how to get term position of a document?

[Li Li]

I want to override TermScorer.score()  method to take  position info
into scoring. e.g. any occurence whose position is less than 100 will
get a boost
The original score method:

  public float score() {
    assert doc != -1;
    int f = freqs[pointer];
    float raw =                                   // compute tf(f)*weight
      f < SCORE_CACHE_SIZE                        // check cache
      ? scoreCache[f]                             // cache hit
      : getSimilarity().tf(f)*weightValue;        // cache miss

    return norms == null ? raw : raw * SIM_NORM_DECODER[norms[doc] &
0xFF]; // normalize for field
  }

I want to modify it to:

  public float score() {
    assert doc != -1;
    int f = freqs[pointer];
    float raw =                                   // compute tf(f)*weight
      f < SCORE_CACHE_SIZE                        // check cache
      ? scoreCache[f]                             // cache hit
      : getSimilarity().tf(f)*weightValue;        // cache miss

    //modified
    int[] termPositions=getPositionOfDoc(doc);
    float positionBoost=1.0;
    for(int pos:termPositions){
        positionBoost*=getPositionScore(pos);
    }
    raw*=positionBoost;
    //end

    return norms == null ? raw : raw * SIM_NORM_DECODER[norms[doc] &
0xFF]; // normalize for field
  }

So I need to get position info of a term given a doc id by       int[]
getPositionOfDoc(doc);
I read some codes of SpanQuery

TermPositions positions;

while(positions.next()){//enum all docs containing the term
      doc = positions.doc();
      freq = positions.freq();  //current doc's tf
      for(int i=0;i<freq;i++){
           position = positions.nextPosition();  // get all the
position information of current doc
      }
}

But to get the doc's position info, I have to sequentially call
positions.next() like this
int[] getPositionOfDoc(doc){
    while(positions.next()){
           currentDoc = positions.doc();
           if(currentDoc==doc){// find it
                      freq = positions.freq();  //current doc's tf
                      for(int i=0;i<freq;i++){
                          //collect info
                      }
                      return result;
           }
    }
}

is there any method like random access to the position of doc? as far
as I know, lucene's invert index is implemented by skip list which
support log(n) search

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