sbool structure_showTiming;

static String structure(Object o) {
  new structure_Data d;
  structure_1(o, d);
  long time = now();
  S s = str(d.out);
  int nOrig = l(s);
  d.out = null;
  s = structure_2(s, d.refd);
  if (structure_showTiming)
    done_always("structure_2 (" + nOrig + " => " + l(s) + ")", time);
  ret s;
}

// leave to false, unless unstructure() breaks
static boolean structure_allowShortening = false;

static int structure_shareStringsLongerThan = 20;

static class structure_Data {
  new StringBuilder out;
  int stringSizeLimit;
  new IdentityHashMap<O, Integer> seen;
  new BitSet refd;
  new HashMap<S, Int> strings;
  new HashSet<S> concepts;
  HashMap<Class, L<Field>> fieldsByClass = new HashMap;
  Class conceptClass = findClass("Concept");
}

static void structure_1(Object o, structure_Data d) {
  StringBuilder out = d.out;
  
  if (o == null) { out.append("null"); ret; }
  
  Class c = o.getClass();
  S name = c.getName();
  S dynName = shortDynamicClassName(o);
  bool concept = d.conceptClass != null && d.conceptClass.isInstance(o);
  L<Field> lFields = d.fieldsByClass.get(c);
  
  if (lFields == null) {
    // these are never back-referenced (for readability)
    
    if (o instanceof Number) {
      if (o instanceof Integer) { out.append((int) o); ret; }
      if (o instanceof Long) { out.append((long) o).append("L"); ret; }
      if (o instanceof Float) { out.append("fl "); quote_impl(str(o), out); ret; }
      if (o instanceof Double) { out.append("d("); quote_impl(str(o), out); out.append(")"); ret; }
      if (o instanceof BigInteger) { out.append("bigint(").append(o).append(")"); ret; }
    }
  
    if (o instanceof Boolean) {
      out.append(((Boolean) o).booleanValue() ? "t" : "f"); ret;
    }
      
    if (o instanceof Character) {
      out.append(quoteCharacter((Character) o)); ret;
    }
      
    if (o instanceof File) {
      out.append("File ").append(quote(((File) o).getPath())); ret;
    }
      
    // referencable objects follow
    
    Integer ref = null;
    if (!(o instanceof S)) {
      ref = d.seen.get(o);
      if (ref != null) {
        d.refd.set(ref);
        out.append("r").append(ref); ret;
      }
    }
    
    if (o instanceof S && (ref = d.strings.get((S) o)) != null) {
      d.refd.set(ref);
      out.append("r").append(ref); ret;
    }
    
    ref = d.seen.size()+1;
    d.seen.put(o, ref);
    out.append("m").append(ref).append(" "); // marker
  
    if (o instanceof S) {
      S s = d.stringSizeLimit != 0 ? shorten((S) o, d.stringSizeLimit) : (S) o;
      if (l(s) >= structure_shareStringsLongerThan)
        d.strings.put(s, ref);
      quote_impl(s, out); ret;
    }
      
    if (o instanceof HashSet) {
      out.append("hashset ");
      structure_1(new ArrayList((Set) o), d);
      ret;
    }
  
    if (o instanceof TreeSet) {
      out.append("treeset ");
      structure_1(new ArrayList((Set) o), d);
      ret;
    }
    
    if (o instanceof Collection && neq(name, "main$Concept$RefL")) {
      out.append("[");
      int l = out.length();
      for (Object x : (Collection) o) {
        if (out.length() != l) out.append(", ");
        structure_1(x, d);
      }
      out.append("]");
      ret;
    }
    
    if (o instanceof Map) {
      if (o instanceof HashMap) out.append("hm");
      out.append("{");
      int l = out.length();
      for (Object e : ((Map) o).entrySet()) {
        if (out.length() != l) out.append(", ");
        structure_1(((Map.Entry) e).getKey(), d);
        out.append("=");
        structure_1(((Map.Entry) e).getValue(), d);
      }
      out.append("}");
      ret;
    }
    
    if (c.isArray()) {
      if (o instanceof byte[]) {
        out.append("ba ").append(quote(bytesToHex((byte[]) o))); ret;
      }
  
      int n = Array.getLength(o);
  
      if (o instanceof bool[]) {
        S hex = boolArrayToHex((bool[]) o);
        int i = l(hex);
        while (i > 0 && hex.charAt(i-1) == '0' && hex.charAt(i-2) == '0') i -= 2;
        out.append("boolarray ").append(n).append(" ").append(quote(substring(hex, 0, i))); ret;
      }
      
      S atype = "array", sep = ", ";
  
      if (o instanceof int[]) {
        //ret "intarray " + quote(intArrayToHex((int[]) o));
        atype = "intarray";
        sep = " ";
      }
      
      out.append(atype).append("{");
      for (int i = 0; i < n; i++) {
        if (i != 0) out.append(sep);
        structure_1(Array.get(o, i), d);
      }
      out.append("}"); ret;
    }
  
    if (o instanceof Class) {
      out.append("class(").append(quote(((Class) o).getName())).append(")"); ret;
    }
      
    if (o instanceof Throwable) {
      out.append("exception(").append(quote(((Throwable) o).getMessage())).append(")"); ret;
    }
      
    if (o instanceof BitSet) {
      BitSet bs = (BitSet) o;
      out.append("bitset{");
      int l = out.length();
      for (int i = bs.nextSetBit(0); i >= 0; i = bs.nextSetBit(i+1)) {
        if (out.length() != l) out.append(", ");
        out.append(i);
      }
      out.append("}"); ret;
    }
      
    // Need more cases? This should cover all library classes...
    if (name.startsWith("java.") || name.startsWith("javax.")) {
      out.append(str(o)); ret;
    }
      
    if (name.equals("main$Lisp")) {
      out.append("l(");
      structure_1(getOpt(o, "head"), d);
      L args = cast getOpt(o, "args");
      if (nempty(args))
        for (int i = 0; i < l(args); i++) {
          out.append(", ");
          O arg = args.get(i);
          
          // sweet shortening
          if (arg != null && eq(arg.getClass().getName(), "main$Lisp") && isTrue(call(arg, "isEmpty")))
            arg = get(arg, "head");
            
          structure_1(arg, d);
        }
      out.append(")"); ret;
    }
    
    if (concept && !d.concepts.contains(dynName)) {
      d.concepts.add(dynName);
      out.append("c ");
    }
    
    // serialize an object with fields.
    // first, collect all fields and values in fv.
    
    TreeSet<Field> fields = new TreeSet<Field>(new Comparator<Field>() {
      public int compare(Field a, Field b) {
        ret stdcompare(a.getName(), b.getName());
      }
    });
    
    while (c != Object.class) {
      for (Field field : getDeclaredFields_cached(c)) {
        if ((field.getModifiers() & (Modifier.STATIC | Modifier.TRANSIENT)) != 0)
          continue;
        S fieldName = field.getName();
        
        fields.add(field);
        
        // put special cases here...
      }
        
      c = c.getSuperclass();
    }
    
    lFields = asList(fields);
    
    // put this$1 first
    for (int i = 0; i < l(lFields); i++) {
      Field f = lFields.get(i);
      if (f.getName().equals("this$1")) {
        lFields.remove(i);
        lFields.add(0, f);
        break;
      }
    }
  
    d.fieldsByClass.put(c, lFields);
  }

  new LinkedHashMap<S, O> fv;
  for (Field f : lFields) {
    Object value;
    try {
      value = f.get(o);
    } catch (Exception e) {
      value = "?";
    }
      
    if (value != null)
      fv.put(f.getName(), value);
  }
  
  String shortName = dropPrefix("main$", name);
    
  // Now we have fields & values. Process fieldValues if it's a DynamicObject.
  
  // omit field "className" if equal to class's name
  if (concept && eq(fv.get("className"), shortName))
    fv.remove("className");
          
  if (o instanceof DynamicObject) {
    fv.putAll((Map) fv.get("fieldValues"));
    fv.remove("fieldValues");
    shortName = dynName;
    fv.remove("className");
  }
  
  S singleField = fv.size() == 1 ? first(fv.keySet()) : null;
  
  // Render this$1 first because unstructure needs it for constructor call.
  
  out.append(shortName).append("(");
  int l = out.length();

  for (S fieldName : fv.keySet()) {
    if (out.length() != l) out.append(", ");
    out.append(fieldName).append("=");
    structure_1(fv.get(fieldName), d);
  }

  if (out.length() == l) out.setLength(l-1); // drop "("
  else out.append(")");
}

// drop unused markers
static S structure_2(final S s, final BitSet refd) {
  final new StringBuilder out;
  
  javaTok_streaming(s, new JavaTok_Stream {
    int n = -1;
    bool drop;
    
    void add(int i, int j) {
      ++n;
      if ((n & 1) != 0 && structure_isMarker(s, i, j)) {
        S t = s.substring(i+1, j);
        if (!refd.get(parseInt(t))) {
          drop = true;
          ret;
        }
      }
      if (drop)
        drop = false;
      else
        out.append(s, i, j);
    }
  });
  
  ret str(out);
}

Began life as a copy of #1005593