Exponential Backoff And Jitter

RandomBackoff.swift

extension CountableClosedRange {
    var randomElement: Element {
        let distance = self.distance(from: startIndex, to: endIndex)
        let offset = arc4random_uniform(UInt32(distance))
        return self[index(startIndex, offsetBy: Bound.Stride(offset))]
    }
}

enum Random {
    enum Backoff {
        // From https://www.awsarchitectureblog.com/2015/03/backoff.html
        static func fullJitter(cap: Int, attempt: Int) -> Int {
            precondition(cap >= 0)
            precondition(attempt >= 0)
            return (0...Int(min(Double(cap), pow(2, Double(attempt))))).randomElement
        }
    }
}