Commit

Carthage/Checkouts/TrueTime.swift/Sources/NTPResponse.swift

@@ -0,0 +1,68 @@
+//
+//  NTPResponse.swift
+//  TrueTime
+//
+//  Created by Michael Sanders on 10/14/16.
+//  Copyright © 2016 Instacart. All rights reserved.
+//
+
+import Foundation
+
+struct NTPResponse {
+    let packet: ntp_packet_t
+    let responseTime: Int64
+    let receiveTime: timeval
+    init?(packet: ntp_packet_t, responseTime: Int64, receiveTime: timeval = .now()) {
+        self.packet = packet
+        self.responseTime = responseTime
+        self.receiveTime = receiveTime
+        guard isValidResponse else { return nil }
+    }
+
+    // See https://en.wikipedia.org/wiki/Network_Time_Protocol#Clock_synchronization_algorithm
+    var offset: Int64 {
+        let T = offsetValues
+        return ((T[1] - T[0]) + (T[2] - T[3])) / 2
+    }
+
+    var delay: Int64 {
+        let T = offsetValues
+        return (T[3] - T[0]) - (T[2] - T[1])
+    }
+
+    var networkDate: Date {
+        let interval = TimeInterval(milliseconds: responseTime + offset)
+        return Date(timeIntervalSince1970: interval)
+    }
+}
+
+func bestTime(fromResponses times: [[FrozenNetworkTime]]) -> FrozenNetworkTime? {
+    let bestTimes = times.map { serverTimes -> FrozenNetworkTime? in
+        serverTimes.min { $0.serverResponse.delay < $1.serverResponse.delay }
+    }.compactMap { $0 }.sorted { $0.serverResponse.offset < $1.serverResponse.offset }
+
+    return bestTimes.isEmpty ? nil : bestTimes[bestTimes.count / 2]
+}
+
+private extension NTPResponse {
+    var isValidResponse: Bool {
+        return packet.stratum > 0 && packet.stratum < 16 &&
+               packet.root_delay.durationInMilliseconds < maxRootDispersion &&
+               packet.root_dispersion.durationInMilliseconds < maxRootDispersion &&
+               packet.client_mode == ntpModeServer &&
+               packet.leap_indicator != leapIndicatorUnknown &&
+               abs(receiveTime.milliseconds - packet.originate_time.milliseconds - delay) < maxDelayDelta
+    }
+
+    var offsetValues: [Int64] {
+        return [packet.originate_time.milliseconds,
+                packet.receive_time.milliseconds,
+                packet.transmit_time.milliseconds,
+                responseTime]
+    }
+}
+
+private let maxRootDispersion: Int64 = 100
+private let maxDelayDelta: Int64 = 100
+private let ntpModeServer: UInt8 = 4
+private let leapIndicatorUnknown: UInt8 = 3