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Alternate take at fixing P1.memo cache issues #108 and #106 #127

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Alternate take at fixing P1.memo cache issues #108 and #106 #127

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67 changes: 47 additions & 20 deletions core/src/main/java/fj/P1.java
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Original file line number Diff line number Diff line change
Expand Up @@ -207,32 +207,59 @@ public <X> P1<X> map(final F<A, X> f) {
return P.lazy(u -> f.f(self._1()));
}

private static final class MemoizingP1<A> extends P1<A> {

private final F0<A> delegate;

// 0: uninitialized, -1: initialized but value is null, 1: initialized and value is non-null
private volatile byte state = 0;

// "value" does not need to be volatile; visibility piggy-backs
// on volatile read of "state". Contains a non-null value from delegate.
private SoftReference<A> value;

MemoizingP1(final F0<A> delegate) {
this.delegate = delegate;
}

@Override
public A _1() {
final byte s = state;
if (s == -1) {
return null;
}
A a;
// A 2-field variant of Double Checked Locking.
if ((s == 0) || ((a = value.get()) == null)) {
synchronized (this) {
if ((state == 0) || ((a = value.get()) == null)) {
a = delegate.f();
if (a == null) {
state = -1;
} else {
final SoftReference<A> tmp = new SoftReference<A>(a);
value = tmp;
state = 1;
}
}
}
}
return a;
}

@Override
public P1<A> memo() {
return this;
}
}

/**
* Provides a memoising P1 that remembers its value.
*
* @return A P1 that calls this P1 once and remembers the value for subsequent calls.
*/
public P1<A> memo() {
final P1<A> self = this;
return new P1<A>() {
private final Object latch = new Object();
@SuppressWarnings({"InstanceVariableMayNotBeInitialized"})
private volatile SoftReference<A> v;

public A _1() {
A a = v != null ? v.get() : null;
if (a == null)
synchronized (latch) {
if (v == null || v.get() == null) {
a = self._1();
v = new SoftReference<A>(a);
} else {
a = v.get();
}
}
return a;
}
};
return new MemoizingP1<>(this);
}

static <A> P1<A> memo(F<Unit, A> f) {
Expand Down