/**
 * Recursive list of objects.
 * @author Jeff Raab
 */
public class RecursiveList implements IList, Position {

    /** First object in this. */
    protected Object first;

    /** Rest of this. */
    protected IList rest;
    
    /** 
     * Constructs a recursive list containing the given object.
     * @param anObject object to be stored in the list
     */
    public RecursiveList(Object anObject) {
        this(anObject, EmptyRecursiveList.EMPTY);
    }

    /** 
     * Constructs a recursive list containing the given object,
     * with the given rest of the list.
     * @param anObject object to be stored in the list
     * @param aList rest of the list
     */
    public RecursiveList(Object anObject, IList aList) {
        first = anObject;
        rest = aList;
    }

    /** Returns the first element in this. */
    public Object element() {
        return first;
    }

    /** Returns the first position in this. */
    public Position first() {
        return this;
    }
    
    /** Returns the last position in this. */
    public Position last() {
        if (isLast(this)) {
            return this;
        }
        
        return rest.last();
    }
    
    /** 
     * Returns the position in this 
     * that comes before the given position.
     * @param aPosition position for which previous position is desired
     */
    public Position before(Position aPosition) {
        if ((aPosition == this) && isFirst(this)) {
            throw new RuntimeException("Position is first");
        }

        if (aPosition == rest) {
            return this;
        }
        
        return rest.before(aPosition);
    }

    /** 
     * Returns the position in this 
     * that comes before the given position.
     * @param aPosition position for which previous position is desired
     */
    public Position after(Position aPosition) {
        if (aPosition == this) {
            if (isLast(this)) {
                throw new RuntimeException("Position is last");
            }
            else {
                return (RecursiveList)rest;
            }
        }
        
        return rest.after(aPosition);
    }
    
    /**
     * Returns whether or not the given position is first in this.
     * @param aPosition position to test
     */
    public boolean isFirst(Position aPosition) {
        return (this == aPosition);
    }

    /**
     * Returns whether or not the given position is last in this.
     * @param aPosition position to test
     */
    public boolean isLast(Position aPosition) {
        if (aPosition == this) {
            return rest.isEmpty();
        }
        
        return rest.isLast(aPosition);
    }

    /**
     * Inserts the given object at the beginning of this,
     * increasing the size of this by 1.
     * @param anObject object to insert
     */
    public IList insertFirst(Object anObject) {
        return new RecursiveList(anObject, this);
    }
    
    /**
     * Inserts the given object at the end of this,
     * increasing the size of this by 1.
     * @param anObject object to insert
     */
    public IList insertLast(Object anObject) {
        rest = rest.insertLast(anObject);
        
        return this;
    }
    
    /**
     * Inserts the given object before the given position in this, 
     * increasing the size of this by 1.
     * @param aPosition position to insert before
     * @param anObject object to insert
     */
    public IList insertBefore(Position aPosition, Object anObject) {
        if (aPosition == this) {
            return new RecursiveList(anObject, this);
        }

        rest = rest.insertBefore(aPosition, anObject);
        
        return this;
    }

    /**
     * Inserts the given object after the given position in this, 
     * increasing the size of this by 1.
     * @param aPosition position to insert before
     * @param anObject object to insert
     */
    public IList insertAfter(Position aPosition, Object anObject) {
        if (aPosition == this) {
            rest = new RecursiveList(anObject, rest);
            return rest;
        }
        
        rest = rest.insertAfter(aPosition, anObject);
        
        return this;
    }
    
    /** Removes the first element of this. */
    public IList removeFirst() {
        return rest;
    }

    /** Removes the last element of this. */
    public IList removeLast() {
        if (isLast(this)) {
            return EmptyRecursiveList.EMPTY;
        }
        
        rest = rest.removeLast();
        
        return this;
    }
    
    /** 
     * Removes the given position from this.
     * @param aPosition position to remove
     */
    public IList remove(Position aPosition) {
        if (aPosition == this) {
            return rest;
        }

        rest = rest.remove(aPosition);
        
        return this;
    }
    
    /**
     * Returns the element in the given position of this
     * and replaces it with the given object.
     * @param aPosition position of element to replace
     * @param anObject object to store in the given position
     */
    public Object replaceElement(Position aPosition, Object anObject) {
        if (aPosition == this) {
            Object temp = first;
            first = anObject;
            return first;
        }
        
        return rest.replaceElement(aPosition, anObject);
    }

    /** Returns the size of this. */
    public int size() {
        return 1 + rest.size();
    }
    
    /** Returns whether or not this is empty. */
    public boolean isEmpty() {
        return false;
    }
}