sclass AdaptiveIdentifierCompression {
  new SS shortenings;
  new MultiSet<S> tokenCount;
  new SS expansions;
  S escapeWord = "xx";
  S codeAlphabet = lowerCaseAlphabet();
  ItIt<S> newCodeMaker;
  int conflicts;
  settable int minCountToCompress = 2;

  void init {
    newCodeMaker if null = allWordsOfAlphabet(codeAlphabet);
  }
  
  S encode(S token) {
    if (!isIdentifier(token)) ret token;
    
    init();
    tokenCount.add(token);
    
    S code = shortenings.get(token);
    if (code != null)
      ret code;
      
    int count = tokenCount.get(token);
    if (count < minCountToCompress)
      ret token;
      
    S conflict = expansions.get(code);
    if (conflict != null) {
      warn("conflict: " + token);
      ++conflicts;
      // TODO
    }

    code = newCode();
    if (code != null) {
      shortenings.put(token, code);
      expansions.put(code, token);
    }
    ret token;
  }
  
  S newCode() {
    S code;
    do {
      if (!newCodeMaker.hasNext())
        null;
      code = newCodeMaker.next();
    } while (shortenings.containsKey(code));
    ret code;
  }
}