// read or create index
  private void readOrCreateIndex(CollectionManager.Force ff, Formatter f) throws IOException {

    // force new index or test for new index needed
    boolean force =
        ((ff == CollectionManager.Force.always)
            || (ff == CollectionManager.Force.test && needsUpdate()));

    // otherwise, we're good as long as the index file exists
    File idx = gc.getIndexFile();
    if (force || !idx.exists() || !readIndex(idx.getPath())) {
      logger.info("GribCollection {}: createIndex {}", gc.getName(), idx.getPath());
      createIndex(idx, ff, f); // write out index
      gc.rafLocation = idx.getPath();
      gc.setRaf(new RandomAccessFile(idx.getPath(), "r"));
      readIndex(gc.getRaf()); // read back in index
    }
  }
  private void createIndex(
      File indexFile, List<Group> groups, ArrayList<String> filenames, Formatter f)
      throws IOException {
    Grib2Record first = null; // take global metadata from here

    if (indexFile.exists()) indexFile.delete(); // replace it
    f.format(" createIndex for %s%n", indexFile.getPath());

    RandomAccessFile raf = new RandomAccessFile(indexFile.getPath(), "rw");
    raf.order(RandomAccessFile.BIG_ENDIAN);
    try {
      //// header message
      raf.write(MAGIC_START.getBytes("UTF-8"));
      raf.writeInt(version);
      long lenPos = raf.getFilePointer();
      raf.writeLong(0); // save space to write the length of the record section
      long countBytes = 0;
      int countRecords = 0;
      for (Group g : groups) {
        g.fileSet = new HashSet<Integer>();
        for (Grib2Rectilyser.VariableBag vb : g.rect.getGribvars()) {
          if (first == null) first = vb.first;
          GribCollectionProto.VariableRecords vr = writeRecordsProto(vb, g.fileSet);
          byte[] b = vr.toByteArray();
          vb.pos = raf.getFilePointer();
          vb.length = b.length;
          raf.write(b);
          countBytes += b.length;
          countRecords += vb.recordMap.length;
        }
      }
      long bytesPerRecord = countBytes / ((countRecords == 0) ? 1 : countRecords);
      f.format(
          "  write RecordMaps: bytes = %d record = %d bytesPerRecord=%d%n",
          countBytes, countRecords, bytesPerRecord);

      if (first == null) {
        logger.error("GribCollection {}: has no files\n{}", gc.getName(), f.toString());
        throw new IllegalArgumentException("GribCollection " + gc.getName() + " has no files");
      }

      long pos = raf.getFilePointer();
      raf.seek(lenPos);
      raf.writeLong(countBytes);
      raf.seek(pos); // back to the output.

      GribCollectionProto.GribCollectionIndex.Builder indexBuilder =
          GribCollectionProto.GribCollectionIndex.newBuilder();
      indexBuilder.setName(gc.getName());

      for (String fn : filenames) indexBuilder.addFiles(fn);

      for (Group g : groups) indexBuilder.addGroups(writeGroupProto(g));

      /* int count = 0;
      for (DatasetCollectionManager dcm : collections) {
        indexBuilder.addParams(makeParamProto(new Parameter("spec" + count, dcm.())));
        count++;
      } */

      // what about just storing first ??
      Grib2SectionIdentification ids = first.getId();
      indexBuilder.setCenter(ids.getCenter_id());
      indexBuilder.setSubcenter(ids.getSubcenter_id());
      indexBuilder.setMaster(ids.getMaster_table_version());
      indexBuilder.setLocal(ids.getLocal_table_version());

      Grib2Pds pds = first.getPDS();
      indexBuilder.setGenProcessType(pds.getGenProcessType());
      indexBuilder.setGenProcessId(pds.getGenProcessId());
      indexBuilder.setBackProcessId(pds.getBackProcessId());

      GribCollectionProto.GribCollectionIndex index = indexBuilder.build();
      byte[] b = index.toByteArray();
      NcStream.writeVInt(raf, b.length); // message size
      raf.write(b); // message  - all in one gulp
      f.format("  write GribCollectionIndex= %d bytes%n", b.length);

    } finally {
      f.format("  file size =  %d bytes%n", raf.length());
      raf.close();
      if (raf != null) raf.close();
    }
  }
 public boolean needsUpdate() {
   File idx = gc.getIndexFile();
   return !idx.exists() || needsUpdate(idx.lastModified());
 }