Pairwise(not tested)

src/R3/Operators/Pairwise.cs

@@ -0,0 +1,47 @@
+namespace R3;
+
+public static partial class ObservableExtensions
+{
+    public static Observable<(T Previous, T Current)> Pairwise<T>(this Observable<T> source)
+    {
+        return new Pairwise<T>(source);
+    }
+}
+
+internal sealed class Pairwise<T>(Observable<T> source) : Observable<(T Previous, T Current)>
+{
+    protected override IDisposable SubscribeCore(Observer<(T Previous, T Current)> observer)
+    {
+        return source.Subscribe(new _Pairwise(observer));
+    }
+
+    sealed class _Pairwise(Observer<(T Previous, T Current)> observer) : Observer<T>
+    {
+        T? previous;
+        bool isFirst = true;
+
+        protected override void OnNextCore(T value)
+        {
+            if (isFirst)
+            {
+                isFirst = false;
+                previous = value;
+                return;
+            }
+
+            var pair = (previous!, value);
+            previous = value;
+            observer.OnNext(pair);
+        }
+
+        protected override void OnErrorResumeCore(Exception error)
+        {
+            observer.OnErrorResume(error);
+        }
+
+        protected override void OnCompletedCore(Result result)
+        {
+            observer.OnCompleted(result);
+        }
+    }
+}

src/R3/Operators/_Operators.cs

@@ -7,8 +7,7 @@ public static partial class ObservableExtensions
 // TODO: this is working space, will remove this file after complete.
 
 // Rx Merging:
-// TODO:not-tested:CombineLatest, Zip, ZipLatest, WithLatestFrom, Switch
-// , Pairwise
+// TODO:not-tested:CombineLatest, Zip, ZipLatest, WithLatestFrom, Switch, Pairwise
 
 // Standard Query:
 // Distinct, DistinctBy, DistinctUntilChanged, Scan, DefaultIfEmpty