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    package org.pcollections;

import java.util.AbstractSequentialList;
import java.util.Collection;
import java.util.Iterator;
import java.util.ListIterator;



/**
 * 
 * A simple persistent stack of non-null values.
 * 
    
 * This implementation is thread-safe (assuming Java's AbstractSequentialList is thread-safe),
 * although its iterators may not be.
 * 
 * @author harold
 *
 * @param 
 */
public final class ConsPStack extends AbstractSequentialList implements PStack {
//// STATIC FACTORY METHODS ////
	private static final ConsPStack EMPTY = new ConsPStack();
	
	/**
	 * @param 
	 * @return an empty stack
	 */
	@SuppressWarnings("unchecked")
	public static  ConsPStack empty() {
		return (ConsPStack)EMPTY; }
	
	/**
	 * @param 
	 * @param e
	 * @return empty().plus(e)
	 */
	public static  ConsPStack singleton(final E e) {
		return ConsPStack.empty().plus(e); }
	
	/**
	 * @param 
	 * @param list
	 * @return a stack consisting of the elements of list in the order of list.iterator()
	 */
	@SuppressWarnings("unchecked")
	public static  ConsPStack from(final Collection list) {
		if(list instanceof ConsPStack)
			return (ConsPStack)list; //(actually we only know it's ConsPStack)
									// but that's good enough for an immutable
									// (i.e. we can't mess someone else up by adding the wrong type to it)
		return from(list.iterator());
	}
	
	private static  ConsPStack from(final Iterator i) {
		if(!i.hasNext()) return empty();
		E e = i.next();
		return from(i).plus(e);
	}

	
//// PRIVATE CONSTRUCTORS ////
	private final E first; private final ConsPStack rest;
	private final int size;
	// not externally instantiable (or subclassable):
	private ConsPStack() { // EMPTY constructor
		if(EMPTY!=null)
			throw new RuntimeException("empty constructor should only be used once");
		size = 0; first=null; rest=null;
	}
	private ConsPStack(final E first, final ConsPStack rest) {
		this.first = first; this.rest = rest;
		
		size = 1 + rest.size;
	}
	
	
//// REQUIRED METHODS FROM AbstractSequentialList ////
	@Override
	public int size() {
		return size; }
	
	@Override
	public ListIterator listIterator(final int index) {
		if(index<0 || index>size) throw new IndexOutOfBoundsException();
		
		return new ListIterator() {
			int i = index;
			ConsPStack next = subList(index);

			public boolean hasNext() {
				return next.size>0; }
			public boolean hasPrevious() {
				return i>0; }
			public int nextIndex() {
				return index; }
			public int previousIndex() {
				return index-1; }
			public E next() {
				E e = next.first;
				next = next.rest;
				return e;
			}
			public E previous() {
				System.err.println("ConsPStack.listIterator().previous() is inefficient, don't use it!");
				next = subList(index-1); // go from beginning...
				return next.first;
			}

			public void add(final E o) {
				throw new UnsupportedOperationException(); }
			public void remove() {
				throw new UnsupportedOperationException(); }
			public void set(final E o) {
				throw new UnsupportedOperationException(); }
		};
	}


//// OVERRIDDEN METHODS FROM AbstractSequentialList ////
	@Override
	public ConsPStack subList(final int start, final int end) {
		if(start<0 || end>size || start>end)
			throw new IndexOutOfBoundsException();
		if(end==size) // want a substack
			return subList(start); // this is faster
		if(start==end) // want nothing
			return empty();
		if(start==0) // want the current element
			return new ConsPStack(first, rest.subList(0, end-1));
		// otherwise, don't want the current element:
		return rest.subList(start-1, end-1);
	}
	
	
//// IMPLEMENTED METHODS OF PStack ////
	public ConsPStack plus(final E e) {
		return new ConsPStack(e, this);
	}
	
	public ConsPStack plusAll(final Collection list) {
		ConsPStack result = this;
		for(E e : list)
			result = result.plus(e);
		return result;
	}

	public ConsPStack plus(final int i, final E e) {
		if(i<0 || i>size)
			throw new IndexOutOfBoundsException();
		if(i==0) // insert at beginning
			return plus(e);
		return new ConsPStack(first, rest.plus(i-1, e));
	}

	public ConsPStack plusAll(final int i, final Collection list) {
		// TODO inefficient if list.isEmpty()
		if(i<0 || i>size)
			throw new IndexOutOfBoundsException();
		if(i==0)
			return plusAll(list);
		return new ConsPStack(first, rest.plusAll(i-1, list));
	}
	
	public ConsPStack minus(final Object e) {
		if(size==0)
			return this;
		if(first.equals(e)) // found it
			return rest; // don't recurse (only remove one)
		// otherwise keep looking:
		ConsPStack newRest = rest.minus(e);
		if(newRest==rest) return this;
		return new ConsPStack(first, newRest);
	}

	public ConsPStack minus(final int i) {
		return minus(get(i));
	}

	public ConsPStack minusAll(final Collection list) {
		if(size==0)
			return this;
		if(list.contains(first)) // get rid of current element
			return rest.minusAll(list); // recursively delete all
		// either way keep looking:
		ConsPStack newRest = rest.minusAll(list);
		if(newRest==rest) return this;
		return new ConsPStack(first, newRest);
	}
	
	public ConsPStack with(final int i, final E e) {
		if(i<0 || i>=size)
			throw new IndexOutOfBoundsException();
		if(i==0) {
			if(first.equals(e)) return this;
			return new ConsPStack(e, rest);
		}
		ConsPStack newRest = rest.with(i-1, e);
		if(newRest==rest) return this;
		return new ConsPStack(first, newRest);
	}

	public ConsPStack subList(final int start) {
		if(start<0 || start>size)
			throw new IndexOutOfBoundsException();
		if(start==0)
			return this;
		return rest.subList(start-1);
	}
}
    
    
    
    

    




    
    
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