Class LazySeq

src/jvm/clojure/lang/LazySeq.java:21–300

import java.util.concurrent.locks.ReentrantLock;

public final class LazySeq extends Obj implements ISeq, Sequential, List, IPending, IHashEq{

private static final long serialVersionUID = -7531333024710395876L;

private transient IFn fn;
private Object sv;
private ISeq s;
private volatile Lock lock;

public LazySeq(IFn f){
	fn = f;
	lock = new ReentrantLock();
}

private LazySeq(IPersistentMap meta, ISeq seq){
	super(meta);
	fn = null;
	s = seq;
}

public Obj withMeta(IPersistentMap meta){
	if(meta() == meta)
		return this;
	return new LazySeq(meta, seq());
}

// MUST be locked when called!
final private void force() {
	if (fn != null) {
		sv = fn.invoke();
		fn = null;
	}
}

final private Object sval() {
    Lock l = lock;
    if(l != null) {
        l.lock();
        try {
            //must re-examine under lock
            if(lock != null) { //unrealized
                force();
                return sv;
            }
        } finally {
            l.unlock();
        }
    }
    // realized, read of lock above guarantees visibility of s
    return s;
}

final private Object unwrap(Object ls){
    while(ls instanceof LazySeq) {
        ls = ((LazySeq) ls).sval();
        }
    return ls;
}