AsyncMerge2Sequence.swift

//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift Async Algorithms open source project
//
// Copyright (c) 2022 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
//
//===----------------------------------------------------------------------===//

import DequeModule

/// Creates an asynchronous sequence of elements from two underlying asynchronous sequences
public func merge<Base1: AsyncSequence, Base2: AsyncSequence>(_ base1: Base1, _ base2: Base2) -> AsyncMerge2Sequence<Base1, Base2>
    where
    Base1.Element == Base2.Element,
    Base1: Sendable, Base2: Sendable,
    Base1.Element: Sendable
{
    return AsyncMerge2Sequence(base1, base2)
}

/// An ``Swift/AsyncSequence`` that takes two upstream ``Swift/AsyncSequence``s and combines their elements.
public struct AsyncMerge2Sequence<
    Base1: AsyncSequence,
    Base2: AsyncSequence
>: Sendable where
    Base1.Element == Base2.Element,
    Base1: Sendable, Base2: Sendable,
    Base1.Element: Sendable
{
    public typealias Element = Base1.Element

    private let base1: Base1
    private let base2: Base2

    /// Initializes a new ``AsyncMerge2Sequence``.
    ///
    /// - Parameters:
    ///     - base1: The first upstream ``Swift/AsyncSequence``.
    ///     - base2: The second upstream ``Swift/AsyncSequence``.
    init(
        _ base1: Base1,
        _ base2: Base2
    ) {
        self.base1 = base1
        self.base2 = base2
    }
}

extension AsyncMerge2Sequence: AsyncSequence {
    public func makeAsyncIterator() -> Iterator {
        let storage = MergeStorage<Base1, Base2, Base1>(
            base1: base1,
            base2: base2,
            base3: nil
        )
        return Iterator(storage: storage)
    }
}

extension AsyncMerge2Sequence {
    public struct Iterator: AsyncIteratorProtocol {
        /// This class is needed to hook the deinit to observe once all references to the ``AsyncIterator`` are dropped.
        ///
        /// If we get move-only types we should be able to drop this class and use the `deinit` of the ``AsyncIterator`` struct itself.
        final class InternalClass: Sendable {
            private let storage: MergeStorage<Base1, Base2, Base1>

            fileprivate init(storage: MergeStorage<Base1, Base2, Base1>) {
                self.storage = storage
            }

            deinit {
                self.storage.iteratorDeinitialized()
            }

            func next() async rethrows -> Element? {
                try await storage.next()
            }
        }

        let internalClass: InternalClass

        fileprivate init(storage: MergeStorage<Base1, Base2, Base1>) {
            internalClass = InternalClass(storage: storage)
        }

        public mutating func next() async rethrows -> Element? {
            try await internalClass.next()
        }
    }
}

@available(*, unavailable)
extension AsyncMerge2Sequence.Iterator: Sendable { }