feat(topic): add Topic.latest() for a single-read value + timestamp (#31)

* feat(topic): add Topic.latest() for a single-read value + timestamp

Checking whether a published value is stale needs two calls:

    long stamp = topic.latestPublishNanos();
    T v = topic.latestValueOr(null);
    long age = System.nanoTime() - stamp;

Both accessors are individually correct, but a publish can land between
them, leaving the caller holding a value from one publish and a timestamp
from another. Reading the timestamp first makes that safe in one
direction -- the age can only be over-reported, never under-reported --
but over-reporting still discards a fresh value as stale, by up to a full
publish interval.

How often that happens depends on how long the two reads take. Normally
they are nanoseconds apart and a publish landing between them is
vanishingly rare. It stops being rare when the reader is preempted
between the two calls by a GC pause or the scheduler, since the window
becomes milliseconds. This library configures a small young gen, so young
GCs are part of the deployment rather than a hypothetical.

`latest()` returns the value and its timestamp from one snapshot read, so
the pair provably comes from a single publish and the window does not
exist:

    Optional<Topic.Latest<T>> snap = topic.latest();
    T v = snap.map(Topic.Latest::value).orElse(defaultValue);
    long age = snap.map(Topic.Latest::ageNanos).orElse(Long.MAX_VALUE);

Additive only. No existing signature or behaviour changes. Callers that
never check an age keep using `latestValueOr` -- the same single volatile
read, and allocation-free.

`Topic.Latest` becomes public with `value()`, `publishNanos()` and
`ageNanos()`. `ageNanos()` is the age of *this* value computed from its
own stamp, which also removes the pre-publish `0` sentinel that the
two-call form has to guard by hand.

`latestPublishNanos()` still returns 0 before the first publish, so that
guard stays necessary for anyone composing by hand, and it is documented
as a heuristic rather than a proof -- System.nanoTime() is permitted to
return 0 too. It is stated as such in the javadoc, the CHANGELOG and the
topics guide.

The concurrency test's reader now uses `latest()`, which is the stronger
property: no straddle is representable, so the "timestamp first"
ordering rule has nothing left to guard. Its cross-check between
`latest()` being empty and `latestPublishNanos()` being nonzero is gone,
because a publish can legitimately land between those two reads.

Suite: 86 tests, 0 failures. javadoc: no errors, no new warnings.

* test(topic): drop absolute time bounds; document empty-read allocation

- Remove the one-second age ceiling. A GC pause between the publish and
  the ageNanos() read legitimately makes the age arbitrarily large, so the
  bound imposed a scheduling deadline on the test. Both bounds are now
  relative to the clock.
- Remove the nonzero publishNanos() assertion. System.nanoTime() has an
  arbitrary origin, so a published snapshot may carry 0; the present
  Optional already proves a publish happened.
- latest() allocates an Optional wrapper only when it returns a value.
  Empty reads return the shared Optional.empty() and allocate nothing,
  which is now what the javadoc and the topics guide say.

Suite 86/86, javadoc clean.

* test(topic): bracket ageNanos() between two clock reads

Three review findings on the snapshot tests:

- ageNanos() was only bounded from above, so an implementation returning a
  constant 0 satisfied the check while reporting nothing about the real age.
  Bracket the call between two clock reads in both the unit test and the
  concurrency reader, and require the age to fall between both deltas from
  the snapshot's own stamp.
- second.ageNanos() < first.ageNanos() sampled at two different moments, so
  a pause between the publish and the second read could make the newer value
  look older. Derive both ages from a single clock reading: from one now, a
  strictly later stamp is a strictly smaller age.

The one-second wall-clock ceiling and the nonzero-stamp assertion reported
earlier were already removed in the previous commit; these reviews landed
on the older SHA.

Suite 86/86, tear test green on a 2-core-pinned runner, javadoc clean.

* docs(topic): unwrap the snapshot once in every example

The value-and-age example called Optional.map() once per field, so each
non-empty read allocated two more Optional wrappers and boxed the long
age -- in a guide aimed at periodic loops, teaching the allocating form
three times over (the latest() wrapper plus two map() calls).

Every example now unwraps with isPresent()/get() and calls value() and
ageNanos() directly: the topics guide, the javadoc and the CHANGELOG.

* test(topic): bracket ageNanos() between clock reads taken around the call

The previous bounds sampled a clock reading outside the ageNanos() call, so
they could not bound the result in either direction: a pause between the
sampling and the call breaks the lower bound, and the age correctly
exceeds a reading taken before it.

Both the unit test and the concurrency reader now take one clock reading
immediately before the call and one immediately after, and require the age
to fall between the two deltas from the snapshot's own stamp. That is the
only bracketing that constrains the value, and it holds regardless of how
long the call itself takes.

Suite 86/86 across 8 consecutive runs, tear test green pinned to two
cores, javadoc clean.

Author: IamCoder18

CHANGELOG.md

@@ -96,13 +96,32 @@ listeners registered, `publish` costs a single volatile read.
   `0`, so a genuine publish can carry it too. The resulting over-reported age rejects
   a fresh value rather than admitting a stale one, so the failure direction is safe.
 
-  **Read the timestamp before the value** when you use the two together as a
-  staleness check. Each accessor now returns a self-consistent pair, but two separate
-  calls can still straddle a publish, so the ordering rule remains — it is now
-  documented on the public accessors and in the topics guide:
+  **Prefer `Topic.latest()` when you need both the value and its age.** It returns the
+  two from one snapshot read, so they provably come from the same publish:
 
   ```java
-  long stamp = topic.latestPublishNanos();  // first
+  Optional<Topic.Latest<T>> snap = topic.latest();
+  T v;
+  long age;
+  if (snap.isPresent()) {
+      Topic.Latest<T> s = snap.get();
+      v = s.value();
+      age = s.ageNanos();
+  } else {
+      v = defaultValue;
+      age = Long.MAX_VALUE;
+  }
+  ```
+
+  Unwrap once rather than calling `map()` per field: each `map()` allocates its own
+  `Optional` and boxes the `long`, which is avoidable garbage in a periodic loop.
+
+  `latestPublishNanos()` and `latestValueOr()` are unchanged and still correct. Composing
+  them takes two calls, which can straddle a publish and leave the pair describing two
+  different publishes; if you do compose them, **read the timestamp first**:
+
+  ```java
+  long stamp = topic.latestPublishNanos();  // read the timestamp FIRST
   T v = topic.latestValueOr(null);         // then
   long age = stamp == 0L ? Long.MAX_VALUE : System.nanoTime() - stamp;
   ```

src/main/java/com/aaravlabs/synapse/Topic.java

@@ -46,15 +46,59 @@ public final class Topic<T> {
         this.boxedType = box(type);
     }
 
-    /** One publish's value and the {@link System#nanoTime()} at which it was recorded. */
-    private static final class Latest<T> {
-        final T value;
-        final long publishNanos;
+    /**
+     * One publish's value and the {@link System#nanoTime()} at which it was recorded.
+     *
+     * <p>Immutable. Returned by {@link Topic#latest()} so a caller can read the value and
+     * its timestamp without risking a publish landing between two calls.
+     *
+     * @param <T> the message type carried by this topic
+     */
+    public static final class Latest<T> {
+
+        private final T value;
+        private final long publishNanos;
 
         Latest(T value, long publishNanos) {
             this.value = value;
             this.publishNanos = publishNanos;
         }
+
+        /**
+         * The published value.
+         *
+         * @return the value published in this snapshot
+         */
+        public T value() {
+            return value;
+        }
+
+        /**
+         * When this value was recorded.
+         *
+         * @return the {@link System#nanoTime()} at which {@link #value()} was recorded, on
+         *         the monotonic clock (arbitrary origin — only differences are meaningful)
+         */
+        public long publishNanos() {
+            return publishNanos;
+        }
+
+        /**
+         * How long ago this value was recorded, measured from its own stamp.
+         *
+         * <p>Unlike {@code System.nanoTime() - publishNanos()}, this is always the age of
+         * <i>this</i> value and needs no {@code 0} sentinel handling.
+         *
+         * @return nanos elapsed since {@link #publishNanos()}, on the monotonic clock
+         */
+        public long ageNanos() {
+            return System.nanoTime() - publishNanos;
+        }
+
+        @Override
+        public String toString() {
+            return "Latest[" + value + " @" + publishNanos + "]";
+        }
     }
 
     /**
@@ -74,27 +118,26 @@ public Class<T> type() {
     /**
      * The most recently published value, or empty if nothing has been published yet.
      *
-     * <p><b>Reading a value together with its age:</b> each accessor reads a
-     * self-consistent pair, but two separate calls can still straddle a publish. Sample
-     * the timestamp <i>first</i>, which can only over-report the value's age:
+     * <p><b>Reading a value together with its age:</b> prefer {@link #latest()}, which
+     * returns both from a single snapshot read. Composing them from two accessors works
+     * only if you sample the timestamp <i>first</i>, which can only over-report the age:
      *
      * <pre>{@code
      * long stamp = topic.latestPublishNanos();  // read the timestamp FIRST
-     * T v = topic.latestValueOr(null);         // then the value
+     * T v = topic.latestValueOr(null);         // then
      * long age = stamp == 0L ? Long.MAX_VALUE : System.nanoTime() - stamp;
      * }</pre>
      *
-     * <p>Reading the timestamp first is what makes the pair safe: a publish landing
-     * between the two calls can only over-report the age, never under-report it.
-     *
      * <p>The {@code 0} guard handles "nothing published yet", where the stamp is still
      * its initial {@code 0} and subtracting it would yield the raw
      * {@link System#nanoTime()} reading. It is a heuristic, not a proof: the JLS permits
      * {@link System#nanoTime()} to return {@code 0}, so a genuine publish can carry a
      * {@code 0} stamp too. That case only over-reports the age, which rejects a fresh
      * value rather than admitting a stale one, so the failure direction is safe.
      *
-     * Reading the value first is the unsafe order: a publish landing between the two
+     * <p>{@link #latest()} removes the question entirely: one snapshot read, no sentinel.
+     *
+     * <p>Reading the value first is the unsafe order: a publish landing between the two
      * calls pairs the older value with the newer timestamp, so an age check on that
      * pair passes even though the value is stale.
      *
@@ -119,7 +162,7 @@ public Optional<T> latestValue() {
      */
     public long latestPublishNanos() {
         Latest<T> snap = latest;
-        return snap == null ? 0L : snap.publishNanos;
+        return snap == null ? 0L : snap.publishNanos();
     }
 
     /**
@@ -134,7 +177,8 @@ public long latestPublishNanos() {
      * publishes. The <b>write path takes the topic monitor</b>, around the clock sample,
      * one short-lived allocation and the store — see below. Each publish therefore
      * allocates; on the read side {@link #latestValueOr(Object)} allocates nothing,
-     * while {@link #latestValue()} wraps its result in an {@link Optional}.
+     * while {@link #latestValue()} and {@link #latest()} each wrap their result in an
+     * {@link Optional}.
      *
      * @param value the value to record
      */
@@ -177,14 +221,70 @@ boolean acceptsType(Class<?> other) {
      * Returns the most recent value, falling back to {@code defaultValue} if nothing has
      * been published yet. Convenience for {@code topic.latestValue().orElse(default)}.
      *
-     * <p>Allocation-free — prefer this over {@link #latestValue()} on hot paths.
+     * <p>Allocation-free — prefer this over {@link #latestValue()} on hot paths, and over
+     * {@link #latest()} if you do not need the timestamp.
      *
      * @param defaultValue the value to return before the first publish
      * @return the latest value, or {@code defaultValue}
      */
     public T latestValueOr(T defaultValue) {
         Latest<T> snap = latest;
-        return snap == null ? defaultValue : snap.value;
+        return snap == null ? defaultValue : snap.value();
+    }
+
+    /**
+     * The most recent value and the {@link System#nanoTime()} at which it was published,
+     * as one consistent pair.
+     *
+     * <p><b>Prefer this over {@link #latestValue()} plus {@link #latestPublishNanos()}
+     * when you are checking whether a value is stale.</b> The two accessors each read the
+     * snapshot correctly, but a caller making two calls can straddle a publish between
+     * them and end up holding a value from one publish and a timestamp from another:
+     *
+     * <pre>{@code
+     * long stamp = topic.latestPublishNanos();   // publish A
+     *                                                  ...publish B lands...
+     * T v = topic.latestValueOr(null);             // publish B's value, with A's stamp
+     * long age = System.nanoTime() - stamp;        // over-reports by a full publish interval
+     * }</pre>
+     *
+     * <p>Reading the timestamp first makes that safe in one direction — the age can only
+     * be over-reported, never under-reported — but over-reporting still costs you: a
+     * fresh value gets rejected as stale. How often that happens depends on how long the
+     * two reads take. Normally they are nanoseconds apart and a publish landing between
+     * them is vanishingly rare. It stops being rare when the reader is preempted between
+     * the two calls by a GC pause or the scheduler, since the window becomes
+     * milliseconds. This accessor removes the window entirely.
+     *
+     * <p>Unwrap once rather than calling {@code map()} per field: each {@code map()} call
+     * allocates its own {@link Optional} and boxes the {@code long} age, which is
+     * avoidable garbage in a periodic loop.
+     *
+     * <pre>{@code
+     * Optional<Topic.Latest<T>> snap = topic.latest();
+     * T v;
+     * long age;
+     * if (snap.isPresent()) {
+     *     Topic.Latest<T> s = snap.get();
+     *     v = s.value();
+     *     age = s.ageNanos();
+     * } else {
+     *     v = defaultValue;
+     *     age = Long.MAX_VALUE;
+     * }
+     * }</pre>
+     *
+     * <p>Returns empty before the first publish.
+     *
+     * <p><b>Non-empty calls allocate one small {@link Optional} wrapper.</b> Calls before
+     * the first publish return the shared {@link Optional#empty()} instance and allocate
+     * nothing. If you only want the value and never check its age, use
+     * {@link #latestValueOr(Object)}, which never allocates.
+     *
+     * @return an {@link Optional} holding the latest value and its publish timestamp
+     */
+    public Optional<Latest<T>> latest() {
+        return Optional.ofNullable(latest);
     }
 
     /** Treat primitive {@code double.class} and wrapper {@code Double.class} as the same type. */

src/test/java/com/aaravlabs/synapse/TopicLockFreeTest.java

@@ -78,6 +78,9 @@ void concurrentPublishAndReadNeverLosesOrTearsTheLatestValue() throws Exception
         // not evidence that the code is correct. What this test does establish is that no
         // reader ever observes a value outside the published range, and that the final
         // state is some publisher's last value.
+        // Timestamp and value from ONE snapshot read, so there is no window for a
+        // publish to land in between. This is the property the two-call accessors cannot
+        // offer, and the reason latest() exists.
         final int publishers = 4;
         final int perPublisher = 25_000;
         final int ids = publishers * perPublisher;
@@ -106,32 +109,44 @@ void concurrentPublishAndReadNeverLosesOrTearsTheLatestValue() throws Exception
         Thread reader = new Thread(() -> {
             try {
                 while (readersRunning.get()) {
-                    // Timestamp FIRST, then value. On this order a publish landing
-                    // between the two reads can only make the reported age too large,
-                    // never too small, so a staleness check can never pass a stale value.
-                    long ts = topic.latestPublishNanos();
-                    Integer v = topic.latestValueOr(null);
-                    if (v == null) {
-                        if (ts != 0L) {
-                            failure.compareAndSet(null, new AssertionError(
-                                    "timestamp recorded with no value: " + ts));
+                    // ONE snapshot read, so the value and the timestamp provably come
+                    // from the same publish. There is no window for a publish to land in,
+                    // so this cannot over-report age the way two calls can.
+                    Topic.Latest<Integer> snap = topic.latest().orElse(null);
+                    if (snap == null) {
+                        // Nothing published yet. There is deliberately no cross-check
+                        // against latestPublishNanos() here: a publish can land between
+                        // the two calls, so a nonzero stamp here would be that publish,
+                        // not an inconsistency.
+                    } else {
+                        Integer v = snap.value();
+                        if (v < 0 || v >= ids) {
+                            failure.compareAndSet(null,
+                                    new AssertionError("torn/garbage latest value: " + v));
                             return;
                         }
-                    } else if (v < 0 || v >= ids) {
-                        failure.compareAndSet(null,
-                                new AssertionError("torn/garbage latest value: " + v));
-                        return;
-                    } else {
-                        // Only a timestamp AFTER the window ends is a tear: the reader
-                        // would be holding an older value beside a newer timestamp. A
-                        // timestamp before the window is the safe, expected straddle --
-                        // the reader sampled the timestamp, a publish landed, then it read
-                        // the newer value.
+                        // The pair is self-consistent by construction, so the only way
+                        // to fail is a stamp that predates the value it came with --
+                        // which the publish-window check bounds.
                         long hi = windowHi.get(v);
-                        if (hi != Long.MIN_VALUE && ts > hi) {
+                        if (hi != Long.MIN_VALUE && snap.publishNanos() > hi) {
+                            failure.compareAndSet(null, new AssertionError(
+                                    "value " + v + " paired with stamp " + snap.publishNanos()
+                                            + " later than its publish end " + hi));
+                            return;
+                        }
+                        // ageNanos must agree with the stamp it was taken from, so bracket
+                        // the call between two clock reads and require the age to fall
+                        // between both deltas. A one-sided check lets an implementation
+                        // returning a constant 0 pass. Zero is legitimate (same tick).
+                        long ageBefore = System.nanoTime();
+                        long age = snap.ageNanos();
+                        long ageAfter = System.nanoTime();
+                        if (age < 0
+                                || age < ageBefore - snap.publishNanos()
+                                || age > ageAfter - snap.publishNanos()) {
                             failure.compareAndSet(null, new AssertionError(
-                                    "value " + v + " is older than the timestamp " + ts
-                                            + " the reader sampled; its publish ended at " + hi));
+                                    "implausible age " + age + " for stamp " + snap.publishNanos()));
                             return;
                         }
                     }
@@ -240,6 +255,76 @@ void acceptsValueClassTreatsPrimitivesAndWrappersAsEquivalent() {
         assertFalse(i.acceptsValueClass(double.class), "Double must not match Integer topic");
     }
 
+    @Test
+    void latestReturnsValueAndTimestampFromOneConsistentPublish() throws Exception {
+        // latest() exists so a staleness check reads one snapshot instead of two. It must
+        // be empty before the first publish, agree with both single-field accessors, and
+        // never need a 0 sentinel.
+        Topic<String> t = orchestrator.getOrCreateTopic("snap", String.class);
+        assertFalse(t.latest().isPresent(), "latest() must be empty before the first publish");
+
+        orchestrator.publish("snap", "a");
+        Topic.Latest<String> first = t.latest().orElseThrow();
+        assertEquals("a", first.value());
+        assertEquals(first.publishNanos(), t.latestPublishNanos(),
+                "latest() and latestPublishNanos() must describe the same publish");
+        assertEquals("a", t.latestValueOr(null));
+
+        // No assertion that publishNanos() is nonzero: System.nanoTime() has an arbitrary
+        // origin, so a published snapshot may legitimately carry 0. The present Optional
+        // already proves a publish happened, which is the property worth checking here.
+
+        // ageNanos must be the age of THIS value, from ITS OWN stamp.
+        //
+        // Comparing ageNanos() against `now - publishNanos()` cannot establish that:
+        // both sides recompute the same subtraction, so the relation holds for any pair
+        // and a Latest carrying a foreign stamp still passes. What actually pins the
+        // stamp to this value is the cross-check above -- latest().publishNanos() equals
+        // latestPublishNanos() equals the stamp of the "a" publish -- combined with a
+        // non-negative age and a stamp that is not in the future.
+        //
+        // Both bounds are relative to the clock, not absolute wall-clock deadlines: a
+        // snapshot read in the same tick as its own publish legitimately reports age 0,
+        // and a GC pause between the publish and this read can legitimately make the
+        // age arbitrarily large.
+        // The clock reads must bracket the ageNanos() call itself; a reading taken
+        // outside it cannot bound the result in either direction, because a pause
+        // between the reading and the call would break the relation.
+        long ageBefore = System.nanoTime();
+        long age = first.ageNanos();
+        long ageAfter = System.nanoTime();
+        assertTrue(age >= 0, "ageNanos must not be negative: " + age);
+        assertTrue(age >= ageBefore - first.publishNanos()
+                        && age <= ageAfter - first.publishNanos(),
+                "ageNanos " + age + " is not the age of stamp " + first.publishNanos()
+                        + " measured between " + ageBefore + " and " + ageAfter);
+
+        // A later publish replaces both halves together; there is no observable state in
+        // which one half is from this publish and the other from the previous one.
+        //
+        // Wait for the clock to pass first.publishNanos() rather than sleeping a fixed
+        // interval: System.nanoTime() may return the same tick for adjacent reads, so
+        // Thread.sleep(2) does not guarantee the next publish gets a later stamp. Same
+        // bounded-wait pattern as latestValueAndTimestampAdvanceTogether, so a broken
+        // clock fails the assert instead of hanging.
+        long deadline = System.nanoTime() + 50_000_000L;
+        while (first.publishNanos() >= System.nanoTime() && System.nanoTime() <= deadline) {
+            Thread.sleep(1);
+        }
+        orchestrator.publish("snap", "b");
+        Topic.Latest<String> second = t.latest().orElseThrow();
+        assertEquals("b", second.value());
+        assertTrue(second.publishNanos() > first.publishNanos(),
+                "a later publish must carry a later stamp");
+        // Derive both ages from ONE clock reading rather than calling ageNanos() twice.
+        // Sampled at different moments the comparison depends on how long each call took,
+        // so a pause between the publish and the second read could make the newer value
+        // look older. From a single now, a strictly later stamp is a strictly smaller age.
+        long laterNow = System.nanoTime();
+        assertTrue(laterNow - second.publishNanos() < laterNow - first.publishNanos(),
+                "the newer value must report a younger age");
+    }
+
     @Test
     void acceptsTypeIsEquivalentToTheOldBoxedAssignableFromCheck() {
         // acceptsType() replaced `boxed(t.type()).isAssignableFrom(boxed(type))` at