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Pure Functional Programming Semantics For Java

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/**
 *   Copyright (c) Rich Hickey. All rights reserved.
 *   The use and distribution terms for this software are covered by the
 *   Eclipse Public License 1.0 (http://opensource.org/licenses/eclipse-1.0.php)
 *   which can be found in the file epl-v10.html at the root of this distribution.
 *   By using this software in any fashion, you are agreeing to be bound by
 * 	 the terms of this license.
 *   You must not remove this notice, or any other, from this software.
 **/

package org.pure4j.collections;

import java.io.IOException;
import java.io.NotSerializableException;
import java.util.Iterator;

/**
 * Presents a Seq from an Iterable object (e.g. a collection).
 * By using {@link Iterable}, this class is able to own the iterator instance.
 * 
 * @author robmoffat
 *
 * @param  Element type
 */
public class IterableSeq extends ASeq {
	final Iterator iter;
	final State state;

	static class State {
		volatile Object val;
		volatile Object _rest;
	}

	private static  IterableSeq create(Iterator iter) {
		if (iter.hasNext())
			return new IterableSeq(iter);
		return null;
	}
	
	public IterableSeq(Iterable iterable) {
		this(iterable.iterator());
	}

	public IterableSeq(Iterator iter) {
		this.iter = iter;
		state = new State();
		this.state.val = state;
		this.state._rest = state;
	}

	IterableSeq(Iterator iter, State state) {
		this.iter = iter;
		this.state = state;
	}

	@SuppressWarnings("unchecked")
	public K first() {
		if (state.val == state)
			synchronized (state) {
				if (state.val == state)
					state.val = iter.next();
			}
		return (K) state.val;
	}

	@SuppressWarnings("unchecked")
	public ISeq next() {
		if (state._rest == state)
			synchronized (state) {
				if (state._rest == state) {
					first();
					state._rest = create(iter);
				}
			}
		return (ISeq) state._rest;
	}

	private void writeObject(java.io.ObjectOutputStream out) throws IOException {
		throw new NotSerializableException(getClass().getName());
	}
}