[Combine] PublisherのArrayをCombineLatestする。

Combineで2つや3つのPublisherをCombineLatestするPublishers.CombineLatest2・Publishers.CombineLatest3はありますが、自由長の配列をCombineLatestすることはできません。

ここにCombineLatestCollectionの実装がありますが、この実装はCombine的に間違っているため、他のOperatorと組み合わせるとクラッシュしたりStreamが崩壊したりします。

なので、Combine的に（多分）正しいCombineLatestCollectionの実装を置いておきます。

（速度に敏感・メインスレッドのみな要件で作ったので@inlinableだったり、Lockしてなかったりしますが、そこは適当に調整してください。）

extension Collection where Element: Publisher {
    public var combineLatest: Publishers.CombineLatestCollection<Self> { Publishers.CombineLatestCollection(upstreams: self) }
}

extension Publishers {
    public struct CombineLatestCollection<Upstreams: Collection>: Publisher where Upstreams.Element: Combine.Publisher {
        public typealias Output = [Upstreams.Element.Output]
        public typealias Failure = Upstreams.Element.Failure

        public let upstreams: Upstreams

        public init(upstreams: Upstreams) { self.upstreams = upstreams }

        public func receive<Downstream: Subscriber>(subscriber downstream: Downstream)
            where Downstream.Input == Output, Downstream.Failure == Self.Failure
        {
            let inner = Inner<Downstream>(downstream: downstream, upstreamCount: upstreams.count)
            self.upstreams.enumerated().forEach{ index, upstream in upstream.map{ (index: index, value: $0) }.subscribe(inner) }
        }
    }
}

extension Publishers.CombineLatestCollection {
    public final class Inner<Downstream: Combine.Subscriber>: Combine.Subscriber where Downstream.Input == [Upstreams.Element.Output] {
        public typealias Input = (index: Int, value: Upstreams.Element.Output)
        public typealias Failure = Downstream.Failure

        @usableFromInline let downstream: Downstream
        @usableFromInline let upstreamCount: Int
        @usableFromInline let subscription = Subscription()
        @usableFromInline var prebuildStorage: [Upstreams.Element.Output?]
        @usableFromInline var completeStorage: [Upstreams.Element.Output]? = nil
        @usableFromInline var isValueCompleted = false
        @usableFromInline var isCompleted = false

        @inlinable init(downstream: Downstream, upstreamCount: Int) {
            self.downstream = downstream
            self.upstreamCount = upstreamCount
            self.prebuildStorage = [Upstreams.Element.Output?](repeating: nil, count: upstreamCount)
        }

        @inlinable public func receive(subscription: Combine.Subscription) {
            self.subscription.subscriptions.append(subscription)
            guard self.subscription.subscriptions.count == upstreamCount else { return }
            self.downstream.receive(subscription: self.subscription)
        }

        @inlinable public func receive(_ input: (index: Int, value: Upstreams.Element.Output)) -> Subscribers.Demand {

            if isValueCompleted {
                self.completeStorage![input.index] = input.value
                return self.downstream.receive(self.completeStorage!)
            } else {
                self.prebuildStorage[input.index] = input.value
            }

            if self.prebuildStorage.allSatisfy({ $0 != nil }) {
                self.isValueCompleted = true
                self.completeStorage = self.prebuildStorage as? [Upstreams.Element.Output]
                return self.downstream.receive(self.completeStorage!)
            }

            return .none
        }

        @inlinable public func receive(completion: Subscribers.Completion<Downstream.Failure>) {
            guard !isCompleted else { return }
            self.isCompleted = true
            self.downstream.receive(completion: completion)
        }
    }

    public final class Subscription: Combine.Subscription {
        @usableFromInline var subscriptions = [Combine.Subscription]()

        public func request(_ demand: Subscribers.Demand) {
            for subscription in subscriptions {
                subscription.request(demand)
            }
        }

        public func cancel() {
            for subscription in subscriptions {
                subscription.cancel()
            }
        }
    }
}