sqlite/ext/fts3/fts3.c

/*
** 2006 Oct 10
**
** 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 is an SQLite module implementing full-text search.
*/

/*
** The code in this file is only compiled if:
**
**     * The FTS3 module is being built as an extension
**       (in which case SQLITE_CORE is not defined), or
**
**     * The FTS3 module is being built into the core of
**       SQLite (in which case SQLITE_ENABLE_FTS3 is defined).
*/

/* TODO(shess) Consider exporting this comment to an HTML file or the
** wiki.
*/
/* The full-text index is stored in a series of b+tree (-like)
** structures called segments which map terms to doclists.  The
** structures are like b+trees in layout, but are constructed from the
** bottom up in optimal fashion and are not updatable.  Since trees
** are built from the bottom up, things will be described from the
** bottom up.
**
**
**** Varints ****
** The basic unit of encoding is a variable-length integer called a
** varint.  We encode variable-length integers in little-endian order
** using seven bits * per byte as follows:
**
** KEY:
**         A = 0xxxxxxx    7 bits of data and one flag bit
**         B = 1xxxxxxx    7 bits of data and one flag bit
**
**  7 bits - A
** 14 bits - BA
** 21 bits - BBA
** and so on.
**
** This is identical to how sqlite encodes varints (see util.c).
**
**
**** Document lists ****
** A doclist (document list) holds a docid-sorted list of hits for a
** given term.  Doclists hold docids, and can optionally associate
** token positions and offsets with docids.
**
** A DL_POSITIONS_OFFSETS doclist is stored like this:
**
** array {
**   varint docid;
**   array {                (position list for column 0)
**     varint position;     (delta from previous position plus POS_BASE)
**     varint startOffset;  (delta from previous startOffset)
**     varint endOffset;    (delta from startOffset)
**   }
**   array {
**     varint POS_COLUMN;   (marks start of position list for new column)
**     varint column;       (index of new column)
**     array {
**       varint position;   (delta from previous position plus POS_BASE)
**       varint startOffset;(delta from previous startOffset)
**       varint endOffset;  (delta from startOffset)
**     }
**   }
**   varint POS_END;        (marks end of positions for this document.
** }
**
** Here, array { X } means zero or more occurrences of X, adjacent in
** memory.  A "position" is an index of a token in the token stream
** generated by the tokenizer, while an "offset" is a byte offset,
** both based at 0.  Note that POS_END and POS_COLUMN occur in the
** same logical place as the position element, and act as sentinals
** ending a position list array.
**
** A DL_POSITIONS doclist omits the startOffset and endOffset
** information.  A DL_DOCIDS doclist omits both the position and
** offset information, becoming an array of varint-encoded docids.
**
** On-disk data is stored as type DL_DEFAULT, so we don't serialize
** the type.  Due to how deletion is implemented in the segmentation
** system, on-disk doclists MUST store at least positions.
**
**
**** Segment leaf nodes ****
** Segment leaf nodes store terms and doclists, ordered by term.  Leaf
** nodes are written using LeafWriter, and read using LeafReader (to
** iterate through a single leaf node's data) and LeavesReader (to
** iterate through a segment's entire leaf layer).  Leaf nodes have
** the format:
**
** varint iHeight;             (height from leaf level, always 0)
** varint nTerm;               (length of first term)
** char pTerm[nTerm];          (content of first term)
** varint nDoclist;            (length of term's associated doclist)
** char pDoclist[nDoclist];    (content of doclist)
** array {
**                             (further terms are delta-encoded)
**   varint nPrefix;           (length of prefix shared with previous term)
**   varint nSuffix;           (length of unshared suffix)
**   char pTermSuffix[nSuffix];(unshared suffix of next term)
**   varint nDoclist;          (length of term's associated doclist)
**   char pDoclist[nDoclist];  (content of doclist)
** }
**
** Here, array { X } means zero or more occurrences of X, adjacent in
** memory.
**
** Leaf nodes are broken into blocks which are stored contiguously in
** the %_segments table in sorted order.  This means that when the end
** of a node is reached, the next term is in the node with the next
** greater node id.
**
** New data is spilled to a new leaf node when the current node
** exceeds LEAF_MAX bytes (default 2048).  New data which itself is
** larger than STANDALONE_MIN (default 1024) is placed in a standalone
** node (a leaf node with a single term and doclist).  The goal of
** these settings is to pack together groups of small doclists while
** making it efficient to directly access large doclists.  The
** assumption is that large doclists represent terms which are more
** likely to be query targets.
**
** TODO(shess) It may be useful for blocking decisions to be more
** dynamic.  For instance, it may make more sense to have a 2.5k leaf
** node rather than splitting into 2k and .5k nodes.  My intuition is
** that this might extend through 2x or 4x the pagesize.
**
**
**** Segment interior nodes ****
** Segment interior nodes store blockids for subtree nodes and terms
** to describe what data is stored by the each subtree.  Interior
** nodes are written using InteriorWriter, and read using
** InteriorReader.  InteriorWriters are created as needed when
** SegmentWriter creates new leaf nodes, or when an interior node
** itself grows too big and must be split.  The format of interior
** nodes:
**
** varint iHeight;           (height from leaf level, always >0)
** varint iBlockid;          (block id of node's leftmost subtree)
** optional {
**   varint nTerm;           (length of first term)
**   char pTerm[nTerm];      (content of first term)
**   array {
**                                (further terms are delta-encoded)
**     varint nPrefix;            (length of shared prefix with previous term)
**     varint nSuffix;            (length of unshared suffix)
**     char pTermSuffix[nSuffix]; (unshared suffix of next term)
**   }
** }
**
** Here, optional { X } means an optional element, while array { X }
** means zero or more occurrences of X, adjacent in memory.
**
** An interior node encodes n terms separating n+1 subtrees.  The
** subtree blocks are contiguous, so only the first subtree's blockid
** is encoded.  The subtree at iBlockid will contain all terms less
** than the first term encoded (or all terms if no term is encoded).
** Otherwise, for terms greater than or equal to pTerm[i] but less
** than pTerm[i+1], the subtree for that term will be rooted at
** iBlockid+i.  Interior nodes only store enough term data to
** distinguish adjacent children (if the rightmost term of the left
** child is "something", and the leftmost term of the right child is
** "wicked", only "w" is stored).
**
** New data is spilled to a new interior node at the same height when
** the current node exceeds INTERIOR_MAX bytes (default 2048).
** INTERIOR_MIN_TERMS (default 7) keeps large terms from monopolizing
** interior nodes and making the tree too skinny.  The interior nodes
** at a given height are naturally tracked by interior nodes at
** height+1, and so on.
**
**
**** Segment directory ****
** The segment directory in table %_segdir stores meta-information for
** merging and deleting segments, and also the root node of the
** segment's tree.
**
** The root node is the top node of the segment's tree after encoding
** the entire segment, restricted to ROOT_MAX bytes (default 1024).
** This could be either a leaf node or an interior node.  If the top
** node requires more than ROOT_MAX bytes, it is flushed to %_segments
** and a new root interior node is generated (which should always fit
** within ROOT_MAX because it only needs space for 2 varints, the
** height and the blockid of the previous root).
**
** The meta-information in the segment directory is:
**   level               - segment level (see below)
**   idx                 - index within level
**                       - (level,idx uniquely identify a segment)
**   start_block         - first leaf node
**   leaves_end_block    - last leaf node
**   end_block           - last block (including interior nodes)
**   root                - contents of root node
**
** If the root node is a leaf node, then start_block,
** leaves_end_block, and end_block are all 0.
**
**
**** Segment merging ****
** To amortize update costs, segments are grouped into levels and
** merged in batches.  Each increase in level represents exponentially
** more documents.
**
** New documents (actually, document updates) are tokenized and
** written individually (using LeafWriter) to a level 0 segment, with
** incrementing idx.  When idx reaches MERGE_COUNT (default 16), all
** level 0 segments are merged into a single level 1 segment.  Level 1
** is populated like level 0, and eventually MERGE_COUNT level 1
** segments are merged to a single level 2 segment (representing
** MERGE_COUNT^2 updates), and so on.
**
** A segment merge traverses all segments at a given level in
** parallel, performing a straightforward sorted merge.  Since segment
** leaf nodes are written in to the %_segments table in order, this
** merge traverses the underlying sqlite disk structures efficiently.
** After the merge, all segment blocks from the merged level are
** deleted.
**
** MERGE_COUNT controls how often we merge segments.  16 seems to be
** somewhat of a sweet spot for insertion performance.  32 and 64 show
** very similar performance numbers to 16 on insertion, though they're
** a tiny bit slower (perhaps due to more overhead in merge-time
** sorting).  8 is about 20% slower than 16, 4 about 50% slower than
** 16, 2 about 66% slower than 16.
**
** At query time, high MERGE_COUNT increases the number of segments
** which need to be scanned and merged.  For instance, with 100k docs
** inserted:
**
**    MERGE_COUNT   segments
**       16           25
**        8           12
**        4           10
**        2            6
**
** This appears to have only a moderate impact on queries for very
** frequent terms (which are somewhat dominated by segment merge
** costs), and infrequent and non-existent terms still seem to be fast
** even with many segments.
**
** TODO(shess) That said, it would be nice to have a better query-side
** argument for MERGE_COUNT of 16.  Also, it is possible/likely that
** optimizations to things like doclist merging will swing the sweet
** spot around.
**
**
**
**** Handling of deletions and updates ****
** Since we're using a segmented structure, with no docid-oriented
** index into the term index, we clearly cannot simply update the term
** index when a document is deleted or updated.  For deletions, we
** write an empty doclist (varint(docid) varint(POS_END)), for updates
** we simply write the new doclist.  Segment merges overwrite older
** data for a particular docid with newer data, so deletes or updates
** will eventually overtake the earlier data and knock it out.  The
** query logic likewise merges doclists so that newer data knocks out
** older data.
**
** TODO(shess) Provide a VACUUM type operation to clear out all
** deletions and duplications.  This would basically be a forced merge
** into a single segment.
*/

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

#if defined(SQLITE_ENABLE_FTS3) && !defined(SQLITE_CORE)
# define SQLITE_CORE 1
#endif

#include "fts3Int.h"

#include <assert.h>
#include <stdlib.h>
#include <stddef.h>
#include <stdio.h>
#include <string.h>

#include "fts3.h"
#ifndef SQLITE_CORE 
# include "sqlite3ext.h"
  SQLITE_EXTENSION_INIT1
#endif

/* 
** Write a 64-bit variable-length integer to memory starting at p[0].
** The length of data written will be between 1 and FTS3_VARINT_MAX bytes.
** The number of bytes written is returned.
*/
int sqlite3Fts3PutVarint(char *p, sqlite_int64 v){
  unsigned char *q = (unsigned char *) p;
  sqlite_uint64 vu = v;
  do{
    *q++ = (unsigned char) ((vu & 0x7f) | 0x80);
    vu >>= 7;
  }while( vu!=0 );
  q[-1] &= 0x7f;  /* turn off high bit in final byte */
  assert( q - (unsigned char *)p <= FTS3_VARINT_MAX );
  return (int) (q - (unsigned char *)p);
}

/* 
** Read a 64-bit variable-length integer from memory starting at p[0].
** Return the number of bytes read, or 0 on error.
** The value is stored in *v.
*/
int sqlite3Fts3GetVarint(const char *p, sqlite_int64 *v){
  const unsigned char *q = (const unsigned char *) p;
  sqlite_uint64 x = 0, y = 1;
  while( (*q&0x80)==0x80 && q-(unsigned char *)p<FTS3_VARINT_MAX ){
    x += y * (*q++ & 0x7f);
    y <<= 7;
  }
  x += y * (*q++);
  *v = (sqlite_int64) x;
  return (int) (q - (unsigned char *)p);
}

/*
** Similar to sqlite3Fts3GetVarint(), except that the output is truncated to a
** 32-bit integer before it is returned.
*/
int sqlite3Fts3GetVarint32(const char *p, int *pi){
 sqlite_int64 i;
 int ret = sqlite3Fts3GetVarint(p, &i);
 *pi = (int) i;
 return ret;
}

/*
** Return the number of bytes required to store the value passed as the
** first argument in varint form.
*/
int sqlite3Fts3VarintLen(sqlite3_uint64 v){
  int i = 0;
  do{
    i++;
    v >>= 7;
  }while( v!=0 );
  return i;
}

/*
** Convert an SQL-style quoted string into a normal string by removing
** the quote characters.  The conversion is done in-place.  If the
** input does not begin with a quote character, then this routine
** is a no-op.
**
** Examples:
**
**     "abc"   becomes   abc
**     'xyz'   becomes   xyz
**     [pqr]   becomes   pqr
**     `mno`   becomes   mno
**
*/
void sqlite3Fts3Dequote(char *z){
  char quote;                     /* Quote character (if any ) */

  quote = z[0];
  if( quote=='[' || quote=='\'' || quote=='"' || quote=='`' ){
    int iIn = 1;                  /* Index of next byte to read from input */
    int iOut = 0;                 /* Index of next byte to write to output */

    /* If the first byte was a '[', then the close-quote character is a ']' */
    if( quote=='[' ) quote = ']';  

    while( ALWAYS(z[iIn]) ){
      if( z[iIn]==quote ){
        if( z[iIn+1]!=quote ) break;
        z[iOut++] = quote;
        iIn += 2;
      }else{
        z[iOut++] = z[iIn++];
      }
    }
    z[iOut] = '\0';
  }
}

static void fts3GetDeltaVarint(char **pp, sqlite3_int64 *pVal){
  sqlite3_int64 iVal;
  *pp += sqlite3Fts3GetVarint(*pp, &iVal);
  *pVal += iVal;
}

static void fts3GetDeltaVarint2(char **pp, char *pEnd, sqlite3_int64 *pVal){
  if( *pp>=pEnd ){
    *pp = 0;
  }else{
    fts3GetDeltaVarint(pp, pVal);
  }
}

/*
** The xDisconnect() virtual table method.
*/
static int fts3DisconnectMethod(sqlite3_vtab *pVtab){
  Fts3Table *p = (Fts3Table *)pVtab;
  int i;

  assert( p->nPendingData==0 );

  /* Free any prepared statements held */
  for(i=0; i<SizeofArray(p->aStmt); i++){
    sqlite3_finalize(p->aStmt[i]);
  }
  for(i=0; i<p->nLeavesStmt; i++){
    sqlite3_finalize(p->aLeavesStmt[i]);
  }
  sqlite3_free(p->zSelectLeaves);
  sqlite3_free(p->aLeavesStmt);

  /* Invoke the tokenizer destructor to free the tokenizer. */
  p->pTokenizer->pModule->xDestroy(p->pTokenizer);

  sqlite3_free(p);
  return SQLITE_OK;
}

/*
** The xDestroy() virtual table method.
*/
static int fts3DestroyMethod(sqlite3_vtab *pVtab){
  int rc;                         /* Return code */
  Fts3Table *p = (Fts3Table *)pVtab;

  /* Create a script to drop the underlying three storage tables. */
  char *zSql = sqlite3_mprintf(
      "DROP TABLE IF EXISTS %Q.'%q_content';"
      "DROP TABLE IF EXISTS %Q.'%q_segments';"
      "DROP TABLE IF EXISTS %Q.'%q_segdir';", 
      p->zDb, p->zName, p->zDb, p->zName, p->zDb, p->zName
  );

  /* If malloc has failed, set rc to SQLITE_NOMEM. Otherwise, try to
  ** execute the SQL script created above.
  */
  if( zSql ){
    rc = sqlite3_exec(p->db, zSql, 0, 0, 0);
    sqlite3_free(zSql);
  }else{
    rc = SQLITE_NOMEM;
  }

  /* If everything has worked, invoke fts3DisconnectMethod() to free the
  ** memory associated with the Fts3Table structure and return SQLITE_OK.
  ** Otherwise, return an SQLite error code.
  */
  return (rc==SQLITE_OK ? fts3DisconnectMethod(pVtab) : rc);
}


/*
** Invoke sqlite3_declare_vtab() to declare the schema for the FTS3 table
** passed as the first argument. This is done as part of the xConnect()
** and xCreate() methods.
*/
static int fts3DeclareVtab(Fts3Table *p){
  int i;                          /* Iterator variable */
  int rc;                         /* Return code */
  char *zSql;                     /* SQL statement passed to declare_vtab() */
  char *zCols;                    /* List of user defined columns */

  /* Create a list of user columns for the virtual table */
  zCols = sqlite3_mprintf("%Q, ", p->azColumn[0]);
  for(i=1; zCols && i<p->nColumn; i++){
    zCols = sqlite3_mprintf("%z%Q, ", zCols, p->azColumn[i]);
  }

  /* Create the whole "CREATE TABLE" statement to pass to SQLite */
  zSql = sqlite3_mprintf(
      "CREATE TABLE x(%s %Q HIDDEN, docid HIDDEN)", zCols, p->zName
  );

  if( !zCols || !zSql ){
    rc = SQLITE_NOMEM;
  }else{
    rc = sqlite3_declare_vtab(p->db, zSql);
  }

  sqlite3_free(zSql);
  sqlite3_free(zCols);
  return rc;
}

/*
** Create the backing store tables (%_content, %_segments and %_segdir)
** required by the FTS3 table passed as the only argument. This is done
** as part of the vtab xCreate() method.
*/
static int fts3CreateTables(Fts3Table *p){
  int rc;                         /* Return code */
  int i;                          /* Iterator variable */
  char *zContentCols;             /* Columns of %_content table */
  char *zSql;                     /* SQL script to create required tables */

  /* Create a list of user columns for the content table */
  zContentCols = sqlite3_mprintf("docid INTEGER PRIMARY KEY");
  for(i=0; zContentCols && i<p->nColumn; i++){
    char *z = p->azColumn[i];
    zContentCols = sqlite3_mprintf("%z, 'c%d%q'", zContentCols, i, z);
  }

  /* Create the whole SQL script */
  zSql = sqlite3_mprintf(
      "CREATE TABLE %Q.'%q_content'(%s);"
      "CREATE TABLE %Q.'%q_segments'(blockid INTEGER PRIMARY KEY, block BLOB);"
      "CREATE TABLE %Q.'%q_segdir'("
        "level INTEGER,"
        "idx INTEGER,"
        "start_block INTEGER,"
        "leaves_end_block INTEGER,"
        "end_block INTEGER,"
        "root BLOB,"
        "PRIMARY KEY(level, idx)"
      ");",
      p->zDb, p->zName, zContentCols, p->zDb, p->zName, p->zDb, p->zName
  );

  /* Unless a malloc() failure has occurred, execute the SQL script to 
  ** create the tables used to store data for this FTS3 virtual table.
  */
  if( zContentCols==0 || zSql==0 ){
    rc = SQLITE_NOMEM;
  }else{
    rc = sqlite3_exec(p->db, zSql, 0, 0, 0);
  }

  sqlite3_free(zSql);
  sqlite3_free(zContentCols);
  return rc;
}

/*
** This function is the implementation of both the xConnect and xCreate
** methods of the FTS3 virtual table.
**
** The argv[] array contains the following:
**
**   argv[0]   -> module name
**   argv[1]   -> database name
**   argv[2]   -> table name
**   argv[...] -> "column name" and other module argument fields.
*/
static int fts3InitVtab(
  int isCreate,                   /* True for xCreate, false for xConnect */
  sqlite3 *db,                    /* The SQLite database connection */
  void *pAux,                     /* Hash table containing tokenizers */
  int argc,                       /* Number of elements in argv array */
  const char * const *argv,       /* xCreate/xConnect argument array */
  sqlite3_vtab **ppVTab,          /* Write the resulting vtab structure here */
  char **pzErr                    /* Write any error message here */
){
  Fts3Hash *pHash = (Fts3Hash *)pAux;
  Fts3Table *p;                   /* Pointer to allocated vtab */
  int rc;                         /* Return code */
  int i;                          /* Iterator variable */
  int nByte;                      /* Size of allocation used for *p */
  int iCol;
  int nString = 0;
  int nCol = 0;
  char *zCsr;
  int nDb;
  int nName;

  const char *zTokenizer = 0;               /* Name of tokenizer to use */
  sqlite3_tokenizer *pTokenizer = 0;        /* Tokenizer for this table */

  nDb = (int)strlen(argv[1]) + 1;
  nName = (int)strlen(argv[2]) + 1;
  for(i=3; i<argc; i++){
    char const *z = argv[i];
    rc = sqlite3Fts3InitTokenizer(pHash, z, &pTokenizer, &zTokenizer, pzErr);
    if( rc!=SQLITE_OK ){
      return rc;
    }
    if( z!=zTokenizer ){
      nString += (int)(strlen(z) + 1);
    }
  }
  nCol = argc - 3 - (zTokenizer!=0);
  if( zTokenizer==0 ){
    rc = sqlite3Fts3InitTokenizer(pHash, 0, &pTokenizer, 0, pzErr);
    if( rc!=SQLITE_OK ){
      return rc;
    }
    assert( pTokenizer );
  }

  if( nCol==0 ){
    nCol = 1;
  }

  /* Allocate and populate the Fts3Table structure. */
  nByte = sizeof(Fts3Table) +              /* Fts3Table */
          nCol * sizeof(char *) +              /* azColumn */
          nName +                              /* zName */
          nDb +                                /* zDb */
          nString;                             /* Space for azColumn strings */
  p = (Fts3Table*)sqlite3_malloc(nByte);
  if( p==0 ){
    rc = SQLITE_NOMEM;
    goto fts3_init_out;
  }
  memset(p, 0, nByte);

  p->db = db;
  p->nColumn = nCol;
  p->nPendingData = 0;
  p->azColumn = (char **)&p[1];
  p->pTokenizer = pTokenizer;
  p->nNodeSize = 1000;
  p->nMaxPendingData = FTS3_MAX_PENDING_DATA;
  zCsr = (char *)&p->azColumn[nCol];

  fts3HashInit(&p->pendingTerms, FTS3_HASH_STRING, 1);

  /* Fill in the zName and zDb fields of the vtab structure. */
  p->zName = zCsr;
  memcpy(zCsr, argv[2], nName);
  zCsr += nName;
  p->zDb = zCsr;
  memcpy(zCsr, argv[1], nDb);
  zCsr += nDb;

  /* Fill in the azColumn array */
  iCol = 0;
  for(i=3; i<argc; i++){
    if( argv[i]!=zTokenizer ){
      char *z; 
      int n;
      z = (char *)sqlite3Fts3NextToken(argv[i], &n);
      memcpy(zCsr, z, n);
      zCsr[n] = '\0';
      sqlite3Fts3Dequote(zCsr);
      p->azColumn[iCol++] = zCsr;
      zCsr += n+1;
      assert( zCsr <= &((char *)p)[nByte] );
    }
  }
  if( iCol==0 ){
    assert( nCol==1 );
    p->azColumn[0] = "content";
  }

  /* If this is an xCreate call, create the underlying tables in the 
  ** database. TODO: For xConnect(), it could verify that said tables exist.
  */
  if( isCreate ){
    rc = fts3CreateTables(p);
    if( rc!=SQLITE_OK ) goto fts3_init_out;
  }

  rc = fts3DeclareVtab(p);
  if( rc!=SQLITE_OK ) goto fts3_init_out;

  *ppVTab = &p->base;

fts3_init_out:
  assert( p || (pTokenizer && rc!=SQLITE_OK) );
  if( rc!=SQLITE_OK ){
    if( p ){
      fts3DisconnectMethod((sqlite3_vtab *)p);
    }else{
      pTokenizer->pModule->xDestroy(pTokenizer);
    }
  }
  return rc;
}

/*
** The xConnect() and xCreate() methods for the virtual table. All the
** work is done in function fts3InitVtab().
*/
static int fts3ConnectMethod(
  sqlite3 *db,                    /* Database connection */
  void *pAux,                     /* Pointer to tokenizer hash table */
  int argc,                       /* Number of elements in argv array */
  const char * const *argv,       /* xCreate/xConnect argument array */
  sqlite3_vtab **ppVtab,          /* OUT: New sqlite3_vtab object */
  char **pzErr                    /* OUT: sqlite3_malloc'd error message */
){
  return fts3InitVtab(0, db, pAux, argc, argv, ppVtab, pzErr);
}
static int fts3CreateMethod(
  sqlite3 *db,                    /* Database connection */
  void *pAux,                     /* Pointer to tokenizer hash table */
  int argc,                       /* Number of elements in argv array */
  const char * const *argv,       /* xCreate/xConnect argument array */
  sqlite3_vtab **ppVtab,          /* OUT: New sqlite3_vtab object */
  char **pzErr                    /* OUT: sqlite3_malloc'd error message */
){
  return fts3InitVtab(1, db, pAux, argc, argv, ppVtab, pzErr);
}

/* 
** Implementation of the xBestIndex method for FTS3 tables. There
** are three possible strategies, in order of preference:
**
**   1. Direct lookup by rowid or docid. 
**   2. Full-text search using a MATCH operator on a non-docid column.
**   3. Linear scan of %_content table.
*/
static int fts3BestIndexMethod(sqlite3_vtab *pVTab, sqlite3_index_info *pInfo){
  Fts3Table *p = (Fts3Table *)pVTab;
  int i;                          /* Iterator variable */
  int iCons = -1;                 /* Index of constraint to use */

  /* By default use a full table scan. This is an expensive option,
  ** so search through the constraints to see if a more efficient 
  ** strategy is possible.
  */
  pInfo->idxNum = FTS3_FULLSCAN_SEARCH;
  pInfo->estimatedCost = 500000;
  for(i=0; i<pInfo->nConstraint; i++){
    struct sqlite3_index_constraint *pCons = &pInfo->aConstraint[i];
    if( pCons->usable==0 ) continue;

    /* A direct lookup on the rowid or docid column. Assign a cost of 1.0. */
    if( pCons->op==SQLITE_INDEX_CONSTRAINT_EQ 
     && (pCons->iColumn<0 || pCons->iColumn==p->nColumn+1 )
    ){
      pInfo->idxNum = FTS3_DOCID_SEARCH;
      pInfo->estimatedCost = 1.0;
      iCons = i;
    }

    /* A MATCH constraint. Use a full-text search.
    **
    ** If there is more than one MATCH constraint available, use the first
    ** one encountered. If there is both a MATCH constraint and a direct
    ** rowid/docid lookup, prefer the MATCH strategy. This is done even 
    ** though the rowid/docid lookup is faster than a MATCH query, selecting
    ** it would lead to an "unable to use function MATCH in the requested 
    ** context" error.
    */
    if( pCons->op==SQLITE_INDEX_CONSTRAINT_MATCH 
     && pCons->iColumn>=0 && pCons->iColumn<=p->nColumn
    ){
      pInfo->idxNum = FTS3_FULLTEXT_SEARCH + pCons->iColumn;
      pInfo->estimatedCost = 2.0;
      iCons = i;
      break;
    }
  }

  if( iCons>=0 ){
    pInfo->aConstraintUsage[iCons].argvIndex = 1;
    pInfo->aConstraintUsage[iCons].omit = 1;
  } 
  return SQLITE_OK;
}

/*
** Implementation of xOpen method.
*/
static int fts3OpenMethod(sqlite3_vtab *pVTab, sqlite3_vtab_cursor **ppCsr){
  sqlite3_vtab_cursor *pCsr;               /* Allocated cursor */

  UNUSED_PARAMETER(pVTab);

  /* Allocate a buffer large enough for an Fts3Cursor structure. If the
  ** allocation succeeds, zero it and return SQLITE_OK. Otherwise, 
  ** if the allocation fails, return SQLITE_NOMEM.
  */
  *ppCsr = pCsr = (sqlite3_vtab_cursor *)sqlite3_malloc(sizeof(Fts3Cursor));
  if( !pCsr ){
    return SQLITE_NOMEM;
  }
  memset(pCsr, 0, sizeof(Fts3Cursor));
  return SQLITE_OK;
}

/****************************************************************/
/****************************************************************/
/****************************************************************/
/****************************************************************/


/*
** Close the cursor.  For additional information see the documentation
** on the xClose method of the virtual table interface.
*/
static int fulltextClose(sqlite3_vtab_cursor *pCursor){
  Fts3Cursor *pCsr = (Fts3Cursor *)pCursor;
  sqlite3_finalize(pCsr->pStmt);
  sqlite3Fts3ExprFree(pCsr->pExpr);
  sqlite3_free(pCsr->aDoclist);
  sqlite3_free(pCsr->aMatchinfo);
  sqlite3_free(pCsr);
  return SQLITE_OK;
}

static int fts3CursorSeek(sqlite3_context *pContext, Fts3Cursor *pCsr){
  if( pCsr->isRequireSeek ){
    pCsr->isRequireSeek = 0;
    sqlite3_bind_int64(pCsr->pStmt, 1, pCsr->iPrevId);
    if( SQLITE_ROW==sqlite3_step(pCsr->pStmt) ){
      return SQLITE_OK;
    }else{
      int rc = sqlite3_reset(pCsr->pStmt);
      if( rc==SQLITE_OK ){
        /* If no row was found and no error has occured, then the %_content
        ** table is missing a row that is present in the full-text index.
        ** The data structures are corrupt.
        */
        rc = SQLITE_CORRUPT;
      }
      pCsr->isEof = 1;
      if( pContext ){
        sqlite3_result_error_code(pContext, rc);
      }
      return rc;
    }
  }else{
    return SQLITE_OK;
  }
}

static int fts3NextMethod(sqlite3_vtab_cursor *pCursor){
  int rc = SQLITE_OK;             /* Return code */
  Fts3Cursor *pCsr = (Fts3Cursor *)pCursor;

  if( pCsr->aDoclist==0 ){
    if( SQLITE_ROW!=sqlite3_step(pCsr->pStmt) ){
      pCsr->isEof = 1;
      rc = sqlite3_reset(pCsr->pStmt);
    }
  }else if( pCsr->pNextId>=&pCsr->aDoclist[pCsr->nDoclist] ){
    pCsr->isEof = 1;
  }else{
    sqlite3_reset(pCsr->pStmt);
    fts3GetDeltaVarint(&pCsr->pNextId, &pCsr->iPrevId);
    pCsr->isRequireSeek = 1;
    pCsr->isMatchinfoOk = 1;
  }
  return rc;
}


/*
** The buffer pointed to by argument zNode (size nNode bytes) contains the
** root node of a b-tree segment. The segment is guaranteed to be at least
** one level high (i.e. the root node is not also a leaf). If successful,
** this function locates the leaf node of the segment that may contain the 
** term specified by arguments zTerm and nTerm and writes its block number 
** to *piLeaf.
**
** It is possible that the returned leaf node does not contain the specified
** term. However, if the segment does contain said term, it is stored on
** the identified leaf node. Because this function only inspects interior
** segment nodes (and never loads leaf nodes into memory), it is not possible
** to be sure.
**
** If an error occurs, an error code other than SQLITE_OK is returned.
*/ 
static int fts3SelectLeaf(
  Fts3Table *p,                   /* Virtual table handle */
  const char *zTerm,              /* Term to select leaves for */
  int nTerm,                      /* Size of term zTerm in bytes */
  const char *zNode,              /* Buffer containing segment interior node */
  int nNode,                      /* Size of buffer at zNode */
  sqlite3_int64 *piLeaf           /* Selected leaf node */
){
  int rc = SQLITE_OK;             /* Return code */
  const char *zCsr = zNode;       /* Cursor to iterate through node */
  const char *zEnd = &zCsr[nNode];/* End of interior node buffer */
  char *zBuffer = 0;              /* Buffer to load terms into */
  int nAlloc = 0;                 /* Size of allocated buffer */

  while( 1 ){
    int isFirstTerm = 1;          /* True when processing first term on page */
    int iHeight;                  /* Height of this node in tree */
    sqlite3_int64 iChild;         /* Block id of child node to descend to */
    int nBlock;                   /* Size of child node in bytes */

    zCsr += sqlite3Fts3GetVarint32(zCsr, &iHeight);
    zCsr += sqlite3Fts3GetVarint(zCsr, &iChild);
  
    while( zCsr<zEnd ){
      int cmp;                    /* memcmp() result */
      int nSuffix;                /* Size of term suffix */
      int nPrefix = 0;            /* Size of term prefix */
      int nBuffer;                /* Total term size */
  
      /* Load the next term on the node into zBuffer */
      if( !isFirstTerm ){
        zCsr += sqlite3Fts3GetVarint32(zCsr, &nPrefix);
      }
      isFirstTerm = 0;
      zCsr += sqlite3Fts3GetVarint32(zCsr, &nSuffix);
      if( nPrefix+nSuffix>nAlloc ){
        char *zNew;
        nAlloc = (nPrefix+nSuffix) * 2;
        zNew = (char *)sqlite3_realloc(zBuffer, nAlloc);
        if( !zNew ){
          sqlite3_free(zBuffer);
          return SQLITE_NOMEM;
        }
        zBuffer = zNew;
      }
      memcpy(&zBuffer[nPrefix], zCsr, nSuffix);
      nBuffer = nPrefix + nSuffix;
      zCsr += nSuffix;
  
      /* Compare the term we are searching for with the term just loaded from
      ** the interior node. If the specified term is greater than or equal
      ** to the term from the interior node, then all terms on the sub-tree 
      ** headed by node iChild are smaller than zTerm. No need to search 
      ** iChild.
      **
      ** If the interior node term is larger than the specified term, then
      ** the tree headed by iChild may contain the specified term.
      */
      cmp = memcmp(zTerm, zBuffer, (nBuffer>nTerm ? nTerm : nBuffer));
      if( cmp<0 || (cmp==0 && nBuffer>nTerm) ) break;
      iChild++;
    };

    /* If (iHeight==1), the children of this interior node are leaves. The
    ** specified term may be present on leaf node iChild.
    */
    if( iHeight==1 ){
      *piLeaf = iChild;
      break;
    }

    /* Descend to interior node iChild. */
    rc = sqlite3Fts3ReadBlock(p, iChild, &zCsr, &nBlock);
    if( rc!=SQLITE_OK ) break;
    zEnd = &zCsr[nBlock];
  }
  sqlite3_free(zBuffer);
  return rc;
}

/*
** This function is used to create delta-encoded serialized lists of FTS3 
** varints. Each call to this function appends a single varint to a list.
*/
static void fts3PutDeltaVarint(
  char **pp,                      /* IN/OUT: Output pointer */
  sqlite3_int64 *piPrev,          /* IN/OUT: Previous value written to list */
  sqlite3_int64 iVal              /* Write this value to the list */
){
  assert( iVal-*piPrev > 0 || (*piPrev==0 && iVal==0) );
  *pp += sqlite3Fts3PutVarint(*pp, iVal-*piPrev);
  *piPrev = iVal;
}

/*
** When this function is called, *ppPoslist is assumed to point to the 
** start of a position-list.
*/
static void fts3PoslistCopy(char **pp, char **ppPoslist){
  char *pEnd = *ppPoslist;
  char c = 0;

  /* The end of a position list is marked by a zero encoded as an FTS3 
  ** varint. A single 0x00 byte. Except, if the 0x00 byte is preceded by
  ** a byte with the 0x80 bit set, then it is not a varint 0, but the tail
  ** of some other, multi-byte, value.
  **
  ** The following block moves pEnd to point to the first byte that is not 
  ** immediately preceded by a byte with the 0x80 bit set. Then increments
  ** pEnd once more so that it points to the byte immediately following the
  ** last byte in the position-list.
  */
  while( *pEnd | c ) c = *pEnd++ & 0x80;
  pEnd++;

  if( pp ){
    int n = (int)(pEnd - *ppPoslist);
    char *p = *pp;
    memcpy(p, *ppPoslist, n);
    p += n;
    *pp = p;
  }
  *ppPoslist = pEnd;
}

static void fts3ColumnlistCopy(char **pp, char **ppPoslist){
  char *pEnd = *ppPoslist;
  char c = 0;

  /* A column-list is terminated by either a 0x01 or 0x00. */
  while( 0xFE & (*pEnd | c) ) c = *pEnd++ & 0x80;
  if( pp ){
    int n = (int)(pEnd - *ppPoslist);
    char *p = *pp;
    memcpy(p, *ppPoslist, n);
    p += n;
    *pp = p;
  }
  *ppPoslist = pEnd;
}

/*
** Value used to signify the end of an offset-list. This is safe because
** it is not possible to have a document with 2^31 terms.
*/
#define OFFSET_LIST_END 0x7fffffff

/*
** This function is used to help parse offset-lists. When this function is
** called, *pp may point to the start of the next varint in the offset-list
** being parsed, or it may point to 1 byte past the end of the offset-list
** (in which case **pp will be 0x00 or 0x01).
**
** If *pp points past the end of the current offset list, set *pi to 
** OFFSET_LIST_END and return. Otherwise, read the next varint from *pp,
** increment the current value of *pi by the value read, and set *pp to
** point to the next value before returning.
*/
static void fts3ReadNextPos(
  char **pp,                      /* IN/OUT: Pointer into offset-list buffer */
  sqlite3_int64 *pi               /* IN/OUT: Value read from offset-list */
){
  if( **pp&0xFE ){
    fts3GetDeltaVarint(pp, pi);
    *pi -= 2;
  }else{
    *pi = OFFSET_LIST_END;
  }
}

/*
** If parameter iCol is not 0, write an 0x01 byte followed by the value of
** iCol encoded as a varint to *pp. 
**
** Set *pp to point to the byte just after the last byte written before 
** returning (do not modify it if iCol==0). Return the total number of bytes
** written (0 if iCol==0).
*/
static int fts3PutColNumber(char **pp, int iCol){
  int n = 0;                      /* Number of bytes written */
  if( iCol ){
    char *p = *pp;                /* Output pointer */
    n = 1 + sqlite3Fts3PutVarint(&p[1], iCol);
    *p = 0x01;
    *pp = &p[n];
  }
  return n;
}

/*
**
*/
static void fts3PoslistMerge(
  char **pp,                      /* Output buffer */
  char **pp1,                     /* Left input list */
  char **pp2                      /* Right input list */
){
  char *p = *pp;
  char *p1 = *pp1;
  char *p2 = *pp2;

  while( *p1 || *p2 ){
    int iCol1;
    int iCol2;

    if( *p1==0x01 ) sqlite3Fts3GetVarint32(&p1[1], &iCol1);
    else if( *p1==0x00 ) iCol1 = OFFSET_LIST_END;
    else iCol1 = 0;

    if( *p2==0x01 ) sqlite3Fts3GetVarint32(&p2[1], &iCol2);
    else if( *p2==0x00 ) iCol2 = OFFSET_LIST_END;
    else iCol2 = 0;

    if( iCol1==iCol2 ){
      sqlite3_int64 i1 = 0;
      sqlite3_int64 i2 = 0;
      sqlite3_int64 iPrev = 0;
      int n = fts3PutColNumber(&p, iCol1);
      p1 += n;
      p2 += n;

      /* At this point, both p1 and p2 point to the start of offset-lists.
      ** An offset-list is a list of non-negative delta-encoded varints, each 
      ** incremented by 2 before being stored. Each list is terminated by a 0 
      ** or 1 value (0x00 or 0x01). The following block merges the two lists
      ** and writes the results to buffer p. p is left pointing to the byte
      ** after the list written. No terminator (0x00 or 0x01) is written to
      ** the output.
      */
      fts3GetDeltaVarint(&p1, &i1);
      fts3GetDeltaVarint(&p2, &i2);
      do {
        fts3PutDeltaVarint(&p, &iPrev, (i1<i2) ? i1 : i2); 
        iPrev -= 2;
        if( i1==i2 ){
          fts3ReadNextPos(&p1, &i1);
          fts3ReadNextPos(&p2, &i2);
        }else if( i1<i2 ){
          fts3ReadNextPos(&p1, &i1);
        }else{
          fts3ReadNextPos(&p2, &i2);
        }
      }while( i1!=OFFSET_LIST_END || i2!=OFFSET_LIST_END );
    }else if( iCol1<iCol2 ){
      p1 += fts3PutColNumber(&p, iCol1);
      fts3ColumnlistCopy(&p, &p1);
    }else{
      p2 += fts3PutColNumber(&p, iCol2);
      fts3ColumnlistCopy(&p, &p2);
    }
  }

  *p++ = '\0';
  *pp = p;
  *pp1 = p1 + 1;
  *pp2 = p2 + 1;
}

/*
** nToken==1 searches for adjacent positions.
*/
static int fts3PoslistPhraseMerge(
  char **pp,                      /* Output buffer */
  int nToken,                     /* Maximum difference in token positions */
  int isSaveLeft,                 /* Save the left position */
  char **pp1,                     /* Left input list */
  char **pp2                      /* Right input list */
){
  char *p = (pp ? *pp : 0);
  char *p1 = *pp1;
  char *p2 = *pp2;

  int iCol1 = 0;
  int iCol2 = 0;
  assert( *p1!=0 && *p2!=0 );
  if( *p1==0x01 ){ 
    p1++;
    p1 += sqlite3Fts3GetVarint32(p1, &iCol1);
  }
  if( *p2==0x01 ){ 
    p2++;
    p2 += sqlite3Fts3GetVarint32(p2, &iCol2);
  }

  while( 1 ){
    if( iCol1==iCol2 ){
      char *pSave = p;
      sqlite3_int64 iPrev = 0;
      sqlite3_int64 iPos1 = 0;
      sqlite3_int64 iPos2 = 0;

      if( pp && iCol1 ){
        *p++ = 0x01;
        p += sqlite3Fts3PutVarint(p, iCol1);
      }

      assert( *p1!=0x00 && *p2!=0x00 && *p1!=0x01 && *p2!=0x01 );
      fts3GetDeltaVarint(&p1, &iPos1); iPos1 -= 2;
      fts3GetDeltaVarint(&p2, &iPos2); iPos2 -= 2;

      while( 1 ){
        if( iPos2>iPos1 && iPos2<=iPos1+nToken ){
          sqlite3_int64 iSave;
          if( !pp ){
            fts3PoslistCopy(0, &p2);
            fts3PoslistCopy(0, &p1);
            *pp1 = p1;
            *pp2 = p2;
            return 1;
          }
          iSave = isSaveLeft ? iPos1 : iPos2;
          fts3PutDeltaVarint(&p, &iPrev, iSave+2); iPrev -= 2;
          pSave = 0;
        }
        if( (!isSaveLeft && iPos2<=(iPos1+nToken)) || iPos2<=iPos1 ){
          if( (*p2&0xFE)==0 ) break;
          fts3GetDeltaVarint(&p2, &iPos2); iPos2 -= 2;
        }else{
          if( (*p1&0xFE)==0 ) break;
          fts3GetDeltaVarint(&p1, &iPos1); iPos1 -= 2;
        }
      }

      if( pSave ){
        assert( pp && p );
        p = pSave;
      }

      fts3ColumnlistCopy(0, &p1);
      fts3ColumnlistCopy(0, &p2);
      assert( (*p1&0xFE)==0 && (*p2&0xFE)==0 );
      if( 0==*p1 || 0==*p2 ) break;

      p1++;
      p1 += sqlite3Fts3GetVarint32(p1, &iCol1);
      p2++;
      p2 += sqlite3Fts3GetVarint32(p2, &iCol2);
    }

    /* Advance pointer p1 or p2 (whichever corresponds to the smaller of
    ** iCol1 and iCol2) so that it points to either the 0x00 that marks the
    ** end of the position list, or the 0x01 that precedes the next 
    ** column-number in the position list. 
    */
    else if( iCol1<iCol2 ){
      fts3ColumnlistCopy(0, &p1);
      if( 0==*p1 ) break;
      p1++;
      p1 += sqlite3Fts3GetVarint32(p1, &iCol1);
    }else{
      fts3ColumnlistCopy(0, &p2);
      if( 0==*p2 ) break;
      p2++;
      p2 += sqlite3Fts3GetVarint32(p2, &iCol2);
    }
  }

  fts3PoslistCopy(0, &p2);
  fts3PoslistCopy(0, &p1);
  *pp1 = p1;
  *pp2 = p2;
  if( !pp || *pp==p ){
    return 0;
  }
  *p++ = 0x00;
  *pp = p;
  return 1;
}

/*
** Merge two position-lists as required by the NEAR operator.
*/
static int fts3PoslistNearMerge(
  char **pp,                      /* Output buffer */
  char *aTmp,                     /* Temporary buffer space */
  int nRight,                     /* Maximum difference in token positions */
  int nLeft,                      /* Maximum difference in token positions */
  char **pp1,                     /* IN/OUT: Left input list */
  char **pp2                      /* IN/OUT: Right input list */
){
  char *p1 = *pp1;
  char *p2 = *pp2;

  if( !pp ){
    if( fts3PoslistPhraseMerge(0, nRight, 0, pp1, pp2) ) return 1;
    *pp1 = p1;
    *pp2 = p2;
    return fts3PoslistPhraseMerge(0, nLeft, 0, pp2, pp1);
  }else{
    char *pTmp1 = aTmp;
    char *pTmp2;
    char *aTmp2;
    int res = 1;

    fts3PoslistPhraseMerge(&pTmp1, nRight, 0, pp1, pp2);
    aTmp2 = pTmp2 = pTmp1;
    *pp1 = p1;
    *pp2 = p2;
    fts3PoslistPhraseMerge(&pTmp2, nLeft, 1, pp2, pp1);
    if( pTmp1!=aTmp && pTmp2!=aTmp2 ){
      fts3PoslistMerge(pp, &aTmp, &aTmp2);
    }else if( pTmp1!=aTmp ){
      fts3PoslistCopy(pp, &aTmp);
    }else if( pTmp2!=aTmp2 ){
      fts3PoslistCopy(pp, &aTmp2);
    }else{
      res = 0;
    }

    return res;
  }
}

/*
** Values that may be used as the first parameter to fts3DoclistMerge().
*/
#define MERGE_NOT        2        /* D + D -> D */
#define MERGE_AND        3        /* D + D -> D */
#define MERGE_OR         4        /* D + D -> D */
#define MERGE_POS_OR     5        /* P + P -> P */
#define MERGE_PHRASE     6        /* P + P -> D */
#define MERGE_POS_PHRASE 7        /* P + P -> P */
#define MERGE_NEAR       8        /* P + P -> D */
#define MERGE_POS_NEAR   9        /* P + P -> P */

/*
** Merge the two doclists passed in buffer a1 (size n1 bytes) and a2
** (size n2 bytes). The output is written to pre-allocated buffer aBuffer,
** which is guaranteed to be large enough to hold the results. The number
** of bytes written to aBuffer is stored in *pnBuffer before returning.
**
** If successful, SQLITE_OK is returned. Otherwise, if a malloc error
** occurs while allocating a temporary buffer as part of the merge operation,
** SQLITE_NOMEM is returned.
*/
static int fts3DoclistMerge(
  int mergetype,                  /* One of the MERGE_XXX constants */
  int nParam1,                    /* Used by MERGE_NEAR and MERGE_POS_NEAR */
  int nParam2,                    /* Used by MERGE_NEAR and MERGE_POS_NEAR */
  char *aBuffer,                  /* Pre-allocated output buffer */
  int *pnBuffer,                  /* OUT: Bytes written to aBuffer */
  char *a1,                       /* Buffer containing first doclist */
  int n1,                         /* Size of buffer a1 */
  char *a2,                       /* Buffer containing second doclist */
  int n2                          /* Size of buffer a2 */
){
  sqlite3_int64 i1 = 0;
  sqlite3_int64 i2 = 0;
  sqlite3_int64 iPrev = 0;

  char *p = aBuffer;
  char *p1 = a1;
  char *p2 = a2;
  char *pEnd1 = &a1[n1];
  char *pEnd2 = &a2[n2];

  assert( mergetype==MERGE_OR     || mergetype==MERGE_POS_OR 
       || mergetype==MERGE_AND    || mergetype==MERGE_NOT
       || mergetype==MERGE_PHRASE || mergetype==MERGE_POS_PHRASE
       || mergetype==MERGE_NEAR   || mergetype==MERGE_POS_NEAR
  );

  if( !aBuffer ){
    *pnBuffer = 0;
    return SQLITE_NOMEM;
  }

  /* Read the first docid from each doclist */
  fts3GetDeltaVarint2(&p1, pEnd1, &i1);
  fts3GetDeltaVarint2(&p2, pEnd2, &i2);

  switch( mergetype ){
    case MERGE_OR:
    case MERGE_POS_OR:
      while( p1 || p2 ){
        if( p2 && p1 && i1==i2 ){
          fts3PutDeltaVarint(&p, &iPrev, i1);
          if( mergetype==MERGE_POS_OR ) fts3PoslistMerge(&p, &p1, &p2);
          fts3GetDeltaVarint2(&p1, pEnd1, &i1);
          fts3GetDeltaVarint2(&p2, pEnd2, &i2);
        }else if( !p2 || (p1 && i1<i2) ){
          fts3PutDeltaVarint(&p, &iPrev, i1);
          if( mergetype==MERGE_POS_OR ) fts3PoslistCopy(&p, &p1);
          fts3GetDeltaVarint2(&p1, pEnd1, &i1);
        }else{
          fts3PutDeltaVarint(&p, &iPrev, i2);
          if( mergetype==MERGE_POS_OR ) fts3PoslistCopy(&p, &p2);
          fts3GetDeltaVarint2(&p2, pEnd2, &i2);
        }
      }
      break;

    case MERGE_AND:
      while( p1 && p2 ){
        if( i1==i2 ){
          fts3PutDeltaVarint(&p, &iPrev, i1);
          fts3GetDeltaVarint2(&p1, pEnd1, &i1);
          fts3GetDeltaVarint2(&p2, pEnd2, &i2);
        }else if( i1<i2 ){
          fts3GetDeltaVarint2(&p1, pEnd1, &i1);
        }else{
          fts3GetDeltaVarint2(&p2, pEnd2, &i2);
        }
      }
      break;

    case MERGE_NOT:
      while( p1 ){
        if( p2 && i1==i2 ){
          fts3GetDeltaVarint2(&p1, pEnd1, &i1);
          fts3GetDeltaVarint2(&p2, pEnd2, &i2);
        }else if( !p2 || i1<i2 ){
          fts3PutDeltaVarint(&p, &iPrev, i1);
          fts3GetDeltaVarint2(&p1, pEnd1, &i1);
        }else{
          fts3GetDeltaVarint2(&p2, pEnd2, &i2);
        }
      }
      break;

    case MERGE_POS_PHRASE:
    case MERGE_PHRASE: {
      char **ppPos = (mergetype==MERGE_PHRASE ? 0 : &p);
      while( p1 && p2 ){
        if( i1==i2 ){
          char *pSave = p;
          sqlite3_int64 iPrevSave = iPrev;
          fts3PutDeltaVarint(&p, &iPrev, i1);
          if( 0==fts3PoslistPhraseMerge(ppPos, 1, 0, &p1, &p2) ){
            p = pSave;
            iPrev = iPrevSave;
          }
          fts3GetDeltaVarint2(&p1, pEnd1, &i1);
          fts3GetDeltaVarint2(&p2, pEnd2, &i2);
        }else if( i1<i2 ){
          fts3PoslistCopy(0, &p1);
          fts3GetDeltaVarint2(&p1, pEnd1, &i1);
        }else{
          fts3PoslistCopy(0, &p2);
          fts3GetDeltaVarint2(&p2, pEnd2, &i2);
        }
      }
      break;
    }

    default: assert( mergetype==MERGE_POS_NEAR || mergetype==MERGE_NEAR ); {
      char *aTmp = 0;
      char **ppPos = 0;
      if( mergetype==MERGE_POS_NEAR ){
        ppPos = &p;
        aTmp = sqlite3_malloc(2*(n1+n2+1));
        if( !aTmp ){
          return SQLITE_NOMEM;
        }
      }

      while( p1 && p2 ){
        if( i1==i2 ){
          char *pSave = p;
          sqlite3_int64 iPrevSave = iPrev;
          fts3PutDeltaVarint(&p, &iPrev, i1);

          if( !fts3PoslistNearMerge(ppPos, aTmp, nParam1, nParam2, &p1, &p2) ){
            iPrev = iPrevSave;
            p = pSave;
          }

          fts3GetDeltaVarint2(&p1, pEnd1, &i1);
          fts3GetDeltaVarint2(&p2, pEnd2, &i2);
        }else if( i1<i2 ){
          fts3PoslistCopy(0, &p1);
          fts3GetDeltaVarint2(&p1, pEnd1, &i1);
        }else{
          fts3PoslistCopy(0, &p2);
          fts3GetDeltaVarint2(&p2, pEnd2, &i2);
        }
      }
      sqlite3_free(aTmp);
      break;
    }
  }

  *pnBuffer = (int)(p-aBuffer);
  return SQLITE_OK;
}

/* 
** A pointer to an instance of this structure is used as the context 
** argument to sqlite3Fts3SegReaderIterate()
*/
typedef struct TermSelect TermSelect;
struct TermSelect {
  int isReqPos;
  char *aOutput;                  /* Malloc'd output buffer */
  int nOutput;                    /* Size of output in bytes */
};

/*
** This function is used as the sqlite3Fts3SegReaderIterate() callback when
** querying the full-text index for a doclist associated with a term or
** term-prefix.
*/
static int fts3TermSelectCb(
  Fts3Table *p,                   /* Virtual table object */
  void *pContext,                 /* Pointer to TermSelect structure */
  char *zTerm,
  int nTerm,
  char *aDoclist,
  int nDoclist
){
  TermSelect *pTS = (TermSelect *)pContext;
  int nNew = pTS->nOutput + nDoclist;
  char *aNew = sqlite3_malloc(nNew);

  UNUSED_PARAMETER(p);
  UNUSED_PARAMETER(zTerm);
  UNUSED_PARAMETER(nTerm);

  if( !aNew ){
    return SQLITE_NOMEM;
  }

  if( pTS->nOutput==0 ){
    /* If this is the first term selected, copy the doclist to the output
    ** buffer using memcpy(). TODO: Add a way to transfer control of the
    ** aDoclist buffer from the caller so as to avoid the memcpy().
    */
    memcpy(aNew, aDoclist, nDoclist);
  }else{
    /* The output buffer is not empty. Merge doclist aDoclist with the
    ** existing output. This can only happen with prefix-searches (as
    ** searches for exact terms return exactly one doclist).
    */
    int mergetype = (pTS->isReqPos ? MERGE_POS_OR : MERGE_OR);
    fts3DoclistMerge(mergetype, 0, 0,
        aNew, &nNew, pTS->aOutput, pTS->nOutput, aDoclist, nDoclist
    );
  }

  sqlite3_free(pTS->aOutput);
  pTS->aOutput = aNew;
  pTS->nOutput = nNew;

  return SQLITE_OK;
}

/*
** This function retreives the doclist for the specified term (or term
** prefix) from the database. 
**
** The returned doclist may be in one of two formats, depending on the 
** value of parameter isReqPos. If isReqPos is zero, then the doclist is
** a sorted list of delta-compressed docids. If isReqPos is non-zero, 
** then the returned list is in the same format as is stored in the
** database without the found length specifier at the start of on-disk
** doclists.
*/
static int fts3TermSelect(
  Fts3Table *p,                   /* Virtual table handle */
  int iColumn,                    /* Column to query (or -ve for all columns) */
  const char *zTerm,              /* Term to query for */
  int nTerm,                      /* Size of zTerm in bytes */
  int isPrefix,                   /* True for a prefix search */
  int isReqPos,                   /* True to include position lists in output */
  int *pnOut,                     /* OUT: Size of buffer at *ppOut */
  char **ppOut                    /* OUT: Malloced result buffer */
){
  int i;
  TermSelect tsc;
  Fts3SegFilter filter;           /* Segment term filter configuration */
  Fts3SegReader **apSegment;      /* Array of segments to read data from */
  int nSegment = 0;               /* Size of apSegment array */
  int nAlloc = 16;                /* Allocated size of segment array */
  int rc;                         /* Return code */
  sqlite3_stmt *pStmt = 0;        /* SQL statement to scan %_segdir table */
  int iAge = 0;                   /* Used to assign ages to segments */

  apSegment = (Fts3SegReader **)sqlite3_malloc(sizeof(Fts3SegReader*)*nAlloc);
  if( !apSegment ) return SQLITE_NOMEM;
  rc = sqlite3Fts3SegReaderPending(p, zTerm, nTerm, isPrefix, &apSegment[0]);
  if( rc!=SQLITE_OK ) goto finished;
  if( apSegment[0] ){
    nSegment = 1;
  }

  /* Loop through the entire %_segdir table. For each segment, create a
  ** Fts3SegReader to iterate through the subset of the segment leaves
  ** that may contain a term that matches zTerm/nTerm. For non-prefix
  ** searches, this is always a single leaf. For prefix searches, this
  ** may be a contiguous block of leaves.
  **
  ** The code in this loop does not actually load any leaves into memory
  ** (unless the root node happens to be a leaf). It simply examines the
  ** b-tree structure to determine which leaves need to be inspected.
  */
  rc = sqlite3Fts3AllSegdirs(p, &pStmt);
  while( rc==SQLITE_OK && SQLITE_ROW==(rc = sqlite3_step(pStmt)) ){
    Fts3SegReader *pNew = 0;
    int nRoot = sqlite3_column_bytes(pStmt, 4);
    char const *zRoot = sqlite3_column_blob(pStmt, 4);
    if( sqlite3_column_int64(pStmt, 1)==0 ){
      /* The entire segment is stored on the root node (which must be a
      ** leaf). Do not bother inspecting any data in this case, just
      ** create a Fts3SegReader to scan the single leaf. 
      */
      rc = sqlite3Fts3SegReaderNew(p, iAge, 0, 0, 0, zRoot, nRoot, &pNew);
    }else{
      int rc2;                    /* Return value of sqlite3Fts3ReadBlock() */
      sqlite3_int64 i1;           /* Blockid of leaf that may contain zTerm */
      rc = fts3SelectLeaf(p, zTerm, nTerm, zRoot, nRoot, &i1);
      if( rc==SQLITE_OK ){
        sqlite3_int64 i2 = sqlite3_column_int64(pStmt, 2);
        rc = sqlite3Fts3SegReaderNew(p, iAge, i1, i2, 0, 0, 0, &pNew);
      }

      /* The following call to ReadBlock() serves to reset the SQL statement
      ** used to retrieve blocks of data from the %_segments table. If it is
      ** not reset here, then it may remain classified as an active statement 
      ** by SQLite, which may lead to "DROP TABLE" or "DETACH" commands 
      ** failing.
      */ 
      rc2 = sqlite3Fts3ReadBlock(p, 0, 0, 0);
      if( rc==SQLITE_OK ){
        rc = rc2;
      }
    }
    iAge++;

    /* If a new Fts3SegReader was allocated, add it to the apSegment array. */
    assert( pNew!=0 || rc!=SQLITE_OK );
    if( pNew ){
      if( nSegment==nAlloc ){
        Fts3SegReader **pArray;
        nAlloc += 16;
        pArray = (Fts3SegReader **)sqlite3_realloc(
            apSegment, nAlloc*sizeof(Fts3SegReader *)
        );
        if( !pArray ){
          sqlite3Fts3SegReaderFree(p, pNew);
          rc = SQLITE_NOMEM;
          goto finished;
        }
        apSegment = pArray;
      }
      apSegment[nSegment++] = pNew;
    }
  }
  if( rc!=SQLITE_DONE ){
    assert( rc!=SQLITE_OK );
    goto finished;
  }

  memset(&tsc, 0, sizeof(TermSelect));
  tsc.isReqPos = isReqPos;

  filter.flags = FTS3_SEGMENT_IGNORE_EMPTY 
        | (isPrefix ? FTS3_SEGMENT_PREFIX : 0)
        | (isReqPos ? FTS3_SEGMENT_REQUIRE_POS : 0)
        | (iColumn<p->nColumn ? FTS3_SEGMENT_COLUMN_FILTER : 0);
  filter.iCol = iColumn;
  filter.zTerm = zTerm;
  filter.nTerm = nTerm;

  rc = sqlite3Fts3SegReaderIterate(p, apSegment, nSegment, &filter,
      fts3TermSelectCb, (void *)&tsc
  );

  if( rc==SQLITE_OK ){
    *ppOut = tsc.aOutput;
    *pnOut = tsc.nOutput;
  }else{
    sqlite3_free(tsc.aOutput);
  }

finished:
  sqlite3_reset(pStmt);
  for(i=0; i<nSegment; i++){
    sqlite3Fts3SegReaderFree(p, apSegment[i]);
  }
  sqlite3_free(apSegment);
  return rc;
}


/* 
** Return a DocList corresponding to the phrase *pPhrase.
*/
static int fts3PhraseSelect(
  Fts3Table *p,                   /* Virtual table handle */
  Fts3Phrase *pPhrase,            /* Phrase to return a doclist for */
  int isReqPos,                   /* True if output should contain positions */
  char **paOut,                   /* OUT: Pointer to malloc'd result buffer */
  int *pnOut                      /* OUT: Size of buffer at *paOut */
){
  char *pOut = 0;
  int nOut = 0;
  int rc = SQLITE_OK;
  int ii;
  int iCol = pPhrase->iColumn;
  int isTermPos = (pPhrase->nToken>1 || isReqPos);

  for(ii=0; ii<pPhrase->nToken; ii++){
    struct PhraseToken *pTok = &pPhrase->aToken[ii];
    char *z = pTok->z;            /* Next token of the phrase */
    int n = pTok->n;              /* Size of z in bytes */
    int isPrefix = pTok->isPrefix;/* True if token is a prefix */
    char *pList;                  /* Pointer to token doclist */
    int nList;                    /* Size of buffer at pList */

    rc = fts3TermSelect(p, iCol, z, n, isPrefix, isTermPos, &nList, &pList);
    if( rc!=SQLITE_OK ) break;

    if( ii==0 ){
      pOut = pList;
      nOut = nList;
    }else{
      /* Merge the new term list and the current output. If this is the
      ** last term in the phrase, and positions are not required in the
      ** output of this function, the positions can be dropped as part
      ** of this merge. Either way, the result of this merge will be
      ** smaller than nList bytes. The code in fts3DoclistMerge() is written
      ** so that it is safe to use pList as the output as well as an input
      ** in this case.
      */
      int mergetype = MERGE_POS_PHRASE;
      if( ii==pPhrase->nToken-1 && !isReqPos ){
        mergetype = MERGE_PHRASE;
      }
      fts3DoclistMerge(mergetype, 0, 0, pList, &nOut, pOut, nOut, pList, nList);
      sqlite3_free(pOut);
      pOut = pList;
    }
    assert( nOut==0 || pOut!=0 );
  }

  if( rc==SQLITE_OK ){
    *paOut = pOut;
    *pnOut = nOut;
  }else{
    sqlite3_free(pOut);
  }
  return rc;
}

static int fts3NearMerge(
  int mergetype,                  /* MERGE_POS_NEAR or MERGE_NEAR */
  int nNear,                      /* Parameter to NEAR operator */
  int nTokenLeft,                 /* Number of tokens in LHS phrase arg */
  char *aLeft,                    /* Doclist for LHS (incl. positions) */
  int nLeft,                      /* Size of LHS doclist in bytes */
  int nTokenRight,                /* As nTokenLeft */
  char *aRight,                   /* As aLeft */
  int nRight,                     /* As nRight */
  char **paOut,                   /* OUT: Results of merge (malloced) */
  int *pnOut                      /* OUT: Sized of output buffer */
){
  char *aOut;
  int rc;

  assert( mergetype==MERGE_POS_NEAR || MERGE_NEAR );

  aOut = sqlite3_malloc(nLeft+nRight+1);
  if( aOut==0 ){
    rc = SQLITE_NOMEM;
  }else{
    rc = fts3DoclistMerge(mergetype, nNear+nTokenRight, nNear+nTokenLeft, 
      aOut, pnOut, aLeft, nLeft, aRight, nRight
    );
    if( rc!=SQLITE_OK ){
      sqlite3_free(aOut);
      aOut = 0;
    }
  }

  *paOut = aOut;
  return rc;
}

int sqlite3Fts3ExprNearTrim(Fts3Expr *pLeft, Fts3Expr *pRight, int nNear){
  int rc;
  if( pLeft->aDoclist==0 || pRight->aDoclist==0 ){
    sqlite3_free(pLeft->aDoclist);
    sqlite3_free(pRight->aDoclist);
    pRight->aDoclist = 0;
    pLeft->aDoclist = 0;
    rc = SQLITE_OK;
  }else{
    char *aOut;
    int nOut;

    rc = fts3NearMerge(MERGE_POS_NEAR, nNear, 
        pLeft->pPhrase->nToken, pLeft->aDoclist, pLeft->nDoclist,
        pRight->pPhrase->nToken, pRight->aDoclist, pRight->nDoclist,
        &aOut, &nOut
    );
    if( rc!=SQLITE_OK ) return rc;
    sqlite3_free(pRight->aDoclist);
    pRight->aDoclist = aOut;
    pRight->nDoclist = nOut;

    rc = fts3NearMerge(MERGE_POS_NEAR, nNear, 
        pRight->pPhrase->nToken, pRight->aDoclist, pRight->nDoclist,
        pLeft->pPhrase->nToken, pLeft->aDoclist, pLeft->nDoclist,
        &aOut, &nOut
    );
    sqlite3_free(pLeft->aDoclist);
    pLeft->aDoclist = aOut;
    pLeft->nDoclist = nOut;
  }
  return rc;
}

/*
** Evaluate the full-text expression pExpr against fts3 table pTab. Store
** the resulting doclist in *paOut and *pnOut.
*/
static int evalFts3Expr(
  Fts3Table *p,                   /* Virtual table handle */
  Fts3Expr *pExpr,                /* Parsed fts3 expression */
  char **paOut,                   /* OUT: Pointer to malloc'd result buffer */
  int *pnOut,                     /* OUT: Size of buffer at *paOut */
  int isReqPos                    /* Require positions in output buffer */
){
  int rc = SQLITE_OK;             /* Return code */

  /* Zero the output parameters. */
  *paOut = 0;
  *pnOut = 0;

  if( pExpr ){
    assert( pExpr->eType==FTSQUERY_PHRASE 
         || pExpr->eType==FTSQUERY_NEAR 
         || isReqPos==0
    );
    if( pExpr->eType==FTSQUERY_PHRASE ){
      rc = fts3PhraseSelect(p, pExpr->pPhrase, 
          isReqPos || (pExpr->pParent && pExpr->pParent->eType==FTSQUERY_NEAR),
          paOut, pnOut
      );
    }else{
      char *aLeft;
      char *aRight;
      int nLeft;
      int nRight;

      if( 0==(rc = evalFts3Expr(p, pExpr->pRight, &aRight, &nRight, isReqPos))
       && 0==(rc = evalFts3Expr(p, pExpr->pLeft, &aLeft, &nLeft, isReqPos))
      ){
        assert( pExpr->eType==FTSQUERY_NEAR || pExpr->eType==FTSQUERY_OR     
            || pExpr->eType==FTSQUERY_AND  || pExpr->eType==FTSQUERY_NOT
        );
        switch( pExpr->eType ){
          case FTSQUERY_NEAR: {
            Fts3Expr *pLeft;
            Fts3Expr *pRight;
            int mergetype = isReqPos ? MERGE_POS_NEAR : MERGE_NEAR;
           
            if( pExpr->pParent && pExpr->pParent->eType==FTSQUERY_NEAR ){
              mergetype = MERGE_POS_NEAR;
            }
            pLeft = pExpr->pLeft;
            while( pLeft->eType==FTSQUERY_NEAR ){ 
              pLeft=pLeft->pRight;
            }
            pRight = pExpr->pRight;
            assert( pRight->eType==FTSQUERY_PHRASE );
            assert( pLeft->eType==FTSQUERY_PHRASE );

            rc = fts3NearMerge(mergetype, pExpr->nNear, 
                pLeft->pPhrase->nToken, aLeft, nLeft,
                pRight->pPhrase->nToken, aRight, nRight,
                paOut, pnOut
            );
            sqlite3_free(aLeft);
            break;
          }

          case FTSQUERY_OR: {
            /* Allocate a buffer for the output. The maximum size is the
            ** sum of the sizes of the two input buffers. The +1 term is
            ** so that a buffer of zero bytes is never allocated - this can
            ** cause fts3DoclistMerge() to incorrectly return SQLITE_NOMEM.
            */
            char *aBuffer = sqlite3_malloc(nRight+nLeft+1);
            rc = fts3DoclistMerge(MERGE_OR, 0, 0, aBuffer, pnOut,
                aLeft, nLeft, aRight, nRight
            );
            *paOut = aBuffer;
            sqlite3_free(aLeft);
            break;
          }

          default: {
            assert( FTSQUERY_NOT==MERGE_NOT && FTSQUERY_AND==MERGE_AND );
            fts3DoclistMerge(pExpr->eType, 0, 0, aLeft, pnOut,
                aLeft, nLeft, aRight, nRight
            );
            *paOut = aLeft;
            break;
          }
        }
      }
      sqlite3_free(aRight);
    }
  }

  return rc;
}

/*
** This is the xFilter interface for the virtual table.  See
** the virtual table xFilter method documentation for additional
** information.
**
** If idxNum==FTS3_FULLSCAN_SEARCH then do a full table scan against
** the %_content table.
**
** If idxNum==FTS3_DOCID_SEARCH then do a docid lookup for a single entry
** in the %_content table.
**
** If idxNum>=FTS3_FULLTEXT_SEARCH then use the full text index.  The
** column on the left-hand side of the MATCH operator is column
** number idxNum-FTS3_FULLTEXT_SEARCH, 0 indexed.  argv[0] is the right-hand
** side of the MATCH operator.
*/
/* TODO(shess) Upgrade the cursor initialization and destruction to
** account for fts3FilterMethod() being called multiple times on the
** same cursor. The current solution is very fragile. Apply fix to
** fts3 as appropriate.
*/
static int fts3FilterMethod(
  sqlite3_vtab_cursor *pCursor,   /* The cursor used for this query */
  int idxNum,                     /* Strategy index */
  const char *idxStr,             /* Unused */
  int nVal,                       /* Number of elements in apVal */
  sqlite3_value **apVal           /* Arguments for the indexing scheme */
){
  const char *azSql[] = {
    "SELECT * FROM %Q.'%q_content' WHERE docid = ?", /* non-full-table-scan */
    "SELECT * FROM %Q.'%q_content'",                 /* full-table-scan */
  };
  int rc;                         /* Return code */
  char *zSql;                     /* SQL statement used to access %_content */
  Fts3Table *p = (Fts3Table *)pCursor->pVtab;
  Fts3Cursor *pCsr = (Fts3Cursor *)pCursor;

  UNUSED_PARAMETER(idxStr);
  UNUSED_PARAMETER(nVal);

  assert( idxNum>=0 && idxNum<=(FTS3_FULLTEXT_SEARCH+p->nColumn) );
  assert( nVal==0 || nVal==1 );
  assert( (nVal==0)==(idxNum==FTS3_FULLSCAN_SEARCH) );

  /* In case the cursor has been used before, clear it now. */
  sqlite3_finalize(pCsr->pStmt);
  sqlite3_free(pCsr->aDoclist);
  sqlite3Fts3ExprFree(pCsr->pExpr);
  memset(&pCursor[1], 0, sizeof(Fts3Cursor)-sizeof(sqlite3_vtab_cursor));

  /* Compile a SELECT statement for this cursor. For a full-table-scan, the
  ** statement loops through all rows of the %_content table. For a
  ** full-text query or docid lookup, the statement retrieves a single
  ** row by docid.
  */
  zSql = sqlite3_mprintf(azSql[idxNum==FTS3_FULLSCAN_SEARCH], p->zDb, p->zName);
  if( !zSql ){
    rc = SQLITE_NOMEM;
  }else{
    rc = sqlite3_prepare_v2(p->db, zSql, -1, &pCsr->pStmt, 0);
    sqlite3_free(zSql);
  }
  if( rc!=SQLITE_OK ) return rc;
  pCsr->eSearch = (i16)idxNum;

  if( idxNum==FTS3_DOCID_SEARCH ){
    rc = sqlite3_bind_value(pCsr->pStmt, 1, apVal[0]);
  }else if( idxNum!=FTS3_FULLSCAN_SEARCH ){
    int iCol = idxNum-FTS3_FULLTEXT_SEARCH;
    const char *zQuery = (const char *)sqlite3_value_text(apVal[0]);

    if( zQuery==0 && sqlite3_value_type(apVal[0])!=SQLITE_NULL ){
      return SQLITE_NOMEM;
    }

    rc = sqlite3Fts3ExprParse(p->pTokenizer, p->azColumn, p->nColumn, 
        iCol, zQuery, -1, &pCsr->pExpr
    );
    if( rc!=SQLITE_OK ) return rc;

    rc = evalFts3Expr(p, pCsr->pExpr, &pCsr->aDoclist, &pCsr->nDoclist, 0);
    pCsr->pNextId = pCsr->aDoclist;
    pCsr->iPrevId = 0;
  }

  if( rc!=SQLITE_OK ) return rc;
  return fts3NextMethod(pCursor);
}

/* 
** This is the xEof method of the virtual table. SQLite calls this 
** routine to find out if it has reached the end of a result set.
*/
static int fts3EofMethod(sqlite3_vtab_cursor *pCursor){
  return ((Fts3Cursor *)pCursor)->isEof;
}

/* 
** This is the xRowid method. The SQLite core calls this routine to
** retrieve the rowid for the current row of the result set. fts3
** exposes %_content.docid as the rowid for the virtual table. The
** rowid should be written to *pRowid.
*/
static int fts3RowidMethod(sqlite3_vtab_cursor *pCursor, sqlite_int64 *pRowid){
  Fts3Cursor *pCsr = (Fts3Cursor *) pCursor;
  if( pCsr->aDoclist ){
    *pRowid = pCsr->iPrevId;
  }else{
    *pRowid = sqlite3_column_int64(pCsr->pStmt, 0);
  }
  return SQLITE_OK;
}

/* 
** This is the xColumn method, called by SQLite to request a value from
** the row that the supplied cursor currently points to.
*/
static int fts3ColumnMethod(
  sqlite3_vtab_cursor *pCursor,   /* Cursor to retrieve value from */
  sqlite3_context *pContext,      /* Context for sqlite3_result_xxx() calls */
  int iCol                        /* Index of column to read value from */
){
  int rc;                         /* Return Code */
  Fts3Cursor *pCsr = (Fts3Cursor *) pCursor;
  Fts3Table *p = (Fts3Table *)pCursor->pVtab;

  /* The column value supplied by SQLite must be in range. */
  assert( iCol>=0 && iCol<=p->nColumn+1 );

  if( iCol==p->nColumn+1 ){
    /* This call is a request for the "docid" column. Since "docid" is an 
    ** alias for "rowid", use the xRowid() method to obtain the value.
    */
    sqlite3_int64 iRowid;
    rc = fts3RowidMethod(pCursor, &iRowid);
    sqlite3_result_int64(pContext, iRowid);
  }else if( iCol==p->nColumn ){
    /* The extra column whose name is the same as the table.
    ** Return a blob which is a pointer to the cursor.
    */
    sqlite3_result_blob(pContext, &pCsr, sizeof(pCsr), SQLITE_TRANSIENT);
    rc = SQLITE_OK;
  }else{
    rc = fts3CursorSeek(0, pCsr);
    if( rc==SQLITE_OK ){
      sqlite3_result_value(pContext, sqlite3_column_value(pCsr->pStmt, iCol+1));
    }
  }
  return rc;
}

/* 
** This function is the implementation of the xUpdate callback used by 
** FTS3 virtual tables. It is invoked by SQLite each time a row is to be
** inserted, updated or deleted.
*/
static int fts3UpdateMethod(
  sqlite3_vtab *pVtab,            /* Virtual table handle */
  int nArg,                       /* Size of argument array */
  sqlite3_value **apVal,          /* Array of arguments */
  sqlite_int64 *pRowid            /* OUT: The affected (or effected) rowid */
){
  return sqlite3Fts3UpdateMethod(pVtab, nArg, apVal, pRowid);
}

/*
** Implementation of xSync() method. Flush the contents of the pending-terms
** hash-table to the database.
*/
static int fts3SyncMethod(sqlite3_vtab *pVtab){
  return sqlite3Fts3PendingTermsFlush((Fts3Table *)pVtab);
}

/*
** Implementation of xBegin() method. This is a no-op.
*/
static int fts3BeginMethod(sqlite3_vtab *pVtab){
  UNUSED_PARAMETER(pVtab);
  assert( ((Fts3Table *)pVtab)->nPendingData==0 );
  return SQLITE_OK;
}

/*
** Implementation of xCommit() method. This is a no-op. The contents of
** the pending-terms hash-table have already been flushed into the database
** by fts3SyncMethod().
*/
static int fts3CommitMethod(sqlite3_vtab *pVtab){
  UNUSED_PARAMETER(pVtab);
  assert( ((Fts3Table *)pVtab)->nPendingData==0 );
  return SQLITE_OK;
}

/*
** Implementation of xRollback(). Discard the contents of the pending-terms
** hash-table. Any changes made to the database are reverted by SQLite.
*/
static int fts3RollbackMethod(sqlite3_vtab *pVtab){
  sqlite3Fts3PendingTermsClear((Fts3Table *)pVtab);
  return SQLITE_OK;
}

/*
** Load the doclist associated with expression pExpr to pExpr->aDoclist.
** The loaded doclist contains positions as well as the document ids.
** This is used by the matchinfo(), snippet() and offsets() auxillary
** functions.
*/
int sqlite3Fts3ExprLoadDoclist(Fts3Table *pTab, Fts3Expr *pExpr){
  return evalFts3Expr(pTab, pExpr, &pExpr->aDoclist, &pExpr->nDoclist, 1);
}

/*
** After ExprLoadDoclist() (see above) has been called, this function is
** used to iterate through the position lists that make up the doclist
** stored in pExpr->aDoclist.
*/
char *sqlite3Fts3FindPositions(
  Fts3Expr *pExpr,                /* Access this expressions doclist */
  sqlite3_int64 iDocid,           /* Docid associated with requested pos-list */
  int iCol                        /* Column of requested pos-list */
){
  assert( pExpr->isLoaded );
  if( pExpr->aDoclist ){
    char *pEnd = &pExpr->aDoclist[pExpr->nDoclist];
    char *pCsr = pExpr->pCurrent;

    assert( pCsr );
    while( pCsr<pEnd ){
      if( pExpr->iCurrent<iDocid ){
        fts3PoslistCopy(0, &pCsr);
        fts3GetDeltaVarint(&pCsr, &pExpr->iCurrent);
        pExpr->pCurrent = pCsr;
      }else{
        if( pExpr->iCurrent==iDocid ){
          int iThis = 0;
          if( iCol<0 ){
            /* If iCol is negative, return a pointer to the start of the
            ** position-list (instead of a pointer to the start of a list
            ** of offsets associated with a specific column).
            */
            return pCsr;
          }
          while( iThis<iCol ){
            fts3ColumnlistCopy(0, &pCsr);
            if( *pCsr==0x00 ) return 0;
            pCsr++;
            pCsr += sqlite3Fts3GetVarint32(pCsr, &iThis);
          }
          if( iCol==iThis && (*pCsr&0xFE) ) return pCsr;
        }
        return 0;
      }
    }
  }

  return 0;
}

/*
** Helper function used by the implementation of the overloaded snippet(),
** offsets() and optimize() SQL functions.
**
** If the value passed as the third argument is a blob of size
** sizeof(Fts3Cursor*), then the blob contents are copied to the 
** output variable *ppCsr and SQLITE_OK is returned. Otherwise, an error
** message is written to context pContext and SQLITE_ERROR returned. The
** string passed via zFunc is used as part of the error message.
*/
static int fts3FunctionArg(
  sqlite3_context *pContext,      /* SQL function call context */
  const char *zFunc,              /* Function name */
  sqlite3_value *pVal,            /* argv[0] passed to function */
  Fts3Cursor **ppCsr         /* OUT: Store cursor handle here */
){
  Fts3Cursor *pRet;
  if( sqlite3_value_type(pVal)!=SQLITE_BLOB 
   || sqlite3_value_bytes(pVal)!=sizeof(Fts3Cursor *)
  ){
    char *zErr = sqlite3_mprintf("illegal first argument to %s", zFunc);
    sqlite3_result_error(pContext, zErr, -1);
    sqlite3_free(zErr);
    return SQLITE_ERROR;
  }
  memcpy(&pRet, sqlite3_value_blob(pVal), sizeof(Fts3Cursor *));
  *ppCsr = pRet;
  return SQLITE_OK;
}

/*
** Implementation of the snippet() function for FTS3
*/
static void fts3SnippetFunc(
  sqlite3_context *pContext,      /* SQLite function call context */
  int nVal,                       /* Size of apVal[] array */
  sqlite3_value **apVal           /* Array of arguments */
){
  Fts3Cursor *pCsr;               /* Cursor handle passed through apVal[0] */
  const char *zStart = "<b>";
  const char *zEnd = "</b>";
  const char *zEllipsis = "<b>...</b>";
  int iCol = -1;
  int nToken = 15;

  /* There must be at least one argument passed to this function (otherwise
  ** the non-overloaded version would have been called instead of this one).
  */
  assert( nVal>=1 );

  if( nVal>6 ){
    sqlite3_result_error(pContext, 
        "wrong number of arguments to function snippet()", -1);
    return;
  }
  if( fts3FunctionArg(pContext, "snippet", apVal[0], &pCsr) ) return;

  switch( nVal ){
    case 6: nToken = sqlite3_value_int(apVal[5]);
    case 5: iCol = sqlite3_value_int(apVal[4]);
    case 4: zEllipsis = (const char*)sqlite3_value_text(apVal[3]);
    case 3: zEnd = (const char*)sqlite3_value_text(apVal[2]);
    case 2: zStart = (const char*)sqlite3_value_text(apVal[1]);
  }
  if( !zEllipsis || !zEnd || !zStart ){
    sqlite3_result_error_nomem(pContext);
  }else if( SQLITE_OK==fts3CursorSeek(pContext, pCsr) ){
    sqlite3Fts3Snippet(pContext, pCsr, zStart, zEnd, zEllipsis, iCol, nToken);
  }
}

/*
** Implementation of the offsets() function for FTS3
*/
static void fts3OffsetsFunc(
  sqlite3_context *pContext,      /* SQLite function call context */
  int nVal,                       /* Size of argument array */
  sqlite3_value **apVal           /* Array of arguments */
){
  Fts3Cursor *pCsr;               /* Cursor handle passed through apVal[0] */

  UNUSED_PARAMETER(nVal);

  assert( nVal==1 );
  if( fts3FunctionArg(pContext, "offsets", apVal[0], &pCsr) ) return;
  assert( pCsr );
  if( SQLITE_OK==fts3CursorSeek(pContext, pCsr) ){
    sqlite3Fts3Offsets(pContext, pCsr);
  }
}

/* 
** Implementation of the special optimize() function for FTS3. This 
** function merges all segments in the database to a single segment.
** Example usage is:
**
**   SELECT optimize(t) FROM t LIMIT 1;
**
** where 't' is the name of an FTS3 table.
*/
static void fts3OptimizeFunc(
  sqlite3_context *pContext,      /* SQLite function call context */
  int nVal,                       /* Size of argument array */
  sqlite3_value **apVal           /* Array of arguments */
){
  int rc;                         /* Return code */
  Fts3Table *p;                   /* Virtual table handle */
  Fts3Cursor *pCursor;            /* Cursor handle passed through apVal[0] */

  UNUSED_PARAMETER(nVal);

  assert( nVal==1 );
  if( fts3FunctionArg(pContext, "optimize", apVal[0], &pCursor) ) return;
  p = (Fts3Table *)pCursor->base.pVtab;
  assert( p );

  rc = sqlite3Fts3Optimize(p);

  switch( rc ){
    case SQLITE_OK:
      sqlite3_result_text(pContext, "Index optimized", -1, SQLITE_STATIC);
      break;
    case SQLITE_DONE:
      sqlite3_result_text(pContext, "Index already optimal", -1, SQLITE_STATIC);
      break;
    default:
      sqlite3_result_error_code(pContext, rc);
      break;
  }
}

/*
** Implementation of the matchinfo() function for FTS3
*/
static void fts3MatchinfoFunc(
  sqlite3_context *pContext,      /* SQLite function call context */
  int nVal,                       /* Size of argument array */
  sqlite3_value **apVal           /* Array of arguments */
){
  Fts3Cursor *pCsr;               /* Cursor handle passed through apVal[0] */

  if( nVal!=1 ){
    sqlite3_result_error(pContext,
        "wrong number of arguments to function matchinfo()", -1);
    return;
  }

  if( SQLITE_OK==fts3FunctionArg(pContext, "matchinfo", apVal[0], &pCsr) ){
    sqlite3Fts3Matchinfo(pContext, pCsr);
  }
}

/*
** This routine implements the xFindFunction method for the FTS3
** virtual table.
*/
static int fts3FindFunctionMethod(
  sqlite3_vtab *pVtab,            /* Virtual table handle */
  int nArg,                       /* Number of SQL function arguments */
  const char *zName,              /* Name of SQL function */
  void (**pxFunc)(sqlite3_context*,int,sqlite3_value**), /* OUT: Result */
  void **ppArg                    /* Unused */
){
  struct Overloaded {
    const char *zName;
    void (*xFunc)(sqlite3_context*,int,sqlite3_value**);
  } aOverload[] = {
    { "snippet", fts3SnippetFunc },
    { "offsets", fts3OffsetsFunc },
    { "optimize", fts3OptimizeFunc },
    { "matchinfo", fts3MatchinfoFunc },
  };
  int i;                          /* Iterator variable */

  UNUSED_PARAMETER(pVtab);
  UNUSED_PARAMETER(nArg);
  UNUSED_PARAMETER(ppArg);

  for(i=0; i<SizeofArray(aOverload); i++){
    if( strcmp(zName, aOverload[i].zName)==0 ){
      *pxFunc = aOverload[i].xFunc;
      return 1;
    }
  }

  /* No function of the specified name was found. Return 0. */
  return 0;
}

/*
** Implementation of FTS3 xRename method. Rename an fts3 table.
*/
static int fts3RenameMethod(
  sqlite3_vtab *pVtab,            /* Virtual table handle */
  const char *zName               /* New name of table */
){
  Fts3Table *p = (Fts3Table *)pVtab;     
  int rc = SQLITE_NOMEM;          /* Return Code */
  char *zSql;                     /* SQL script to run to rename tables */
 
  zSql = sqlite3_mprintf(
    "ALTER TABLE %Q.'%q_content'  RENAME TO '%q_content';"
    "ALTER TABLE %Q.'%q_segments' RENAME TO '%q_segments';"
    "ALTER TABLE %Q.'%q_segdir'   RENAME TO '%q_segdir';"
    , p->zDb, p->zName, zName 
    , p->zDb, p->zName, zName 
    , p->zDb, p->zName, zName
  );
  if( zSql ){
    rc = sqlite3_exec(p->db, zSql, 0, 0, 0);
    sqlite3_free(zSql);
  }
  return rc;
}

static const sqlite3_module fts3Module = {
  /* iVersion      */ 0,
  /* xCreate       */ fts3CreateMethod,
  /* xConnect      */ fts3ConnectMethod,
  /* xBestIndex    */ fts3BestIndexMethod,
  /* xDisconnect   */ fts3DisconnectMethod,
  /* xDestroy      */ fts3DestroyMethod,
  /* xOpen         */ fts3OpenMethod,
  /* xClose        */ fulltextClose,
  /* xFilter       */ fts3FilterMethod,
  /* xNext         */ fts3NextMethod,
  /* xEof          */ fts3EofMethod,
  /* xColumn       */ fts3ColumnMethod,
  /* xRowid        */ fts3RowidMethod,
  /* xUpdate       */ fts3UpdateMethod,
  /* xBegin        */ fts3BeginMethod,
  /* xSync         */ fts3SyncMethod,
  /* xCommit       */ fts3CommitMethod,
  /* xRollback     */ fts3RollbackMethod,
  /* xFindFunction */ fts3FindFunctionMethod,
  /* xRename */       fts3RenameMethod,
};

/*
** This function is registered as the module destructor (called when an
** FTS3 enabled database connection is closed). It frees the memory
** allocated for the tokenizer hash table.
*/
static void hashDestroy(void *p){
  Fts3Hash *pHash = (Fts3Hash *)p;
  sqlite3Fts3HashClear(pHash);
  sqlite3_free(pHash);
}

/*
** The fts3 built-in tokenizers - "simple" and "porter" - are implemented
** in files fts3_tokenizer1.c and fts3_porter.c respectively. The following
** two forward declarations are for functions declared in these files
** used to retrieve the respective implementations.
**
** Calling sqlite3Fts3SimpleTokenizerModule() sets the value pointed
** to by the argument to point a the "simple" tokenizer implementation.
** Function ...PorterTokenizerModule() sets *pModule to point to the
** porter tokenizer/stemmer implementation.
*/
void sqlite3Fts3SimpleTokenizerModule(sqlite3_tokenizer_module const**ppModule);
void sqlite3Fts3PorterTokenizerModule(sqlite3_tokenizer_module const**ppModule);
void sqlite3Fts3IcuTokenizerModule(sqlite3_tokenizer_module const**ppModule);

/*
** Initialise the fts3 extension. If this extension is built as part
** of the sqlite library, then this function is called directly by
** SQLite. If fts3 is built as a dynamically loadable extension, this
** function is called by the sqlite3_extension_init() entry point.
*/
int sqlite3Fts3Init(sqlite3 *db){
  int rc = SQLITE_OK;
  Fts3Hash *pHash = 0;
  const sqlite3_tokenizer_module *pSimple = 0;
  const sqlite3_tokenizer_module *pPorter = 0;

#ifdef SQLITE_ENABLE_ICU
  const sqlite3_tokenizer_module *pIcu = 0;
  sqlite3Fts3IcuTokenizerModule(&pIcu);
#endif

  sqlite3Fts3SimpleTokenizerModule(&pSimple);
  sqlite3Fts3PorterTokenizerModule(&pPorter);

  /* Allocate and initialise the hash-table used to store tokenizers. */
  pHash = sqlite3_malloc(sizeof(Fts3Hash));
  if( !pHash ){
    rc = SQLITE_NOMEM;
  }else{
    sqlite3Fts3HashInit(pHash, FTS3_HASH_STRING, 1);
  }

  /* Load the built-in tokenizers into the hash table */
  if( rc==SQLITE_OK ){
    if( sqlite3Fts3HashInsert(pHash, "simple", 7, (void *)pSimple)
     || sqlite3Fts3HashInsert(pHash, "porter", 7, (void *)pPorter) 
#ifdef SQLITE_ENABLE_ICU
     || (pIcu && sqlite3Fts3HashInsert(pHash, "icu", 4, (void *)pIcu))
#endif
    ){
      rc = SQLITE_NOMEM;
    }
  }

#ifdef SQLITE_TEST
  if( rc==SQLITE_OK ){
    rc = sqlite3Fts3ExprInitTestInterface(db);
  }
#endif

  /* Create the virtual table wrapper around the hash-table and overload 
  ** the two scalar functions. If this is successful, register the
  ** module with sqlite.
  */
  if( SQLITE_OK==rc 
   && SQLITE_OK==(rc = sqlite3Fts3InitHashTable(db, pHash, "fts3_tokenizer"))
   && SQLITE_OK==(rc = sqlite3_overload_function(db, "snippet", -1))
   && SQLITE_OK==(rc = sqlite3_overload_function(db, "offsets", 1))
   && SQLITE_OK==(rc = sqlite3_overload_function(db, "matchinfo", -1))
   && SQLITE_OK==(rc = sqlite3_overload_function(db, "optimize", 1))
  ){
    return sqlite3_create_module_v2(
        db, "fts3", &fts3Module, (void *)pHash, hashDestroy
    );
  }

  /* An error has occurred. Delete the hash table and return the error code. */
  assert( rc!=SQLITE_OK );
  if( pHash ){
    sqlite3Fts3HashClear(pHash);
    sqlite3_free(pHash);
  }
  return rc;
}

#if !SQLITE_CORE
int sqlite3_extension_init(
  sqlite3 *db, 
  char **pzErrMsg,
  const sqlite3_api_routines *pApi
){
  SQLITE_EXTENSION_INIT2(pApi)
  return sqlite3Fts3Init(db);
}
#endif

#endif