import Foundation

/// Whether anyone is looking at the app: the phone UI in the foreground, or CarPlay connected.
/// Work that only feeds a screen (stemd polling, a shallow output buffer for snappy faders)
/// checks this, so a locked phone playing music does as little as it can.
enum AppActivity {
    private static let lock = UnfairLock()
    nonisolated(unsafe) private static var phone = true
    nonisolated(unsafe) private static var car = false
    nonisolated(unsafe) private static var waiters: [CheckedContinuation<Void, Never>] = []

    static var isActive: Bool { lock.with { phone || car } }

    static func setPhone(_ on: Bool) { update { phone = on } }
    static func setCar(_ on: Bool) { update { car = on } }

    private static func update(_ change: () -> Void) {
        let ready = lock.with { () -> [CheckedContinuation<Void, Never>] in
            change()
            guard phone || car else { return [] }
            defer { waiters = [] }
            return waiters
        }
        ready.forEach { $0.resume() }
    }

    /// Returns once the app is active, at once if it already is.
    static func untilActive() async {
        await withCheckedContinuation { (c: CheckedContinuation<Void, Never>) in
            let now = lock.with { () -> Bool in
                if phone || car { return true }
                waiters.append(c)
                return false
            }
            if now { c.resume() }
        }
    }
}