static bool structure_showTiming, structure_checkTokenCount;

static S structure(O o) {
  ret structure(o, new structure_Data);
}

static S structure(O o, structure_Data d) {
  new StringWriter sw;
  d.out = new PrintWriter(sw);
  structure_go(o, d);
  S s = str(sw);
  if (structure_checkTokenCount) {
    print("token count=" + d.n);
    assertEquals("token count", l(javaTokC(s)), d.n);
  }
  ret s;
}

static void structure_go(O o, structure_Data d) {
  structure_1(o, d);
  while (nempty(d.stack))
    popLast(d.stack).run();
}

static void structureToPrintWriter(O o, PrintWriter out) {
  new structure_Data d;
  d.out = out;
  structure_go(o, d);
}

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

static class structure_Data {
  PrintWriter out;
  int stringSizeLimit;
  int shareStringsLongerThan = 20;

  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");
  int n; // token count
  new L<Runnable> stack;
  
  // append single token
  structure_Data append(S token) { out.print(token); ++n; ret this; }
  structure_Data append(int i) { out.print(i); ++n; ret this; }
  
  // append multiple tokens
  structure_Data append(S token, int tokCount) { out.print(token); n += tokCount; ret this; }
  
  // extend last token
  structure_Data app(S token) { out.print(token); ret this; }
  structure_Data app(int i) { out.print(i); ret this; }
}

static void structure_1(final Object o, final structure_Data d) {
  final PrintWriter out = d.out;
  
  if (o == null) { d.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) { int i = ((Int) o).intValue(); out.print(i); d.n += i < 0 ? 2 : 1; ret; }
      if (o instanceof Long) { long l = ((Long) o).longValue(); out.print(l); out.print("L"); d.n += l < 0 ? 2 : 1; ret; }
      if (o instanceof Float) { d.append("fl ", 2); quoteToPrintWriter(str(o), out); ret; }
      if (o instanceof Double) { d.append("d(", 3); quoteToPrintWriter(str(o), out); d.append(")"); ret; }
      if (o instanceof BigInteger) { out.print("bigint("); out.print(o); out.print(")"); d.n += ((BigInteger) o).signum() < 0 ? 5 : 4; ret; }
    }
  
    if (o instanceof Boolean) {
      d.append(((Boolean) o).booleanValue() ? "t" : "f"); ret;
    }
      
    if (o instanceof Character) {
      d.append(quoteCharacter((Character) o)); ret;
    }
      
    if (o instanceof File) {
      d.append("File ").append(quote(((File) o).getPath())); ret;
    }
      
    // referencable objects follow
    
    Integer ref = d.seen.get(o);
    if (o instanceof S && ref == null)
      ref = d.strings.get((S) o);
      
    if (ref != null) {
      d.refd.set(ref);
      d.append("t").app(ref); ret;
    }
    
    ref = d.n; //d.seen.size()+1;
    if (d.shareStringsLongerThan < Int.MAX_VALUE || !o instanceof S)
      d.seen.put(o, ref);
    //d.append("m").app(ref).app(" "); // marker
    
    if (o instanceof S) {
      S s = d.stringSizeLimit != 0 ? shorten((S) o, d.stringSizeLimit) : (S) o;
      if (l(s) >= d.shareStringsLongerThan)
        d.strings.put(s, ref);
      quoteToPrintWriter(s, out); d.n++; ret;
    }
      
    if (o instanceof HashSet) {
      d.append("hashset ");
      structure_1(new ArrayList((Set) o), d);
      ret;
    }
  
    if (o instanceof TreeSet) {
      d.append("treeset ");
      structure_1(new ArrayList((Set) o), d);
      ret;
    }
    
    if (o instanceof Collection && neq(name, "main$Concept$RefL")) {
      d.append("[");
      final int l = d.n;
      final Iterator it = ((Collection) o).iterator();
      d.stack.add(r {
        if (!it.hasNext())
          d.append("]");
        else {
          d.stack.add(this);
          if (d.n != l) d.append(", ");
          structure_1(it.next(), d);
        }
      });
      ret;
    }
    
    if (o instanceof Map) {
      if (o instanceof LinkedHashMap) d.append("lhm");
      else if (o instanceof HashMap) d.append("hm");
      d.append("{");
      final int l = d.n;
      final Iterator it = ((Map) o).entrySet().iterator();
      
      d.stack.add(new Runnable() {
        bool v;
        Map.Entry e;
        
        public void run() {
          if (v) {
            d.append("=");
            v = false;
            d.stack.add(this);
            structure_1(e.getValue(), d);
          } else {
            if (!it.hasNext())
              d.append("}");
            else {
              e = (Map.Entry) it.next();
              v = true;
              d.stack.add(this);
              if (d.n != l) d.append(", ");
              structure_1(e.getKey(), d);
            }
          }
        }
      });
      ret;
    }
    
    if (c.isArray()) {
      if (o instanceof byte[]) {
        d.append("ba ").append(quote(bytesToHex((byte[]) o))); ret;
      }
  
      final 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;
        d.append("boolarray ").append(n).app(" ").append(quote(substring(hex, 0, i))); ret;
      }
      
      S atype = "array", sep = ", ";
  
      if (o instanceof int[]) {
        //ret "intarray " + quote(intArrayToHex((int[]) o));
        atype = "intarray";
        sep = " ";
      }
      
      d.append(atype).append("{");
      d.stack.add(new Runnable() {
        int i;
        public void run() {
          if (i >= n)
            d.append("}");
          else {
            d.stack.add(this);
            if (i > 0) d.append(", ");
            structure_1(Array.get(o, i++), d);
          }
        }
      });
      ret;
    }
  
    if (o instanceof Class) {
      d.append("class(", 2).append(quote(((Class) o).getName())).append(")"); ret;
    }
      
    if (o instanceof Throwable) {
      d.append("exception(", 2).append(quote(((Throwable) o).getMessage())).append(")"); ret;
    }
      
    if (o instanceof BitSet) {
      BitSet bs = (BitSet) o;
      d.append("bitset{", 2);
      int l = d.n;
      for (int i = bs.nextSetBit(0); i >= 0; i = bs.nextSetBit(i+1)) {
        if (d.n != l) d.append(", ");
        d.append(i);
      }
      d.append("}"); ret;
    }
      
    // Need more cases? This should cover all library classes...
    if (name.startsWith("java.") || name.startsWith("javax.")) {
      d.append("j ").append(quote(str(o))); ret; // Hm. this is not unstructure-able
    }
    
    ifclass Lisp
      if (o << Lisp) {
        d.append("l(", 2);
        final Lisp lisp = cast o;
        structure_1(lisp.head, d);
        final Iterator it = lisp.args.iterator();
        d.stack.add(r {
          if (!it.hasNext())
            d.append(")");
          else {
            d.stack.add(this);
            d.append(", ");
            structure_1(it.next(), d);
          }
        });
        ret;
      }
    endif
      
    /*if (name.equals("main$Lisp")) {
      fail("lisp not supported right now");
    }*/
    
    if (concept && !d.concepts.contains(dynName)) {
      d.concepts.add(dynName);
      d.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);
    
    // Render this$1 first because unstructure needs it for constructor call.
    
    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;
  
  d.append(shortName);

  final int l = d.n;
  final Iterator it = fv.entrySet().iterator();
  
  d.stack.add(r {
    if (!it.hasNext()) {
      if (d.n != l)
        d.append(")");
    } else {
      Map.Entry e = (Map.Entry) it.next();
      d.append(d.n == l ? "(" : ", ");
      d.append((S) e.getKey()).append("=");
      d.stack.add(this);
      structure_1(e.getValue(), d);
    }
  });
}

Began life as a copy of #1005656