1 files changed, 189 insertions, 0 deletions
diff --git a/src/main/java/bjc/esodata/DoubleTape.java b/src/main/java/bjc/esodata/DoubleTape.java
new file mode 100644
index 0000000..c36fcff
--- /dev/null
+++ b/src/main/java/bjc/esodata/DoubleTape.java
@@ -0,0 +1,189 @@
+package bjc.esodata;
+
+/**
+ * Double-sided tape is essentially two tapes stuck together with a shared
+ * cursor.
+ *
+ * The main way a double-sided tape differs is that it can be flipped, allowing
+ * access to another set of data.
+ *
+ * However, there is only one cursor, and the position of the cursor on one side
+ * is the inverse of the position on the other side.
+ *
+ * When one side is extended, a null will be inserted into the inactive side
+ * regardless of the auto-extension policy of the tape. The policy will still be
+ * respected for the active side.
+ *
+ * All operations that refer to the tape refer to the currently active side of
+ * the tape, except for flip.
+ *
+ * Flip refers to the entire tape for 'obvious' reasons.
+ *
+ * @param <T>
+ *        The element type of the tape.
+ *
+ * @author bjculkin
+ */
+public class DoubleTape<T> implements Tape<T>, DoubleSided {
+	private boolean frontActive;
+
+	/* The front-side of the tape. */
+	private Tape<T> front;
+	/* The back-side of the tape. */
+	private Tape<T> back;
+
+	/** Create a new empty double-sided tape that doesn't autoextend. */
+	public DoubleTape() {
+		this(false);
+	}
+
+	/**
+	 * Create a new empty double-sided tape that follows the specified
+	 * auto-extension policy.
+	 *
+	 * @param autoExtnd
+	 *        Whether or not to auto-extend the tape to the right w/ nulls.
+	 */
+	public DoubleTape(final boolean autoExtnd) {
+		front = new SingleTape<>(autoExtnd);
+		back = new SingleTape<>(autoExtnd);
+
+		frontActive = true;
+	}
+
+	@Override
+	public T item() {
+		return front.item();
+	}
+
+	@Override
+	public void item(final T itm) {
+		front.item(itm);
+	}
+
+	@Override
+	public int size() {
+		return front.size();
+	}
+
+	@Override
+	public int position() {
+		return front.position();
+	}
+
+	@Override
+	public void insertBefore(final T itm) {
+		front.insertBefore(itm);
+		back.insertAfter(null);
+	}
+
+	@Override
+	public void insertAfter(final T itm) {
+		front.insertAfter(itm);
+		back.insertBefore(itm);
+	}
+
+	@Override
+	public T remove() {
+		back.remove();
+
+		return front.remove();
+	}
+
+	@Override
+	public void first() {
+		front.first();
+		back.last();
+	}
+
+	@Override
+	public void last() {
+		front.last();
+		back.first();
+	}
+
+	@Override
+	public boolean left() {
+		return left(1);
+	}
+
+	@Override
+	public boolean left(final int amt) {
+		final boolean succ = front.left(amt);
+
+		if(succ) {
+			back.right(amt);
+		}
+
+		return succ;
+	}
+
+	@Override
+	public boolean right() {
+		return right(1);
+	}
+
+	@Override
+	public boolean right(final int amt) {
+		final boolean succ = front.right(amt);
+
+		if(succ) {
+			back.left(amt);
+		}
+
+		return succ;
+	}
+
+	public boolean seekTo(int tgtPos) {
+		return front.seekTo(tgtPos);
+	}
+
+	@Override
+	public void flip() {
+		final Tape<T> tmp = front;
+
+		front = back;
+
+		back = tmp;
+
+		frontActive = !frontActive;
+	}
+
+	@Override
+	public boolean currentSide() {
+		return frontActive;
+	}
+
+	@Override
+	public int hashCode() {
+		final int prime = 31;
+		int result = 1;
+		result = prime * result + (back == null ? 0 : back.hashCode());
+		result = prime * result + (front == null ? 0 : front.hashCode());
+		return result;
+	}
+
+	@Override
+	public boolean equals(final Object obj) {
+		if(this == obj) return true;
+		if(obj == null) return false;
+		if(!(obj instanceof DoubleTape<?>)) return false;
+
+		final DoubleTape<?> other = (DoubleTape<?>) obj;
+
+		if(back == null) {
+			if(other.back != null) return false;
+		} else if(!back.equals(other.back)) return false;
+
+		if(front == null) {
+			if(other.front != null) return false;
+		} else if(!front.equals(other.front)) return false;
+
+		return true;
+	}
+
+	@Override
+	public String toString() {
+		return String.format("DoubleTape [front=%s, back=%s]", front, back);
+	}
+}