[WIP] Add combineLatestMany

Author: czechboy0

Sources/AsyncAlgorithms/CombineLatest/AsyncCombineLatestManySequence.swift

@@ -0,0 +1,86 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Async Algorithms open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+/// Creates an asynchronous sequence that combines the latest values from many `AsyncSequence` types
+/// by emitting a tuple of the values. ``combineLatestMany(_:)`` only emits a value whenever any of the base `AsyncSequence`s
+/// emit a value (so long as each of the bases have emitted at least one value).
+///
+/// Finishes:
+/// ``combineLatestMany(_:)`` finishes when one of the bases finishes before emitting any value or
+/// when all bases finished.
+///
+/// Throws:
+/// ``combineLatestMany(_:)`` throws when one of the bases throws. If one of the bases threw any buffered and not yet consumed
+/// values will be dropped.
+@available(AsyncAlgorithms 1.1, *)
+public func combineLatestMany<Element: Sendable>(_ bases: [any CombineLatestManyBase<Element>]) -> AsyncCombineLatestManySequence<Element>
+{
+  AsyncCombineLatestManySequence(bases)
+}
+
+// TODO: Can we get rid of this typealias?
+@available(AsyncAlgorithms 1.1, *)
+public typealias CombineLatestManyBase<Element: Sendable> = AsyncSequence<Element, any Error> & Sendable
+
+/// An `AsyncSequence` that combines the latest values produced from many asynchronous sequences into an asynchronous sequence of tuples.
+@available(AsyncAlgorithms 1.1, *)
+public struct AsyncCombineLatestManySequence<Element: Sendable>: AsyncSequence, Sendable {
+  public typealias AsyncIterator = Iterator
+    
+  typealias Base = AsyncSequence<Element, any Error> & Sendable
+  let bases: [any Base]
+
+  init(_ bases: [any Base]) {
+    self.bases = bases
+  }
+
+  public func makeAsyncIterator() -> AsyncIterator {
+    Iterator(
+      storage: .init(self.bases)
+    )
+  }
+
+  public struct Iterator: AsyncIteratorProtocol {
+    final class InternalClass {
+      private let storage: CombineLatestManyStorage<Element>
+
+      fileprivate init(storage: CombineLatestManyStorage<Element>) {
+        self.storage = storage
+      }
+
+      deinit {
+        self.storage.iteratorDeinitialized()
+      }
+
+      func next() async throws -> [Element]? {
+        guard let element = try await self.storage.next() else {
+          return nil
+        }
+
+        // This force unwrap is safe since there must be a third element.
+        return element
+      }
+    }
+
+    let internalClass: InternalClass
+
+    fileprivate init(storage: CombineLatestManyStorage<Element>) {
+      self.internalClass = InternalClass(storage: storage)
+    }
+
+    public mutating func next() async throws -> [Element]? {
+      try await self.internalClass.next()
+    }
+  }
+}
+
+@available(*, unavailable)
+extension AsyncCombineLatestManySequence.Iterator: Sendable {}