Add sample operator

source/concurrency/sequence.d

@@ -1085,6 +1085,118 @@ private auto getSome(T)(ref T t) {
     return t.get();
 }
 
+auto proxyNext(Sequence, Receiver)(Sequence sequence, Receiver receiver) {
+    return ProxyNextSequence!(Sequence, Receiver)(sequence, receiver);
+}
+
+struct ProxyNextSequence(Sequence, NextReceiver) {
+    alias Value = Sequence.Value;
+
+    Sequence sequence;
+    NextReceiver nextReceiver;
+
+    auto connect(Receiver)(return Receiver receiver) @safe return scope {
+        // ensure NRVO
+        auto op = sequence.connect(ProxyNextReceiver!(NextReceiver, Receiver)(nextReceiver, receiver));
+        return op;
+    }
+}
+
+struct ProxyNextReceiver(NextReceiver, Receiver) {
+    NextReceiver nextReceiver;
+    Receiver receiver;
+
+    auto setNext(Sender)(Sender sender) {
+        return nextReceiver.setNext(sender);
+    }
+    auto setValue() {
+        receiver.setValue();
+    }
+    auto setDone() nothrow @safe {
+        receiver.setDone();
+    }
+    auto setError(Throwable t) nothrow @safe {
+        receiver.setError(t);
+    }
+    import concurrency.receiver : ForwardExtensionPoints;
+    mixin ForwardExtensionPoints!receiver;
+}
+
+auto sample(BaseSequence, TriggerSequence)(BaseSequence base, TriggerSequence trigger) {
+    return SampleSequence!(BaseSequence, TriggerSequence)(base, trigger);
+}
+
+struct SampleSequence(BaseSequence, TriggerSequence) {
+    alias Value = void;
+    alias Element = BaseSequence.Element;
+
+    BaseSequence base;
+    TriggerSequence trigger;
+
+    auto connect(Receiver)(return Receiver receiver) @safe return scope {
+        // ensure NRVO
+        auto op = SampleSequenceOp!(BaseSequence, TriggerSequence, Receiver)(base, trigger, receiver);
+        return op;
+    }
+}
+
+struct SampleSequenceOp(BaseSequence, TriggerSequence, Receiver) {
+	import concurrency.bitfield : SharedBitField;
+	import concurrency.sender : OpType;
+    import concurrency.operations : RaceSender;
+
+    import std.typecons : Nullable;
+	enum Flags : size_t {
+		locked = 0x1,
+		valid = 0x2
+	}
+	shared SharedBitField!Flags state;
+    alias Element = BaseSequence.Element;
+	Element item;
+    alias RaceAllSender = RaceSender!(
+        SequenceCollect!(BaseSequence, void delegate(Element) shared @safe nothrow @nogc),
+        ProxyNextSequence!(FilterMapSequence!(TriggerSequence, Nullable!Element delegate() shared @safe nothrow @nogc), Receiver)
+        );
+    alias Op = OpType!(RaceAllSender, Receiver);
+
+    Op op;
+
+    @disable this(ref return scope typeof(this) rhs);
+    @disable this(this);
+
+    @disable void opAssign(typeof(this) rhs) nothrow @safe @nogc;
+    @disable void opAssign(ref typeof(this) rhs) nothrow @safe @nogc;
+
+    this(BaseSequence base, TriggerSequence trigger, return Receiver receiver) @trusted return scope {
+        import concurrency.operations : raceAll;
+        op = raceAll(
+            base.collect(&(cast(shared)this).produced),
+            trigger.filterMap(&(cast(shared)this).triggered).proxyNext(receiver)
+        ).connect(receiver);
+    }
+
+    void start() {
+        op.start();
+    }
+
+    private void produced(Element item) shared @safe nothrow @nogc {
+        with (state.lock(Flags.valid)) {
+            this.item = item;
+        }
+    }
+
+    private Nullable!Element triggered() shared @safe nothrow @nogc{
+        with (state.lock()) {
+            if (was(Flags.valid)) {
+                auto localElement = item;
+                release(Flags.valid);
+                return Nullable!Element(localElement);
+            }
+            return Nullable!Element.init;
+        }
+    }
+}
+
 
 // cron - create a sequence like interval but using cron spec
 

tests/ut/concurrency/sequence.d

@@ -173,3 +173,27 @@ import unit_threaded;
         return Nullable!int(1);
     }).take(4).toList().syncWait.value.should == [1,1,1,1];
 }
+
+@("sample")
+@safe unittest {
+    import core.time : msecs;
+	import concurrency.scheduler : ManualTimeWorker;
+    import concurrency.operations : then, whenAll, withScheduler;
+
+	auto worker = new shared ManualTimeWorker();
+	auto driver = just(worker).then((shared ManualTimeWorker worker) shared {
+        for(;;) {
+            auto span = worker.timeUntilNextEvent();
+            if (span.isNull)
+                break;
+            worker.advance(span.get());
+        }
+    });
+    whenAll(
+        sample(
+            interval(1.msecs, false).scan((int acc) => acc + 1, 0),
+            interval(2.msecs, false)
+        ).take(4).toList().withScheduler(worker.getScheduler()),
+        driver,
+    ).syncWait.value.should == [1,3,5,7];
+}