// l + r
private R connectEmpty() {
synchronized (root) {
Connector<T, R> l = left, r = right;
if (l == null) {
return pushRight(none(), none());
}
left = right = null;
l.rhs = null;
T laright = l.right;
l.right = none();
if (l.acc == NONE) {
if (r == null) l.drain();
else {
if (l.lhs != null) {
l.lhs.right = r;
r.lhs = l.lhs;
}
}
return none();
}
if (r == null) {
return l.drainLeft();
}
r.lhs = null;
if (r.acc == NONE) {
if (r.rhs != null) {
r.rhs.left = l;
l.rhs = r.rhs;
l.right = laright;
}
return none();
}
T raleft = r.left;
r.left = none();
if (mergeable.test(laright, raleft)) {
R acc = combiner.apply(l.acc, r.acc);
if (l.left == NONE && r.right == NONE) {
l.drain();
r.drain();
return acc;
}
l.acc = acc;
l.right = r.right;
if (r.rhs != null) {
r.rhs.left = l;
l.rhs = r.rhs;
}
return none();
}
if (l.left == NONE) {
if (r.right == NONE) right = new Connector<>(this, r.drain(), null);
return l.drain();
}
return r.drainRight();
}
}
// l + <first|acc|?>
private R pushLeft(T first, R acc) {
synchronized (root) {
Connector<T, R> l = left;
if (l == null) return acc;
left = null;
l.rhs = null;
T laright = l.right;
l.right = none();
if (l.acc == NONE) {
l.acc = acc;
l.left = first;
return none();
}
if (this.mergeable.test(laright, first)) {
l.acc = this.combiner.apply(l.acc, acc);
return l.drainLeft();
}
if (l.left == NONE) {
left = new Connector<>(null, acc, this);
return l.drain();
}
}
return acc;
}
