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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);
}
}
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