static String structure(Object o) {
  new HashSet refd;
  return structure_2(structure_1(o, new structure_Data(refd)), refd);
}

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

static int structure_shareStringsLongerThan = 20;

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

static String structure_1(Object o, structure_Data d) {
  if (o == null) return "null";
  
  // these are never back-referenced (for readability)
  
  if (o instanceof BigInteger)
    return "bigint(" + o + ")";
  
  if (o instanceof Double)
    return "d(" + quote(str(o)) + ")";
    
  if (o instanceof Float)
    return "fl " + quote(str(o));
    
  if (o instanceof Long)
    return o + "L";
  
  if (o instanceof Integer)
    return str(o);
    
  if (o instanceof Boolean)
    return ((Boolean) o).booleanValue() ? "t" : "f";
    
  if (o instanceof Character)
    ret quoteCharacter((Character) o);
    
  if (o instanceof File)
    ret "File " + quote(((File) o).getPath());
    
  // referencable objects follow
  
  Integer ref = d.seen.get(o);
  if (ref != null) {
    d.refd.add(ref);
    return "r" + ref;
  }
      
  if (o instanceof S && (ref = d.strings.get((S) o)) != null) {
    d.refd.add(ref);
    return "r" + ref;
  }
  
  ref = d.seen.size()+1;
  d.seen.put(o, ref);
  S r = "m" + ref + " "; // 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);
    return r + quote(s);
  }
    
  String name = o.getClass().getName();

  StringBuilder buf = new StringBuilder();
  
  if (o instanceof HashSet)
    return r + "hashset " + structure_1(new ArrayList((Set) o), d);

  if (o instanceof TreeSet)
    return r + "treeset " + structure_1(new ArrayList((Set) o), d);
  
  if (o instanceof Collection && neq(name, "main$Concept$RefL")) {
    for (Object x : (Collection) o) {
      if (buf.length() != 0) buf.append(", ");
      buf.append(structure_1(x, d));
    }
    return r + "[" + buf + "]";
  }
  
  if (o instanceof Map) {
    for (Object e : ((Map) o).entrySet()) {
      if (buf.length() != 0) buf.append(", ");
      buf.append(structure_1(((Map.Entry) e).getKey(), d));
      buf.append("=");
      buf.append(structure_1(((Map.Entry) e).getValue(), d));
    }
    return r + (o instanceof HashMap ? "hm" : "") + "{" + buf + "}";
  }
  
  if (o.getClass().isArray()) {
    if (o instanceof byte[])
      ret "ba " + quote(bytesToHex((byte[]) o));

    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;
      ret "boolarray " + n + " " + quote(substring(hex, 0, i));
    }
    
    S atype = "array", sep = ", ";

    if (o instanceof int[]) {
      //ret "intarray " + quote(intArrayToHex((int[]) o));
      atype = "intarray";
      sep = " ";
    }

    for (int i = 0; i < n; i++) {
      if (buf.length() != 0) buf.append(sep);
      buf.append(structure_1(Array.get(o, i), d));
    }
    return r + atype + "{" + buf + "}";
  }

  if (o instanceof Class)
    return r + "class(" + quote(((Class) o).getName()) + ")";
    
  if (o instanceof Throwable)
    return r + "exception(" + quote(((Throwable) o).getMessage()) + ")";
    
  if (o instanceof BitSet) {
    BitSet bs = (BitSet) o;
    for (int i = bs.nextSetBit(0); i >= 0; i = bs.nextSetBit(i+1)) {
      if (buf.length() != 0) buf.append(", ");
       buf.append(i);
    }
    return "bitset{" + buf + "}";
  }
    
  // Need more cases? This should cover all library classes...
  if (name.startsWith("java.") || name.startsWith("javax."))
    return r + String.valueOf(o);
    
  String shortName = o.getClass().getName().replaceAll("^main\\$", "");
  
  if (shortName.equals("Lisp")) {
    buf.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++) {
        buf.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");
          
        buf.append(structure_1(arg, d));
      }
    buf.append(")");
    ret r + str(buf);
  }
  
  Class c = o.getClass();
  bool concept = d.conceptClass != null && d.conceptClass.isInstance(o);
  if (concept && !d.concepts.contains(c.getName())) {
    d.concepts.add(c.getName());
    r += "c ";
  }
  
  // serialize an object with fields.
  // first, collect all fields and values in fv.
  
  new TreeMap<S, O> fv;
  while (c != Object.class) {
    for (Field field : c.getDeclaredFields()) {
      if ((field.getModifiers() & (Modifier.STATIC | Modifier.TRANSIENT)) != 0)
        continue;
      S fieldName = field.getName();
      
      Object value;
      try {
        field.setAccessible(true);
        value = field.get(o);
      } catch (Exception e) {
        value = "?";
      }
      
      // omit field "className" if equal to class's name
      if (concept && eq(fieldName, "className")
        && eq(value, shortName)) value = null;
      
      // put special cases here...
      
      if (value != null)
        fv.put(fieldName, value);
    }
    c = c.getSuperclass();
  }
  
  // Now we have fields & values. Process fieldValues if it's a DynamicObject.
  
  if (o instanceof DynamicObject) {
    fv.putAll((Map) fv.get("fieldValues"));
    fv.remove("fieldValues");
    shortName = (S) fv.get("className");
    fv.remove("className");
  }
  
  S singleField = fv.size() == 1 ? first(fv.keySet()) : null;
  
  // Render this$1 first because unstructure needs it for constructor call.
  
  if (fv.containsKey("this$1")) {
    buf.append("this$1=" + structure_1(fv.get("this$1"), d));
    fv.remove("this$1");
  }
  
  // Render the other fields.

  for (S fieldName : fv.keySet()) {
    if (buf.length() != 0) buf.append(", ");
    buf.append(fieldName + "=" + structure_1(fv.get(fieldName), d));
  }

  String b = buf.toString();
  
  if (structure_allowShortening && singleField != null)
    b = b.replaceAll("^" + singleField + "=", ""); // drop field name if only one
  String s = shortName;
  if (buf.length() != 0)
    s += "(" + b + ")";
  return r + s;
}

// drop unused markers
static S structure_2(S s, HashSet<Integer> refd) {
  L<S> tok = javaTok(s);
  new StringBuilder out;
  for (int i = 1; i < l(tok); i += 2) {
    S t = tok.get(i);
    if (t.startsWith("m") && isInteger(t.substring(1))
      && !refd.contains(parseInt(t.substring(1))))
      continue;
    out.append(t).append(tok.get(i+1));
  }
  ret str(out);
}

Began life as a copy of #1002825