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package bjc.utils.esodata;
import java.util.ArrayList;
/**
* A tape is a one-dimensional array that can only be accessed in one position
* at a time.
*
* A tape is essentially a 1D array with a cursor attached to it, and you can
* only affect elements at that cursor. The size of the array is theoretically
* unbounded to the right, but in practice bounded by available memory.
*
* You can choose whether or not you want the tape to automatically extend
* itself to the right with null elements by specifiying its auto-extension
* policy.
*
* @param T
* The element type of the tape.
* @author bjculkin
*/
public class SingleTape<T> implements Tape<T> {
protected ArrayList<T> backing;
protected int pos;
protected boolean autoExtend;
/**
* Create a new empty tape that doesn't autoextend.
*/
public SingleTape() {
this(false);
}
/**
* Create a new empty tape that follows the specified auto-extension
* policy.
*
* @param autoExtnd
* Whether or not to auto-extend the tape to the right w/
* nulls.
*/
public SingleTape(boolean autoExtnd) {
autoExtend = autoExtnd;
backing = new ArrayList<>();
}
/**
* Get the item the tape is currently on.
*
* @return The item the tape is on.
*/
@Override
public T item() {
return backing.get(pos);
}
/**
* Set the item the tape is currently on.
*
* @param itm
* The new value for the tape item.
*/
@Override
public void item(T itm) {
backing.set(pos, itm);
}
/**
* Get the current number of elements in the tape.
*
* @return The current number of elements in the tape.
*/
@Override
public int size() {
return backing.size();
}
/**
* Insert an element before the current item.
*
* @param itm
* The item to add.
*/
@Override
public void insertBefore(T itm) {
backing.add(pos, itm);
}
/**
* Insert an element after the current item.
*/
@Override
public void insertAfter(T itm) {
if(pos == backing.size() - 1) {
backing.add(itm);
} else {
backing.add(pos + 1, itm);
}
}
/**
* Remove the current element.
*
* Also moves the cursor back one step if possible to maintain relative
* position.
*
* @return The removed item.
*/
@Override
public T remove() {
T res = backing.remove(pos);
if(pos != 0) {
pos -= 1;
}
return res;
}
/**
* Move the cursor to the left-most position.
*/
@Override
public void first() {
pos = 0;
}
/**
* Move the cursor the right-most position.
*/
@Override
public void last() {
pos = backing.size() - 1;
}
/**
* Move the cursor one space left.
*
* The cursor can't go past zero.
*
* @return True if the cursor was moved left.
*/
@Override
public boolean left() {
return left(1);
}
/**
* Move the cursor the specified amount left.
*
* The cursor can't go past zero. Attempts to move the cursor by amounts
* that would exceed zero don't move the cursor at all.
*
* @param amt
* The amount to attempt to move the cursor left.
*
* @return True if the cursor was moved left.
*/
@Override
public boolean left(int amt) {
if(pos - amt < 0) return false;
pos -= amt;
return true;
}
/**
* Move the cursor one space right.
*
* Moving the cursor right will auto-extend the tape if that is enabled.
*
* @return Whether the cursor was moved right.
*/
@Override
public boolean right() {
return right(1);
}
/**
* Move the cursor the specified amount right.
*
* Moving the cursor right will auto-extend the tape if that is enabled.
*
* @param amt
* The amount to move the cursor right by.
*
* @return Whether the cursor was moved right.
*/
@Override
public boolean right(int amt) {
if(pos + amt >= backing.size() - 1) {
if(autoExtend) {
while(pos + amt >= backing.size() - 1) {
backing.add(null);
}
} else
return false;
}
pos += amt;
return true;
}
@Override
public boolean isDoubleSided() {
return false;
}
}
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