ZLib decompression fails on large byte array

Question

When experimenting with ZLib compression, I have run across a strange problem. Decompressing a zlib-compressed byte array with random data fails reproducibly if the source array is at least 32752 bytes long. Here's a little program that reproduces the problem, you can see it in action on IDEOne. The compression and decompression methods are standard code picked off tutorials.

public class ZlibMain {

    private static byte[] compress(final byte[] data) {
        final Deflater deflater = new Deflater();
        deflater.setInput(data);

        deflater.finish();
        final byte[] bytesCompressed = new byte[Short.MAX_VALUE];
        final int numberOfBytesAfterCompression = deflater.deflate(bytesCompressed);
        final byte[] returnValues = new byte[numberOfBytesAfterCompression];
        System.arraycopy(bytesCompressed, 0, returnValues, 0, numberOfBytesAfterCompression);
        return returnValues;

    }

    private static byte[] decompress(final byte[] data) {
        final Inflater inflater = new Inflater();
        inflater.setInput(data);
        try (ByteArrayOutputStream outputStream = new ByteArrayOutputStream(data.length)) {
            final byte[] buffer = new byte[Math.max(1024, data.length / 10)];
            while (!inflater.finished()) {
                final int count = inflater.inflate(buffer);
                outputStream.write(buffer, 0, count);
            }
            outputStream.close();
            final byte[] output = outputStream.toByteArray();
            return output;
        } catch (DataFormatException | IOException e) {
            throw new RuntimeException(e);
        }
    }

    public static void main(final String[] args) {
        roundTrip(100);
        roundTrip(1000);
        roundTrip(10000);
        roundTrip(20000);
        roundTrip(30000);
        roundTrip(32000);
        for (int i = 32700; i < 33000; i++) {
            if(!roundTrip(i))break;
        }
    }

    private static boolean roundTrip(final int i) {
        System.out.printf("Starting round trip with size %d: ", i);
        final byte[] data = new byte[i];
        for (int j = 0; j < data.length; j++) {
            data[j]= (byte) j;
        }
        shuffleArray(data);

        final byte[] compressed = compress(data);
        try {
            final byte[] decompressed = CompletableFuture.supplyAsync(() -> decompress(compressed))
                                                         .get(2, TimeUnit.SECONDS);
            System.out.printf("Success (%s)%n", Arrays.equals(data, decompressed) ? "matching" : "non-matching");
            return true;
        } catch (InterruptedException | ExecutionException | TimeoutException e) {
            System.out.println("Failure!");
            return false;
        }
    }

    // Implementing Fisher–Yates shuffle
    // source: https://stackoverflow.com/a/1520212/342852
    static void shuffleArray(byte[] ar) {
        Random rnd = ThreadLocalRandom.current();
        for (int i = ar.length - 1; i > 0; i--) {
            int index = rnd.nextInt(i + 1);
            // Simple swap
            byte a = ar[index];
            ar[index] = ar[i];
            ar[i] = a;
        }
    }
}

Is this a known bug in ZLib? Or do I have an error in my compress / decompress routines?

Sean Patrick Floyd Sean Patrick Floyd · Accepted Answer · 2017-05-31T13:01:18

Apparently the compress() method was faulty. This one works:

public static byte[] compress(final byte[] data) {
    try (final ByteArrayOutputStream outputStream = 
                                     new ByteArrayOutputStream(data.length);) {

        final Deflater deflater = new Deflater();
        deflater.setInput(data);
        deflater.finish();
        final byte[] buffer = new byte[1024];
        while (!deflater.finished()) {
            final int count = deflater.deflate(buffer);
            outputStream.write(buffer, 0, count);
        }

        final byte[] output = outputStream.toByteArray();
        return output;
    } catch (IOException e) {
        throw new IllegalStateException(e);
    }
}

ZLib decompression fails on large byte array

2 Answers