Why Do Java and Go Produce Different GZIP Compression Results?

Why Does Gzip Compression Differ Between Java and Go?

When compressing data using gzip in Java and Go, you may encounter different results. This disparity stems from fundamental differences in data representation and compression implementation.

Byte Representation

Java's byte type is signed, ranging from -128 to 127. In Go, the byte type is an alias for uint8, representing unsigned integers from 0 to 255. This means that negative values in Java must be shifted by 256 to match the range of Go bytes.

Compression Differences

Even after accounting for byte representation, compression results may still diverge between Java and Go. The gzip algorithm, which employs LZ77 and Huffman coding, is influenced by the frequency of input characters. Variations in character frequencies can lead to different output codes and bit patterns.

Additionally, different implementations may employ different default compression levels. While Java and Go both nominally use a default level of 6, slight variations in implementation can account for residual differences.

Achieving Similar Output

To eliminate these differences and obtain matching gzip outputs, you can set the compression level to 0 in both languages. Java offers the Deflater.NO_COMPRESSION option, while Go provides gzip.NoCompression.

Example Java Code:

ByteArrayOutputStream buf = new ByteArrayOutputStream();
GZIPOutputStream gz = new GZIPOutputStream(buf) {
  {
    def.setLevel(Deflater.NO_COMPRESSION);
  }
};
gz.write("helloworld".getBytes("UTF-8"));
gz.close();
for (byte b : buf.toByteArray())
  System.out.print((b & 0xff) + " ");

Example Go Code:

var buf bytes.Buffer
gz, _ := gzip.NewWriterLevel(&buf, gzip.NoCompression)
gz.Write([]byte("helloworld"))
gz.Close()
fmt.Println(buf.Bytes())

Header Fields

It's worth noting that gzip includes optional header fields, such as modification time and file name. Java does not add these fields by default, while Go does. Therefore, even with the same compression level, exact output may not be achieved due to these additional headers.

Practical Considerations

Although the compressed outputs may not match between Java and Go, the data can still be decompressed using any compatible gzip decoder. Decompressed data will be identical irrespective of the compression implementation. Therefore, the differences in output are not practically significant.

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