Design the client of a Tinder-style swipe app: card prefetching, instant feel, and swipes that must not be lost or duplicated on a bad network.

Predict first

What does the deck and outbox report?

struct Card: Equatable { let id: Int }
struct Swipe: Equatable { let card: Int, like: Bool, key: String }

struct Deck {
    private(set) var cards: [Card] = []
    private(set) var position = 0
    private(set) var outbox: [Swipe] = []                 // swipes the server has not acknowledged yet
    let lowWater = 3
    var needsMore: Bool { cards.count - position <= lowWater }
    mutating func append(_ more: [Card]) { cards += more }
    mutating func swipe(like: Bool) {
        guard position < cards.count else { return }
        let c = cards[position]; position += 1
        outbox.append(Swipe(card: c.id, like: like, key: "swipe-\(c.id)"))   // key makes a retry harmless
    }
    mutating func acknowledged(_ keys: Set<String>) { outbox.removeAll { keys.contains($0.key) } }
}
var deck = Deck()
deck.append((1...5).map(Card.init))
print("needs more:", deck.needsMore)
deck.swipe(like: true); deck.swipe(like: false)
print("position \(deck.position), needs more: \(deck.needsMore)")
deck.append((6...10).map(Card.init))
print("after refill needs more:", deck.needsMore)
print("outbox:", deck.outbox.map(\.key))
var server = Set<String>()                                 // server dedupes by idempotency key
func send(_ batch: [Swipe]) -> Set<String> { for s in batch { server.insert(s.key) }; return Set(batch.map(\.key)) }
_ = send(deck.outbox)                                       // first attempt: reply lost, client does not know
_ = send(deck.outbox)                                       // retry after reconnect
print("server stored:", server.count)
deck.acknowledged(send(deck.outbox))
print("outbox after ack:", deck.outbox.count)

• needs more: false / position 2, needs more: true / after refill needs more: false / outbox: ["swipe-1", "swipe-2"] / server stored: 2 / outbox after ack: 0
• needs more: true at the start
• server stored: 4 after the retry