Herman hermanbanken

Redis SCAN cursor

Have you ever wondered what the cursor value that Redis returns for SCAN commands means? I did, and set out to make sense of these seemingly arbitrary numbers.

Some time ago I was reading a StackOverflow article, that linked the documentation inside the Redis SCAN implementation. At the time, I only quickly skimmed the text. I remember thinking that it should be possible to come up with an estimate of how far along the SCAN is.

Today I 'cracked' the algorithm. It is not really that much of an achievement, given it is thoroughly documented, but figuring it out without the source code documentation still gave me a Eureka-moment. The result:

	extension Observable : Publisher {
	@available(iOS 13.0, *)
	public func receive<S>(subscriber: S) where S : Subscriber, Observable.Failure == S.Failure, Observable.Output == S.Input {
	_ = self.subscribe(onNext: { (next) in
	let demand = subscriber.receive(next)
	}, onError: { (err) in
	subscriber.receive(completion: .failure(err))
	}, onCompleted: {
	subscriber.receive(completion: .finished)
	})

	func run () {
	if #available(iOS 13.0, *) {

	// Create
	let p1 = Publishers.Empty<Int, Error>()
	let p2 = Publishers.Just(42)
	let p3 = Publishers.Sequence<[Int], Error>(sequence: [1,2,3,4])

	// Operators
	let p4 = p1

	@available(iOS 13.0, *)
	let p1 = Publishers.Empty<Any, Error>()

	@available(iOS 13.0, *)
	let p2 = Publishers.Just(42)

	@available(iOS 13.0, *)
	let p3 = Publishers.Sequence<[Int], Error>(sequence: [1,2,3,4])

	@available(iOS 13.0, *)
	let p = Publishers.Future { (subscriber: @escaping (Result<Int,Error>) -> Void) in
	DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(2), execute: {
	subscriber(.success(42))
	})
	}

	struct MyPublisher : Publisher {
	public typealias Failure = Error
	public typealias Output = Int

	@available(iOS 13.0, *)
	public func receive<S>(subscriber: S) where S : Subscriber, Failure == S.Failure, Output == S.Input {
	DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(2), execute: {
	_ = subscriber.receive(42)
	subscriber.receive(completion: .finished)
	})

	let p3 = Publishers.Just(42)
	p3.sink { (value) in
	debugPrint(value)
	}

	// Using RxSwift Observables
	let observable: Observable<Int> = Observable.create { observer in
	// could be anything async, e.g. a network call
	DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(2), execute: {
	observer.onNext(42)
	observer.onCompleted()
	})
	return Disposables.create()
	}

	Publishers.Future { (n: @escaping (Result<_, _>) -> Void) in
	Timer.init(timeInterval: 100, repeats: false) { (_) in
	n(.value(1))
	}
	}