_int(pNextIdx, 0);
    }
    rc = sqlite3_reset(pNextIdx);
  }

  if( rc==SQLITE_OK ){
    /* If iNext is FTS3_MERGE_COUNT, indicating that level iLevel is already
    ** full, merge all segments in level iLevel into a single iLevel+1
    ** segment and allocate (newly freed) index 0 at level iLevel. Otherwise,
    ** if iNext is less than FTS3_MERGE_COUNT, allocate index iNext.
    */
    if( iNext>=FTS3_MERGE_COUNT ){
      rc = fts3SegmentMerge(p, iLevel);
      *piIdx = 0;
    }else{
      *piIdx = iNext;
    }
  }

  return rc;
}

/*
** The %_segments table is declared as follows:
**
**   CREATE TABLE %_segments(blockid INTEGER PRIMARY KEY, block BLOB)
**
** This function reads data from a single row of the %_segments table. The
** specific row is identified by the iBlockid parameter. If paBlob is not
** NULL, then a buffer is allocated using sqlite3_malloc() and populated
** with the contents of the blob stored in the "block" column of the 
** identified table row is. Whether or not paBlob is NULL, *pnBlob is set
** to the size of the blob in bytes before returning.
**
** If an error occurs, or the table does not contain the specified row,
** an SQLite error code is returned. Otherwise, SQLITE_OK is returned. If
** paBlob is non-NULL, then it is the responsibility of the caller to
** eventually free the returned buffer.
**
** This function may leave an open sqlite3_blob* handle in the
** Fts3Table.pSegments variable. This handle is reused by subsequent calls
** to this function. The handle may be closed by calling the
** sqlite3Fts3SegmentsClose() function. Reusing a blob handle is a handy
** performance improvement, but the blob handle should always be closed
** before control is returned to the user (to prevent a lock being held
** on the database file for longer than necessary). Thus, any virtual table
** method (xFilter etc.) that may directly or indirectly call this function
** must call sqlite3Fts3SegmentsClose() before returning.
*/
int sqlite3Fts3ReadBlock(
  Fts3Table *p,                   /* FTS3 table handle */
  sqlite3_int64 iBlockid,         /* Access the row with blockid=$iBlockid */
  char **paBlob,                  /* OUT: Blob data in malloc'd buffer */
  int *pnBlob                     /* OUT: Size of blob data */
){
  int rc;                         /* Return code */

  /* pnBlob must be non-NULL. paBlob may be NULL or non-NULL. */
  assert( pnBlob);

  if( p->pSegments ){
    rc = sqlite3_blob_reopen(p->pSegments, iBlockid);
  }else{
    if( 0==p->zSegmentsTbl ){
      p->zSegmentsTbl = sqlite3_mprintf("%s_segments", p->zName);
      if( 0==p->zSegmentsTbl ) return SQLITE_NOMEM;
    }
    rc = sqlite3_blob_open(
       p->db, p->zDb, p->zSegmentsTbl, "block", iBlockid, 0, &p->pSegments
    );
  }

  if( rc==SQLITE_OK ){
    int nByte = sqlite3_blob_bytes(p->pSegments);
    if( paBlob ){
      char *aByte = sqlite3_malloc(nByte + FTS3_NODE_PADDING);
      if( !aByte ){
        rc = SQLITE_NOMEM;
      }else{
        rc = sqlite3_blob_read(p->pSegments, aByte, nByte, 0);
        memset(&aByte[nByte], 0, FTS3_NODE_PADDING);
        if( rc!=SQLITE_OK ){
          sqlite3_free(aByte);
          aByte = 0;
        }
      }
      *paBlob = aByte;
    }
    *pnBlob = nByte;
  }

  return rc;
}

/*
** Close the blob handle at p->pSegments, if it is open. See comments above
** the sqlite3Fts3ReadBlock() function for details.
*/
void sqlite3Fts3SegmentsClose(Fts3Table *p){
  sqlite3_blob_close(p->pSegments);
  p->pSegments = 0;
}

/*
** Move the iterator passed as the first argument to the next term in the
** segment. If successful, SQLITE_OK is returned. If there is no next term,
** SQLITE_DONE. Otherwise, an SQLite error code.
*/
static int fts3SegReaderNext(Fts3Table *p, Fts3SegReader *pReader){
  char *pNext;                    /* Cursor variable */
  int nPrefix;                    /* Number of bytes in term prefix */
  int nSuffix;                    /* Number of bytes in term suffix */

  if( !pReader->aDoclist ){
    pNext = pReader->aNode;
  }else{
    pNext = &pReader->aDoclist[pReader->nDoclist];
  }

  if( !pNext || pNext>=&pReader->aNode[pReader->nNode] ){
    int rc;                       /* Return code from Fts3ReadBlock() */

    if( fts3SegReaderIsPending(pReader) ){
      Fts3HashElem *pElem = *(pReader->ppNextElem);
      if( pElem==0 ){
        pReader->aNode = 0;
      }else{
        PendingList *pList = (PendingList *)fts3HashData(pElem);
        pReader->zTerm = (char *)fts3HashKey(pElem);
        pReader->nTerm = fts3HashKeysize(pElem);
        pReader->nNode = pReader->nDoclist = pList->nData + 1;
        pReader->aNode = pReader->aDoclist = pList->aData;
        pReader->ppNextElem++;
        assert( pReader->aNode );
      }
      return SQLITE_OK;
    }

    if( !fts3SegReaderIsRootOnly(pReader) ){
      sqlite3_free(pReader->aNode);
    }
    pReader->aNode = 0;

    /* If iCurrentBlock>=iLeafEndBlock, this is an EOF condition. All leaf 
    ** blocks have already been traversed.  */
    assert( pReader->iCurrentBlock<=pReader->iLeafEndBlock );
    if( pReader->iCurrentBlock>=pReader->iLeafEndBlock ){
      return SQLITE_OK;
    }

    rc = sqlite3Fts3ReadBlock(
        p, ++pReader->iCurrentBlock, &pReader->aNode, &pReader->nNode
    );
    if( rc!=SQLITE_OK ) return rc;
    pNext = pReader->aNode;
  }
  
  /* Because of the FTS3_NODE_PADDING bytes of padding, the following is 
  ** safe (no risk of overread) even if the node data is corrupted.  
  */
  pNext += sqlite3Fts3GetVarint32(pNext, &nPrefix);
  pNext += sqlite3Fts3GetVarint32(pNext, &nSuffix);
  if( nPrefix<0 || nSuffix<=0 
   || &pNext[nSuffix]>&pReader->aNode[pReader->nNode] 
  ){
    return SQLITE_CORRUPT;
  }

  if( nPrefix+nSuffix>pReader->nTermAlloc ){
    int nNew = (nPrefix+nSuffix)*2;
    char *zNew = sqlite3_realloc(pReader->zTerm, nNew);
    if( !zNew ){
      return SQLITE_NOMEM;
    }
    pReader->zTerm = zNew;
    pReader->nTermAlloc = nNew;
  }
  memcpy(&pReader->zTerm[nPrefix], pNext, nSuffix);
  pReader->nTerm = nPrefix+nSuffix;
  pNext += nSuffix;
  pNext += sqlite3Fts3GetVarint32(pNext, &pReader->nDoclist);
  pReader->aDoclist = pNext;
  pReader->pOffsetList = 0;

  /* Check that the doclist does not appear to extend past the end of the
  ** b-tree node. And that the final byte of the doclist is 0x00. If either 
  ** of these statements is untrue, then the data structure is corrupt.
  */
  if( &pReader->aDoclist[pReader->nDoclist]>&pReader->aNode[pReader->nNode] 
   || pReader->aDoclist[pReader->nDoclist-1]
  ){
    return SQLITE_CORRUPT;
  }
  return SQLITE_OK;
}

/*
** Set the SegReader to point to the first docid in the doclist associated
** with the current term.
*/
static void fts3SegReaderFirstDocid(Fts3SegReader *pReader){
  int n;
  assert( pReader->aDoclist );
  assert( !pReader->pOffsetList );
  n = sqlite3Fts3GetVarint(pReader->aDoclist, &pReader->iDocid);
  pReader->pOffsetList = &pReader->aDoclist[n];
}

/*
** Advance the SegReader to point to the next docid in the doclist
** associated with the current term.
** 
** If arguments ppOffsetList and pnOffsetList are not NULL, then 
** *ppOffsetList is set to point to the first column-offset list
** in the doclist entry (i.e. immediately past the docid varint).
** *pnOffsetList is set to the length of the set of column-offset
** lists, not including the nul-terminator byte. For example:
*/
static void fts3SegReaderNextDocid(
  Fts3SegReader *pReader,
  char **ppOffsetList,
  int *pnOffsetList
){
  char *p = pReader->pOffsetList;
  char c = 0;

  /* Pointer p currently points at the first byte of an offset list. The
  ** following two lines advance it to point one byte past the end of
  ** the same offset list.
  */
  while( *p | c ) c = *p++ & 0x80;
  p++;

  /* If required, populate the output variables with a pointer to and the
  ** size of the previous offset-list.
  */
  if( ppOffsetList ){
    *ppOffsetList = pReader->pOffsetList;
    *pnOffsetList = (int)(p - pReader->pOffsetList - 1);
  }

  /* If there are no more entries in the doclist, set pOffsetList to
  ** NULL. Otherwise, set Fts3SegReader.iDocid to the next docid and
  ** Fts3SegReader.pOffsetList to point to the next offset list before
  ** returning.
  */
  if( p>=&pReader->aDoclist[pReader->nDoclist] ){
    pReader->pOffsetList = 0;
  }else{
    sqlite3_int64 iDelta;
    pReader->pOffsetList = p + sqlite3Fts3GetVarint(p, &iDelta);
    pReader->iDocid += iDelta;
  }
}

/*
** This function is called to estimate the amount of data that will be 
** loaded from the disk If SegReaderIterate() is called on this seg-reader,
** in units of average document size.
** 
** This can be used as follows: If the caller has a small doclist that 
** contains references to N documents, and is considering merging it with
** a large doclist (size X "average documents"), it may opt not to load
** the large doclist if X>N.
*/
int sqlite3Fts3SegReaderCost(
  Fts3Cursor *pCsr,               /* FTS3 cursor handle */
  Fts3SegReader *pReader,         /* Segment-reader handle */
  int *pnCost                     /* IN/OUT: Number of bytes read */
){
  Fts3Table *p = (Fts3Table*)pCsr->base.pVtab;
  int rc = SQLITE_OK;             /* Return code */
  int nCost = 0;                  /* Cost in bytes to return */
  int pgsz = p->nPgsz;            /* Database page size */

  /* If this seg-reader is reading the pending-terms table, or if all data
  ** for the segment is stored on the root page of the b-tree, then the cost
  ** is zero. In this case all required data is already in main memory.
  */
  if( p->bHasStat 
   && !fts3SegReaderIsPending(pReader) 
   && !fts3SegReaderIsRootOnly(pReader) 
  ){
    int nBlob = 0;
    sqlite3_int64 iBlock;

    if( pCsr->nRowAvg==0 ){
      /* The average document size, which is required to calculate the cost
      ** of each doclist, has not yet been determined. Read the required 
      ** data from the %_stat table to calculate it.
      **
      ** Entry 0 of the %_stat table is a blob containing (nCol+1) FTS3 
      ** varints, where nCol is the number of columns in the FTS3 table.
      ** The first varint is the number of documents currently stored in
      ** the table. The following nCol varints contain the total amount of
      ** data stored in all rows of each column of the table, from left
      ** to right.
      */
      sqlite3_stmt *pStmt;
      sqlite3_int64 nDoc = 0;
      sqlite3_int64 nByte = 0;
      const char *a;
      rc = sqlite3Fts3SelectDoctotal(p, &pStmt);
      if( rc ) return rc;
      a = sqlite3_column_blob(pStmt, 0);
      if( a ){
        const char *pEnd = &a[sqlite3_column_bytes(pStmt, 0)];
        a += sqlite3Fts3GetVarint(a, &nDoc);
        while( a<pEnd ){
          a += sqlite3Fts3GetVarint(a, &nByte);
        }
      }
      if( nDoc==0 || nByte==0 ){
        sqlite3_reset(pStmt);
        return SQLITE_CORRUPT;
      }

      pCsr->nRowAvg = (int)(((nByte / nDoc) + pgsz) / pgsz);
      assert( pCsr->nRowAvg>0 ); 
      rc = sqlite3_reset(pStmt);
      if( rc!=SQLITE_OK ) return rc;
    }

    /* Assume that a blob flows over onto overflow pages if it is larger
    ** than (pgsz-35) bytes in size (the file-format documentation
    ** confirms this).
    */
    for(iBlock=pReader->iStartBlock; iBlock<=pReader->iLeafEndBlock; iBlock++){
      rc = sqlite3Fts3ReadBlock(p, iBlock, 0, &nBlob);
      if( rc!=SQLITE_OK ) break;
      if( (nBlob+35)>pgsz ){
        int nOvfl = (nBlob + 34)/pgsz;
        nCost += ((nOvfl + pCsr->nRowAvg - 1)/pCsr->nRowAvg);
      }
    }
  }

  *pnCost += nCost;
  return rc;
}

/*
** Free all allocations associated with the iterator passed as the 
** second argument.
*/
void sqlite3Fts3SegReaderFree(Fts3SegReader *pReader){
  if( pReader && !fts3SegReaderIsPending(pReader) ){
    sqlite3_free(pReader->zTerm);
    if( !fts3SegReaderIsRootOnly(pReader) ){
      sqlite3_free(pReader->aNode);
    }
  }
  sqlite3_free(pReader);
}

/*
** Allocate a new SegReader object.
*/
int sqlite3Fts3SegReaderNew(
  int iAge,                       /* Segment "age". */
  sqlite3_int64 iStartLeaf,       /* First leaf to traverse */
  sqlite3_int64 iEndLeaf,         /* Final leaf to traverse */
  sqlite3_int64 iEndBlock,        /* Final block of segment */
  const char *zRoot,              /* Buffer containing root node */
  int nRoot,                      /* Size of buffer containing root node */
  Fts3SegReader **ppReader        /* OUT: Allocated Fts3SegReader */
){
  int rc = SQLITE_OK;             /* Return code */
  Fts3SegReader *pReader;         /* Newly allocated SegReader object */
  int nExtra = 0;                 /* Bytes to allocate segment root node */

  assert( iStartLeaf<=iEndLeaf );
  if( iStartLeaf==0 ){
    nExtra = nRoot + FTS3_NODE_PADDING;
  }

  pReader = (Fts3SegReader *)sqlite3_malloc(sizeof(Fts3SegReader) + nExtra);
  if( !pReader ){
    return SQLITE_NOMEM;
  }
  memset(pReader, 0, sizeof(Fts3SegReader));
  pReader->iIdx = iAge;
  pReader->iStartBlock = iStartLeaf;
  pReader->iLeafEndBlock = iEndLeaf;
  pReader->iEndBlock = iEndBlock;

  if( nExtra ){
    /* The entire segment is stored in the root node. */
    pReader->aNode = (char *)&pReader[1];
    pReader->nNode = nRoot;
    memcpy(pReader->aNode, zRoot, nRoot);
    memset(&pReader->aNode[nRoot], 0, FTS3_NODE_PADDING);
  }else{
    pReader->iCurrentBlock = iStartLeaf-1;
  }

  if( rc==SQLITE_OK ){
    *ppReader = pReader;
  }else{
    sqlite3Fts3SegReaderFree(pReader);
  }
  return rc;
}

/*
** This is a comparison function used as a qsort() callback when sorting
** an array of pending terms by term. This occurs as part of flushing
** the contents of the pending-terms hash table to the database.
*/
static int fts3CompareElemByTerm(const void *lhs, const void *rhs){
  char *z1 = fts3HashKey(*(Fts3HashElem **)lhs);
  char *z2 = fts3HashKey(*(Fts3HashElem **)rhs);
  int n1 = fts3HashKeysize(*(Fts3HashElem **)lhs);
  int n2 = fts3HashKeysize(*(Fts3HashElem **)rhs);

  int n = (n1<n2 ? n1 : n2);
  int c = memcmp(z1, z2, n);
  if( c==0 ){
    c = n1 - n2;
  }
  return c;
}

/*
** This function is used to allocate an Fts3SegReader that iterates through
** a subset of the terms stored in the Fts3Table.pendingTerms array.
*/
int sqlite3Fts3SegReaderPending(
  Fts3Table *p,                   /* Virtual table handle */
  const char *zTerm,              /* Term to search for */
  int nTerm,                      /* Size of buffer zTerm */
  int isPrefix,                   /* True for a term-prefix query */
  Fts3SegReader **ppReader        /* OUT: SegReader for pending-terms */
){
  Fts3SegReader *pReader = 0;     /* Fts3SegReader object to return */
  Fts3HashElem **aElem = 0;       /* Array of term hash entries to scan */
  int nElem = 0;                  /* Size of array at aElem */
  int rc = SQLITE_OK;             /* Return Code */

  if( isPrefix ){
    int nAlloc = 0;               /* Size of allocated array at aElem */
    Fts3HashElem *pE = 0;         /* Iterator variable */

    for(pE=fts3HashFirst(&p->pendingTerms); pE; pE=fts3HashNext(pE)){
      char *zKey = (char *)fts3HashKey(pE);
      int nKey = fts3HashKeysize(pE);
      if( nTerm==0 || (nKey>=nTerm && 0==memcmp(zKey, zTerm, nTerm)) ){
        if( nElem==nAlloc ){
          Fts3HashElem **aElem2;
          nAlloc += 16;
          aElem2 = (Fts3HashElem **)sqlite3_realloc(
              aElem, nAlloc*sizeof(Fts3HashElem *)
          );
          if( !aElem2 ){
            rc = SQLITE_NOMEM;
            nElem = 0;
            break;
          }
          aElem = aElem2;
        }
        aElem[nElem++] = pE;
      }
    }

    /* If more than one term matches the prefix, sort the Fts3HashElem
    ** objects in term order using qsort(). This uses the same comparison
    ** callback as is used when flushing terms to disk.
    */
    if( nElem>1 ){
      qsort(aElem, nElem, sizeof(Fts3HashElem *), fts3CompareElemByTerm);
    }

  }else{
    Fts3HashElem *pE = fts3HashFindElem(&p->pendingTerms, zTerm, nTerm);
    if( pE ){
      aElem = &pE;
      nElem = 1;
    }
  }

  if( nElem>0 ){
    int nByte = sizeof(Fts3SegReader) + (nElem+1)*sizeof(Fts3HashElem *);
    pReader = (Fts3SegReader *)sqlite3_malloc(nByte);
    if( !pReader ){
      rc = SQLITE_NOMEM;
    }else{
      memset(pReader, 0, nByte);
      pReader->iIdx = 0x7FFFFFFF;
      pReader->ppNextElem = (Fts3HashElem **)&pReader[1];
      memcpy(pReader->ppNextElem, aElem, nElem*sizeof(Fts3HashElem *));
    }
  }

  if( isPrefix ){
    sqlite3_free(aElem);
  }
  *ppReader = pReader;
  return rc;
}

/*
** Compare the entries pointed to by two Fts3SegReader structures. 
** Comparison is as follows:
**
**   1) EOF is greater than not EOF.
**
**   2) The current terms (if any) are compared using memcmp(). If one
**      term is a prefix of another, the longer term is considered the
**      larger.
**
**   3) By segment age. An older segment is considered larger.
*/
static int fts3SegReaderCmp(Fts3SegReader *pLhs, Fts3SegReader *pRhs){
  int rc;
  if( pLhs->aNode && pRhs->aNode ){
    int rc2 = pLhs->nTerm - pRhs->nTerm;
    if( rc2<0 ){
      rc = memcmp(pLhs->zTerm, pRhs->zTerm, pLhs->nTerm);
    }else{
      rc = memcmp(pLhs->zTerm, pRhs->zTerm, pRhs->nTerm);
    }
    if( rc==0 ){
      rc = rc2;
    }
  }else{
    rc = (pLhs->aNode==0) - (pRhs->aNode==0);
  }
  if( rc==0 ){
    rc = pRhs->iIdx - pLhs->iIdx;
  }
  assert( rc!=0 );
  return rc;
}

/*
** A different comparison function for SegReader structures. In this
** version, it is assumed that each SegReader points to an entry in
** a doclist for identical terms. Comparison is made as follows:
**
**   1) EOF (end of doclist in this case) is greater than not EOF.
**
**   2) By current docid.
**
**   3) By segment age. An older segment is considered larger.
*/
static int fts3SegReaderDoclistCmp(Fts3SegReader *pLhs, Fts3SegReader *pRhs){
  int rc = (pLhs->pOffsetList==0)-(pRhs->pOffsetList==0);
  if( rc==0 ){
    if( pLhs->iDocid==pRhs->iDocid ){
      rc = pRhs->iIdx - pLhs->iIdx;
    }else{
      rc = (pLhs->iDocid > pRhs->iDocid) ? 1 : -1;
    }
  }
  assert( pLhs->aNode && pRhs->aNode );
  return rc;
}

/*
** Compare the term that the Fts3SegReader object passed as the first argument
** points to with the term specified by arguments zTerm and nTerm. 
**
** If the pSeg iterator is already at EOF, return 0. Otherwise, return
** -ve if the pSeg term is less than zTerm/nTerm, 0 if the two terms are
** equal, or +ve if the pSeg term is greater than zTerm/nTerm.
*/
static int fts3SegReaderTermCmp(
  Fts3SegReader *pSeg,            /* Segment reader object */
  const char *zTerm,              /* Term to compare to */
  int nTerm                       /* Size of term zTerm in bytes */
){
  int res = 0;
  if( pSeg->aNode ){
    if( pSeg->nTerm>nTerm ){
      res = memcmp(pSeg->zTerm, zTerm, nTerm);
    }else{
      res = memcmp(pSeg->zTerm, zTerm, pSeg->nTerm);
    }
    if( res==0 ){
      res = pSeg->nTerm-nTerm;
    }
  }
  return res;
}

/*
** Argument apSegment is an array of nSegment elements. It is known that
** the final (nSegment-nSuspect) members are already in sorted order
** (according to the comparison function provided). This function shuffles
** the array around until all entries are in sorted order.
*/
static void fts3SegReaderSort(
  Fts3SegReader **apSegment,                     /* Array to sort entries of */
  int nSegment,                                  /* Size of apSegment array */
  int nSuspect,                                  /* Unsorted entry count */
  int (*xCmp)(Fts3SegReader *, Fts3SegReader *)  /* Comparison function */
){
  int i;                          /* Iterator variable */

  assert( nSuspect<=nSegment );

  if( nSuspect==nSegment ) nSuspect--;
  for(i=nSuspect-1; i>=0; i--){
    int j;
    for(j=i; j<(nSegment-1); j++){
      Fts3SegReader *pTmp;
      if( xCmp(apSegment[j], apSegment[j+1])<0 ) break;
      pTmp = apSegment[j+1];
      apSegment[j+1] = apSegment[j];
      apSegment[j] = pTmp;
    }
  }

#ifndef NDEBUG
  /* Check that the list really is sorted now. */
  for(i=0; i<(nSuspect-1); i++){
    assert( xCmp(apSegment[i], apSegment[i+1])<0 );
  }
#endif
}

/* 
** Insert a record into the %_segments table.
*/
static int fts3WriteSegment(
  Fts3Table *p,                   /* Virtual table handle */
  sqlite3_int64 iBlock,           /* Block id for new block */
  char *z,                        /* Pointer to buffer containing block data */
  int n                           /* Size of buffer z in bytes */
){
  sqlite3_stmt *pStmt;
  int rc = fts3SqlStmt(p, SQL_INSERT_SEGMENTS, &pStmt, 0);
  if( rc==SQLITE_OK ){
    sqlite3_bind_int64(pStmt, 1, iBlock);
    sqlite3_bind_blob(pStmt, 2, z, n, SQLITE_STATIC);
    sqlite3_step(pStmt);
    rc = sqlite3_reset(pStmt);
  }
  return rc;
}

/* 
** Insert a record into the %_segdir table.
*/
static int fts3WriteSegdir(
  Fts3Table *p,                   /* Virtual table handle */
  int iLevel,                     /* Value for "level" field */
  int iIdx,                       /* Value for "idx" field */
  sqlite3_int64 iStartBlock,      /* Value for "start_block" field */
  sqlite3_int64 iLeafEndBlock,    /* Value for "leaves_end_block" field */
  sqlite3_int64 iEndBlock,        /* Value for "end_block" field */
  char *zRoot,                    /* Blob value for "root" field */
  int nRoot                       /* Number of bytes in buffer zRoot */
){
  sqlite3_stmt *pStmt;
  int rc = fts3SqlStmt(p, SQL_INSERT_SEGDIR, &pStmt, 0);
  if( rc==SQLITE_OK ){
    sqlite3_bind_int(pStmt, 1, iLevel);
    sqlite3_bind_int(pStmt, 2, iIdx);
    sqlite3_bind_int64(pStmt, 3, iStartBlock);
    sqlite3_bind_int64(pStmt, 4, iLeafEndBlock);
    sqlite3_bind_int64(pStmt, 5, iEndBlock);
    sqlite3_bind_blob(pStmt, 6, zRoot, nRoot, SQLITE_STATIC);
    sqlite3_step(pStmt);
    rc = sqlite3_reset(pStmt);
  }
  return rc;
}

/*
** Return the size of the common prefix (if any) shared by zPrev and
** zNext, in bytes. For example, 
**
**   fts3PrefixCompress("abc", 3, "abcdef", 6)   // returns 3
**   fts3PrefixCompress("abX", 3, "abcdef", 6)   // returns 2
**   fts3PrefixCompress("abX", 3, "Xbcdef", 6)   // returns 0
*/
static int fts3PrefixCompress(
  const char *zPrev,              /* Buffer containing previous term */
  int nPrev,                      /* Size of buffer zPrev in bytes */
  const char *zNext,              /* Buffer containing next term */
  int nNext                       /* Size of buffer zNext in bytes */
){
  int n;
  UNUSED_PARAMETER(nNext);
  for(n=0; n<nPrev && zPrev[n]==zNext[n]; n++);
  return n;
}

/*
** Add term zTerm to the SegmentNode. It is guaranteed that zTerm is larger
** (according to memcmp) than the previous term.
*/
static int fts3NodeAddTerm(
  Fts3Table *p,                   /* Virtual table handle */
  SegmentNode **ppTree,           /* IN/OUT: SegmentNode handle */ 
  int isCopyTerm,                 /* True if zTerm/nTerm is transient */
  const char *zTerm,              /* Pointer to buffer containing term */
  int nTerm                       /* Size of term in bytes */
){
  SegmentNode *pTree = *ppTree;
  int rc;
  SegmentNode *pNew;

  /* First try to append the term to the current node. Return early if 
  ** this is possible.
  */
  if( pTree ){
    int nData = pTree->nData;     /* Current size of node in bytes */
    int nReq = nData;             /* Required space after adding zTerm */
    int nPrefix;                  /* Number of bytes of prefix compression */
    int nSuffix;                  /* Suffix length */

    nPrefix = fts3PrefixCompress(pTree->zTerm, pTree->nTerm, zTerm, nTerm);
    nSuffix = nTerm-nPrefix;

    nReq += sqlite3Fts3VarintLen(nPrefix)+sqlite3Fts3VarintLen(nSuffix)+nSuffix;
    if( nReq<=p->nNodeSize || !pTree->zTerm ){

      if( nReq>p->nNodeSize ){
        /* An unusual case: this is the first term to be added to the node
        ** and the static node buffer (p->nNodeSize bytes) is not large
        ** enough. Use a separately malloced buffer instead This wastes
        ** p->nNodeSize bytes, but since this scenario only comes about when
        ** the database contain two terms that share a prefix of almost 2KB, 
        ** this is not expected to be a serious problem. 
        */
        assert( pTree->aData==(char *)&pTree[1] );
        pTree->aData = (char *)sqlite3_malloc(nReq);
        if( !pTree->aData ){
          return SQLITE_NOMEM;
        }
      }

      if( pTree->zTerm ){
        /* There is no prefix-length field for first term in a node */
        nData += sqlite3Fts3PutVarint(&pTree->aData[nData], nPrefix);
      }

      nData += sqlite3Fts3PutVarint(&pTree->aData[nData], nSuffix);
      memcpy(&pTree->aData[nData], &zTerm[nPrefix], nSuffix);
      pTree->nData = nData + nSuffix;
      pTree->nEntry++;

      if( isCopyTerm ){
        if( pTree->nMalloc<nTerm ){
          char *zNew = sqlite3_realloc(pTree->zMalloc, nTerm*2);
          if( !zNew ){
            return SQLITE_NOMEM;
          }
          pTree->nMalloc = nTerm*2;
          pTree->zMalloc = zNew;
        }
        pTree->zTerm = pTree->zMalloc;
        memcpy(pTree->zTerm, zTerm, nTerm);
        pTree->nTerm = nTerm;
      }else{
        pTree->zTerm = (char *)zTerm;
        pTree->nTerm = nTerm;
      }
      return SQLITE_OK;
    }
  }

  /* If control flows to here, it was not possible to append zTerm to the
  ** current node. Create a new node (a right-sibling of the current node).
  ** If this is the first node in the tree, the term is added to it.
  **
  ** Otherwise, the term is not added to the new node, it is left empty for
  ** now. Instead, the term is inserted into the parent of pTree. If pTree 
  ** has no parent, one is created here.
  */
  pNew = (SegmentNode *)sqlite3_malloc(sizeof(SegmentNode) + p->nNodeSize);
  if( !pNew ){
    return SQLITE_NOMEM;
  }
  memset(pNew, 0, sizeof(SegmentNode));
  pNew->nData = 1 + FTS3_VARINT_MAX;
  pNew->aData = (char *)&pNew[1];

  if( pTree ){
    SegmentNode *pParent = pTree->pParent;
    rc = fts3NodeAddTerm(p, &pParent, isCopyTerm, zTerm, nTerm);
    if( pTree->pParent==0 ){
      pTree->pParent = pParent;
    }
    pTree->pRight = pNew;
    pNew->pLeftmost = pTree->pLeftmost;
    pNew->pParent = pParent;
    pNew->zMalloc = pTree->zMalloc;
    pNew->nMalloc = pTree->nMalloc;
    pTree->zMalloc = 0;
  }else{
    pNew->pLeftmost = pNew;
    rc = fts3NodeAddTerm(p, &pNew, isCopyTerm, zTerm, nTerm); 
  }

  *ppTree = pNew;
  return rc;
}

/*
** Helper function for fts3NodeWrite().
*/
static int fts3TreeFinishNode(
  SegmentNode *pTree, 
  int iHeight, 
  sqlite3_int64 iLeftChild
){
  int nStart;
  assert( iHeight>=1 && iHeight<128 );
  nStart = FTS3_VARINT_MAX - sqlite3Fts3VarintLen(iLeftChild);
  pTree->aData[nStart] = (char)iHeight;
  sqlite3Fts3PutVarint(&pTree->aData[nStart+1], iLeftChild);
  return nStart;
}

/*
** Write the buffer for the segment node pTree and all of its peers to the
** database. Then call this function recursively to write the parent of 
** pTree and its peers to the database. 
**
** Except, if pTree is a root node, do not write it to the database. Instead,
** set output variables *paRoot and *pnRoot to contain the root node.
**
** If successful, SQLITE_OK is returned and output variable *piLast is
** set to the largest blockid written to the database (or zero if no
** blocks were written to the db). Otherwise, an SQLite error code is 
** returned.
*/
static int fts3NodeWrite(
  Fts3Table *p,                   /* Virtual table handle */
  SegmentNode *pTree,             /* SegmentNode handle */
  int iHeight,                    /* Height of this node in tree */
  sqlite3_int64 iLeaf,            /* Block id of first leaf node */
  sqlite3_int64 iFree,            /* Block id of next free slot in %_segments */
  sqlite3_int64 *piLast,          /* OUT: Block id of last entry written */
  char **paRoot,                  /* OUT: Data for root node */
  int *pnRoot                     /* OUT: Size of root node in bytes */
){
  int rc = SQLITE_OK;

  if( !pTree->pParent ){
    /* Root node of the tree. */
    int nStart = fts3TreeFinishNode(pTree, iHeight, iLeaf);
    *piLast = iFree-1;
    *pnRoot = pTree->nData - nStart;
    *paRoot = &pTree->aData[nStart];
  }else{
    SegmentNode *pIter;
    sqlite3_int64 iNextFree = iFree;
    sqlite3_int64 iNextLeaf = iLeaf;
    for(pIter=pTree->pLeftmost; pIter && rc==SQLITE_OK; pIter=pIter->pRight){
      int nStart = fts3TreeFinishNode(pIter, iHeight, iNextLeaf);
      int nWrite = pIter->nData - nStart;
  
      rc = fts3WriteSegment(p, iNextFree, &pIter->aData[nStart], nWrite);
      iNextFree++;
      iNextLeaf += (pIter->nEntry+1);
    }
    if( rc==SQLITE_OK ){
      assert( iNextLeaf==iFree );
      rc = fts3NodeWrite(
          p, pTree->pParent, iHeight+1, iFree, iNextFree, piLast, paRoot, pnRoot
      );
    }
  }

  return rc;
}

/*
** Free all memory allocations associated with the tree pTree.
*/
static void fts3NodeFree(SegmentNode *pTree){
  if( pTree ){
    SegmentNode *p = pTree->pLeftmost;
    fts3NodeFree(p->pParent);
    while( p ){
      SegmentNode *pRight = p->pRight;
      if( p->aData!=(char *)&p[1] ){
        sqlite3_free(p->aData);
      }
      assert( pRight==0 || p->zMalloc==0 );
      sqlite3_free(p->zMalloc);
      sqlite3_free(p);
      p = pRight;
    }
  }
}

/*
** Add a term to the segment being constructed by the SegmentWriter object
** *ppWriter. When adding the first term to a segment, *ppWriter should
** be passed NULL. This function will allocate a new SegmentWriter object
** and return it via the input/output variable *ppWriter in this case.
**
** If successful, SQLITE_OK is returned. Otherwise, an SQLite error code.
*/
static int fts3SegWriterAdd(
  Fts3Table *p,                   /* Virtual table handle */
  SegmentWriter **ppWriter,       /* IN/OUT: SegmentWriter handle */ 
  int isCopyTerm,                 /* True if buffer zTerm must be copied */
  const char *zTerm,              /* Pointer to buffer containing term */
  int nTerm,                      /* Size of term in bytes */
  const char *aDoclist,           /* Pointer to buffer containing doclist */
  int nDoclist                    /* Size of doclist in bytes */
){
  int nPrefix;                    /* Size of term prefix in bytes */
  int nSuffix;                    /* Size of term suffix in bytes */
  int nReq;                       /* Number of bytes required on leaf page */
  int nData;
  SegmentWriter *pWriter = *ppWriter;

  if( !pWriter ){
    int rc;
    sqlite3_stmt *pStmt;

    /* Allocate the SegmentWriter structure */
    pWriter = (SegmentWriter *)sqlite3_malloc(sizeof(SegmentWriter));
    if( !pWriter ) return SQLITE_NOMEM;
    memset(pWriter, 0, sizeof(SegmentWriter));
    *ppWriter = pWriter;

    /* Allocate a buffer in which to accumulate data */
    pWriter->aData = (char *)sqlite3_malloc(p->nNodeSize);
    if( !pWriter->aData ) return SQLITE_NOMEM;
    pWriter->nSize = p->nNodeSize;

    /* Find the next free blockid in the %_segments table */
    rc = fts3SqlStmt(p, SQL_NEXT_SEGMENTS_ID, &pStmt, 0);
    if( rc!=SQLITE_OK ) return rc;
    if( SQLITE_ROW==sqlite3_step(pStmt) ){
      pWriter->iFree = sqlite3_column_int64(pStmt, 0);
      pWriter->iFirst = pWriter->iFree;
    }
    rc = sqlite3_reset(pStmt);
    if( rc!=SQLITE_OK ) return rc;
  }
  nData = pWriter->nData;

  nPrefix = fts3PrefixCompress(pWriter->zTerm, pWriter->nTerm, zTerm, nTerm);
  nSuffix = nTerm-nPrefix;

  /* Figure out how many bytes are required by this new entry */
  nReq = sqlite3Fts3VarintLen(nPrefix) +    /* varint containing prefix size */
    sqlite3Fts3VarintLen(nSuffix) +         /* varint containing suffix size */
    nSuffix +                               /* Term suffix */
    sqlite3Fts3VarintLen(nDoclist) +        /* Size of doclist */
    nDoclist;                               /* Doclist data */

  if( nData>0 && nData+nReq>p->nNodeSize ){
    int rc;

    /* The current leaf node is full. Write it out to the database. */
    rc = fts3WriteSegment(p, pWriter->iFree++, pWriter->aData, nData);
    if( rc!=SQLITE_OK ) return rc;

    /* Add the current term to the interior node tree. The term added to
    ** the interior tree must:
    **
    **   a) be greater than the largest term on the leaf node just written
    **      to the database (still available in pWriter->zTerm), and
    **
    **   b) be less than or equal to the term about to be added to the new
    **      leaf node (zTerm/nTerm).
    **
    ** In other words, it must be the prefix of zTerm 1 byte longer than
    ** the common prefix (if any) of zTerm and pWriter->zTerm.
    */
    assert( nPrefix<nTerm );
    rc = fts3NodeAddTerm(p, &pWriter->pTree, isCopyTerm, zTerm, nPrefix+1);
    if( rc!=SQLITE_OK ) return rc;

    nData = 0;
    pWriter->nTerm = 0;

    nPrefix = 0;
    nSuffix = nTerm;
    nReq = 1 +                              /* varint containing prefix size */
      sqlite3Fts3VarintLen(nTerm) +         /* varint containing suffix size */
      nTerm +                               /* Term suffix */
      sqlite3Fts3VarintLen(nDoclist) +      /* Size of doclist */
      nDoclist;                             /* Doclist data */
  }

  /* If the buffer currently allocated is too small for this entry, realloc
  ** the buffer to make it large enough.
  */
  if( nReq>pWriter->nSize ){
    char *aNew = sqlite3_realloc(pWriter->aData, nReq);
    if( !aNew ) return SQLITE_NOMEM;
    pWriter->aData = aNew;
    pWriter->nSize = nReq;
  }
  assert( nData+nReq<=pWriter->nSize );

  /* Append the prefix-compressed term and doclist to the buffer. */
  nData += sqlite3Fts3PutVarint(&pWriter->aData[nData], nPrefix);
  nData += sqlite3Fts3PutVarint(&pWriter->aData[nData], nSuffix);
  memcpy(&pWriter->aData[nData], &zTerm[nPrefix], nSuffix);
  nData += nSuffix;
  nData += sqlite3Fts3PutVarint(&pWriter->aData[nData], nDoclist);
  memcpy(&pWriter->aData[nData], aDoclist, nDoclist);
  pWriter->nData = nData + nDoclist;

  /* Save the current term so that it can be used to prefix-compress the next.
  ** If the isCopyTerm parameter is true, then the buffer pointed to by
  ** zTerm is transient, so take a copy of the term data. Otherwise, just
  ** store a copy of the pointer.
  */
  if( isCopyTerm ){
    if( nTerm>pWriter->nMalloc ){
      char *zNew = sqlite3_realloc(pWriter->zMalloc, nTerm*2);
      if( !zNew ){
        return SQLITE_NOMEM;
      }
      pWriter->nMalloc = nTerm*2;
      pWriter->zMalloc = zNew;
      pWriter->zTerm = zNew;
    }
    assert( pWriter->zTerm==pWriter->zMalloc );
    memcpy(pWriter->zTerm, zTerm, nTerm);
  }else{
    pWriter->zTerm = (char *)zTerm;
  }
  pWriter->nTerm = nTerm;

  return SQLITE_OK;
}

/*
** Flush all data associated with the SegmentWriter object pWriter to the
** database. This function must be called after all terms have been added
** to the segment using fts3SegWriterAdd(). If successful, SQLITE_OK is
** returned. Otherwise, an SQLite error code.
*/
static int fts3SegWriterFlush(
  Fts3Table *p,                   /* Virtual table handle */
  SegmentWriter *pWriter,         /* SegmentWriter to flush to the db */
  int iLevel,                     /* Value for 'level' column of %_segdir */
  int iIdx                        /* Value for 'idx' column of %_segdir */
){
  int rc;                         /* Return code */
  if( pWriter->pTree ){
    sqlite3_int64 iLast = 0;      /* Largest block id written to database */
    sqlite3_int64 iLastLeaf;      /* Largest leaf block id written to db */
    char *zRoot = NULL;           /* Pointer to buffer containing root node */
    int nRoot = 0;                /* Size of buffer zRoot */

    iLastLeaf = pWriter->iFree;
    rc = fts3WriteSegment(p, pWriter->iFree++, pWriter->aData, pWriter->nData);
    if( rc==SQLITE_OK ){
      rc = fts3NodeWrite(p, pWriter->pTree, 1,
          pWriter->iFirst, pWriter->iFree, &iLast, &zRoot, &nRoot);
    }
    if( rc==SQLITE_OK ){
      rc = fts3WriteSegdir(
          p, iLevel, iIdx, pWriter->iFirst, iLastLeaf, iLast, zRoot, nRoot);
    }
  }else{
    /* The entire tree fits on the root node. Write it to the segdir table. */
    rc = fts3WriteSegdir(
        p, iLevel, iIdx, 0, 0, 0, pWriter->aData, pWriter->nData);
  }
  return rc;
}

/*
** Release all memory held by the SegmentWriter object passed as the 
** first argument.
*/
static void fts3SegWriterFree(SegmentWriter *pWriter){
  if( pWriter ){
    sqlite3_free(pWriter->aData);
    sqlite3_free(pWriter->zMalloc);
    fts3NodeFree(pWriter->pTree);
    sqlite3_free(pWriter);
  }
}

/*
** The first value in the apVal[] array is assumed to contain an integer.
** This function tests if there exist any documents with docid values that
** are different from that integer. i.e. if deleting the document with docid
** apVal[0] would mean the FTS3 table were empty.
**
** If successful, *pisEmpty is set to true if the table is empty except for
** document apVal[0], or false otherwise, and SQLITE_OK is returned. If an
** error occurs, an SQLite error code is returned.
*/
static int fts3IsEmpty(Fts3Table *p, sqlite3_value **apVal, int *pisEmpty){
  sqlite3_stmt *pStmt;
  int rc;
  rc = fts3SqlStmt(p, SQL_IS_EMPTY, &pStmt, apVal);
  if( rc==SQLITE_OK ){
    if( SQLITE_ROW==sqlite3_step(pStmt) ){
      *pisEmpty = sqlite3_column_int(pStmt, 0);
    }
    rc = sqlite3_reset(pStmt);
  }
  return rc;
}

/*
** Set *pnSegment to the total number of segments in the database. Set
** *pnMax to the largest segment level in the database (segment levels
** are stored in the 'level' column of the %_segdir table).
**
** Return SQLITE_OK if successful, or an SQLite error code if not.
*/
static int fts3SegmentCountMax(Fts3Table *p, int *pnSegment, int *pnMax){
  sqlite3_stmt *pStmt;
  int rc;

  rc = fts3SqlStmt(p, SQL_SELECT_SEGDIR_COUNT_MAX, &pStmt, 0);
  if( rc!=SQLITE_OK ) return rc;
  if( SQLITE_ROW==sqlite3_step(pStmt) ){
    *pnSegment = sqlite3_column_int(pStmt, 0);
    *pnMax = sqlite3_column_int(pStmt, 1);
  }
  return sqlite3_reset(pStmt);
}

/*
** This function is used after merging multiple segments into a single large
** segment to delete the old, now redundant, segment b-trees. Specifically,
** it:
** 
**   1) Deletes all %_segments entries for the segments associated with 
**      each of the SegReader objects in the array passed as the third 
**      argument, and
**
**   2) deletes all %_segdir entries with level iLevel, or all %_segdir
**      entries regardless of level if (iLevel<0).
**
** SQLITE_OK is returned if successful, otherwise an SQLite error code.
*/
static int fts3DeleteSegdir(
  Fts3Table *p,                   /* Virtual table handle */
  int iLevel,                     /* Level of %_segdir entries to delete */
  Fts3SegReader **apSegment,      /* Array of SegReader objects */
  int nReader                     /* Size of array apSegment */
){
  int rc;                         /* Return Code */
  int i;                          /* Iterator variable */
  sqlite3_stmt *pDelete;          /* SQL statement to delete rows */

  rc = fts3SqlStmt(p, SQL_DELETE_SEGMENTS_RANGE, &pDelete, 0);
  for(i=0; rc==SQLITE_OK && i<nReader; i++){
    Fts3SegReader *pSegment = apSegment[i];
    if( pSegment->iStartBlock ){
      sqlite3_bind_int64(pDelete, 1, pSegment->iStartBlock);
      sqlite3_bind_int64(pDelete, 2, pSegment->iEndBlock);
      sqlite3_step(pDelete);
      rc = sqlite3_reset(pDelete);
    }
  }
  if( rc!=SQLITE_OK ){
    return rc;
  }

  if( iLevel==FTS3_SEGCURSOR_ALL ){
    fts3SqlExec(&rc, p, SQL_DELETE_ALL_SEGDIR, 0);
  }else if( iLevel==FTS3_SEGCURSOR_PENDING ){
    sqlite3Fts3PendingTermsClear(p);
  }else{
    assert( iLevel>=0 );
    rc = fts3SqlStmt(p, SQL_DELETE_SEGDIR_BY_LEVEL, &pDelete, 0);
    if( rc==SQLITE_OK ){
      sqlite3_bind_int(pDelete, 1, iLevel);
      sqlite3_step(pDelete);
      rc = sqlite3_reset(pDelete);
    }
  }

  return rc;
}

/*
** When this function is called, buffer *ppList (size *pnList bytes) contains 
** a position list that may (or may not) feature multiple columns. This
** function adjusts the pointer *ppList and the length *pnList so that they
** identify the subset of the position list that corresponds to column iCol.
**
** If there are no entries in the input position list for column iCol, then
** *pnList is set to zero before returning.
*/
static void fts3ColumnFilter(
  int iCol,                       /* Column to filter on */
  char **ppList,                  /* IN/OUT: Pointer to position list */
  int *pnList                     /* IN/OUT: Size of buffer *ppList in bytes */
){
  char *pList = *ppList;
  int nList = *pnList;
  char *pEnd = &pList[nList];
  int iCurrent = 0;
  char *p = pList;

  assert( iCol>=0 );
  while( 1 ){
    char c = 0;
    while( p<pEnd && (c | *p)&0xFE ) c = *p++ & 0x80;
  
    if( iCol==iCurrent ){
      nList = (int)(p - pList);
      break;
    }

    nList -= (int)(p - pList);
    pList = p;
    if( nList==0 ){
      break;
    }
    p = &pList[1];
    p += sqlite3Fts3GetVarint32(p, &iCurrent);
  }

  *ppList = pList;
  *pnList = nList;
}

int sqlite3Fts3SegReaderStart(
  Fts3Table *p,                   /* Virtual table handle */
  Fts3SegReaderCursor *pCsr,      /* Cursor object */
  Fts3SegFilter *pFilter          /* Restrictions on range of iteration */
){
  int i;

  /* Initialize the cursor object */
  pCsr->pFilter = pFilter;

  /* If the Fts3SegFilter defines a specific term (or term prefix) to search 
  ** for, then advance each segment iterator until it points to a term of
  ** equal or greater value than the specified term. This prevents many
  ** unnecessary merge/sort operations for the case where single segment
  ** b-tree leaf nodes contain more than one term.
  */
  for(i=0; i<pCsr->nSegment; i++){
    int nTerm = pFilter->nTerm;
    const char *zTerm = pFilter->zTerm;
    Fts3SegReader *pSeg = pCsr->apSegment[i];
    do {
      int rc = fts3SegReaderNext(p, pSeg);
      if( rc!=SQLITE_OK ) return rc;
    }while( zTerm && fts3SegReaderTermCmp(pSeg, zTerm, nTerm)<0 );
  }
  fts3SegReaderSort(
      pCsr->apSegment, pCsr->nSegment, pCsr->nSegment, fts3SegReaderCmp);

  return SQLITE_OK;
}

int sqlite3Fts3SegReaderStep(
  Fts3Table *p,                   /* Virtual table handle */
  Fts3SegReaderCursor *pCsr       /* Cursor object */
){
  int rc = SQLITE_OK;

  int isIgnoreEmpty =  (pCsr->pFilter->flags & FTS3_SEGMENT_IGNORE_EMPTY);
  int isRequirePos =   (pCsr->pFilter->flags & FTS3_SEGMENT_REQUIRE_POS);
  int isColFilter =    (pCsr->pFilter->flags & FTS3_SEGMENT_COLUMN_FILTER);
  int isPrefix =       (pCsr->pFilter->flags & FTS3_SEGMENT_PREFIX);

  Fts3SegReader **apSegment = pCsr->apSegment;
  int nSegment = pCsr->nSegment;
  Fts3SegFilter *pFilter = pCsr->pFilter;

  if( pCsr->nSegment==0 ) return SQLITE_OK;

  do {
    int nMerge;
    int i;
  
    /* Advance the first pCsr->nAdvance entries in the apSegment[] array
    ** forward. Then sort the list in order of current term again.  
    */
    for(i=0; i<pCsr->nAdvance; i++){
      rc = fts3SegReaderNext(p, apSegment[i]);
      if( rc!=SQLITE_OK ) return rc;
    }
    fts3SegReaderSort(apSegment, nSegment, pCsr->nAdvance, fts3SegReaderCmp);
    pCsr->nAdvance = 0;

    /* If all the seg-readers are at EOF, we're finished. return SQLITE_OK. */
    assert( rc==SQLITE_OK );
    if( apSegment[0]->aNode==0 ) break;

    pCsr->nTerm = apSegment[0]->nTerm;
    pCsr->zTerm = apSegment[0]->zTerm;

    /* If this is a prefix-search, and if the term that apSegment[0] points
    ** to does not share a suffix with pFilter->zTerm/nTerm, then all 
    ** required callbacks have been made. In this case exit early.
    **
    ** Similarly, if this is a search for an exact match, and the first term
    ** of segment apSegment[0] is not a match, exit early.
    */
    if( pFilter->zTerm ){
      if( pCsr->nTerm<pFilter->nTerm 
       || (!isPrefix && pCsr->nTerm>pFilter->nTerm)
       || memcmp(pCsr->zTerm, pFilter->zTerm, pFilter->nTerm) 
      ){
        break;
      }
    }

    nMerge = 1;
    while( nMerge<nSegment 
        && apSegment[nMerge]->aNode
        && apSegment[nMerge]->nTerm==pCsr->nTerm 
        && 0==memcmp(pCsr->zTerm, apSegment[nMerge]->zTerm, pCsr->nTerm)
    ){
      nMerge++;
    }

    assert( isIgnoreEmpty || (isRequirePos && !isColFilter) );
    if( nMerge==1 && !isIgnoreEmpty ){
      pCsr->aDoclist = apSegment[0]->aDoclist;
      pCsr->nDoclist = apSegment[0]->nDoclist;
      rc = SQLITE_ROW;
    }else{
      int nDoclist = 0;           /* Size of doclist */
      sqlite3_int64 iPrev = 0;    /* Previous docid stored in doclist */

      /* The current term of the first nMerge entries in the array
      ** of Fts3SegReader objects is the same. The doclists must be merged
      ** and a single term returned with the merged doclist.
      */
      for(i=0; i<nMerge; i++){
        fts3SegReaderFirstDocid(apSegment[i]);
      }
      fts3SegReaderSort(apSegment, nMerge, nMerge, fts3SegReaderDoclistCmp);
      while( apSegment[0]->pOffsetList ){
        int j;                    /* Number of segments that share a docid */
        char *pList;
        int nList;
        int nByte;
        sqlite3_int64 iDocid = apSegment[0]->iDocid;
        fts3SegReaderNextDocid(apSegment[0], &pList, &nList);
        j = 1;
        while( j<nMerge
            && apSegment[j]->pOffsetList
            && apSegment[j]->iDocid==iDocid
        ){
          fts3SegReaderNextDocid(apSegment[j], 0, 0);
          j++;
        }

        if( isColFilter ){
          fts3ColumnFilter(pFilter->iCol, &pList, &nList);
        }

        if( !isIgnoreEmpty || nList>0 ){
          nByte = sqlite3Fts3VarintLen(iDocid-iPrev) + (isRequirePos?nList+1:0);
          if( nDoclist+nByte>pCsr->nBuffer ){
            char *aNew;
            pCsr->nBuffer = (nDoclist+nByte)*2;
            aNew = sqlite3_realloc(pCsr->aBuffer, pCsr->nBuffer);
            if( !aNew ){
              return SQLITE_NOMEM;
            }
            pCsr->aBuffer = aNew;
          }
          nDoclist += sqlite3Fts3PutVarint(
              &pCsr->aBuffer[nDoclist], iDocid-iPrev
          );
          iPrev = iDocid;
          if( isRequirePos ){
            memcpy(&pCsr->aBuffer[nDoclist], pList, nList);
            nDoclist += nList;
            pCsr->aBuffer[nDoclist++] = '\0';
          }
        }

        fts3SegReaderSort(apSegment, nMerge, j, fts3SegReaderDoclistCmp);
      }
      if( nDoclist>0 ){
        pCsr->aDoclist = pCsr->aBuffer;
        pCsr->nDoclist = nDoclist;
        rc = SQLITE_ROW;
      }
    }
    pCsr->nAdvance = nMerge;
  }while( rc==SQLITE_OK );

  return rc;
}

void sqlite3Fts3SegReaderFinish(
  Fts3SegReaderCursor *pCsr       /* Cursor object */
){
  if( pCsr ){
    int i;
    for(i=0; i<pCsr->nSegment; i++){
      sqlite3Fts3SegReaderFree(pCsr->apSegment[i]);
    }
    sqlite3_free(pCsr->apSegment);
    sqlite3_free(pCsr->aBuffer);

    pCsr->nSegment = 0;
    pCsr->apSegment = 0;
    pCsr->aBuffer = 0;
  }
}

/*
** Merge all level iLevel segments in the database into a single 
** iLevel+1 segment. Or, if iLevel<0, merge all segments into a
** single segment with a level equal to the numerically largest level 
** currently present in the database.
**
** If this function is called with iLevel<0, but there is only one
** segment in the database, SQLITE_DONE is returned immediately. 
** Otherwise, if successful, SQLITE_OK is returned. If an error occurs, 
** an SQLite error code is returned.
*/
static int fts3SegmentMerge(Fts3Table *p, int iLevel){
  int rc;                         /* Return code */
  int iIdx;                       /* Index of new segment */
  int iNewLevel = 0;              /* Level to create new segment at */
  SegmentWriter *pWriter = 0;     /* Used to write the new, merged, segment */
  Fts3SegFilter filter;           /* Segment term filter condition */
  Fts3SegReaderCursor csr;        /* Cursor to iterate through level(s) */

  rc = sqlite3Fts3SegReaderCursor(p, iLevel, 0, 0, 1, &csr);
  if( rc!=SQLITE_OK || csr.nSegment==0 ) goto finished;

  if( iLevel==FTS3_SEGCURSOR_ALL ){
    /* This call is to merge all segments in the database to a single
    ** segment. The level of the new segment is equal to the the numerically 
    ** greatest segment level currently present in the database. The index
    ** of the new segment is always 0.  */
    int nDummy; /* TODO: Remove this */
    if( csr.nSegment==1 ){
      rc = SQLITE_DONE;
      goto finished;
    }
    iIdx = 0;
    rc = fts3SegmentCountMax(p, &nDummy, &iNewLevel);
  }else{
    /* This call is to merge all segments at level iLevel. Find the next
    ** available segment index at level iLevel+1. The call to
    ** fts3AllocateSegdirIdx() will merge the segments at level iLevel+1 to 
    ** a single iLevel+2 segment if necessary.  */
    iNewLevel = iLevel+1;
    rc = fts3AllocateSegdirIdx(p, iNewLevel, &iIdx);
  }
  if( rc!=SQLITE_OK ) goto finished;
  assert( csr.nSegment>0 );
  assert( iNewLevel>=0 );

  memset(&filter, 0, sizeof(Fts3SegFilter));
  filter.flags = FTS3_SEGMENT_REQUIRE_POS;
  filter.flags |= (iLevel==FTS3_SEGCURSOR_ALL ? FTS3_SEGMENT_IGNORE_EMPTY : 0);

  rc = sqlite3Fts3SegReaderStart(p, &csr, &filter);
  while( SQLITE_OK==rc ){
    rc = sqlite3Fts3SegReaderStep(p, &csr);
    if( rc!=SQLITE_ROW ) break;
    rc = fts3SegWriterAdd(p, &pWriter, 1, 
        csr.zTerm, csr.nTerm, csr.aDoclist, csr.nDoclist);
  }
  if( rc!=SQLITE_OK ) goto finished;
  assert( pWriter );

  rc = fts3DeleteSegdir(p, iLevel, csr.apSegment, csr.nSegment);
  if( rc!=SQLITE_OK ) goto finished;
  rc = fts3SegWriterFlush(p, pWriter, iNewLevel, iIdx);

 finished:
  fts3SegWriterFree(pWriter);
  sqlite3Fts3SegReaderFinish(&csr);
  return rc;
}


/* 
** Flush the contents of pendingTerms to a level 0 segment.
*/
int sqlite3Fts3PendingTermsFlush(Fts3Table *p){
  return fts3SegmentMerge(p, FTS3_SEGCURSOR_PENDING);
}

/*
** Encode N integers as varints into a blob.
*/
static void fts3EncodeIntArray(
  int N,             /* The number of integers to encode */
  u32 *a,            /* The integer values */
  char *zBuf,        /* Write the BLOB here */
  int *pNBuf         /* Write number of bytes if zBuf[] used here */
){
  int i, j;
  for(i=j=0; i<N; i++){
    j += sqlite3Fts3PutVarint(&zBuf[j], (sqlite3_int64)a[i]);
  }
  *pNBuf = j;
}

/*
** Decode a blob of varints into N integers
*/
static void fts3DecodeIntArray(
  int N,             /* The number of integers to decode */
  u32 *a,            /* Write the integer values */
  const char *zBuf,  /* The BLOB containing the varints */
  int nBuf           /* size of the BLOB */
){
  int i, j;
  UNUSED_PARAMETER(nBuf);
  for(i=j=0; i<N; i++){
    sqlite3_int64 x;
    j += sqlite3Fts3GetVarint(&zBuf[j], &x);
    assert(j<=nBuf);
    a[i] = (u32)(x & 0xffffffff);
  }
}

/*
** Insert the sizes (in tokens) for each column of the document
** with docid equal to p->iPrevDocid.  The sizes are encoded as
** a blob of varints.
*/
static void fts3InsertDocsize(
  int *pRC,         /* Result code */
  Fts3Table *p,     /* Table into which to insert */
  u32 *aSz          /* Sizes of each column */
){
  char *pBlob;             /* The BLOB encoding of the document size */
  int nBlob;               /* Number of bytes in the BLOB */
  sqlite3_stmt *pStmt;     /* Statement used to insert the encoding */
  int rc;                  /* Result code from subfunctions */

  if( *pRC ) return;
  pBlob = sqlite3_malloc( 10*p->nColumn );
  if( pBlob==0 ){
    *pRC = SQLITE_NOMEM;
    return;
  }
  fts3EncodeIntArray(p->nColumn, aSz, pBlob, &nBlob);
  rc = fts3SqlStmt(p, SQL_REPLACE_DOCSIZE, &pStmt, 0);
  if( rc ){
    sqlite3_free(pBlob);
    *pRC = rc;
    return;
  }
  sqlite3_bind_int64(pStmt, 1, p->iPrevDocid);
  sqlite3_bind_blob(pStmt, 2, pBlob, nBlob, sqlite3_free);
  sqlite3_step(pStmt);
  *pRC = sqlite3_reset(pStmt);
}

/*
** Record 0 of the %_stat table contains a blob consisting of N varints,
** where N is the number of user defined columns in the fts3 table plus
** two. If nCol is the number of user defined columns, then values of the 
** varints are set as follows:
**
**   Varint 0:       Total number of rows in the table.
**
**   Varint 1..nCol: For each column, the total number of tokens stored in
**                   the column for all rows of the table.
**
**   Varint 1+nCol:  The total size, in bytes, of all text values in all
**                   columns of all rows of the table.
**
*/
static void fts3UpdateDocTotals(
  int *pRC,                       /* The result code */
  Fts3Table *p,                   /* Table being updated */
  u32 *aSzIns,                    /* Size increases */
  u32 *aSzDel,                    /* Size decreases */
  int nChng                       /* Change in the number of documents */
){
  char *pBlob;             /* Storage for BLOB written into %_stat */
  int nBlob;               /* Size of BLOB written into %_stat */
  u32 *a;                  /* Array of integers that becomes the BLOB */
  sqlite3_stmt *pStmt;     /* Statement for reading and writing */
  int i;                   /* Loop counter */
  int rc;                  /* Result code from subfunctions */

  const int nStat = p->nColumn+2;

  if( *pRC ) return;
  a = sqlite3_malloc( (sizeof(u32)+10)*nStat );
  if( a==0 ){
    *pRC = SQLITE_NOMEM;
    return;
  }
  pBlob = (char*)&a[nStat];
  rc = fts3SqlStmt(p, SQL_SELECT_DOCTOTAL, &pStmt, 0);
  if( rc ){
    sqlite3_free(a);
    *pRC = rc;
    return;
  }
  if( sqlite3_step(pStmt)==SQLITE_ROW ){
    fts3DecodeIntArray(nStat, a,
         sqlite3_column_blob(pStmt, 0),
         sqlite3_column_bytes(pStmt, 0));
  }else{
    memset(a, 0, sizeof(u32)*(nStat) );
  }
  sqlite3_reset(pStmt);
  if( nChng<0 && a[0]<(u32)(-nChng) ){
    a[0] = 0;
  }else{
    a[0] += nChng;
  }
  for(i=0; i<p->nColumn+1; i++){
    u32 x = a[i+1];
    if( x+aSzIns[i] < aSzDel[i] ){
      x = 0;
    }else{
      x = x + aSzIns[i] - aSzDel[i];
    }
    a[i+1] = x;
  }
  fts3EncodeIntArray(nStat, a, pBlob, &nBlob);
  rc = fts3SqlStmt(p, SQL_REPLACE_DOCTOTAL, &pStmt, 0);
  if( rc ){
    sqlite3_free(a);
    *pRC = rc;
    return;
  }
  sqlite3_bind_blob(pStmt, 1, pBlob, nBlob, SQLITE_STATIC);
  sqlite3_step(pStmt);
  *pRC = sqlite3_reset(pStmt);
  sqlite3_free(a);
}

/*
** Handle a 'special' INSERT of the form:
**
**   "INSERT INTO tbl(tbl) VALUES(<expr>)"
**
** Argument pVal contains the result of <expr>. Currently the only 
** meaningful value to insert is the text 'optimize'.
*/
static int fts3SpecialInsert(Fts3Table *p, sqlite3_value *pVal){
  int rc;                         /* Return Code */
  const char *zVal = (const char *)sqlite3_value_text(pVal);
  int nVal = sqlite3_value_bytes(pVal);

  if( !zVal ){
    return SQLITE_NOMEM;
  }else if( nVal==8 && 0==sqlite3_strnicmp(zVal, "optimize", 8) ){
    rc = fts3SegmentMerge(p, FTS3_SEGCURSOR_ALL);
    if( rc==SQLITE_DONE ){
      rc = SQLITE_OK;
    }else{
      sqlite3Fts3PendingTermsClear(p);
    }
#ifdef SQLITE_TEST
  }else if( nVal>9 && 0==sqlite3_strnicmp(zVal, "nodesize=", 9) ){
    p->nNodeSize = atoi(&zVal[9]);
    rc = SQLITE_OK;
  }else if( nVal>11 && 0==sqlite3_strnicmp(zVal, "maxpending=", 9) ){
    p->nMaxPendingData = atoi(&zVal[11]);
    rc = SQLITE_OK;
#endif
  }else{
    rc = SQLITE_ERROR;
  }

  sqlite3Fts3SegmentsClose(p);
  return rc;
}

/*
** Return the deferred doclist associated with deferred token pDeferred.
** This function assumes that sqlite3Fts3CacheDeferredDoclists() has already
** been called to allocate and populate the doclist.
*/
char *sqlite3Fts3DeferredDoclist(Fts3DeferredToken *pDeferred, int *pnByte){
  if( pDeferred->pList ){
    *pnByte = pDeferred->pList->nData;
    return pDeferred->pList->aData;
  }
  *pnByte = 0;
  return 0;
}

/*
** Helper fucntion for FreeDeferredDoclists(). This function removes all
** references to deferred doclists from within the tree of Fts3Expr 
** structures headed by 
*/
static void fts3DeferredDoclistClear(Fts3Expr *pExpr){
  if( pExpr ){
    fts3DeferredDoclistClear(pExpr->pLeft);
    fts3DeferredDoclistClear(pExpr->pRight);
    if( pExpr->isLoaded ){
      sqlite3_free(pExpr->aDoclist);
      pExpr->isLoaded = 0;
      pExpr->aDoclist = 0;
      pExpr->nDoclist = 0;
      pExpr->pCurrent = 0;
      pExpr->iCurrent = 0;
    }
  }
}

/*
** Delete all cached deferred doclists. Deferred doclists are cached
** (allocated) by the sqlite3Fts3CacheDeferredDoclists() function.
*/
void sqlite3Fts3FreeDeferredDoclists(Fts3Cursor *pCsr){
  Fts3DeferredToken *pDef;
  for(pDef=pCsr->pDeferred; pDef; pDef=pDef->pNext){
    sqlite3_free(pDef->pList);
    pDef->pList = 0;
  }
  if( pCsr->pDeferred ){
    fts3DeferredDoclistClear(pCsr->pExpr);
  }
}

/*
** Free all entries in the pCsr->pDeffered list. Entries are added to 
** this list using sqlite3Fts3DeferToken().
*/
void sqlite3Fts3FreeDeferredTokens(Fts3Cursor *pCsr){
  Fts3DeferredToken *pDef;
  Fts3DeferredToken *pNext;
  for(pDef=pCsr->pDeferred; pDef; pDef=pNext){
    pNext = pDef->pNext;
    sqlite3_free(pDef->pList);
    sqlite3_free(pDef);
  }
  pCsr->pDeferred = 0;
}

/*
** Generate deferred-doclists for all tokens in the pCsr->pDeferred list
** based on the row that pCsr currently points to.
**
** A deferred-doclist is like any other doclist with position information
** included, except that it only contains entries for a single row of the
** table, not for all rows.
*/
int sqlite3Fts3CacheDeferredDoclists(Fts3Cursor *pCsr){
  int rc = SQLITE_OK;             /* Return code */
  if( pCsr->pDeferred ){
    int i;                        /* Used to iterate through table columns */
    sqlite3_int64 iDocid;         /* Docid of the row pCsr points to */
    Fts3DeferredToken *pDef;      /* Used to iterate through deferred tokens */
  
    Fts3Table *p = (Fts3Table *)pCsr->base.pVtab;
    sqlite3_tokenizer *pT = p->pTokenizer;
    sqlite3_tokenizer_module const *pModule = pT->pModule;
   
    assert( pCsr->isRequireSeek==0 );
    iDocid = sqlite3_column_int64(pCsr->pStmt, 0);
  
    for(i=0; i<p->nColumn && rc==SQLITE_OK; i++){
      const char *zText = (const char *)sqlite3_column_text(pCsr->pStmt, i+1);
      sqlite3_tokenizer_cursor *pTC = 0;
  
      rc = pModule->xOpen(pT, zText, -1, &pTC);
      while( rc==SQLITE_OK ){
        char const *zToken;       /* Buffer containing token */
        int nToken;               /* Number of bytes in token */
        int iDum1, iDum2;         /* Dummy variables */
        int iPos;                 /* Position of token in zText */
  
        pTC->pTokenizer = pT;
        rc = pModule->xNext(pTC, &zToken, &nToken, &iDum1, &iDum2, &iPos);
        for(pDef=pCsr->pDeferred; pDef && rc==SQLITE_OK; pDef=pDef->pNext){
          Fts3PhraseToken *pPT = pDef->pToken;
          if( (pDef->iCol>=p->nColumn || pDef->iCol==i)
           && (pPT->n==nToken || (pPT->isPrefix && pPT->n<nToken))
           && (0==memcmp(zToken, pPT->z, pPT->n))
          ){
            fts3PendingListAppend(&pDef->pList, iDocid, i, iPos, &rc);
          }
        }
      }
      if( pTC ) pModule->xClose(pTC);
      if( rc==SQLITE_DONE ) rc = SQLITE_OK;
    }
  
    for(pDef=pCsr->pDeferred; pDef && rc==SQLITE_OK; pDef=pDef->pNext){
      if( pDef->pList ){
        rc = fts3PendingListAppendVarint(&pDef->pList, 0);
      }
    }
  }

  return rc;
}

/*
** Add an entry for token pToken to the pCsr->pDeferred list.
*/
int sqlite3Fts3DeferToken(
  Fts3Cursor *pCsr,               /* Fts3 table cursor */
  Fts3PhraseToken *pToken,        /* Token to defer */
  int iCol                        /* Column that token must appear in (or -1) */
){
  Fts3DeferredToken *pDeferred;
  pDeferred = sqlite3_malloc(sizeof(*pDeferred));
  if( !pDeferred ){
    return SQLITE_NOMEM;
  }
  memset(pDeferred, 0, sizeof(*pDeferred));
  pDeferred->pToken = pToken;
  pDeferred->pNext = pCsr->pDeferred; 
  pDeferred->iCol = iCol;
  pCsr->pDeferred = pDeferred;

  assert( pToken->pDeferred==0 );
  pToken->pDeferred = pDeferred;

  return SQLITE_OK;
}


/*
** This function does the work for the xUpdate method of FTS3 virtual
** tables.
*/
int sqlite3Fts3UpdateMethod(
  sqlite3_vtab *pVtab,            /* FTS3 vtab object */
  int nArg,                       /* Size of argument array */
  sqlite3_value **apVal,          /* Array of arguments */
  sqlite_int64 *pRowid            /* OUT: The affected (or effected) rowid */
){
  Fts3Table *p = (Fts3Table *)pVtab;
  int rc = SQLITE_OK;             /* Return Code */
  int isRemove = 0;               /* True for an UPDATE or DELETE */
  sqlite3_int64 iRemove = 0;      /* Rowid removed by UPDATE or DELETE */
  u32 *aSzIns;                    /* Sizes of inserted documents */
  u32 *aSzDel;                    /* Sizes of deleted documents */
  int nChng = 0;                  /* Net change in number of documents */

  assert( p->pSegments==0 );

  /* Allocate space to hold the change in document sizes */
  aSzIns = sqlite3_malloc( sizeof(aSzIns[0])*(p->nColumn+1)*2 );
  if( aSzIns==0 ) return SQLITE_NOMEM;
  aSzDel = &aSzIns[p->nColumn+1];
  memset(aSzIns, 0, sizeof(aSzIns[0])*(p->nColumn+1)*2);

  /* If this is a DELETE or UPDATE operation, remove the old record. */
  if( sqlite3_value_type(apVal[0])!=SQLITE_NULL ){
    int isEmpty = 0;
    rc = fts3IsEmpty(p, apVal, &isEmpty);
    if( rc==SQLITE_OK ){
      if( isEmpty ){
        /* Deleting this row means the whole table is empty. In this case
        ** delete the contents of all three tables and throw away any
        ** data in the pendingTerms hash table.
        */
        rc = fts3DeleteAll(p);
      }else{
        isRemove = 1;
        iRemove = sqlite3_value_int64(apVal[0]);
        rc = fts3PendingTermsDocid(p, iRemove);
        fts3DeleteTerms(&rc, p, apVal, aSzDel);
        fts3SqlExec(&rc, p, SQL_DELETE_CONTENT, apVal);
        if( p->bHasDocsize ){
          fts3SqlExec(&rc, p, SQL_DELETE_DOCSIZE, apVal);
        }
        nChng--;
      }
    }
  }else if( sqlite3_value_type(apVal[p->nColumn+2])!=SQLITE_NULL ){
    sqlite3_free(aSzIns);
    return fts3SpecialInsert(p, apVal[p->nColumn+2]);
  }
  
  /* If this is an INSERT or UPDATE operation, insert the new record. */
  if( nArg>1 && rc==SQLITE_OK ){
    rc = fts3InsertData(p, apVal, pRowid);
    if( rc==SQLITE_OK && (!isRemove || *pRowid!=iRemove) ){
      rc = fts3PendingTermsDocid(p, *pRowid);
    }
    if( rc==SQLITE_OK ){
      rc = fts3InsertTerms(p, apVal, aSzIns);
    }
    if( p->bHasDocsize ){
      fts3InsertDocsize(&rc, p, aSzIns);
    }
    nChng++;
  }

  if( p->bHasStat ){
    fts3UpdateDocTotals(&rc, p, aSzIns, aSzDel, nChng);
  }

  sqlite3_free(aSzIns);
  sqlite3Fts3SegmentsClose(p);
  return rc;
}

/* 
** Flush any data in the pending-terms hash table to disk. If successful,
** merge all segments in the database (including the new segment, if 
** there was any data to flush) into a single segment. 
*/
int sqlite3Fts3Optimize(Fts3Table *p){
  int rc;
  rc = sqlite3_exec(p->db, "SAVEPOINT fts3", 0, 0, 0);
  if( rc==SQLITE_OK ){
    rc = fts3SegmentMerge(p, FTS3_SEGCURSOR_ALL);
    if( rc==SQLITE_OK ){
      rc = sqlite3_exec(p->db, "RELEASE fts3", 0, 0, 0);
      if( rc==SQLITE_OK ){
        sqlite3Fts3PendingTermsClear(p);
      }
    }else{
      sqlite3_exec(p->db, "ROLLBACK TO fts3", 0, 0, 0);
      sqlite3_exec(p->db, "RELEASE fts3", 0, 0, 0);
    }
  }
  sqlite3Fts3SegmentsClose(p);
  return rc;
}

#endif