public MultiTermDocs(IndexReader[] r,
    int[] s) 
{
      readers = r;
      starts = s;
      readerTermDocs = new TermDocs[r.length];
}

 public  void close() throws IOException 
{
      for (int i = 0; i <  readerTermDocs.length; i++) {
        if (readerTermDocs[i] != null)
          readerTermDocs[i].close();
      }
}

 public int doc() 
{
      return base + current.doc();
}

 public int freq() 
{
      return current.freq();
}

 public boolean next() throws IOException 
{
      for(;;) {
        if (current!=null && current.next()) {
          return true;
        }
        else if (pointer <  readers.length) {
          base = starts[pointer];
          current = termDocs(pointer++);
        } else {
          return false;
        }
      }
}

 public int read(int[] docs,
    int[] freqs) throws IOException 
{
      while (true) {
        while (current == null) {
          if (pointer <  readers.length) {      // try next segment
            base = starts[pointer];
            current = termDocs(pointer++);
          } else {
            return 0;
          }
        }
        int end = current.read(docs, freqs);
        if (end == 0) {          // none left in segment
          current = null;
        } else {            // got some
          final int b = base;        // adjust doc numbers
          for (int i = 0; i <  end; i++)
           docs[i] += b;
          return end;
        }
      }
}

Optimized implementation.

 public  void seek(Term term) 
{
      this.term = term;
      this.base = 0;
      this.pointer = 0;
      this.current = null;
}

 public  void seek(TermEnum termEnum) throws IOException 
{
      seek(termEnum.term());
}

 public boolean skipTo(int target) throws IOException 
{
      for(;;) {
        if (current != null && current.skipTo(target-base)) {
          return true;
        } else if (pointer <  readers.length) {
          base = starts[pointer];
          current = termDocs(pointer++);
        } else
          return false;
      }
}

 protected TermDocs termDocs(IndexReader reader) throws IOException 
{
      return reader.termDocs();
}