Add AsyncSeq.windowed for sliding window over async sequences (#241)

Adds AsyncSeq.windowed : int -> AsyncSeq<'T> -> AsyncSeq<'T[]>

This is a sliding-window combinator analogous to Seq.windowed and
Seq.pairwise (which is a special case with windowSize=2). Each window
is yielded as an immutable array snapshot; the window slides one
element at a time.

Implementation uses a Queue<'T> of capacity windowSize to maintain the
current window with O(1) enqueue/dequeue per element.

Includes 7 tests covering: empty source, source shorter than window,
exact-size source, sliding windows, size-1, size-2, and ArgumentException
for windowSize < 1.

All 201 tests pass.

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <[email protected]>
Co-authored-by: Don Syme <[email protected]>

Author: 3 people

src/FSharp.Control.AsyncSeq/AsyncSeq.fs

@@ -1087,6 +1087,21 @@ module AsyncSeq =
           let! moven = ie.MoveNext()
           b := moven }
 
+  let windowed (windowSize:int) (source:AsyncSeq<'T>) : AsyncSeq<'T[]> =
+    if windowSize < 1 then invalidArg (nameof windowSize) "must be positive"
+    asyncSeq {
+      let window = System.Collections.Generic.Queue<'T>(windowSize)
+      use ie = source.GetEnumerator()
+      let! move = ie.MoveNext()
+      let b = ref move
+      while b.Value.IsSome do
+          window.Enqueue(b.Value.Value)
+          if window.Count = windowSize then
+              yield window.ToArray()
+              window.Dequeue() |> ignore
+          let! moven = ie.MoveNext()
+          b := moven }
+
   let pickAsync (f:'T -> Async<'U option>) (source:AsyncSeq<'T>) = async {
       use ie = source.GetEnumerator()
       let! v = ie.MoveNext()

src/FSharp.Control.AsyncSeq/AsyncSeq.fsi

@@ -198,6 +198,14 @@ module AsyncSeq =
     /// singleton input sequence.
     val pairwise : source:AsyncSeq<'T> -> AsyncSeq<'T * 'T>
 
+    /// Returns an asynchronous sequence that yields sliding windows of the given size
+    /// over the source sequence, each yielded as an array. The first window is emitted
+    /// once <c>windowSize</c> elements have been consumed; subsequent windows slide one
+    /// element at a time. The sequence is empty when the source has fewer than
+    /// <c>windowSize</c> elements. Raises <c>System.ArgumentException</c> if
+    /// <c>windowSize</c> is less than 1.
+    val windowed : windowSize:int -> source:AsyncSeq<'T> -> AsyncSeq<'T []>
+
     /// Asynchronously aggregate the elements of the input asynchronous sequence using the
     /// specified asynchronous 'aggregation' function.
     val foldAsync : folder:('State -> 'T -> Async<'State>) -> state:'State -> source:AsyncSeq<'T> -> Async<'State>

tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs

@@ -2599,6 +2599,46 @@ let ``AsyncSeq.pairwise with three elements should produce two pairs`` () =
   let result = AsyncSeq.pairwise source |> AsyncSeq.toListSynchronously
   Assert.AreEqual([(1, 2); (2, 3)], result)
 
+[<Test>]
+let ``AsyncSeq.windowed empty sequence returns empty`` () =
+  let result = AsyncSeq.windowed 3 AsyncSeq.empty<int> |> AsyncSeq.toListSynchronously
+  Assert.AreEqual([], result)
+
+[<Test>]
+let ``AsyncSeq.windowed fewer elements than window returns empty`` () =
+  let source = asyncSeq { yield 1; yield 2 }
+  let result = AsyncSeq.windowed 3 source |> AsyncSeq.toListSynchronously
+  Assert.AreEqual([], result)
+
+[<Test>]
+let ``AsyncSeq.windowed exact window size returns single window`` () =
+  let source = asyncSeq { yield 1; yield 2; yield 3 }
+  let result = AsyncSeq.windowed 3 source |> AsyncSeq.toListSynchronously
+  Assert.AreEqual([[|1; 2; 3|]], result)
+
+[<Test>]
+let ``AsyncSeq.windowed sliding window produces correct windows`` () =
+  let source = asyncSeq { yield 1; yield 2; yield 3; yield 4; yield 5 }
+  let result = AsyncSeq.windowed 3 source |> AsyncSeq.toListSynchronously
+  Assert.AreEqual([[|1;2;3|]; [|2;3;4|]; [|3;4;5|]], result)
+
+[<Test>]
+let ``AsyncSeq.windowed size 1 returns each element as singleton array`` () =
+  let source = asyncSeq { yield 10; yield 20; yield 30 }
+  let result = AsyncSeq.windowed 1 source |> AsyncSeq.toListSynchronously
+  Assert.AreEqual([[|10|]; [|20|]; [|30|]], result)
+
+[<Test>]
+let ``AsyncSeq.windowed size 2 is equivalent to pairwise as arrays`` () =
+  let source = asyncSeq { yield 1; yield 2; yield 3; yield 4 }
+  let result = AsyncSeq.windowed 2 source |> AsyncSeq.toListSynchronously
+  Assert.AreEqual([[|1;2|]; [|2;3|]; [|3;4|]], result)
+
+[<Test>]
+let ``AsyncSeq.windowed with size 0 raises ArgumentException`` () =
+  Assert.Throws<System.ArgumentException>(fun () ->
+    AsyncSeq.windowed 0 (asyncSeq { yield 1 }) |> AsyncSeq.toListSynchronously |> ignore) |> ignore
+
 [<Test>]
 let ``AsyncSeq.distinctUntilChangedWith should work with custom equality`` () =
   let source = asyncSeq { yield "a"; yield "A"; yield "B"; yield "b"; yield "c" }