1 // Copyright 2023 The Chromium Authors
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
5 #ifndef GIN_TIME_CLAMPER_H_
6 #define GIN_TIME_CLAMPER_H_
10 #include "base/rand_util.h"
11 #include "base/time/time.h"
12 #include "gin/gin_export.h"
16 // This class adds some amount of jitter to time. That is, for every
17 // `kResolutionMicros` microseconds it calculates a threshold (using a hash)
18 // that once exceeded advances to the next threshold. This is done so that
19 // time jumps slightly and does not move smoothly.
21 // NOTE: the implementation assumes it's used for servicing calls from JS,
22 // which uses the unix-epoch at time 0.
23 // TODO(skyostil): Deduplicate this with the clamper in Blink.
24 class GIN_EXPORT TimeClamper {
27 static const int64_t kResolutionMicros;
29 TimeClamper() : secret_(base::RandUint64()) {}
30 // This constructor should only be used in tests.
31 explicit TimeClamper(uint64_t secret) : secret_(secret) {}
33 TimeClamper(const TimeClamper&) = delete;
34 TimeClamper& operator=(const TimeClamper&) = delete;
35 ~TimeClamper() = default;
37 // Clamps a time to millisecond precision. The return value is in milliseconds
38 // relative to unix-epoch (which is what JS uses).
39 inline int64_t ClampToMillis(base::Time time) const {
40 // Adding jitter is non-trivial, only use it if necessary.
41 // ClampTimeResolution() adjusts the time to land on `kResolutionMicros`
42 // boundaries, and either uses the current `kResolutionMicros` boundary, or
43 // the next one. Because `kResolutionMicros` is smaller than 1ms, and this
44 // function returns millisecond accuracy, ClampTimeResolution() is only
45 // necessary when within `kResolutionMicros` of the next millisecond.
46 const int64_t now_micros =
47 (time - base::Time::UnixEpoch()).InMicroseconds();
48 const int64_t micros = now_micros % 1000;
49 // abs() is necessary for devices with times before unix-epoch (most likely
50 // configured incorrectly).
51 if (abs(micros) + kResolutionMicros < 1000) {
52 return now_micros / 1000;
54 return ClampTimeResolution(now_micros) / 1000;
57 // Clamps the time, giving microsecond precision. The return value is in
58 // milliseconds relative to unix-epoch (which is what JS uses).
59 inline double ClampToMillisHighResolution(base::Time now) const {
60 const int64_t clamped_time =
61 ClampTimeResolution((now - base::Time::UnixEpoch()).InMicroseconds());
62 return static_cast<double>(clamped_time) / 1000.0;
66 inline int64_t ClampTimeResolution(int64_t time_micros) const {
67 if (time_micros < 0) {
68 return -ClampTimeResolutionPositiveValue(-time_micros);
70 return ClampTimeResolutionPositiveValue(time_micros);
73 inline int64_t ClampTimeResolutionPositiveValue(int64_t time_micros) const {
74 DCHECK_GE(time_micros, 0u);
75 // For each clamped time interval, compute a pseudorandom transition
76 // threshold. The reported time will either be the start of that interval or
77 // the next one depending on which side of the threshold |time_seconds| is.
78 const int64_t interval = time_micros / kResolutionMicros;
79 const int64_t clamped_time_micros = interval * kResolutionMicros;
80 const int64_t tick_threshold = ThresholdFor(clamped_time_micros);
81 if (time_micros - clamped_time_micros < tick_threshold) {
82 return clamped_time_micros;
84 return clamped_time_micros + kResolutionMicros;
87 inline int64_t ThresholdFor(int64_t clamped_time) const {
88 // Returns a random value between 0 and kResolutionMicros. The distribution
89 // is not necessarily equal, but for a random value it's good enough.
90 // Avoid floating-point math by rewriting:
91 // (random_value * 1.0 / UINT64_MAX) * kResolutionMicros
93 // random_value / (UINT64_MAX / kResolutionMicros)
94 // where we avoid integer overflow by dividing instead of multiplying.
95 const uint64_t random_value = MurmurHash3(clamped_time ^ secret_);
96 return std::min(static_cast<int64_t>(random_value /
97 (std::numeric_limits<uint64_t>::max() /
102 static inline uint64_t MurmurHash3(uint64_t value) {
103 value ^= value >> 33;
104 value *= uint64_t{0xFF51AFD7ED558CCD};
105 value ^= value >> 33;
106 value *= uint64_t{0xC4CEB9FE1A85EC53};
107 value ^= value >> 33;
111 const uint64_t secret_;
116 #endif // GIN_TIME_CLAMPER_H_