code: public int  nonzeros(int [] res){
      if(!isSparse())
        for( int i = 0; i < _len; ++i)
          res[i] = i;
      else {
        int j = 0;
        for (int i = nextNZ(-1); i < _len; i = nextNZ(i))
          res[j++] = i;
        assert res.length == j;
      }
      return _len;
    }
docs: Get chunk-relative indexes of values (nonzeros for sparse, all for dense) stored in this chunk.
  For desne chunks, this will contain indeces of all the rows in this chunk.
  
  @return array of chunk-relative indeces of values stored in this chunk.