SQLite

**
**     * The FTS2 module is being built as an extension
**       (in which case SQLITE_CORE is not defined), or
**
**     * The FTS2 module is being built into the core of
**       SQLite (in which case SQLITE_ENABLE_FTS2 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.
**
** TODO(shess) Delta-encode docids.  This provides a 10% win versus
** DL_POSITIONS_OFFSETS on the first 100,000 documents of the Enron
** corpus, greater versus DL_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)
** array {
**   varint nTerm;           (length of term)
**   char pTerm[nTerm];      (content of term)
** }
**
** Here, 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.
**
** New data is spilled to a new interior node at the same height when
** the current node exceeds INTERIOR_MAX bytes (default 2048).  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 groups into levels and
** merged in matches.  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's 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_FTS2)

#if defined(SQLITE_ENABLE_FTS2) && !defined(SQLITE_CORE)
# define SQLITE_CORE 1
#endif

#include <assert.h>
#if !defined(__APPLE__)
#include <malloc.h>
#endif
#include <stdlib.h>

#include <stdio.h>
#include <string.h>
#include <ctype.h>

#include "fts2.h"
#include "fts2_hash.h"
#include "fts2_tokenizer.h"
#include "sqlite3.h"
#include "sqlite3ext.h"
SQLITE_EXTENSION_INIT1


/* TODO(shess) MAN, this thing needs some refactoring.  At minimum, it
** would be nice to order the file better, perhaps something along the
** lines of:
**
**  - utility functions
**  - table setup functions
**  - table update functions
**  - table query functions
**
** Put the query functions last because they're likely to reference
** typedefs or functions from the table update section.
*/

#if 0
# define TRACE(A)  printf A; fflush(stdout)
#else
# define TRACE(A)
#endif

  sb->len += nFrom;
  sb->s[sb->len] = 0;
}
static void append(StringBuffer *sb, const char *zFrom){
  nappend(sb, zFrom, strlen(zFrom));
}

/* Helper functions for certain common memory-allocation idioms:

**
** data_dup() - malloc+memcpy to duplicate a buffer
** data_replace() - realloc+memcpy to dup a buffer over an existing buffer
** data_append() - realloc+memcpy to append data to an existing buffer
** data_append2() - shorthand for calling data_append() twice.
*/
/* TODO(shess) There is a "block of binary data on the heap" construct
** in here which could be shared with (at least) the StringBuffer and

** DocList constructs.
*/
static void data_replace(char **ppTarget, int *pnTarget,
                         const char *pSource, int nSource){
  *ppTarget = realloc(*ppTarget, nSource);
  memcpy(*ppTarget, pSource, nSource);
  *pnTarget = nSource;
}

static void data_dup(char **ppTarget, int *pnTarget,
                     const char *pSource, int nSource){
  *ppTarget = malloc(nSource);
  memcpy(*ppTarget, pSource, nSource);
  *pnTarget = nSource;
}

static void data_append(char **ppTarget, int *pnTarget,
                        const char *pSource, int nSource){
  *ppTarget = realloc(*ppTarget, *pnTarget+nSource);
  memcpy(*ppTarget+*pnTarget, pSource, nSource);
  *pnTarget += nSource;
}

static void data_append2(char **ppTarget, int *pnTarget,
                         const char *pSource1, int nSource1,
                         const char *pSource2, int nSource2){
  *ppTarget = realloc(*ppTarget, *pnTarget+nSource1+nSource2);
  memcpy(*ppTarget+*pnTarget, pSource1, nSource1);
  memcpy(*ppTarget+*pnTarget+nSource1, pSource2, nSource2);
  *pnTarget += nSource1+nSource2;
}

/* We may need up to VARINT_MAX bytes to store an encoded 64-bit integer. */
#define VARINT_MAX 10

/* Write a 64-bit variable-length integer to memory starting at p[0].
 * The length of data written will be between 1 and VARINT_MAX bytes.
 * The number of bytes written is returned. */

 sqlite_int64 i;
 int ret = getVarint(p, &i);
 *pi = (int) i;
 assert( *pi==i );
 return ret;
}














































typedef enum DocListType {
  DL_DOCIDS,              /* docids only */
  DL_POSITIONS,           /* docids + positions */
  DL_POSITIONS_OFFSETS    /* docids + positions + offsets */
} DocListType;

/*
** By default, only positions and not offsets are stored in the doclists.
** To change this so that offsets are stored too, compile with
**
**          -DDL_DEFAULT=DL_POSITIONS_OFFSETS
**
** If DL_DEFAULT is set to DL_DOCIDS, your table can only be inserted
** into (no deletes or updates).
*/
#ifndef DL_DEFAULT
# define DL_DEFAULT DL_POSITIONS
#endif

typedef struct DocList {
  char *pData;
  int nData;
  DocListType iType;
  int iLastColumn;    /* the last column written */
  int iLastPos;       /* the last position written */
  int iLastOffset;    /* the last start offset written */
} DocList;

enum {
  POS_END = 0,        /* end of this position list */
  POS_COLUMN,         /* followed by new column number */
  POS_BASE
};

/* TODO(shess) I think it might be time to refactor the doclist
** manipulation.  Broadly put, there are four fairly discrete clients,
** tokenization, insert-time segment merging, query-time segment
** merging and query-time analysis.  The breakdown I think might be
** reasonable would be:
**
** DocListReader - Wraps const char *pData, int nData.
**   Used to traverse doclists
** DocListWriter - Starts empty, can add complete doclist elements.
**   Used in merging doclists.
** DocBuilder - Used when tokenizing documents.
*/

static void docListCoreInit(DocList *d, DocListType iType,
                            char *pData, int nData){
  d->nData = nData;




  d->pData = pData;

  d->iType = iType;
  d->iLastColumn = 0;
  d->iLastPos = d->iLastOffset = 0;
}

/* Initialize a new DocList pointing to static data.  Don't call
** docListDestroy() to release, just free(d) (if you originally
** malloced d).
*/
static void docListStaticInit(DocList *d, DocListType iType,
                              const char *pData, int nData){
  docListCoreInit(d, iType, (char *)pData, nData);
}

/* Initialize a new DocList to hold a copy of the given data. */
static void docListInit(DocList *d, DocListType iType,
                        const char *pData, int nData){
  char *p = 0;
  if( nData>0 ){
    p = malloc(nData);
    memcpy(p, pData, nData);
  }
  docListCoreInit(d, iType, p, nData);
}

/* Create a new dynamically-allocated DocList. */
static DocList *docListNew(DocListType iType){
  DocList *d = (DocList *) malloc(sizeof(DocList));
  docListInit(d, iType, 0, 0);
  return d;
}

  sqlite_int64 d = 0;
  while( !atEnd(pReader) && (d=peekDocid(pReader))<iDocid ){
    skipDocument(pReader);
  }
  return !atEnd(pReader) && d==iDocid;
}









#ifdef SQLITE_DEBUG
/*
** This routine is used for debugging purpose only.
**
** Write the content of a doclist to standard output.
*/
static void printDoclist(DocList *p){

    }
  }

  docListDestroy(in);
  *in = out;
}

/* Efficiently merge left and right into out, with duplicated docids
** from right overwriting those in left (left is effectively older
** than right).  The previous code had a memmove() which introduced an
** O(N^2) into merges, while this code should be O(N).
*/
static void docListMerge(DocList *out, DocList *left, DocList *right){

  DocListReader leftReader, rightReader;

  int iData = 0;
#ifndef NDEBUG

  /* Track these to make certain that every byte is processed. */








  int nLeftProcessed = 0, nRightProcessed = 0;





#endif










  assert( left->iType==right->iType );




  /* Handle edge cases. */



  /* TODO(shess) Consider simply forbidding edge cases, in the


  ** interests of saving copies.







  */


  if( left->nData==0 ){


    docListInit(out, right->iType, right->pData, right->nData);
    return;
  }else if(right->nData==0 ){

    docListInit(out, left->iType, left->pData, left->nData);

    return;
  }
  docListInit(out, left->iType, 0, 0);

  /* At this time, the sum of the space of the inputs is a strict
  ** upper bound.  *out can end up smaller if elements of *right
  ** overwrite elements of *left, but never larger.
  */
  out->nData = left->nData+right->nData;
  out->pData = malloc(out->nData);

  readerInit(&leftReader, left);
  readerInit(&rightReader, right);

  while( !atEnd(&leftReader) && !atEnd(&rightReader) ){
    sqlite_int64 iLeftDocid = peekDocid(&leftReader);
    sqlite_int64 iRightDocid = peekDocid(&rightReader);
    const char *pStart, *pEnd;

    if( iLeftDocid<iRightDocid ){
      /* Copy from *left where less than iRightDocid. */
      pStart = leftReader.p;
      skipToDocid(&leftReader, iRightDocid);
      pEnd = leftReader.p;
#ifndef NDEBUG
      nLeftProcessed += pEnd-pStart;
#endif
    }else{
      /* Copy from *right where less than iLeftDocid, plus the element
      ** matching iLeftDocid, if present.  Also drop the matching
      ** element from *left.
      */
      pStart = rightReader.p;
      if( skipToDocid(&rightReader, iLeftDocid) ){
#ifndef NDEBUG
        const char *pLeftStart = leftReader.p;
#endif
        skipDocument(&leftReader);
        skipDocument(&rightReader);
#ifndef NDEBUG
        nLeftProcessed += leftReader.p-pLeftStart;
#endif
      }
      pEnd = rightReader.p;
#ifndef NDEBUG
      nRightProcessed += pEnd-pStart;
#endif
    }
    assert( pEnd>pStart );
    assert( iData+pEnd-pStart<=out->nData );
    memcpy(out->pData+iData, pStart, pEnd-pStart);
    iData += pEnd-pStart;
  }

  if( !atEnd(&leftReader) ){
    int n = left->nData-(leftReader.p-left->pData);
    assert( atEnd(&rightReader) );
    memcpy(out->pData+iData, leftReader.p, n);
    iData += n;
#ifndef NDEBUG
    nLeftProcessed += n;
#endif
  }else if( !atEnd(&rightReader) ){
    int n = right->nData-(rightReader.p-right->pData);
    memcpy(out->pData+iData, rightReader.p, n);
    iData += n;
#ifndef NDEBUG
    nRightProcessed += n;
#endif
  }
  out->nData = iData;
  out->pData = realloc(out->pData, out->nData);

  assert( nLeftProcessed==left->nData );
  assert( nRightProcessed==right->nData );
}

/*
** Read the next docid off of pIn.  Return 0 if we reach the end.
*
* TODO: This assumes that docids are never 0, but they may actually be 0 since
* users can choose docids when inserting into a full-text table.  Fix this.

typedef enum QueryType {
  QUERY_GENERIC,   /* table scan */
  QUERY_ROWID,     /* lookup by rowid */
  QUERY_FULLTEXT   /* QUERY_FULLTEXT + [i] is a full-text search for column i*/
} QueryType;














typedef enum fulltext_statement {
  CONTENT_INSERT_STMT,
  CONTENT_SELECT_STMT,
  CONTENT_UPDATE_STMT,
  CONTENT_DELETE_STMT,

  BLOCK_INSERT_STMT,
  BLOCK_SELECT_STMT,
  BLOCK_DELETE_STMT,

  SEGDIR_MAX_INDEX_STMT,
  SEGDIR_SET_STMT,
  SEGDIR_SELECT_STMT,
  SEGDIR_SPAN_STMT,
  SEGDIR_DELETE_STMT,
  SEGDIR_SELECT_ALL_STMT,

  MAX_STMT                     /* Always at end! */
} fulltext_statement;

/* These must exactly match the enum above. */
/* TODO(shess): Is there some risk that a statement will be used in two

** cursors at once, e.g.  if a query joins a virtual table to itself?
** If so perhaps we should move some of these to the cursor object.
*/
static const char *const fulltext_zStatement[MAX_STMT] = {
  /* CONTENT_INSERT */ NULL,  /* generated in contentInsertStatement() */
  /* CONTENT_SELECT */ "select * from %_content where rowid = ?",
  /* CONTENT_UPDATE */ NULL,  /* generated in contentUpdateStatement() */
  /* CONTENT_DELETE */ "delete from %_content where rowid = ?",

  /* BLOCK_INSERT */ "insert into %_segments values (?)",
  /* BLOCK_SELECT */ "select block from %_segments where rowid = ?",
  /* BLOCK_DELETE */ "delete from %_segments where rowid between ? and ?",

  /* SEGDIR_MAX_INDEX */ "select max(idx) from %_segdir where level = ?",
  /* SEGDIR_SET */ "insert into %_segdir values (?, ?, ?, ?, ?, ?)",
  /* SEGDIR_SELECT */
  "select start_block, leaves_end_block, root from %_segdir "
  " where level = ? order by idx",
  /* SEGDIR_SPAN */

  "select min(start_block), max(end_block) from %_segdir "
  " where level = ? and start_block <> 0",
  /* SEGDIR_DELETE */ "delete from %_segdir where level = ?",
  /* SEGDIR_SELECT_ALL */ "select root from %_segdir order by level desc, idx",
};

/* MERGE_COUNT controls how often we merge segments (see comment at
** top of file).
*/
#define MERGE_COUNT 16

/*
** A connection to a fulltext index is an instance of the following
** structure.  The xCreate and xConnect methods create an instance
** of this structure and xDestroy and xDisconnect free that instance.
** All other methods receive a pointer to the structure as one of their
** arguments.

  char **azContentColumn;          /* column names in content table; malloced */
  sqlite3_tokenizer *pTokenizer;   /* tokenizer for inserts and queries */

  /* Precompiled statements which we keep as long as the table is
  ** open.
  */
  sqlite3_stmt *pFulltextStatements[MAX_STMT];

  /* Precompiled statements used for segment merges.  We run a
  ** separate select across the leaf level of each tree being merged.
  */
  sqlite3_stmt *pLeafSelectStmts[MERGE_COUNT];
  /* The statement used to prepare pLeafSelectStmts. */
#define LEAF_SELECT \
  "select block from %_segments where rowid between ? and ? order by rowid"
};

/*
** When the core wants to do a query, it create a cursor using a
** call to xOpen.  This structure is an instance of a cursor.  It
** is destroyed by xClose.
*/

*/
static int sql_single_step_statement(fulltext_vtab *v,
                                     fulltext_statement iStmt,
                                     sqlite3_stmt **ppStmt){
  int rc = sql_step_statement(v, iStmt, ppStmt);
  return (rc==SQLITE_DONE) ? SQLITE_OK : rc;
}

/* Like sql_get_statement(), but for special replicated LEAF_SELECT
** statements.
*/
/* TODO(shess) Write version for generic statements and then share
** that between the cached-statement functions.
*/
static int sql_get_leaf_statement(fulltext_vtab *v, int idx,
                                  sqlite3_stmt **ppStmt){
  assert( idx>=0 && idx<MERGE_COUNT );
  if( v->pLeafSelectStmts[idx]==NULL ){
    int rc = sql_prepare(v->db, v->zName, &v->pLeafSelectStmts[idx],
                         LEAF_SELECT);
    if( rc!=SQLITE_OK ) return rc;
  }else{
    int rc = sqlite3_reset(v->pLeafSelectStmts[idx]);
    if( rc!=SQLITE_OK ) return rc;
  }

  *ppStmt = v->pLeafSelectStmts[idx];
  return SQLITE_OK;
}

/* Like sql_step_statement(), but for special replicated LEAF_SELECT
** statements.
*/
/* TODO(shess) Write version for generic statements and then share
** that between the cached-statement functions.
*/
static int sql_step_leaf_statement(fulltext_vtab *v, int idx,
                                   sqlite3_stmt **ppStmt){
  int rc;
  sqlite3_stmt *s = *ppStmt;

  while( (rc=sqlite3_step(s))!=SQLITE_DONE && rc!=SQLITE_ROW ){
    sqlite3_stmt *pNewStmt;

    if( rc==SQLITE_BUSY ) continue;
    if( rc!=SQLITE_ERROR ) return rc;

    rc = sqlite3_reset(s);
    if( rc!=SQLITE_SCHEMA ) return SQLITE_ERROR;

    v->pLeafSelectStmts[idx] = NULL;   /* Still in s */
    rc = sql_get_leaf_statement(v, idx, &pNewStmt);
    if( rc!=SQLITE_OK ) goto err;
    *ppStmt = pNewStmt;

    rc = sqlite3_transfer_bindings(s, pNewStmt);
    if( rc!=SQLITE_OK ) goto err;

    rc = sqlite3_finalize(s);
    if( rc!=SQLITE_OK ) return rc;
    s = pNewStmt;
  }
  return rc;

 err:
  sqlite3_finalize(s);
  return rc;
}

/* insert into %_content (rowid, ...) values ([rowid], [pValues]) */
static int content_insert(fulltext_vtab *v, sqlite3_value *rowid,
                          sqlite3_value **pValues){
  sqlite3_stmt *s;
  int i;
  int rc = sql_get_statement(v, CONTENT_INSERT_STMT, &s);

  rc = sqlite3_bind_int64(s, 1, iRow);
  if( rc!=SQLITE_OK ) return rc;

  return sql_single_step_statement(v, CONTENT_DELETE_STMT, &s);
}

/* insert into %_segments values ([pData])
**   returns assigned rowid in *piBlockid
*/
static int block_insert(fulltext_vtab *v, const char *pData, int nData,
                        sqlite_int64 *piBlockid){
  sqlite3_stmt *s;
  int rc = sql_get_statement(v, BLOCK_INSERT_STMT, &s);
  if( rc!=SQLITE_OK ) return rc;

  rc = sqlite3_bind_blob(s, 1, pData, nData, SQLITE_STATIC);
  if( rc!=SQLITE_OK ) return rc;

  rc = sql_step_statement(v, BLOCK_INSERT_STMT, &s);
  if( rc==SQLITE_ROW ) return SQLITE_ERROR;
  if( rc!=SQLITE_DONE ) return rc;

  *piBlockid = sqlite3_last_insert_rowid(v->db);
  return SQLITE_OK;
}

/* delete from %_segments
**   where rowid between [iStartBlockid] and [iEndBlockid]
**
** Deletes the range of blocks, inclusive, used to delete the blocks
** which form a segment.
*/
static int block_delete(fulltext_vtab *v,
                        sqlite_int64 iStartBlockid, sqlite_int64 iEndBlockid){
  sqlite3_stmt *s;
  int rc = sql_get_statement(v, BLOCK_DELETE_STMT, &s);
  if( rc!=SQLITE_OK ) return rc;

  rc = sqlite3_bind_int64(s, 1, iStartBlockid);
  if( rc!=SQLITE_OK ) return rc;

  rc = sqlite3_bind_int64(s, 2, iEndBlockid);
  if( rc!=SQLITE_OK ) return rc;

  return sql_single_step_statement(v, BLOCK_DELETE_STMT, &s);
}

/* Returns SQLITE_ROW with *pidx set to the maximum segment idx found
** at iLevel.  Returns SQLITE_DONE if there are no segments at
** iLevel.  Otherwise returns an error.
*/
static int segdir_max_index(fulltext_vtab *v, int iLevel, int *pidx){
  sqlite3_stmt *s;
  int rc = sql_get_statement(v, SEGDIR_MAX_INDEX_STMT, &s);
  if( rc!=SQLITE_OK ) return rc;

  rc = sqlite3_bind_int(s, 1, iLevel);
  if( rc!=SQLITE_OK ) return rc;

  rc = sql_step_statement(v, SEGDIR_MAX_INDEX_STMT, &s);
  /* Should always get at least one row due to how max() works. */
  if( rc==SQLITE_DONE ) return SQLITE_DONE;
  if( rc!=SQLITE_ROW ) return rc;

  /* NULL means that there were no inputs to max(). */
  if( SQLITE_NULL==sqlite3_column_type(s, 0) ){
    rc = sqlite3_step(s);
    if( rc==SQLITE_ROW ) return SQLITE_ERROR;
    return rc;
  }

  *pidx = sqlite3_column_int(s, 0);

  /* We expect only one row.  We must execute another sqlite3_step()
   * to complete the iteration; otherwise the table will remain locked. */
  rc = sqlite3_step(s);
  if( rc==SQLITE_ROW ) return SQLITE_ERROR;
  if( rc!=SQLITE_DONE ) return rc;
  return SQLITE_ROW;
}

/* insert into %_segdir values (
**   [iLevel], [idx],
**   [iStartBlockid], [iLeavesEndBlockid], [iEndBlockid],
**   [pRootData]
** )
*/
static int segdir_set(fulltext_vtab *v, int iLevel, int idx,
                      sqlite_int64 iStartBlockid,
                      sqlite_int64 iLeavesEndBlockid,
                      sqlite_int64 iEndBlockid,
                      const char *pRootData, int nRootData){
  sqlite3_stmt *s;
  int rc = sql_get_statement(v, SEGDIR_SET_STMT, &s);
  if( rc!=SQLITE_OK ) return rc;

  rc = sqlite3_bind_int(s, 1, iLevel);
  if( rc!=SQLITE_OK ) return rc;

  rc = sqlite3_bind_int(s, 2, idx);
  if( rc!=SQLITE_OK ) return rc;

  rc = sqlite3_bind_int64(s, 3, iStartBlockid);
  if( rc!=SQLITE_OK ) return rc;

  rc = sqlite3_bind_int64(s, 4, iLeavesEndBlockid);
  if( rc!=SQLITE_OK ) return rc;

  rc = sqlite3_bind_int64(s, 5, iEndBlockid);
  if( rc!=SQLITE_OK ) return rc;

  rc = sqlite3_bind_blob(s, 6, pRootData, nRootData, SQLITE_STATIC);
  if( rc!=SQLITE_OK ) return rc;

  return sql_single_step_statement(v, SEGDIR_SET_STMT, &s);
}

/* Queries %_segdir for the block span of the segments in level
** iLevel.  Returns SQLITE_DONE if there are no blocks for iLevel,
** SQLITE_ROW if there are blocks, else an error.
*/
static int segdir_span(fulltext_vtab *v, int iLevel,
                       sqlite_int64 *piStartBlockid,
                       sqlite_int64 *piEndBlockid){
  sqlite3_stmt *s;
  int rc = sql_get_statement(v, SEGDIR_SPAN_STMT, &s);
  if( rc!=SQLITE_OK ) return rc;

  rc = sqlite3_bind_int(s, 1, iLevel);
  if( rc!=SQLITE_OK ) return rc;

  rc = sql_step_statement(v, SEGDIR_SPAN_STMT, &s);
  if( rc==SQLITE_DONE ) return SQLITE_DONE;  /* Should never happen */
  if( rc!=SQLITE_ROW ) return rc;

  /* This happens if all segments at this level are entirely inline. */
  if( SQLITE_NULL==sqlite3_column_type(s, 0) ){
    /* We expect only one row.  We must execute another sqlite3_step()
     * to complete the iteration; otherwise the table will remain locked. */
    int rc2 = sqlite3_step(s);
    if( rc2==SQLITE_ROW ) return SQLITE_ERROR;
    return rc2;
  }

  *piStartBlockid = sqlite3_column_int64(s, 0);

  *piEndBlockid = sqlite3_column_int64(s, 1);

  /* We expect only one row.  We must execute another sqlite3_step()
   * to complete the iteration; otherwise the table will remain locked. */
  rc = sqlite3_step(s);
  if( rc==SQLITE_ROW ) return SQLITE_ERROR;
  if( rc!=SQLITE_DONE ) return rc;
  return SQLITE_ROW;
}

/* Delete the segment blocks and segment directory records for all

** segments at iLevel.







*/

static int segdir_delete(fulltext_vtab *v, int iLevel){






  sqlite3_stmt *s;


  sqlite_int64 iStartBlockid, iEndBlockid;


  int rc = segdir_span(v, iLevel, &iStartBlockid, &iEndBlockid);


























  if( rc!=SQLITE_ROW && rc!=SQLITE_DONE ) return rc;








  if( rc==SQLITE_ROW ){













    rc = block_delete(v, iStartBlockid, iEndBlockid);





    if( rc!=SQLITE_OK ) return rc;
  }





  /* Delete the segment directory itself. */










  rc = sql_get_statement(v, SEGDIR_DELETE_STMT, &s);
  if( rc!=SQLITE_OK ) return rc;




  rc = sqlite3_bind_int64(s, 1, iLevel);
  if( rc!=SQLITE_OK ) return rc;

  return sql_single_step_statement(v, SEGDIR_DELETE_STMT, &s);











}

/*
** Free the memory used to contain a fulltext_vtab structure.
*/
static void fulltext_vtab_destroy(fulltext_vtab *v){
  int iStmt, i;

  TRACE(("FTS2 Destroy %p\n", v));
  for( iStmt=0; iStmt<MAX_STMT; iStmt++ ){
    if( v->pFulltextStatements[iStmt]!=NULL ){
      sqlite3_finalize(v->pFulltextStatements[iStmt]);
      v->pFulltextStatements[iStmt] = NULL;
    }
  }

  for( i=0; i<MERGE_COUNT; i++ ){
    if( v->pLeafSelectStmts[i]!=NULL ){
      sqlite3_finalize(v->pLeafSelectStmts[i]);
      v->pLeafSelectStmts[i] = NULL;
    }
  }

  if( v->pTokenizer!=NULL ){
    v->pTokenizer->pModule->xDestroy(v->pTokenizer);
    v->pTokenizer = NULL;
  }
  
  free(v->azColumn);

  if( rc!=SQLITE_OK ) return rc;

  rc = constructVtab(db, &spec, ppVTab, pzErr);
  clearTableSpec(&spec);
  return rc;
}

/* The %_content table holds the text of each document, with
** the rowid used as the docid.



























*/
/* TODO(shess) This comment needs elaboration to match the updated



** code.  Work it into the top-of-file comment at that time.
*/
static int fulltextCreate(sqlite3 *db, void *pAux,
                          int argc, const char * const *argv,
                          sqlite3_vtab **ppVTab, char **pzErr){
  int rc;
  TableSpec spec;
  StringBuffer schema;
  TRACE(("FTS2 Create\n"));

  rc = parseSpec(&spec, argc, argv, pzErr);
  if( rc!=SQLITE_OK ) return rc;

  initStringBuffer(&schema);
  append(&schema, "CREATE TABLE %_content(");
  appendList(&schema, spec.nColumn, spec.azContentColumn);
  append(&schema, ")");
  rc = sql_exec(db, spec.zName, schema.s);
  free(schema.s);
  if( rc!=SQLITE_OK ) goto out;

  rc = sql_exec(db, spec.zName, "create table %_segments(block blob);");
  if( rc!=SQLITE_OK ) goto out;

  rc = sql_exec(db, spec.zName,
                "create table %_segdir("
                "  level integer,"
                "  idx integer,"
                "  start_block integer,"
                "  leaves_end_block integer,"
                "  end_block integer,"
                "  root blob,"
                "  primary key(level, idx)"
                ");");
  if( rc!=SQLITE_OK ) goto out;

  rc = constructVtab(db, &spec, ppVTab, pzErr);

out:
  clearTableSpec(&spec);
  return rc;

static int fulltextDestroy(sqlite3_vtab *pVTab){
  fulltext_vtab *v = (fulltext_vtab *)pVTab;
  int rc;

  TRACE(("FTS2 Destroy %p\n", pVTab));
  rc = sql_exec(v->db, v->zName,
                "drop table %_content;"
                "drop table %_segments;"
                "drop table %_segdir;"
                );
  if( rc!=SQLITE_OK ) return rc;

  fulltext_vtab_destroy((fulltext_vtab *)pVTab);
  return SQLITE_OK;
}

static int fulltextOpen(sqlite3_vtab *pVTab, sqlite3_vtab_cursor **ppCursor){

      return SQLITE_OK;
    }
    /* an error occurred; abort */
    return rc==SQLITE_DONE ? SQLITE_ERROR : rc;
  }
}


/* TODO(shess) If we pushed LeafReader to the top of the file, or to
** another file, term_select() could be pushed above
** docListOfTerm().
*/
static int termSelect(fulltext_vtab *v, int iColumn,
                      const char *pTerm, int nTerm, DocList *out);

/* Return a DocList corresponding to the query term *pTerm.  If *pTerm
** is the first term of a phrase query, go ahead and evaluate the phrase
** query and return the doclist for the entire phrase query.
**
** The result is stored in pTerm->doclist.
*/
static int docListOfTerm(
  fulltext_vtab *v,     /* The full text index */
  int iColumn,          /* column to restrict to.  No restrition if >=nColumn */
  QueryTerm *pQTerm,    /* Term we are looking for, or 1st term of a phrase */
  DocList **ppResult    /* Write the result here */
){
  DocList *pLeft, *pRight, *pNew;
  int i, rc;

  pLeft = docListNew(DL_POSITIONS);
  rc = termSelect(v, iColumn, pQTerm->pTerm, pQTerm->nTerm, pLeft);
  if( rc ) return rc;
  for(i=1; i<=pQTerm->nPhrase; i++){
    pRight = docListNew(DL_POSITIONS);
    rc = termSelect(v, iColumn, pQTerm[i].pTerm, pQTerm[i].nTerm, pRight);
    if( rc ){
      docListDelete(pLeft);
      return rc;
    }
    pNew = docListNew(i<pQTerm->nPhrase ? DL_POSITIONS : DL_DOCIDS);
    docListPhraseMerge(pLeft, pRight, pNew);
    docListDelete(pLeft);

  ** though one could argue that failure there means that the data is
  ** not durable.  *ponder*
  */
  pTokenizer->pModule->xClose(pCursor);
  return rc;
}




































































/* Add doclists for all terms in [pValues] to the hash table [terms]. */
static int insertTerms(fulltext_vtab *v, fts2Hash *terms, sqlite_int64 iRowid,
                sqlite3_value **pValues){
  int i;
  for(i = 0; i < v->nColumn ; ++i){
    char *zText = (char*)sqlite3_value_text(pValues[i]);
    int rc = buildTerms(v, terms, iRowid, zText, i);
    if( rc!=SQLITE_OK ) return rc;
  }
  return SQLITE_OK;
}

/* Add empty doclists for all terms in the given row's content to the hash
 * table [pTerms]. */
static int deleteTerms(fulltext_vtab *v, fts2Hash *pTerms, sqlite_int64 iRowid){
  const char **pValues;
  int i, rc;

  /* TODO(shess) Should we allow such tables at all? */
  if( DL_DEFAULT==DL_DOCIDS ) return SQLITE_ERROR;

  rc = content_select(v, iRowid, &pValues);
  if( rc!=SQLITE_OK ) return rc;

  for(i = 0 ; i < v->nColumn; ++i) {
    rc = buildTerms(v, pTerms, iRowid, pValues[i], -1);
    if( rc!=SQLITE_OK ) break;
  }

  /* Now add positions for terms which appear in the updated row. */
  rc = insertTerms(v, pTerms, iRow, pValues);
  if( rc!=SQLITE_OK ) return rc;

  return content_update(v, pValues, iRow);  /* execute an SQL UPDATE */
}

/*******************************************************************/
/* InteriorWriter is used to collect terms and block references into
** interior nodes in %_segments.  See commentary at top of file for
** format.
*/

/* How large interior nodes can grow. */
#define INTERIOR_MAX 2048

/* ROOT_MAX controls how much data is stored inline in the segment
** directory.
*/
/* TODO(shess) Push ROOT_MAX down to whoever is writing things.  It's
** only here so that interiorWriterRootInfo() and leafWriterRootInfo()
** can both see it, but if the caller passed it in, we wouldn't even
** need a define.
*/
#define ROOT_MAX 1024
#if ROOT_MAX<VARINT_MAX*2
# error ROOT_MAX must have enough space for a header.
#endif

/* InteriorBlock stores a linked-list of interior blocks while a lower
** layer is being constructed.
*/
typedef struct InteriorBlock {
  char *pTerm;               /* Leftmost term in block's subtree. */
  int nTerm;

  char *pData;
  int nData;

  struct InteriorBlock *next;
} InteriorBlock;

static InteriorBlock *interiorBlockNew(int iHeight, sqlite_int64 iChildBlock,
                                       const char *pTerm, int nTerm){
  InteriorBlock *block = calloc(1, sizeof(InteriorBlock));
  char c[VARINT_MAX+VARINT_MAX];
  int n;

  data_dup(&block->pTerm, &block->nTerm, pTerm, nTerm);

  n = putVarint(c, iHeight);
  n += putVarint(c+n, iChildBlock);
  data_dup(&block->pData, &block->nData, c, n);

  return block;
}

typedef struct InteriorWriter {
  int iHeight;                   /* from 0 at leaves. */
  InteriorBlock *first, *last;
  struct InteriorWriter *parentWriter;

#ifndef NDEBUG
  sqlite_int64 iLastChildBlock;  /* for consistency checks. */
#endif
} InteriorWriter;

/* Initialize an interior node where pTerm[nTerm] marks the leftmost
** term in the tree.  iChildBlock is the leftmost child block at the
** next level down the tree.
*/
static void interiorWriterInit(int iHeight, const char *pTerm, int nTerm,
                               sqlite_int64 iChildBlock,
                               InteriorWriter *pWriter){
  InteriorBlock *block;
  assert( iHeight>0 );
  memset(pWriter, 0, sizeof(*pWriter));

  pWriter->iHeight = iHeight;
#ifndef NDEBUG
  pWriter->iLastChildBlock = iChildBlock;
#endif
  block = interiorBlockNew(iHeight, iChildBlock, pTerm, nTerm);
  pWriter->last = pWriter->first = block;
}

/* Append the child node rooted at iChildBlock to the interior node,
** with pTerm[nTerm] as the leftmost term in iChildBlock's subtree.
*/
static void interiorWriterAppend(InteriorWriter *pWriter,
                                 const char *pTerm, int nTerm,
                                 sqlite_int64 iChildBlock){
  char c[VARINT_MAX+VARINT_MAX];
  int n = putVarint(c, nTerm);

#ifndef NDEBUG
  pWriter->iLastChildBlock++;
#endif
  assert( pWriter->iLastChildBlock==iChildBlock );

  if( pWriter->last->nData+n+nTerm>INTERIOR_MAX ){
    /* Overflow to a new block. */
    pWriter->last->next = interiorBlockNew(pWriter->iHeight, iChildBlock,
                                           pTerm, nTerm);
    pWriter->last = pWriter->last->next;
  }else{
    InteriorBlock *last = pWriter->last;
    data_append2(&last->pData, &last->nData, c, n, pTerm, nTerm);
  }
}

/* Free the space used by pWriter, including the linked-list of
** InteriorBlocks.
*/
static int interiorWriterDestroy(InteriorWriter *pWriter){
  InteriorBlock *block = pWriter->first;

  while( block!=NULL ){
    InteriorBlock *b = block;
    block = block->next;
    free(b->pData);
    free(b->pTerm);
    free(b);
  }
#ifndef NDEBUG
  memset(pWriter, 0x55, sizeof(pWriter));
#endif
  return SQLITE_OK;
}

/* If pWriter can fit entirely in ROOT_MAX, return it as the root info
** directly, leaving *piEndBlockid unchanged.  Otherwise, flush
** pWriter to %_segments, building a new layer of interior nodes, and
** recursively ask for their root into.
*/
static int interiorWriterRootInfo(fulltext_vtab *v, InteriorWriter *pWriter,
                                  char **ppRootInfo, int *pnRootInfo,
                                  sqlite_int64 *piEndBlockid){
  InteriorBlock *block = pWriter->first;
  sqlite_int64 iBlockid = 0;
  int rc;

  /* If we can fit the segment inline */
  if( block==pWriter->last && block->nData<ROOT_MAX ){
    *ppRootInfo = block->pData;
    *pnRootInfo = block->nData;
    return SQLITE_OK;
  }

  /* Flush the first block to %_segments, and create a new level of
  ** interior node.
  */
  rc = block_insert(v, block->pData, block->nData, &iBlockid);
  if( rc!=SQLITE_OK ) return rc;
  *piEndBlockid = iBlockid;

  pWriter->parentWriter = malloc(sizeof(*pWriter->parentWriter));
  interiorWriterInit(pWriter->iHeight+1,
                     block->pTerm, block->nTerm,
                     iBlockid, pWriter->parentWriter);

  /* Flush additional blocks and append to the higher interior
  ** node.
  */
  for(block=block->next; block!=NULL; block=block->next){
    rc = block_insert(v, block->pData, block->nData, &iBlockid);
    if( rc!=SQLITE_OK ) return rc;
    *piEndBlockid = iBlockid;

    interiorWriterAppend(pWriter->parentWriter,
                         block->pTerm, block->nTerm, iBlockid);
  }

  /* Parent node gets the chance to be the root. */
  return interiorWriterRootInfo(v, pWriter->parentWriter,
                                ppRootInfo, pnRootInfo, piEndBlockid);
}

/****************************************************************/
/* InteriorReader is used to read off the data from an interior node
** (see comment at top of file for the format).  InteriorReader does
** not own its data, so interiorReaderDestroy() is a formality.
*/
typedef struct InteriorReader {
  const char *pData;
  int nData;

  sqlite_int64 iBlockid;
} InteriorReader;

static void interiorReaderDestroy(InteriorReader *pReader){
#ifndef NDEBUG
  memset(pReader, 0x55, sizeof(pReader));
#endif
}

static void interiorReaderInit(const char *pData, int nData,
                               InteriorReader *pReader){
  int n;

  /* Require at least the leading flag byte */
  assert( nData>0 );
  assert( pData[0]!='\0' );

  memset(pReader, '\0', sizeof(pReader));

  /* Decode the base blockid, and set the cursor to the first term. */
  n = getVarint(pData+1, &pReader->iBlockid);
  assert( 1+n<=nData );
  pReader->pData = pData+1+n;
  pReader->nData = nData-(1+n);
}

static int interiorReaderAtEnd(InteriorReader *pReader){
  return pReader->nData<=0;
}

static sqlite_int64 interiorReaderCurrentBlockid(InteriorReader *pReader){
  return pReader->iBlockid;
}

static int interiorReaderTermBytes(InteriorReader *pReader){
  int nTerm;
  assert( !interiorReaderAtEnd(pReader) );
  getVarint32(pReader->pData, &nTerm);
  return nTerm;
}
static const char *interiorReaderTerm(InteriorReader *pReader){
  int n, nTerm;
  assert( !interiorReaderAtEnd(pReader) );
  n = getVarint32(pReader->pData, &nTerm);
  return pReader->pData+n;
}

/* Step forward to the next term in the node. */
static void interiorReaderStep(InteriorReader *pReader){
  int n, nTerm;
  assert( !interiorReaderAtEnd(pReader) );
  n = getVarint32(pReader->pData, &nTerm);
  assert( n+nTerm<=pReader->nData );
  pReader->pData += n+nTerm;
  pReader->nData -= n+nTerm;
  pReader->iBlockid++;
}

/* Compare the current term to pTerm[nTerm], returning strcmp-style
** results.
*/
static int interiorReaderTermCmp(InteriorReader *pReader,
                                 const char *pTerm, int nTerm){
  const char *pReaderTerm = interiorReaderTerm(pReader);
  int nReaderTerm = interiorReaderTermBytes(pReader);
  int c, n = nReaderTerm<nTerm ? nReaderTerm : nTerm;

  if( n==0 ){
    if( nReaderTerm>0 ) return -1;
    if( nTerm>0 ) return 1;
    return 0;
  }

  c = memcmp(pReaderTerm, pTerm, n);
  if( c!=0 ) return c;
  return nReaderTerm - nTerm;
}

/****************************************************************/
/* LeafWriter is used to collect terms and associated doclist data
** into leaf blocks in %_segments (see top of file for format info).
*/

/* Put terms with data this big in their own block. */
#define STANDALONE_MIN 1024

/* Keep leaf blocks below this size. */
#define LEAF_MAX 2048

typedef struct LeafWriter {
  int iLevel;
  int idx;
  sqlite_int64 iStartBlockid;     /* needed to create the root info */
  sqlite_int64 iEndBlockid;       /* when we're done writing. */

  char *pTerm;                    /* previous encoded term */
  int nTerm;

  char *pData;                    /* encoding buffer */
  int nData;

  InteriorWriter parentWriter;    /* if we overflow */
  int has_parent;
} LeafWriter;

static void leafWriterInit(int iLevel, int idx, LeafWriter *pWriter){
  memset(pWriter, 0, sizeof(*pWriter));
  pWriter->iLevel = iLevel;
  pWriter->idx = idx;

  /* Start out with a reasonably sized block, though it can grow. */
  pWriter->pData = malloc(LEAF_MAX);
  pWriter->nData = putVarint(pWriter->pData, 0);
}

/* Flush the current leaf node to %_segments, and adding the resulting
** blockid and the starting term to the interior node which will
** contain it.
*/
static int leafWriterInternalFlush(fulltext_vtab *v, LeafWriter *pWriter){
  sqlite_int64 iBlockid = 0;
  const char *pStartingTerm;
  int nStartingTerm, rc, n;

  /* Must have the leading varint(0) flag, plus at least some data. */
  assert( pWriter->nData>2 );

  rc = block_insert(v, pWriter->pData, pWriter->nData, &iBlockid);
  if( rc!=SQLITE_OK ) return rc;
  assert( iBlockid!=0 );

  /* Reconstruct the first term in the leaf for purposes of building
  ** the interior node.
  */
  n = getVarint32(pWriter->pData+1, &nStartingTerm);
  pStartingTerm = pWriter->pData+1+n;
  assert( pWriter->nData>1+n+nStartingTerm );

  if( pWriter->has_parent ){
    interiorWriterAppend(&pWriter->parentWriter,
                         pStartingTerm, nStartingTerm, iBlockid);
  }else{
    interiorWriterInit(1, pStartingTerm, nStartingTerm, iBlockid,
                       &pWriter->parentWriter);
    pWriter->has_parent = 1;
  }

  /* Track the span of this segment's leaf nodes. */
  if( pWriter->iEndBlockid==0 ){
    pWriter->iEndBlockid = pWriter->iStartBlockid = iBlockid;
  }else{
    pWriter->iEndBlockid++;
    assert( iBlockid==pWriter->iEndBlockid );
  }

  /* Re-initialize the output buffer. */
  pWriter->nData = putVarint(pWriter->pData, 0);
  pWriter->nTerm = 0;

  return SQLITE_OK;
}

/* Fetch the root info for the segment.  If the entire leaf fits
** within ROOT_MAX, then it will be returned directly, otherwise it
** will be flushed and the root info will be returned from the
** interior node.  *piEndBlockid is set to the blockid of the last
** interior or leaf node written to disk (0 if none are written at
** all).
*/
static int leafWriterRootInfo(fulltext_vtab *v, LeafWriter *pWriter,
                              char **ppRootInfo, int *pnRootInfo,
                              sqlite_int64 *piEndBlockid){
  /* we can fit the segment entirely inline */
  if( !pWriter->has_parent && pWriter->nData<ROOT_MAX ){
    *ppRootInfo = pWriter->pData;
    *pnRootInfo = pWriter->nData;
    *piEndBlockid = 0;
    return SQLITE_OK;
  }

  /* Flush remaining leaf data. */
  if( pWriter->nData>1 ){
    int rc = leafWriterInternalFlush(v, pWriter);
    if( rc!=SQLITE_OK ) return rc;
  }

  /* We must have flushed a leaf at some point. */
  assert( pWriter->has_parent );

  /* Tenatively set the end leaf blockid as the end blockid.  If the
  ** interior node can be returned inline, this will be the final
  ** blockid, otherwise it will be overwritten by
  ** interiorWriterRootInfo().
  */
  *piEndBlockid = pWriter->iEndBlockid;

  return interiorWriterRootInfo(v, &pWriter->parentWriter,
                                ppRootInfo, pnRootInfo, piEndBlockid);
}

/* Collect the rootInfo data and store it into the segment directory.
** This has the effect of flushing the segment's leaf data to
** %_segments, and also flushing any interior nodes to %_segments.
*/
static int leafWriterFlush(fulltext_vtab *v, LeafWriter *pWriter){
  sqlite_int64 iEndBlockid;
  char *pRootInfo;
  int rc, nRootInfo;

  rc = leafWriterRootInfo(v, pWriter, &pRootInfo, &nRootInfo, &iEndBlockid);
  if( rc!=SQLITE_OK ) return rc;

  return segdir_set(v, pWriter->iLevel, pWriter->idx,
                    pWriter->iStartBlockid, pWriter->iEndBlockid,
                    iEndBlockid, pRootInfo, nRootInfo);
}

static void leafWriterDestroy(LeafWriter *pWriter){
  if( pWriter->has_parent ) interiorWriterDestroy(&pWriter->parentWriter);
  free(pWriter->pTerm);
  free(pWriter->pData);
}

/* Push pTerm[nTerm] along with the doclist data to the leaf layer of
** %_segments.
*/
static int leafWriterStep(fulltext_vtab *v, LeafWriter *pWriter,
                          const char *pTerm, int nTerm, DocList *doclist){
  char c[VARINT_MAX+VARINT_MAX];
  int rc, n;

  /* Flush existing data if this item won't fit well. */
  if( pWriter->nData>1 &&
      (doclist->nData+nTerm>STANDALONE_MIN ||
       pWriter->nData+doclist->nData+nTerm>LEAF_MAX) ){
    rc = leafWriterInternalFlush(v, pWriter);
    if( rc!=SQLITE_OK ) return rc;
  }

  if( pWriter->nTerm==0 ){
    /* Encode the entire leading term as:
    **  varint(nTerm)
    **  char pTerm[nTerm]
    */
    n = putVarint(c, nTerm);
    assert( pWriter->nData==1 );
    data_append2(&pWriter->pData, &pWriter->nData,
                 c, n, pTerm, nTerm);
  }else{
    /* Delta-encode the term as:
    **  varint(nPrefix)
    **  varint(nSuffix)
    **  char pTermSuffix[nSuffix]
    */
    int nPrefix = 0;

    while( nPrefix<nTerm && nPrefix<pWriter->nTerm &&
           pTerm[nPrefix]==pWriter->pTerm[nPrefix] ){
      nPrefix++;
    }

    n = putVarint(c, nPrefix);
    n += putVarint(c+n, nTerm-nPrefix);

    data_append2(&pWriter->pData, &pWriter->nData,
                 c, n, pTerm+nPrefix, nTerm-nPrefix);
  }
  data_replace(&pWriter->pTerm, &pWriter->nTerm, pTerm, nTerm);

  /* Encode the doclist as:
  **  varint(nDoclist)
  **  char pDoclist[nDoclist]
  */
  n = putVarint(c, doclist->nData);
  data_append2(&pWriter->pData, &pWriter->nData,
               c, n, doclist->pData, doclist->nData);

  /* Flush standalone blocks right out */
  if( doclist->nData+nTerm>STANDALONE_MIN ){
    rc = leafWriterInternalFlush(v, pWriter);
    if( rc!=SQLITE_OK ) return rc;
  }

  return SQLITE_OK;
}


/****************************************************************/
/* LeafReader is used to iterate over an individual leaf node. */
typedef struct LeafReader {
  char *pTerm;              /* copy of current term. */
  int nTerm;

  const char *pData;        /* data for current term. */
  int nData;
} LeafReader;

static void leafReaderDestroy(LeafReader *pReader){
  free(pReader->pTerm);
#ifndef NDEBUG
  memset(pReader, 0x55, sizeof(pReader));
#endif
}

static int leafReaderAtEnd(LeafReader *pReader){
  return pReader->nData<=0;
}

/* Access the current term. */
static int leafReaderTermBytes(LeafReader *pReader){
  return pReader->nTerm;
}
static const char *leafReaderTerm(LeafReader *pReader){
  assert( pReader->nTerm>0 );
  return pReader->pTerm;
}

/* Access the doclist data for the current term. */
static int leafReaderDataBytes(LeafReader *pReader){
  int nData;
  assert( pReader->nTerm>0 );
  getVarint32(pReader->pData, &nData);
  return nData;
}
static const char *leafReaderData(LeafReader *pReader){
  int n, nData;
  assert( pReader->nTerm>0 );
  n = getVarint32(pReader->pData, &nData);
  return pReader->pData+n;
}

static void leafReaderInit(const char *pData, int nData,
                           LeafReader *pReader){
  int nTerm, n;

  assert( nData>0 );
  assert( pData[0]=='\0' );

  memset(pReader, '\0', sizeof(pReader));

  /* Read the first term, skipping the header byte. */
  n = getVarint32(pData+1, &nTerm);
  data_dup(&pReader->pTerm, &pReader->nTerm, pData+1+n, nTerm);

  /* Position after the first term. */
  assert( 1+n+nTerm<nData );
  pReader->pData = pData+1+n+nTerm;
  pReader->nData = nData-1-n-nTerm;
}

/* Step the reader forward to the next term. */
static void leafReaderStep(LeafReader *pReader){
  int n, nData, nPrefix, nSuffix;
  assert( !leafReaderAtEnd(pReader) );

  /* Skip previous entry's data block. */
  n = getVarint32(pReader->pData, &nData);
  assert( n+nData<=pReader->nData );
  pReader->pData += n+nData;
  pReader->nData -= n+nData;

  if( !leafReaderAtEnd(pReader) ){
    /* Construct the new term using a prefix from the old term plus a
    ** suffix from the leaf data.
    */
    n = getVarint32(pReader->pData, &nPrefix);
    n += getVarint32(pReader->pData+n, &nSuffix);
    assert( n+nSuffix<pReader->nData );
    pReader->nTerm = nPrefix;
    data_append(&pReader->pTerm, &pReader->nTerm, pReader->pData+n, nSuffix);

    pReader->pData += n+nSuffix;
    pReader->nData -= n+nSuffix;
  }
}

/* strcmp-style comparison of pReader's current term against pTerm. */
static int leafReaderTermCmp(LeafReader *pReader,
                             const char *pTerm, int nTerm){
  int c, n = pReader->nTerm<nTerm ? pReader->nTerm : nTerm;
  if( n==0 ){
    if( pReader->nTerm>0 ) return -1;
    if(nTerm>0 ) return 1;
    return 0;
  }

  c = memcmp(pReader->pTerm, pTerm, n);
  if( c!=0 ) return c;
  return pReader->nTerm - nTerm;
}


/****************************************************************/
/* LeavesReader wraps LeafReader to allow iterating over the entire
** leaf layer of the tree.
*/
typedef struct LeavesReader {
  int idx;                  /* Index within the segment. */

  sqlite3_stmt *pStmt;      /* Statement we're streaming leaves from. */
  int eof;                  /* we've seen SQLITE_DONE from pStmt. */

  LeafReader leafReader;    /* reader for the current leaf. */
  char *pRootData;          /* root data for inline. */
} LeavesReader;

/* Access the current term. */
static int leavesReaderTermBytes(LeavesReader *pReader){
  assert( !pReader->eof );
  return leafReaderTermBytes(&pReader->leafReader);
}
static const char *leavesReaderTerm(LeavesReader *pReader){
  assert( !pReader->eof );
  return leafReaderTerm(&pReader->leafReader);
}

/* Access the doclist data for the current term. */
static int leavesReaderDataBytes(LeavesReader *pReader){
  assert( !pReader->eof );
  return leafReaderDataBytes(&pReader->leafReader);
}
static const char *leavesReaderData(LeavesReader *pReader){
  assert( !pReader->eof );
  return leafReaderData(&pReader->leafReader);
}

static int leavesReaderAtEnd(LeavesReader *pReader){
  return pReader->eof;
}

static void leavesReaderDestroy(LeavesReader *pReader){
  leafReaderDestroy(&pReader->leafReader);
  if( pReader->pRootData!=0 ) free(pReader->pRootData);
#ifndef NDEBUG
  memset(pReader, 0x55, sizeof(pReader));
#endif
}

/* Initialize pReader with the given root data (if iStartBlockid==0
** the leaf data was entirely contained in the root), or from the
** stream of blocks between iStartBlockid and iEndBlockid, inclusive.
*/
static int leavesReaderInit(fulltext_vtab *v,
                            int idx,
                            sqlite_int64 iStartBlockid,
                            sqlite_int64 iEndBlockid,
                            const char *pRootData, int nRootData,
                            LeavesReader *pReader){
  memset(pReader, 0, sizeof(*pReader));
  pReader->idx = idx;

  if( iStartBlockid==0 ){
    /* Entire leaf level fit in root data. */
    int n;
    data_dup(&pReader->pRootData, &n, pRootData, nRootData);
    leafReaderInit(pReader->pRootData, nRootData, &pReader->leafReader);
  }else{
    sqlite3_stmt *s;
    int rc = sql_get_leaf_statement(v, idx, &s);
    if( rc!=SQLITE_OK ) return rc;

    rc = sqlite3_bind_int64(s, 1, iStartBlockid);
    if( rc!=SQLITE_OK ) return rc;

    rc = sqlite3_bind_int64(s, 2, iEndBlockid);
    if( rc!=SQLITE_OK ) return rc;

    rc = sql_step_leaf_statement(v, idx, &s);
    if( rc==SQLITE_DONE ){
      pReader->eof = 1;
      return SQLITE_OK;
    }
    if( rc!=SQLITE_ROW ) return rc;

    pReader->pStmt = s;
    leafReaderInit(sqlite3_column_blob(pReader->pStmt, 0),
                   sqlite3_column_bytes(pReader->pStmt, 0),
                   &pReader->leafReader);
  }
  return SQLITE_OK;
}

/* Step the current leaf forward to the next term.  If we reach the
** end of the current leaf, step forward to the next leaf block.
*/
static int leavesReaderStep(fulltext_vtab *v, LeavesReader *pReader){
  assert( !leavesReaderAtEnd(pReader) );
  leafReaderStep(&pReader->leafReader);

  if( leafReaderAtEnd(&pReader->leafReader) ){
    int rc;
    if( pReader->pRootData ){
      pReader->eof = 1;
      return SQLITE_OK;
    }
    rc = sql_step_leaf_statement(v, pReader->idx, &pReader->pStmt);
    if( rc!=SQLITE_ROW ){
      pReader->eof = 1;
      return rc==SQLITE_DONE ? SQLITE_OK : rc;
    }
    leafReaderDestroy(&pReader->leafReader);
    leafReaderInit(sqlite3_column_blob(pReader->pStmt, 0),
                   sqlite3_column_bytes(pReader->pStmt, 0),
                   &pReader->leafReader);
  }
  return SQLITE_OK;
}

/* Order LeavesReaders by their term, ignoring idx.  Readers at eof
** always sort to the end.
*/
static int leavesReaderTermCmp(LeavesReader *lr1, LeavesReader *lr2){
  if( leavesReaderAtEnd(lr1) ){
    if( leavesReaderAtEnd(lr2) ) return 0;
    return 1;
  }
  if( leavesReaderAtEnd(lr2) ) return -1;

  return leafReaderTermCmp(&lr1->leafReader,
                           leavesReaderTerm(lr2), leavesReaderTermBytes(lr2));
}

/* Similar to leavesReaderTermCmp(), with additional ordering by idx
** so that older segments sort before newer segments.
*/
static int leavesReaderCmp(LeavesReader *lr1, LeavesReader *lr2){
  int c = leavesReaderTermCmp(lr1, lr2);
  if( c!=0 ) return c;
  return lr1->idx-lr2->idx;
}

/* Assume that pLr[1]..pLr[nLr] are sorted.  Bubble pLr[0] into its
** sorted position.
*/
static void leavesReaderReorder(LeavesReader *pLr, int nLr){
  while( nLr>1 && leavesReaderCmp(pLr, pLr+1)>0 ){
    LeavesReader tmp = pLr[0];
    pLr[0] = pLr[1];
    pLr[1] = tmp;
    nLr--;
    pLr++;
  }
}

/* Initializes pReaders with the segments from level iLevel, returning
** the number of segments in *piReaders.  Leaves pReaders in sorted
** order.
*/
static int leavesReadersInit(fulltext_vtab *v, int iLevel,
                             LeavesReader *pReaders, int *piReaders){
  sqlite3_stmt *s;
  int i, rc = sql_get_statement(v, SEGDIR_SELECT_STMT, &s);
  if( rc!=SQLITE_OK ) return rc;

  rc = sqlite3_bind_int(s, 1, iLevel);
  if( rc!=SQLITE_OK ) return rc;

  i = 0;
  while( (rc = sql_step_statement(v, SEGDIR_SELECT_STMT, &s))==SQLITE_ROW ){
    sqlite_int64 iStart = sqlite3_column_int64(s, 0);
    sqlite_int64 iEnd = sqlite3_column_int64(s, 1);
    const char *pRootData = sqlite3_column_blob(s, 2);
    int nRootData = sqlite3_column_bytes(s, 2);

    assert( i<MERGE_COUNT );
    rc = leavesReaderInit(v, i, iStart, iEnd, pRootData, nRootData,
                          &pReaders[i]);
    if( rc!=SQLITE_OK ) break;

    i++;
  }
  if( rc!=SQLITE_DONE ){
    while( i-->0 ){
      leavesReaderDestroy(&pReaders[i]);
    }
    return rc;
  }

  *piReaders = i;

  /* Leave our results sorted by term, then age. */
  while( i-- ){
    leavesReaderReorder(pReaders+i, *piReaders-i);
  }
  return SQLITE_OK;
}

/* Merge doclists from pReaders[nReaders] into a single doclist, which
** is written to pWriter.  Assumes pReaders is ordered oldest to
** newest.
*/
/* TODO(shess) I have a version of this that merges the doclists
** pairwise, and is thus much faster, but is also more intricate.  So
** I'll throw that in as a standalone change.  N-way merge would be
** even faster.
*/
static int leavesReadersMerge(fulltext_vtab *v,
                              LeavesReader *pReaders, int nReaders,
                              LeafWriter *pWriter){
  const char *pTerm = leavesReaderTerm(pReaders);
  int i, rc, nTerm = leavesReaderTermBytes(pReaders);
  DocList doclist;

  /* No need to merge, insert directly. */
  if( nReaders==1 ){
    docListStaticInit(&doclist, DL_DEFAULT,
                      leavesReaderData(pReaders),
                      leavesReaderDataBytes(pReaders));
  }else{
    docListInit(&doclist, DL_DEFAULT,
                leavesReaderData(pReaders),
                leavesReaderDataBytes(pReaders));

    for(i=1; i<nReaders; i++){
      DocList new, merged;
      docListStaticInit(&new, DL_DEFAULT,
                        leavesReaderData(pReaders+i),
                        leavesReaderDataBytes(pReaders+i));
      docListMerge(&merged, &doclist, &new);
      docListDestroy(&doclist);
      doclist = merged;
    }
  }

  /* Insert the new doclist */
  rc = leafWriterStep(v, pWriter, pTerm, nTerm, &doclist);
  if( nReaders>1 ) docListDestroy(&doclist);
  return rc;
}

/* Forward ref due to mutual recursion with segdirNextIndex(). */
static int segmentMerge(fulltext_vtab *v, int iLevel);

/* Put the next available index at iLevel into *pidx.  If iLevel
** already has MERGE_COUNT segments, they are merged to a higher
** level to make room.
*/
static int segdirNextIndex(fulltext_vtab *v, int iLevel, int *pidx){
  int rc = segdir_max_index(v, iLevel, pidx);
  if( rc==SQLITE_DONE ){              /* No segments at iLevel. */
    *pidx = 0;
  }else if( rc==SQLITE_ROW ){
    if( *pidx==(MERGE_COUNT-1) ){
      rc = segmentMerge(v, iLevel);
      if( rc!=SQLITE_OK ) return rc;
      *pidx = 0;
    }else{
      (*pidx)++;
    }
  }else{
    return rc;
  }
  return SQLITE_OK;
}

/* Merge MERGE_COUNT segments at iLevel into a new segment at
** iLevel+1.  If iLevel+1 is already full of segments, those will be
** merged to make room.
*/
static int segmentMerge(fulltext_vtab *v, int iLevel){
  LeafWriter writer;
  LeavesReader lrs[MERGE_COUNT];
  int i, rc, idx = 0;

  /* Determine the next available segment index at the next level,
  ** merging as necessary.
  */
  rc = segdirNextIndex(v, iLevel+1, &idx);
  if( rc!=SQLITE_OK ) return rc;

  /* TODO(shess) This assumes that we'll always see exactly
  ** MERGE_COUNT segments to merge at a given level.  That will be
  ** broken if we allow the developer to request preemptive or
  ** deferred merging.
  */
  memset(&lrs, '\0', sizeof(lrs));
  rc = leavesReadersInit(v, iLevel, lrs, &i);
  if( rc!=SQLITE_OK ) return rc;
  assert( i==MERGE_COUNT );

  leafWriterInit(iLevel+1, idx, &writer);

  /* Since leavesReaderReorder() pushes readers at eof to the end,
  ** when the first reader is empty, all will be empty.
  */
  while( !leavesReaderAtEnd(lrs) ){
    /* Figure out how many readers share their next term. */
    for(i=1; i<MERGE_COUNT && !leavesReaderAtEnd(lrs+i); i++){
      if( 0!=leavesReaderTermCmp(lrs, lrs+i) ) break;
    }

    rc = leavesReadersMerge(v, lrs, i, &writer);
    if( rc!=SQLITE_OK ) goto err;

    /* Step forward those that were merged. */
    while( i-->0 ){
      rc = leavesReaderStep(v, lrs+i);
      if( rc!=SQLITE_OK ) goto err;

      /* Reorder by term, then by age. */
      leavesReaderReorder(lrs+i, MERGE_COUNT-i);
    }
  }

  for(i=0; i<MERGE_COUNT; i++){
    leavesReaderDestroy(&lrs[i]);
  }

  rc = leafWriterFlush(v, &writer);
  leafWriterDestroy(&writer);
  if( rc!=SQLITE_OK ) return rc;

  /* Delete the merged segment data. */
  return segdir_delete(v, iLevel);

 err:
  for(i=0; i<MERGE_COUNT; i++){
    leavesReaderDestroy(&lrs[i]);
  }
  leafWriterDestroy(&writer);
  return rc;
}

/* Read pData[nData] as a leaf node, and if the doclist for
** pTerm[nTerm] is present, merge it over *out (any duplicate doclists
** read from pData will overwrite those in *out).
*/
static int loadSegmentLeaf(fulltext_vtab *v, const char *pData, int nData,
                           const char *pTerm, int nTerm, DocList *out){
  LeafReader reader;
  assert( nData>1 );
  assert( *pData=='\0' );

  leafReaderInit(pData, nData, &reader);
  while( !leafReaderAtEnd(&reader) ){
    int c = leafReaderTermCmp(&reader, pTerm, nTerm);
    if( c==0 ){
      DocList new, doclist;
      docListStaticInit(&new, DL_DEFAULT,
                        leafReaderData(&reader), leafReaderDataBytes(&reader));
      docListMerge(&doclist, out, &new);
      docListDestroy(out);
      *out = doclist;
    }
    if( c>=0 ) break;
    leafReaderStep(&reader);
  }
  leafReaderDestroy(&reader);
  return SQLITE_OK;
}

/* Traverse the tree represented by pData[nData] looking for
** pTerm[nTerm], merging its doclist over *out if found (any duplicate
** doclists read from the segment rooted at pData will overwrite those
** in *out).
*/
static int loadSegment(fulltext_vtab *v, const char *pData, int nData,
                       const char *pTerm, int nTerm, DocList *out){
  int rc;
  sqlite3_stmt *s = NULL;

  assert( nData>1 );

  /* Process data as an interior node until we reach a leaf. */
  while( *pData!='\0' ){
    sqlite_int64 iBlockid;
    InteriorReader reader;

    /* Scan the node data until we find a term greater than our term.
    ** Our target child will be in the blockid under that term, or in
    ** the last blockid in the node if we never find such a term.
    */
    interiorReaderInit(pData, nData, &reader);
    while( !interiorReaderAtEnd(&reader) ){
      if( interiorReaderTermCmp(&reader, pTerm, nTerm)>0 ) break;
      interiorReaderStep(&reader);
    }

    /* Grab the child blockid before calling sql_get_statement(),
    ** because sql_get_statement() may reset our data out from under
    ** us.
    */
    iBlockid = interiorReaderCurrentBlockid(&reader);
    interiorReaderDestroy(&reader);

    rc = sql_get_statement(v, BLOCK_SELECT_STMT, &s);
    if( rc!=SQLITE_OK ) return rc;

    rc = sqlite3_bind_int64(s, 1, iBlockid);
    if( rc!=SQLITE_OK ) return rc;

    rc = sql_step_statement(v, BLOCK_SELECT_STMT, &s);
    if( rc==SQLITE_DONE ) return SQLITE_ERROR;
    if( rc!=SQLITE_ROW ) return rc;

    pData = sqlite3_column_blob(s, 0);
    nData = sqlite3_column_bytes(s, 0);
  }

  rc = loadSegmentLeaf(v, pData, nData, pTerm, nTerm, out);
  if( rc!=SQLITE_OK ) return rc;

  /* If we selected a child node, we need to finish that select. */
  if( s!=NULL ){
    /* We expect only one row.  We must execute another sqlite3_step()
     * to complete the iteration; otherwise the table will remain
     * locked. */
    rc = sqlite3_step(s);
    if( rc==SQLITE_ROW ) return SQLITE_ERROR;
    if( rc!=SQLITE_DONE ) return rc;
  }
  return SQLITE_OK;
}

/* Scan the database and merge together the posting lists for the term
** into *out.
*/
static int termSelect(fulltext_vtab *v, int iColumn,
                      const char *pTerm, int nTerm, DocList *out){
  DocList doclist;
  sqlite3_stmt *s;
  int rc = sql_get_statement(v, SEGDIR_SELECT_ALL_STMT, &s);
  if( rc!=SQLITE_OK ) return rc;

  docListInit(&doclist, DL_DEFAULT, 0, 0);

  /* Traverse the segments from oldest to newest so that newer doclist
  ** elements for given docids overwrite older elements.
  */
  while( (rc=sql_step_statement(v, SEGDIR_SELECT_ALL_STMT, &s))==SQLITE_ROW ){
    rc = loadSegment(v, sqlite3_column_blob(s, 0), sqlite3_column_bytes(s, 0),
                     pTerm, nTerm, &doclist);
    if( rc!=SQLITE_OK ) goto err;
  }
  if( rc==SQLITE_DONE ){
    *out = doclist;

    /* TODO(shess) The old term_select_all() code applied the column
    ** restrict as we merged segments, leading to smaller buffers.
    ** This is probably worthwhile to bring back, once the new storage
    ** system is checked in.
    */
    if( iColumn<v->nColumn ){   /* querying a single column */
      docListRestrictColumn(out, iColumn);
    }
    docListDiscardEmpty(out);
    return SQLITE_OK;
  }

 err:
  docListDestroy(&doclist);
  return rc;
}

/****************************************************************/
/* Used to hold hashtable data for sorting. */
typedef struct TermData {
  const char *pTerm;
  int nTerm;
  DocList *pDoclist;
} TermData;

/* Orders TermData elements in strcmp fashion ( <0 for less-than, 0
** for equal, >0 for greater-than).
*/
static int termDataCmp(const void *av, const void *bv){
  const TermData *a = (const TermData *)av;
  const TermData *b = (const TermData *)bv;
  int n = a->nTerm<b->nTerm ? a->nTerm : b->nTerm;
  int c = memcmp(a->pTerm, b->pTerm, n);
  if( c!=0 ) return c;
  return a->nTerm-b->nTerm;
}

/* Order pTerms data by term, then write a new level 0 segment using
** LeafWriter.
*/
static int writeZeroSegment(fulltext_vtab *v, fts2Hash *pTerms){
  fts2HashElem *e;
  int idx, rc, i, n;
  TermData *pData;
  LeafWriter writer;

  /* Determine the next index at level 0, merging as necessary. */
  rc = segdirNextIndex(v, 0, &idx);
  if( rc!=SQLITE_OK ) return rc;

  n = fts2HashCount(pTerms);
  pData = malloc(n*sizeof(TermData));

  for(i = 0, e = fts2HashFirst(pTerms); e; i++, e = fts2HashNext(e)){
    assert( i<n );
    pData[i].pTerm = fts2HashKey(e);
    pData[i].nTerm = fts2HashKeysize(e);
    pData[i].pDoclist = fts2HashData(e);
  }
  assert( i==n );

  /* TODO(shess) Should we allow user-defined collation sequences,
  ** here?  I think we only need that once we support prefix searches.
  */
  if( n>1 ) qsort(pData, n, sizeof(*pData), termDataCmp);

  leafWriterInit(0, idx, &writer);
  for(i=0; i<n; i++){
    rc = leafWriterStep(v, &writer,
                        pData[i].pTerm, pData[i].nTerm, pData[i].pDoclist);
    if( rc!=SQLITE_OK ) goto err;
  }
  rc = leafWriterFlush(v, &writer);

 err:
  free(pData);
  leafWriterDestroy(&writer);
  return rc;
}

/* This function implements the xUpdate callback; it's the top-level entry
 * point for inserting, deleting or updating a row in a full-text table. */
static int fulltextUpdate(sqlite3_vtab *pVtab, int nArg, sqlite3_value **ppArg,
                   sqlite_int64 *pRowid){
  fulltext_vtab *v = (fulltext_vtab *) pVtab;
  fts2Hash terms;   /* maps term string -> PosList */

     * ppArg[2..2+v->nColumn-1] = values
     * ppArg[2+v->nColumn] = value for magic column (we ignore this)
     */
    assert( nArg==2+v->nColumn+1);
    rc = index_insert(v, ppArg[1], &ppArg[2], pRowid, &terms);
  }

  if( rc==SQLITE_OK ) rc = writeZeroSegment(v, &terms);








  /* clean up */
  for(e=fts2HashFirst(&terms); e; e=fts2HashNext(e)){
    DocList *p = fts2HashData(e);
    docListDelete(p);
  }
  fts2HashClear(&terms);