From 4c8d251f6b2718175aad77131ff044549a97868e Mon Sep 17 00:00:00 2001
From: GitHub Copilot <copilot@github.com>
Date: Sat, 21 Feb 2026 03:35:20 +0000
Subject: [PATCH] Add AsyncSeq.chunkBy and AsyncSeq.chunkByAsync

Groups consecutive elements sharing the same key, yielding
AsyncSeq<'Key * 'T list>. Mirrors the existing chunkBySize pattern.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 src/FSharp.Control.AsyncSeq/AsyncSeq.fs       | 27 ++++++++++++++++
 src/FSharp.Control.AsyncSeq/AsyncSeq.fsi      |  8 +++++
 .../AsyncSeqTests.fs                          | 32 +++++++++++++++++++
 3 files changed, 67 insertions(+)

diff --git a/src/FSharp.Control.AsyncSeq/AsyncSeq.fs b/src/FSharp.Control.AsyncSeq/AsyncSeq.fs
index f749ea7..e73a4d1 100644
--- a/src/FSharp.Control.AsyncSeq/AsyncSeq.fs
+++ b/src/FSharp.Control.AsyncSeq/AsyncSeq.fs
@@ -1558,6 +1558,33 @@ module AsyncSeq =
   let bufferByCount (bufferSize:int) (source:AsyncSeq<'T>) : AsyncSeq<'T[]> =
     chunkBySize bufferSize source
 
+  let chunkByAsync (projection:'T -> Async<'Key>) (source:AsyncSeq<'T>) : AsyncSeq<'Key * 'T list> = asyncSeq {
+    use ie = source.GetEnumerator()
+    let! move = ie.MoveNext()
+    let b = ref move
+    if b.Value.IsSome then
+      let! key0 = projection b.Value.Value
+      let mutable currentKey = key0
+      let buffer = ResizeArray<'T>()
+      buffer.Add b.Value.Value
+      let! moveNext = ie.MoveNext()
+      b := moveNext
+      while b.Value.IsSome do
+        let! key = projection b.Value.Value
+        if key = currentKey then
+          buffer.Add b.Value.Value
+        else
+          yield (currentKey, buffer |> Seq.toList)
+          currentKey <- key
+          buffer.Clear()
+          buffer.Add b.Value.Value
+        let! moveNext = ie.MoveNext()
+        b := moveNext
+      yield (currentKey, buffer |> Seq.toList) }
+
+  let chunkBy (projection:'T -> 'Key) (source:AsyncSeq<'T>) : AsyncSeq<'Key * 'T list> =
+    chunkByAsync (projection >> async.Return) source
+
   #if !FABLE_COMPILER
   let toSortedSeq fn source =
     toArrayAsync source |> Async.map fn |> Async.RunSynchronously
diff --git a/src/FSharp.Control.AsyncSeq/AsyncSeq.fsi b/src/FSharp.Control.AsyncSeq/AsyncSeq.fsi
index 15c1bf5..a759fb6 100644
--- a/src/FSharp.Control.AsyncSeq/AsyncSeq.fsi
+++ b/src/FSharp.Control.AsyncSeq/AsyncSeq.fsi
@@ -484,6 +484,14 @@ module AsyncSeq =
     [<Obsolete("Use AsyncSeq.chunkBySize instead")>]
     val bufferByCount : bufferSize:int -> source:AsyncSeq<'T> -> AsyncSeq<'T []>
 
+    /// Groups consecutive elements of the async sequence that share the same key (as computed by an async projection)
+    /// and yields each group as a pair of the key and a list of elements.
+    val chunkByAsync<'T, 'Key when 'Key : equality> : projection:('T -> Async<'Key>) -> source:AsyncSeq<'T> -> AsyncSeq<'Key * 'T list>
+
+    /// Groups consecutive elements of the async sequence that share the same key (as computed by a projection)
+    /// and yields each group as a pair of the key and a list of elements.
+    val chunkBy<'T, 'Key when 'Key : equality> : projection:('T -> 'Key) -> source:AsyncSeq<'T> -> AsyncSeq<'Key * 'T list>
+
     #if !FABLE_COMPILER
     /// Buffer items from the async sequence until a specified buffer size is reached or a specified amount of time is elapsed.
     val bufferByCountAndTime : bufferSize:int -> timeoutMs:int -> source:AsyncSeq<'T> -> AsyncSeq<'T []>
diff --git a/tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs b/tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs
index 6a44cf4..964722c 100644
--- a/tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs
+++ b/tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs
@@ -2611,3 +2611,35 @@ let ``AsyncSeqOp.FoldAsync with exception in folder should propagate`` () =
       () // Expected
   } |> Async.RunSynchronously
 
+[<Test>]
+let ``AsyncSeq.chunkBy empty sequence returns empty`` () =
+  let result = AsyncSeq.chunkBy id AsyncSeq.empty<int> |> AsyncSeq.toListSynchronously
+  Assert.AreEqual([], result)
+
+[<Test>]
+let ``AsyncSeq.chunkBy single element`` () =
+  let source = asyncSeq { yield 42 }
+  let result = AsyncSeq.chunkBy id source |> AsyncSeq.toListSynchronously
+  Assert.AreEqual([(42, [42])], result)
+
+[<Test>]
+let ``AsyncSeq.chunkBy groups consecutive equal keys`` () =
+  let source = asyncSeq { yield 1; yield 1; yield 2; yield 2; yield 1 }
+  let result = AsyncSeq.chunkBy id source |> AsyncSeq.toListSynchronously
+  Assert.AreEqual([(1, [1;1]); (2, [2;2]); (1, [1])], result)
+
+[<Test>]
+let ``AsyncSeq.chunkBy with projection`` () =
+  let source = asyncSeq { yield 1; yield 3; yield 2; yield 4; yield 5 }
+  let result = AsyncSeq.chunkBy (fun x -> x % 2 = 0) source |> AsyncSeq.toListSynchronously
+  Assert.AreEqual([(false, [1;3]); (true, [2;4]); (false, [5])], result)
+
+[<Test>]
+let ``AsyncSeq.chunkByAsync groups consecutive equal keys`` () =
+  async {
+    let source = asyncSeq { yield 1; yield 1; yield 2; yield 2 }
+    let result = AsyncSeq.chunkByAsync (fun x -> async { return x }) source |> AsyncSeq.toListSynchronously
+    Assert.AreEqual([(1, [1;1]); (2, [2;2])], result)
+  } |> Async.RunSynchronously
+
+