bundleplugin/src/main/java/aQute/libg/gzip/GZipUtils.java

package aQute.libg.gzip;

import java.io.BufferedInputStream;
import java.io.EOFException;
import java.io.IOException;
import java.io.InputStream;
import java.util.zip.GZIPInputStream;

public class GZipUtils {
	
	/**
	 * Determines whether the specified stream contains gzipped data, by
	 * checking for the GZIP magic number, and returns a stream capable of
	 * reading those data.
	 * 
	 * @throws IOException
	 */
	public static InputStream detectCompression(InputStream stream) throws IOException {
		InputStream buffered;
		if (stream.markSupported())
			buffered = stream;
		else
			buffered = new BufferedInputStream(stream);
		
		buffered.mark(2);
		int magic = readUShort(buffered);
		buffered.reset();
		
		InputStream result;
		if (magic == GZIPInputStream.GZIP_MAGIC)
			result = new GZIPInputStream(buffered);
		else
			result = buffered;
		return result;
	}

    /*
     * Reads unsigned short in Intel byte order.
     */
	private static int readUShort(InputStream in) throws IOException {
		int b = readUByte(in);
		return (readUByte(in) << 8) | b;
	}
    
    /*
     * Reads unsigned byte.
     */
    private static int readUByte(InputStream in) throws IOException {
	int b = in.read();
	if (b == -1) {
	    throw new EOFException();
	}
        if (b < -1 || b > 255) {
            // Report on this.in, not argument in; see read{Header, Trailer}.
            throw new IOException(in.getClass().getName() + ".read() returned value out of range -1..255: " + b);
        }
	return b;
    }

}