1 // Copyright 2019 The Chromium Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
4
5 #ifndef CAST_STREAMING_NTP_TIME_H_
6 #define CAST_STREAMING_NTP_TIME_H_
7
8 #include <stdint.h>
9
10 #include "platform/api/time.h"
11
12 namespace openscreen {
13 namespace cast {
14
15 // NTP timestamps are 64-bit timestamps that consist of two 32-bit parts: 1) The
16 // number of seconds since 1 January 1900; and 2) The fraction of the second,
17 // where 0 maps to 0x00000000 and each unit increment represents another 2^-32
18 // seconds.
19 //
20 // Note that it is part of the design of NTP for the seconds part to roll around
21 // on 7 February 2036.
22 using NtpTimestamp = uint64_t;
23
24 // NTP fixed-point time math: Declare two std::chrono::duration types with the
25 // bit-width necessary to reliably perform all conversions to/from NTP format.
26 using NtpSeconds = std::chrono::duration<int64_t, std::chrono::seconds::period>;
27 using NtpFraction =
28 std::chrono::duration<int64_t, std::ratio<1, INT64_C(0x100000000)>>;
29
NtpSecondsPart(NtpTimestamp timestamp)30 constexpr NtpSeconds NtpSecondsPart(NtpTimestamp timestamp) {
31 return NtpSeconds(timestamp >> 32);
32 }
33
NtpFractionPart(NtpTimestamp timestamp)34 constexpr NtpFraction NtpFractionPart(NtpTimestamp timestamp) {
35 return NtpFraction(timestamp & 0xffffffff);
36 }
37
AssembleNtpTimestamp(NtpSeconds seconds,NtpFraction fraction)38 constexpr NtpTimestamp AssembleNtpTimestamp(NtpSeconds seconds,
39 NtpFraction fraction) {
40 return (static_cast<uint64_t>(seconds.count()) << 32) |
41 static_cast<uint32_t>(fraction.count());
42 }
43
44 // Converts between Clock::time_points and NtpTimestamps. The class is
45 // instantiated with the current Clock time and the current wall clock time, and
46 // these are used to determine a fixed origin reference point for all
47 // conversions. Thus, to avoid introducing unintended timing-related behaviors,
48 // only one NtpTimeConverter instance should be used for converting all the NTP
49 // timestamps in the same streaming session.
50 class NtpTimeConverter {
51 public:
52 NtpTimeConverter(
53 Clock::time_point now,
54 std::chrono::seconds since_unix_epoch = GetWallTimeSinceUnixEpoch());
55 ~NtpTimeConverter();
56
57 NtpTimestamp ToNtpTimestamp(Clock::time_point time_point) const;
58 Clock::time_point ToLocalTime(NtpTimestamp timestamp) const;
59
60 private:
61 // The time point on the platform clock's timeline that corresponds to
62 // approximately the same time point on the NTP timeline. Note that it is
63 // acceptable for the granularity of the NTP seconds value to be whole seconds
64 // here: Both a Cast Streaming Sender and Receiver will assume their clocks
65 // can be off (with respect to each other) by even a large amount; and all
66 // that matters is that time ticks forward at a reasonable pace from some
67 // initial point.
68 const Clock::time_point start_time_;
69 const NtpSeconds since_ntp_epoch_;
70 };
71
72 } // namespace cast
73 } // namespace openscreen
74
75 #endif // CAST_STREAMING_NTP_TIME_H_
76