// Example of when backoff is calculated by a client
let rng = SystemRandomNumberGenerator() // or a seeded rng for unit tests
var backoff = Backoff
  .exponential(factor: 2, initial: .seconds(5))
  .fullJitter(using: rng)
  .minimum(.seconds(2))
  .maximum(.seconds(100))

let response = try await retry(maxAttempts: 5) {
  try await URLSession.shared.data(from: url)
} strategy: { error in
  return .backoff(backoff.nextDuration())
}

// Example of when backoff is returned by a server
let response = try await retry(maxAttempts: 5) {
  let (data, response) = try await URLSession.shared.data(from: url)
  if
    let response = response as? HTTPURLResponse,
    response.statusCode == 429,
    let retryAfter = response.value(forHTTPHeaderField: "Retry-After")
  {
   throw TooManyRequestsError(retryAfter: Double(retryAfter)!)
  }
  return (data, response)
} strategy: { error in
  if let error = error as? TooManyRequestsError {
    return .backoff(.seconds(error.retryAfter))
  } else {
    return .stop
  }
}