Add AsyncStream-based serial queue

Sources/Queue/AsyncSerialQueue.swift

@@ -0,0 +1,70 @@
+actor AsyncSerialQueue {
+	public actor QueueTask {
+		var cancelled = false
+		let operation: @Sendable () async throws -> Void
+
+		init(operation: @escaping @Sendable () async throws -> Void) {
+			self.operation = operation
+		}
+
+		public nonisolated func cancel() {
+			Task { await internalCancel() }
+		}
+
+		private func internalCancel() {
+			cancelled = true
+		}
+
+		func run() async throws {
+			if cancelled {
+				return
+			}
+
+			try await operation()
+		}
+	}
+
+	typealias Stream = AsyncStream<QueueTask>
+
+	private let continuation: Stream.Continuation
+
+	public init() {
+		let (stream, continuation) = Stream.makeStream()
+
+		self.continuation = continuation
+
+		Task {
+			for await item in stream {
+				try? await item.run()
+			}
+		}
+	}
+
+	deinit {
+		continuation.finish()
+	}
+
+	/// Submit a throwing operation to the queue.
+	@discardableResult
+	public nonisolated func addOperation(
+		@_inheritActorContext operation: @escaping @Sendable () async throws -> Void
+	) -> QueueTask {
+		let queueTask = QueueTask(operation: operation)
+
+		continuation.yield(queueTask)
+
+		return queueTask
+	}
+
+	/// Submit an operation to the queue.
+	@discardableResult
+	public nonisolated func addOperation(
+		@_inheritActorContext operation: @escaping @Sendable () async -> Void
+	) -> QueueTask {
+		let queueTask = QueueTask(operation: operation)
+
+		continuation.yield(queueTask)
+
+		return queueTask
+	}
+}

Tests/QueueTests/AsyncSerialQueueTests.swift

@@ -0,0 +1,23 @@
+import XCTest
+@testable import Queue
+
+final class AsyncSerialQueueTests: XCTestCase {
+
+	func testEnqueuePerformance() {
+		let queue = AsyncSerialQueue()
+
+		measure {
+			// techincally measuring the actor creation time too, but I don't think that is a big deal
+			let s = Counter()
+
+			for i in 1 ... 1_000 {
+				queue.addOperation { [i] in
+					let result = await s.increment()
+					if i != result {
+						print(i, "does not match", result)
+					}
+				}
+			}
+		}
+	}
+}