// we don't actually write a .fdx-like index, instead we read the
 // stored fields file in entirety up-front and save the offsets
 // so we can seek to the documents later.
 private void readIndex(int size) throws IOException {
   ChecksumIndexInput input = new BufferedChecksumIndexInput(in);
   offsets = new long[size];
   int upto = 0;
   while (!scratch.get().equals(END)) {
     SimpleTextUtil.readLine(input, scratch);
     if (StringHelper.startsWith(scratch.get(), DOC)) {
       offsets[upto] = input.getFilePointer();
       upto++;
     }
   }
   SimpleTextUtil.checkFooter(input);
   assert upto == offsets.length;
 }
 private void readLine() throws IOException {
   SimpleTextUtil.readLine(in, scratch);
 }