SQLite

struct Fts5DoclistIter {
  u8 *aEof;                       /* Pointer to 1 byte past end of doclist */

  /* Output variables. aPoslist==0 at EOF */
  i64 iRowid;
  u8 *aPoslist;
  int nPoslist;

};

/*
** The contents of the "structure" record for each index are represented
** using an Fts5Structure record in memory. Which uses instances of the 
** other Fts5StructureXXX types as components.
*/

** If an error occurs, an error code is left in p->rc. It is assumed
** no error has already occurred when this function is called.
*/
static int fts5MultiIterPoslist(
  Fts5Index *p,
  Fts5IndexIter *pMulti,
  Fts5Colset *pColset,
  int bSz,                        /* Append a size field before the data */
  Fts5Buffer *pBuf
){
  if( p->rc==SQLITE_OK ){
    int iSz;
    int iData;

    Fts5SegIter *pSeg = &pMulti->aSeg[ pMulti->aFirst[1].iFirst ];
    assert( fts5MultiIterEof(p, pMulti)==0 );

    if( bSz ){
      /* WRITEPOSLISTSIZE */
      iSz = pBuf->n;
      fts5BufferAppendVarint(&p->rc, pBuf, pSeg->nPos*2);
      iData = pBuf->n;
    }

    fts5SegiterPoslist(p, pSeg, pColset, pBuf);

    if( bSz && pColset ){
      int nActual = pBuf->n - iData;
      if( nActual!=pSeg->nPos ){
        /* WRITEPOSLISTSIZE */
        if( nActual==0 ){
          return 1;
        }else{
          int nReq = sqlite3Fts5GetVarintLen((u32)(nActual*2));
          while( iSz<(iData-nReq) ){ pBuf->p[iSz++] = 0x80; }
          sqlite3Fts5PutVarint(&pBuf->p[iSz], nActual*2);
        }
      }
    }
  }

  return 0;
}

static void fts5DoclistIterNext(Fts5DoclistIter *pIter){
  u8 *p = pIter->aPoslist + pIter->nPoslist;

  assert( pIter->aPoslist );
  if( p>=pIter->aEof ){
    pIter->aPoslist = 0;
  }else{
    i64 iDelta;

    p += fts5GetVarint(p, (u64*)&iDelta);
    pIter->iRowid += iDelta;

    /* Read position list size */
    if( p[0] & 0x80 ){
      int nPos;
      p += fts5GetVarint32(p, nPos);
      pIter->nPoslist = (nPos>>1);
    }else{
      pIter->nPoslist = ((int)(p[0])) >> 1;
      p++;
    }

    pIter->aPoslist = p;
  }
}

static void fts5DoclistIterInit(

    sqlite3Fts5BufferGrow(&p->rc, &out, p1->n + p2->n);
    fts5DoclistIterInit(p1, &i1);
    fts5DoclistIterInit(p2, &i2);
    while( p->rc==SQLITE_OK && (i1.aPoslist!=0 || i2.aPoslist!=0) ){
      if( i2.aPoslist==0 || (i1.aPoslist && i1.iRowid<i2.iRowid) ){
        /* Copy entry from i1 */
        fts5MergeAppendDocid(&out, iLastRowid, i1.iRowid);
        /* WRITEPOSLISTSIZE */
        fts5BufferSafeAppendVarint(&out, i1.nPoslist * 2);
        fts5BufferSafeAppendBlob(&out, i1.aPoslist, i1.nPoslist);
        fts5DoclistIterNext(&i1);
      }
      else if( i1.aPoslist==0 || i2.iRowid!=i1.iRowid ){
        /* Copy entry from i2 */
        fts5MergeAppendDocid(&out, iLastRowid, i2.iRowid);
        /* WRITEPOSLISTSIZE */
        fts5BufferSafeAppendVarint(&out, i2.nPoslist * 2);
        fts5BufferSafeAppendBlob(&out, i2.aPoslist, i2.nPoslist);
        fts5DoclistIterNext(&i2);
      }
      else{
        i64 iPos1 = 0;
        i64 iPos2 = 0;
        int iOff1 = 0;
        int iOff2 = 0;



        Fts5PoslistWriter writer;
        memset(&writer, 0, sizeof(writer));

        /* Merge the two position lists. */ 
        fts5MergeAppendDocid(&out, iLastRowid, i2.iRowid);
        fts5BufferZero(&tmp);

        sqlite3Fts5PoslistNext64(i1.aPoslist, i1.nPoslist, &iOff1, &iPos1);
        sqlite3Fts5PoslistNext64(i2.aPoslist, i2.nPoslist, &iOff2, &iPos2);

        while( p->rc==SQLITE_OK && (iPos1>=0 || iPos2>=0) ){
          i64 iNew;
          if( iPos2<0 || (iPos1>=0 && iPos1<iPos2) ){
            iNew = iPos1;
            sqlite3Fts5PoslistNext64(i1.aPoslist, i1.nPoslist, &iOff1, &iPos1);
          }else{
            iNew = iPos2;
            sqlite3Fts5PoslistNext64(i2.aPoslist, i2.nPoslist, &iOff2, &iPos2);
            if( iPos1==iPos2 ){
              sqlite3Fts5PoslistNext64(i1.aPoslist, i1.nPoslist, &iOff1,&iPos1);
            }
          }
          p->rc = sqlite3Fts5PoslistWriterAppend(&tmp, &writer, iNew);
        }

        /* WRITEPOSLISTSIZE */
        fts5BufferSafeAppendVarint(&out, tmp.n * 2);

        iLastRowid = 0;
      }

      if( 0==sqlite3Fts5BufferGrow(&p->rc, &doclist, 9) ){
        int iSave = doclist.n;
        assert( doclist.n!=0 || iLastRowid==0 );
        fts5BufferSafeAppendVarint(&doclist, iRowid - iLastRowid);
        if( fts5MultiIterPoslist(p, p1, pColset, 1, &doclist) ){
          doclist.n = iSave;
        }else{
          iLastRowid = iRowid;
        }
      }
    }

    i64 iRowid = fts5MultiIterRowid(pIter);
    char *z = (char*)fts5MultiIterTerm(pIter, &n);

    /* If this is a new term, query for it. Update cksum3 with the results. */
    fts5TestTerm(p, &term, z, n, cksum2, &cksum3);

    poslist.n = 0;
    fts5MultiIterPoslist(p, pIter, 0, 0, &poslist);
    while( 0==sqlite3Fts5PoslistNext64(poslist.p, poslist.n, &iOff, &iPos) ){
      int iCol = FTS5_POS2COLUMN(iPos);
      int iTokOff = FTS5_POS2OFFSET(iPos);
      cksum2 ^= fts5IndexEntryCksum(iRowid, iCol, iTokOff, -1, z, n);
    }
  }
  fts5TestTerm(p, &term, 0, 0, cksum2, &cksum3);