Add ability to report absolute positions (relative to the root stream)

Fix kaitai-io#26

Author: Mingun

src/main/java/io/kaitai/struct/ByteBufferKaitaiStream.java

@@ -63,6 +63,20 @@ public ByteBufferKaitaiStream(String fileName) throws IOException {
      * @param arr byte array to read from or write to
      */
     public ByteBufferKaitaiStream(byte[] arr) {
+        this(arr, 0);
+    }
+
+    /**
+     * Initializes a stream that will get data from the given byte array when read.
+     * Internally, ByteBuffer wrapping given array will be used.
+     *
+     * @param arr byte array to read
+     * @param offset offset from the root stream where this stream begins
+     *
+     * @since 0.11
+     */
+    public ByteBufferKaitaiStream(byte[] arr, long offset) {
+        super(offset);
         fc = null;
         bb = ByteBuffer.wrap(arr);
     }
@@ -73,6 +87,19 @@ public ByteBufferKaitaiStream(byte[] arr) {
      * @param buffer {@link ByteBuffer} to read from or write to
      */
     public ByteBufferKaitaiStream(ByteBuffer buffer) {
+        this(buffer, 0);
+    }
+
+    /**
+     * Initializes a stream that will get data from given {@link ByteBuffer} on read.
+     *
+     * @param buffer ByteBuffer to read from
+     * @param offset offset from the root stream where this stream begins
+     *
+     * @since 0.11
+     */
+    public ByteBufferKaitaiStream(ByteBuffer buffer, long offset) {
+        super(offset);
         fc = null;
         bb = buffer;
     }

src/main/java/io/kaitai/struct/KaitaiStream.java

@@ -60,13 +60,31 @@
 public abstract class KaitaiStream implements Closeable {
     protected int bitsLeft = 0;
     protected long bits = 0;
+    /**
+     * Offset from the root stream where this stream begins.
+     *
+     * @since 0.11
+     */
+    protected final long offset;
     protected boolean bitsLe = false;
     protected boolean bitsWriteMode = false;
 
     protected WriteBackHandler writeBackHandler;
 
     protected List<KaitaiStream> childStreams = new ArrayList<>();
 
+    /** Initializes a stream with zero offset from the root stream. */
+    public KaitaiStream() { this(0); }
+
+    /**
+     * Initializes a stream with specified offset from the root stream.
+     *
+     * @param offset offset from the root stream where this stream begins
+     *
+     * @since 0.11
+     */
+    public KaitaiStream(long offset) { this.offset = offset; }
+
     @Override
     abstract public void close() throws IOException;
 
@@ -90,6 +108,16 @@ public abstract class KaitaiStream implements Closeable {
      */
     abstract public void seek(long newPos);
 
+    /**
+     * Get position of a stream pointer relative to the root stream in the stream hierarchy.
+     * Root stream is a stream without parent stream.
+     *
+     * @return the pointer position, number of bytes from the beginning of the root stream
+     *
+     * @since 0.11
+     */
+    public long offset() { return this.offset; }
+
     /**
      * Get current position of a stream pointer.
      * @return pointer position, number of bytes from the beginning of the stream