String base64 decoded un-gziped from little-endian 4-byte int to java int

Question

I'm trying to implement TMX files in Android and I was hoping someone could help. Based on the TMX guide, in order to get the GID's I have to

first base64 decode the string, then gunzip the resulting data if the compression attribute is set to "gzip" as in the above example. Finally, you can read 4 bytes at a time for each GID from the beginning of the data stream until the end.

I think I've figured out the base64 decoding and 'gunzip' but the result from the code below is 27,0,0,0 repeating. I think the output is supposed to be

(0,0) (1,0) (2,0) (3,0) (0,1) (1,1) (2,1) (3,1) (0,2) (1,2) (2,2) (3,2)

Thanks!

 public static void main( String[] args )
 {
 String myString = "H4sIAAAAAAAAAO3NoREAMAgEsLedAfafE4+s6l0jolNJiif18tt/Fj8AAMC9ARtYg28AEAAA";

 byte[] decode = Base64.decodeBase64(myString);

 ByteArrayInputStream byteArrayInputStream = new ByteArrayInputStream(decode); 
 GZIPInputStream gzipInputStream;

 int read;
 try
 {
      gzipInputStream = new GZIPInputStream(byteArrayInputStream);

      InputStreamReader inputStreamReader = new InputStreamReader(gzipInputStream);
      BufferedReader bufferedReader = new BufferedReader(inputStreamReader, 4);

      while ( ( read = bufferedReader.read() ) != -1 )
      {
           System.out.println("read :" + read);
      }
 }
 catch (IOException e)
 {
      e.printStackTrace();
 }
 }

Answer 1

Don't use Readers for anything but character data!

Use a DataInput to read the integers. Decorate your GZIPInputStream with a DataInputStream and use readInt.

If the ints are little-endian, you'll need to reverse the order of the bytes for the type. Java uses network byte order (big-endian). For integers, this can be done with Integer.reverseBytes.

You can print the hex values using:

System.out.format("%08x", (int) n);

---

How to read all int values from a stream of arbitrary length:

One mechanism would be to use the available() method which estimates the number of remaining bytes:

byte[] ints = {0x00, 0x00, 0x00, (byte) 0xFF,
               (byte) 0xAA, (byte) 0xBB, (byte) 0xEE, (byte) 0xFF};
ByteArrayInputStream array = new ByteArrayInputStream(ints);
DataInputStream data = new DataInputStream(array);
while(data.available() > 0) {
  int reversed = Integer.reverseBytes(data.readInt());
  System.out.format("%08x%n", reversed);
}

In the general case, available() is not a reliable mechanism. But you can augment your stream with a buffer to check data availability:

  public static void main(String[] args) throws IOException {
    byte[] ints = {0x00, 0x00, 0x00, (byte) 0xFF,
                    (byte) 0xAA, (byte) 0xBB, (byte) 0xEE, (byte) 0xFF};
    ByteArrayInputStream array = new ByteArrayInputStream(ints);
    BufferedInputStream buffer = new BufferedInputStream(array);
    DataInputStream data = new DataInputStream(buffer);
    while(hasData(data)) {
      int reversed = Integer.reverseBytes(data.readInt());
      System.out.format("%08x%n", reversed);
    }
  }

  public static boolean hasData(InputStream in) throws IOException {
    in.mark(1);
    try {
      return in.read() != -1;
    } finally {
      in.reset();
    }
  }