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class JsMessage.Builder {
    /**
     * Generates a compact uppercase alphanumeric text representation of a
     * 63-bit fingerprint of the content parts of a message.
     */
    private static String fingerprint(List<CharSequence> messageParts) {
      StringBuilder sb = new StringBuilder();
      for (CharSequence part : messageParts) {
        if (part instanceof JsMessage.PlaceholderReference) {
          sb.append(part.toString());
        } else {
          sb.append(part);
        }
      }
      long nonnegativeHash = Long.MAX_VALUE & Hash.hash64(sb.toString());
      return Long.toString(nonnegativeHash, 36).toUpperCase();
    }

}