** 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:
................................................................................
**
** dataBufferInit - create a buffer with given initial capacity.
** dataBufferReset - forget buffer's data, retaining capacity.
** dataBufferDestroy - free buffer's data.
** dataBufferExpand - expand capacity without adding data.
** dataBufferAppend - append data.
** dataBufferAppend2 - append two pieces of data at once.
** dataBufferAppendLenData - append a varint-encoded length plus data.
** dataBufferReplace - replace buffer's data.
*/
typedef struct DataBuffer {
  char *pData;          /* Pointer to malloc'ed buffer. */
  int nCapacity;        /* Size of pData buffer. */
  int nData;            /* End of data loaded into pData. */
} DataBuffer;
................................................................................
  assert( nSource1>0 && pSource1!=NULL );
  assert( nSource2>0 && pSource2!=NULL );
  dataBufferExpand(pBuffer, nSource1+nSource2);
  memcpy(pBuffer->pData+pBuffer->nData, pSource1, nSource1);
  memcpy(pBuffer->pData+pBuffer->nData+nSource1, pSource2, nSource2);
  pBuffer->nData += nSource1+nSource2;
}
static void dataBufferAppendLenData(DataBuffer *pBuffer,
                                    const char *pSource, int nSource){
  char c[VARINT_MAX];
  int n = putVarint(c, nSource);
  dataBufferAppend2(pBuffer, c, n, pSource, nSource);
}
static void dataBufferReplace(DataBuffer *pBuffer,
                              const char *pSource, int nSource){
  dataBufferReset(pBuffer);
  dataBufferAppend(pBuffer, pSource, nSource);
}

/* StringBuffer is a null-terminated version of DataBuffer. */
................................................................................
}

#ifndef NDEBUG
/* Verify that the doclist can be validly decoded.  Also returns the
** last docid found because it's convenient in other assertions for
** DLWriter.
*/
static int docListValidate(DocListType iType, const char *pData, int nData,
                           sqlite_int64 *pLastDocid){
  sqlite_int64 iPrevDocid = 0;

  assert( pData!=0 );
  assert( nData!=0 );

  while( nData!=0 ){
    sqlite_int64 iDocidDelta;
    int n = getVarint(pData, &iDocidDelta);
    iPrevDocid += iDocidDelta;
    if( iType>DL_DOCIDS ){
      int iDummy;
      while( 1 ){
................................................................................
      }
    }
    assert( n<=nData );
    pData += n;
    nData -= n;
  }
  if( pLastDocid ) *pLastDocid = iPrevDocid;
  return 1;
}



#endif

/*******************************************************************/
/* DLWriter is used to write doclist data to a DataBuffer.  DLWriter
** always appends to the buffer and does not own it.
**
** dlwInit - initialize to write a given type doclistto a buffer.
................................................................................
  assert( nFirstOld<nData || (nFirstOld==nData && pWriter->iType==DL_DOCIDS) );
  nFirstNew = putVarint(c, iFirstDocid-pWriter->iPrevDocid);

  /* Verify that the incoming doclist is valid AND that it ends with
  ** the expected docid.  This is essential because we'll trust this
  ** docid in future delta-encoding.
  */
  assert( docListValidate(pWriter->iType, pData, nData, &iLastDocidDelta) );
  assert( iLastDocid==iFirstDocid-iDocid+iLastDocidDelta );

  /* Append recoded initial docid and everything else.  Rest of docids
  ** should have been delta-encoded from previous initial docid.
  */
  if( nFirstOld<nData ){
    dataBufferAppend2(pWriter->b, c, nFirstNew,
................................................................................
  n = putVarint(c, iHeight);
  n += putVarint(c+n, iChildBlock);
  dataBufferInit(&block->data, INTERIOR_MAX);
  dataBufferReplace(&block->data, c, n);

  return block;
}












































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

  sqlite_int64 iOpeningChildBlock; /* First child block in block "last". */
................................................................................
  pWriter->iHeight = iHeight;
  pWriter->iOpeningChildBlock = iChildBlock;
#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 );

  /* Overflow to a new block if the new term makes the current block
................................................................................
    pWriter->last->next = interiorBlockNew(pWriter->iHeight, iChildBlock,
                                           pTerm, nTerm);
    pWriter->last = pWriter->last->next;
    pWriter->iOpeningChildBlock = iChildBlock;
  }else{
    dataBufferAppend2(&pWriter->last->data, c, n, pTerm, nTerm);
  }

}

/* Free the space used by pWriter, including the linked-list of
** InteriorBlocks, and parentWriter, if present.
*/
static int interiorWriterDestroy(InteriorWriter *pWriter){
  InteriorBlock *block = pWriter->first;
................................................................................
    *pnRootInfo = block->data.nData;
    return SQLITE_OK;
  }

  /* Flush the first block to %_segments, and create a new level of
  ** interior node.
  */

  rc = block_insert(v, block->data.pData, block->data.nData, &iBlockid);
  if( rc!=SQLITE_OK ) return rc;
  *piEndBlockid = iBlockid;

  pWriter->parentWriter = malloc(sizeof(*pWriter->parentWriter));
  interiorWriterInit(pWriter->iHeight+1,
                     block->term.pData, block->term.nData,
                     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->data.pData, block->data.nData, &iBlockid);
    if( rc!=SQLITE_OK ) return rc;
    *piEndBlockid = iBlockid;

    interiorWriterAppend(pWriter->parentWriter,
                         block->term.pData, block->term.nData, iBlockid);
  }
................................................................................
  DataBuffer data;                /* encoding buffer */

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

static void leafWriterInit(int iLevel, int idx, LeafWriter *pWriter){
  char c[VARINT_MAX];
  int n;

  CLEAR(pWriter);
  pWriter->iLevel = iLevel;
  pWriter->idx = idx;

  dataBufferInit(&pWriter->term, 32);

  /* Start out with a reasonably sized block, though it can grow. */
  dataBufferInit(&pWriter->data, LEAF_MAX);
  n = putVarint(c, 0);
  dataBufferReplace(&pWriter->data, c, n);
}

#ifndef NDEBUG
/* Verify that the data is readable as a leaf node. */
static int leafNodeValidate(const char *pData, int nData){
  int n, iDummy;



  assert( pData!=0 );
  assert( nData!=0 );


  /* Must lead with a varint(0) */
  n = getVarint32(pData, &iDummy);
  assert( iDummy==0 );
  if( nData==n ) return 1;

  pData += n;
  nData -= n;

  /* Leading term length and data must fit in buffer. */
  n = getVarint32(pData, &iDummy);



  assert( n+iDummy<nData );
  pData += n+iDummy;
  nData -= n+iDummy;

  /* Leading term's doclist length and data must fit. */
  n = getVarint32(pData, &iDummy);



  assert( n+iDummy<=nData );
  assert( docListValidate(DL_DEFAULT, pData+n, iDummy, NULL) );
  pData += n+iDummy;
  nData -= n+iDummy;

  /* Verify that trailing terms and doclists also are readable. */
  while( nData!=0 ){
    n = getVarint32(pData, &iDummy);





    n += getVarint32(pData+n, &iDummy);



    assert( n+iDummy<nData );
    pData += n+iDummy;
    nData -= n+iDummy;

    n = getVarint32(pData, &iDummy);



    assert( n+iDummy<=nData );
    assert( docListValidate(DL_DEFAULT, pData+n, iDummy, NULL) );
    pData += n+iDummy;
    nData -= n+iDummy;
  }
  return 1;
}



#endif

/* 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,
................................................................................

  /* Must have the leading varint(0) flag, plus at least some
  ** valid-looking data.
  */
  assert( nData>2 );
  assert( iData>=0 );
  assert( iData+nData<=pWriter->data.nData );
  assert( leafNodeValidate(pWriter->data.pData+iData, nData) );

  rc = block_insert(v, pWriter->data.pData+iData, 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.
................................................................................
  return SQLITE_OK;
}
static int leafWriterFlush(fulltext_vtab *v, LeafWriter *pWriter){
  int rc = leafWriterInternalFlush(v, pWriter, 0, pWriter->data.nData);
  if( rc!=SQLITE_OK ) return rc;

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

  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
................................................................................
    *ppRootInfo = pWriter->data.pData;
    *pnRootInfo = pWriter->data.nData;
    *piEndBlockid = 0;
    return SQLITE_OK;
  }

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

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

................................................................................
  char *pRootInfo;
  int rc, nRootInfo;

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

  /* Don't bother storing an entirely empty segment. */
  if( iEndBlockid==0 && nRootInfo==1 ) return SQLITE_OK;

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

static void leafWriterDestroy(LeafWriter *pWriter){
................................................................................
  dataBufferDestroy(&pWriter->term);
  dataBufferDestroy(&pWriter->data);
}

/* Encode a term into the leafWriter, delta-encoding as appropriate. */
static void leafWriterEncodeTerm(LeafWriter *pWriter,
                                 const char *pTerm, int nTerm){



  if( pWriter->term.nData==0 ){
    /* Encode the entire leading term as:

    **  varint(nTerm)
    **  char pTerm[nTerm]
    */
    assert( pWriter->data.nData==1 );
    dataBufferAppendLenData(&pWriter->data, pTerm, nTerm);

  }else{
    /* Delta-encode the term as:
    **  varint(nPrefix)
    **  varint(nSuffix)
    **  char pTermSuffix[nSuffix]
    */
    char c[VARINT_MAX+VARINT_MAX];
    int n, nPrefix = 0;


    while( nPrefix<nTerm && nPrefix<pWriter->term.nData &&
           pTerm[nPrefix]==pWriter->term.pData[nPrefix] ){
      nPrefix++;


    }

    n = putVarint(c, nPrefix);
    n += putVarint(c+n, nTerm-nPrefix);
    dataBufferAppend2(&pWriter->data, c, n, pTerm+nPrefix, nTerm-nPrefix);
  }
  dataBufferReplace(&pWriter->term, pTerm, nTerm);
}

/* Push pTerm[nTerm] along with the doclist data to the leaf layer of
** %_segments.
*/
/* TODO(shess) Revise writeZeroSegment() so that doclists are
** constructed directly in pWriter->data.  That implies refactoring
** leafWriterStep() and leafWriterStepMerge() to share more code.
*/
static int leafWriterStep(fulltext_vtab *v, LeafWriter *pWriter,
                          const char *pTerm, int nTerm,
                          const char *pData, int nData){
  int rc;

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

  leafWriterEncodeTerm(pWriter, pTerm, nTerm);

  /* Encode the doclist as:
  **  varint(nDoclist)
  **  char pDoclist[nDoclist]
  */
  dataBufferAppendLenData(&pWriter->data, pData, nData);

  /* Flush standalone blocks right out */
  if( nData+nTerm>STANDALONE_MIN ){
    rc = leafWriterFlush(v, pWriter);
    if( rc!=SQLITE_OK ) return rc;
  }
  assert( leafNodeValidate(pWriter->data.pData, pWriter->data.nData) );

  return SQLITE_OK;
}

/* Used to avoid a memmove when a large amount of doclist data is in
** the buffer.  This constructs a node and term header before
** iDoclistData and flushes the resulting complete node using
** leafWriterInternalFlush().
*/
static int leafWriterInlineFlush(fulltext_vtab *v, LeafWriter *pWriter,
                                 const char *pTerm, int nTerm,
................................................................................
static int leafWriterStepMerge(fulltext_vtab *v, LeafWriter *pWriter,
                               const char *pTerm, int nTerm,
                               DLReader *pReaders, int nReaders){
  char c[VARINT_MAX+VARINT_MAX];
  int iTermData = pWriter->data.nData, iDoclistData;
  int i, nData, n, nActualData, nActual, rc;

  assert( leafNodeValidate(pWriter->data.pData, pWriter->data.nData) );
  leafWriterEncodeTerm(pWriter, pTerm, nTerm);

  iDoclistData = pWriter->data.nData;

  /* Estimate the length of the merged doclist so we can leave space
  ** to encode it.
  */
................................................................................
  for(i=0, nData=0; i<nReaders; i++){
    nData += dlrAllDataBytes(&pReaders[i]);
  }
  n = putVarint(c, nData);
  dataBufferAppend(&pWriter->data, c, n);

  docListMerge(&pWriter->data, pReaders, nReaders);
  assert( docListValidate(DL_DEFAULT,
                          pWriter->data.pData+iDoclistData+n,
                          pWriter->data.nData-iDoclistData-n, NULL) );

  /* The actual amount of doclist data at this point could be smaller
  ** than the length we encoded.  Additionally, the space required to
  ** encode this length could be smaller.  For small doclists, this is
  ** not a big deal, we can just use memmove() to adjust things.
  */
  nActualData = pWriter->data.nData-(iDoclistData+n);
................................................................................
  /* TODO(shess) This test matches leafWriterStep(), which does this
  ** test before it knows the cost to varint-encode the term and
  ** doclist lengths.  At some point, change to
  ** pWriter->data.nData-iTermData>STANDALONE_MIN.
  */
  if( nTerm+nActualData>STANDALONE_MIN ){
    /* Push leaf node from before this term. */
    if( iTermData>1 ){
      rc = leafWriterInternalFlush(v, pWriter, 0, iTermData);
      if( rc!=SQLITE_OK ) return rc;
    }

    /* Fix the encoded doclist length. */
    iDoclistData += n - nActual;
    memcpy(pWriter->data.pData+iDoclistData, c, nActual);

    /* Push the standalone leaf node. */
    rc = leafWriterInlineFlush(v, pWriter, pTerm, nTerm, iDoclistData);
    if( rc!=SQLITE_OK ) return rc;

    /* Leave the node empty. */
    pWriter->data.nData = putVarint(pWriter->data.pData, 0);
    dataBufferReset(&pWriter->term);
    return rc;
  }

  /* At this point, we know that the doclist was small, so do the
  ** memmove if indicated.
  */
  if( nActual<n ){
................................................................................
    assert( 2*STANDALONE_MIN<=LEAF_MAX );
    assert( n+pWriter->data.nData-iDoclistData<iDoclistData );
    memcpy(pWriter->data.pData+n,
           pWriter->data.pData+iDoclistData,
           pWriter->data.nData-iDoclistData);
    pWriter->data.nData -= iDoclistData-n;
  }
  assert( leafNodeValidate(pWriter->data.pData, pWriter->data.nData) );

  return SQLITE_OK;
}





















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

** 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:
................................................................................
**
** dataBufferInit - create a buffer with given initial capacity.
** dataBufferReset - forget buffer's data, retaining capacity.
** dataBufferDestroy - free buffer's data.
** dataBufferExpand - expand capacity without adding data.
** dataBufferAppend - append data.
** dataBufferAppend2 - append two pieces of data at once.

** dataBufferReplace - replace buffer's data.
*/
typedef struct DataBuffer {
  char *pData;          /* Pointer to malloc'ed buffer. */
  int nCapacity;        /* Size of pData buffer. */
  int nData;            /* End of data loaded into pData. */
} DataBuffer;
................................................................................
  assert( nSource1>0 && pSource1!=NULL );
  assert( nSource2>0 && pSource2!=NULL );
  dataBufferExpand(pBuffer, nSource1+nSource2);
  memcpy(pBuffer->pData+pBuffer->nData, pSource1, nSource1);
  memcpy(pBuffer->pData+pBuffer->nData+nSource1, pSource2, nSource2);
  pBuffer->nData += nSource1+nSource2;
}






static void dataBufferReplace(DataBuffer *pBuffer,
                              const char *pSource, int nSource){
  dataBufferReset(pBuffer);
  dataBufferAppend(pBuffer, pSource, nSource);
}

/* StringBuffer is a null-terminated version of DataBuffer. */
................................................................................
}

#ifndef NDEBUG
/* Verify that the doclist can be validly decoded.  Also returns the
** last docid found because it's convenient in other assertions for
** DLWriter.
*/
static void docListValidate(DocListType iType, const char *pData, int nData,
                            sqlite_int64 *pLastDocid){
  sqlite_int64 iPrevDocid = 0;
  assert( nData>0 );
  assert( pData!=0 );

  assert( pData+nData>pData );
  while( nData!=0 ){
    sqlite_int64 iDocidDelta;
    int n = getVarint(pData, &iDocidDelta);
    iPrevDocid += iDocidDelta;
    if( iType>DL_DOCIDS ){
      int iDummy;
      while( 1 ){
................................................................................
      }
    }
    assert( n<=nData );
    pData += n;
    nData -= n;
  }
  if( pLastDocid ) *pLastDocid = iPrevDocid;

}
#define ASSERT_VALID_DOCLIST(i, p, n, o) docListValidate(i, p, n, o)
#else
#define ASSERT_VALID_DOCLIST(i, p, n, o) assert( 1 )
#endif

/*******************************************************************/
/* DLWriter is used to write doclist data to a DataBuffer.  DLWriter
** always appends to the buffer and does not own it.
**
** dlwInit - initialize to write a given type doclistto a buffer.
................................................................................
  assert( nFirstOld<nData || (nFirstOld==nData && pWriter->iType==DL_DOCIDS) );
  nFirstNew = putVarint(c, iFirstDocid-pWriter->iPrevDocid);

  /* Verify that the incoming doclist is valid AND that it ends with
  ** the expected docid.  This is essential because we'll trust this
  ** docid in future delta-encoding.
  */
  ASSERT_VALID_DOCLIST(pWriter->iType, pData, nData, &iLastDocidDelta);
  assert( iLastDocid==iFirstDocid-iDocid+iLastDocidDelta );

  /* Append recoded initial docid and everything else.  Rest of docids
  ** should have been delta-encoded from previous initial docid.
  */
  if( nFirstOld<nData ){
    dataBufferAppend2(pWriter->b, c, nFirstNew,
................................................................................
  n = putVarint(c, iHeight);
  n += putVarint(c+n, iChildBlock);
  dataBufferInit(&block->data, INTERIOR_MAX);
  dataBufferReplace(&block->data, c, n);

  return block;
}

#ifndef NDEBUG
/* Verify that the data is readable as an interior node. */
static void interiorBlockValidate(InteriorBlock *pBlock){
  const char *pData = pBlock->data.pData;
  int nData = pBlock->data.nData;
  int n, iDummy;
  sqlite_int64 iBlockid;

  assert( nData>0 );
  assert( pData!=0 );
  assert( pData+nData>pData );

  /* Must lead with height of node as a varint(n), n>0 */
  n = getVarint32(pData, &iDummy);
  assert( n>0 );
  assert( iDummy>0 );
  assert( n<nData );
  pData += n;
  nData -= n;

  /* Must contain iBlockid. */
  n = getVarint(pData, &iBlockid);
  assert( n>0 );
  assert( n<=nData );
  pData += n;
  nData -= n;

  /* Zero or more terms of positive length */
  while( nData!=0 ){
    n = getVarint32(pData, &iDummy);
    assert( n>0 );
    assert( iDummy>0 );
    assert( n+iDummy>0);
    assert( n+iDummy<=nData );
    pData += n+iDummy;
    nData -= n+iDummy;
  }
}
#define ASSERT_VALID_INTERIOR_BLOCK(x) interiorBlockValidate(x)
#else
#define ASSERT_VALID_INTERIOR_BLOCK(x) assert( 1 )
#endif

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

  sqlite_int64 iOpeningChildBlock; /* First child block in block "last". */
................................................................................
  pWriter->iHeight = iHeight;
  pWriter->iOpeningChildBlock = iChildBlock;
#ifndef NDEBUG
  pWriter->iLastChildBlock = iChildBlock;
#endif
  block = interiorBlockNew(iHeight, iChildBlock, pTerm, nTerm);
  pWriter->last = pWriter->first = block;
  ASSERT_VALID_INTERIOR_BLOCK(pWriter->last);
}

/* 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);

  ASSERT_VALID_INTERIOR_BLOCK(pWriter->last);

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

  /* Overflow to a new block if the new term makes the current block
................................................................................
    pWriter->last->next = interiorBlockNew(pWriter->iHeight, iChildBlock,
                                           pTerm, nTerm);
    pWriter->last = pWriter->last->next;
    pWriter->iOpeningChildBlock = iChildBlock;
  }else{
    dataBufferAppend2(&pWriter->last->data, c, n, pTerm, nTerm);
  }
  ASSERT_VALID_INTERIOR_BLOCK(pWriter->last);
}

/* Free the space used by pWriter, including the linked-list of
** InteriorBlocks, and parentWriter, if present.
*/
static int interiorWriterDestroy(InteriorWriter *pWriter){
  InteriorBlock *block = pWriter->first;
................................................................................
    *pnRootInfo = block->data.nData;
    return SQLITE_OK;
  }

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

  pWriter->parentWriter = malloc(sizeof(*pWriter->parentWriter));
  interiorWriterInit(pWriter->iHeight+1,
                     block->term.pData, block->term.nData,
                     iBlockid, pWriter->parentWriter);

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

    interiorWriterAppend(pWriter->parentWriter,
                         block->term.pData, block->term.nData, iBlockid);
  }
................................................................................
  DataBuffer data;                /* encoding buffer */

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

static void leafWriterInit(int iLevel, int idx, LeafWriter *pWriter){



  CLEAR(pWriter);
  pWriter->iLevel = iLevel;
  pWriter->idx = idx;

  dataBufferInit(&pWriter->term, 32);

  /* Start out with a reasonably sized block, though it can grow. */
  dataBufferInit(&pWriter->data, LEAF_MAX);


}

#ifndef NDEBUG
/* Verify that the data is readable as a leaf node. */
static void leafNodeValidate(const char *pData, int nData){
  int n, iDummy;

  if( nData==0 ) return;
  assert( nData>0 );
  assert( pData!=0 );

  assert( pData+nData>pData );

  /* Must lead with a varint(0) */
  n = getVarint32(pData, &iDummy);
  assert( iDummy==0 );
  assert( n>0 );
  assert( n<nData );
  pData += n;
  nData -= n;

  /* Leading term length and data must fit in buffer. */
  n = getVarint32(pData, &iDummy);
  assert( n>0 );
  assert( iDummy>0 );
  assert( n+iDummy>0 );
  assert( n+iDummy<nData );
  pData += n+iDummy;
  nData -= n+iDummy;

  /* Leading term's doclist length and data must fit. */
  n = getVarint32(pData, &iDummy);
  assert( n>0 );
  assert( iDummy>0 );
  assert( n+iDummy>0 );
  assert( n+iDummy<=nData );
  ASSERT_VALID_DOCLIST(DL_DEFAULT, pData+n, iDummy, NULL);
  pData += n+iDummy;
  nData -= n+iDummy;

  /* Verify that trailing terms and doclists also are readable. */
  while( nData!=0 ){
    n = getVarint32(pData, &iDummy);
    assert( n>0 );
    assert( iDummy>=0 );
    assert( n<nData );
    pData += n;
    nData -= n;
    n = getVarint32(pData, &iDummy);
    assert( n>0 );
    assert( iDummy>0 );
    assert( n+iDummy>0 );
    assert( n+iDummy<nData );
    pData += n+iDummy;
    nData -= n+iDummy;

    n = getVarint32(pData, &iDummy);
    assert( n>0 );
    assert( iDummy>0 );
    assert( n+iDummy>0 );
    assert( n+iDummy<=nData );
    ASSERT_VALID_DOCLIST(DL_DEFAULT, pData+n, iDummy, NULL);
    pData += n+iDummy;
    nData -= n+iDummy;
  }

}
#define ASSERT_VALID_LEAF_NODE(p, n) leafNodeValidate(p, n)
#else
#define ASSERT_VALID_LEAF_NODE(p, n) assert( 1 )
#endif

/* 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,
................................................................................

  /* Must have the leading varint(0) flag, plus at least some
  ** valid-looking data.
  */
  assert( nData>2 );
  assert( iData>=0 );
  assert( iData+nData<=pWriter->data.nData );
  ASSERT_VALID_LEAF_NODE(pWriter->data.pData+iData, nData);

  rc = block_insert(v, pWriter->data.pData+iData, 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.
................................................................................
  return SQLITE_OK;
}
static int leafWriterFlush(fulltext_vtab *v, LeafWriter *pWriter){
  int rc = leafWriterInternalFlush(v, pWriter, 0, pWriter->data.nData);
  if( rc!=SQLITE_OK ) return rc;

  /* Re-initialize the output buffer. */

  dataBufferReset(&pWriter->data);

  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
................................................................................
    *ppRootInfo = pWriter->data.pData;
    *pnRootInfo = pWriter->data.nData;
    *piEndBlockid = 0;
    return SQLITE_OK;
  }

  /* Flush remaining leaf data. */
  if( pWriter->data.nData>0 ){
    int rc = leafWriterFlush(v, pWriter);
    if( rc!=SQLITE_OK ) return rc;
  }

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

................................................................................
  char *pRootInfo;
  int rc, nRootInfo;

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

  /* Don't bother storing an entirely empty segment. */
  if( iEndBlockid==0 && nRootInfo==0 ) return SQLITE_OK;

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

static void leafWriterDestroy(LeafWriter *pWriter){
................................................................................
  dataBufferDestroy(&pWriter->term);
  dataBufferDestroy(&pWriter->data);
}

/* Encode a term into the leafWriter, delta-encoding as appropriate. */
static void leafWriterEncodeTerm(LeafWriter *pWriter,
                                 const char *pTerm, int nTerm){
  char c[VARINT_MAX+VARINT_MAX];
  int n;

  if( pWriter->data.nData==0 ){
    /* Encode the node header and leading term as:
    **  varint(0)
    **  varint(nTerm)
    **  char pTerm[nTerm]
    */
    n = putVarint(c, '\0');
    n += putVarint(c+n, nTerm);
    dataBufferAppend2(&pWriter->data, c, n, pTerm, nTerm);
  }else{
    /* Delta-encode the term as:
    **  varint(nPrefix)
    **  varint(nSuffix)
    **  char pTermSuffix[nSuffix]
    */

    int nPrefix = 0;

    assert( nTerm>0 );
    while( nPrefix<pWriter->term.nData &&
           pTerm[nPrefix]==pWriter->term.pData[nPrefix] ){
      nPrefix++;
      /* Failing this implies that the terms weren't in order. */
      assert( nPrefix<nTerm );
    }

    n = putVarint(c, nPrefix);
    n += putVarint(c+n, nTerm-nPrefix);
    dataBufferAppend2(&pWriter->data, c, n, pTerm+nPrefix, nTerm-nPrefix);
  }
  dataBufferReplace(&pWriter->term, pTerm, nTerm);
}







































/* Used to avoid a memmove when a large amount of doclist data is in
** the buffer.  This constructs a node and term header before
** iDoclistData and flushes the resulting complete node using
** leafWriterInternalFlush().
*/
static int leafWriterInlineFlush(fulltext_vtab *v, LeafWriter *pWriter,
                                 const char *pTerm, int nTerm,
................................................................................
static int leafWriterStepMerge(fulltext_vtab *v, LeafWriter *pWriter,
                               const char *pTerm, int nTerm,
                               DLReader *pReaders, int nReaders){
  char c[VARINT_MAX+VARINT_MAX];
  int iTermData = pWriter->data.nData, iDoclistData;
  int i, nData, n, nActualData, nActual, rc;

  ASSERT_VALID_LEAF_NODE(pWriter->data.pData, pWriter->data.nData);
  leafWriterEncodeTerm(pWriter, pTerm, nTerm);

  iDoclistData = pWriter->data.nData;

  /* Estimate the length of the merged doclist so we can leave space
  ** to encode it.
  */
................................................................................
  for(i=0, nData=0; i<nReaders; i++){
    nData += dlrAllDataBytes(&pReaders[i]);
  }
  n = putVarint(c, nData);
  dataBufferAppend(&pWriter->data, c, n);

  docListMerge(&pWriter->data, pReaders, nReaders);
  ASSERT_VALID_DOCLIST(DL_DEFAULT,
                       pWriter->data.pData+iDoclistData+n,
                       pWriter->data.nData-iDoclistData-n, NULL);

  /* The actual amount of doclist data at this point could be smaller
  ** than the length we encoded.  Additionally, the space required to
  ** encode this length could be smaller.  For small doclists, this is
  ** not a big deal, we can just use memmove() to adjust things.
  */
  nActualData = pWriter->data.nData-(iDoclistData+n);
................................................................................
  /* TODO(shess) This test matches leafWriterStep(), which does this
  ** test before it knows the cost to varint-encode the term and
  ** doclist lengths.  At some point, change to
  ** pWriter->data.nData-iTermData>STANDALONE_MIN.
  */
  if( nTerm+nActualData>STANDALONE_MIN ){
    /* Push leaf node from before this term. */
    if( iTermData>0 ){
      rc = leafWriterInternalFlush(v, pWriter, 0, iTermData);
      if( rc!=SQLITE_OK ) return rc;
    }

    /* Fix the encoded doclist length. */
    iDoclistData += n - nActual;
    memcpy(pWriter->data.pData+iDoclistData, c, nActual);

    /* Push the standalone leaf node. */
    rc = leafWriterInlineFlush(v, pWriter, pTerm, nTerm, iDoclistData);
    if( rc!=SQLITE_OK ) return rc;

    /* Leave the node empty. */
    dataBufferReset(&pWriter->data);

    return rc;
  }

  /* At this point, we know that the doclist was small, so do the
  ** memmove if indicated.
  */
  if( nActual<n ){
................................................................................
    assert( 2*STANDALONE_MIN<=LEAF_MAX );
    assert( n+pWriter->data.nData-iDoclistData<iDoclistData );
    memcpy(pWriter->data.pData+n,
           pWriter->data.pData+iDoclistData,
           pWriter->data.nData-iDoclistData);
    pWriter->data.nData -= iDoclistData-n;
  }
  ASSERT_VALID_LEAF_NODE(pWriter->data.pData, pWriter->data.nData);

  return SQLITE_OK;
}

/* Push pTerm[nTerm] along with the doclist data to the leaf layer of
** %_segments.
*/
/* TODO(shess) Revise writeZeroSegment() so that doclists are
** constructed directly in pWriter->data.
*/
static int leafWriterStep(fulltext_vtab *v, LeafWriter *pWriter,
                          const char *pTerm, int nTerm,
                          const char *pData, int nData){
  int rc;
  DLReader reader;

  dlrInit(&reader, DL_DEFAULT, pData, nData);
  rc = leafWriterStepMerge(v, pWriter, pTerm, nTerm, &reader, 1);
  dlrDestroy(&reader);

  return rc;
}


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