1 // Copyright 2018 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 #include "third_party/blink/renderer/core/timing/time_clamper.h"
6 
7 #include "base/bit_cast.h"
8 #include "base/rand_util.h"
9 #include "third_party/blink/renderer/platform/wtf/assertions.h"
10 
11 #include <cmath>
12 
13 namespace blink {
14 
TimeClamper()15 TimeClamper::TimeClamper() : secret_(base::RandUint64()) {}
16 
ClampTimeResolution(double time_seconds) const17 double TimeClamper::ClampTimeResolution(double time_seconds) const {
18   bool was_negative = false;
19   if (time_seconds < 0) {
20     was_negative = true;
21     time_seconds = -time_seconds;
22   }
23   double interval = floor(time_seconds / kResolutionSeconds);
24   double clamped_time = interval * kResolutionSeconds;
25   double tick_threshold = ThresholdFor(clamped_time);
26 
27   if (time_seconds >= tick_threshold)
28     clamped_time = (interval + 1) * kResolutionSeconds;
29   if (was_negative)
30     clamped_time = -clamped_time;
31   return clamped_time;
32 }
33 
ThresholdFor(double clamped_time) const34 inline double TimeClamper::ThresholdFor(double clamped_time) const {
35   uint64_t time_hash = MurmurHash3(bit_cast<int64_t>(clamped_time) ^ secret_);
36   return clamped_time + kResolutionSeconds * ToDouble(time_hash);
37 }
38 
39 // static
ToDouble(uint64_t value)40 inline double TimeClamper::ToDouble(uint64_t value) {
41   // Exponent for double values for [1.0 .. 2.0]
42   static const uint64_t kExponentBits = uint64_t{0x3FF0000000000000};
43   static const uint64_t kMantissaMask = uint64_t{0x000FFFFFFFFFFFFF};
44   uint64_t random = (value & kMantissaMask) | kExponentBits;
45   return bit_cast<double>(random) - 1;
46 }
47 
48 // static
MurmurHash3(uint64_t value)49 inline uint64_t TimeClamper::MurmurHash3(uint64_t value) {
50   value ^= value >> 33;
51   value *= uint64_t{0xFF51AFD7ED558CCD};
52   value ^= value >> 33;
53   value *= uint64_t{0xC4CEB9FE1A85EC53};
54   value ^= value >> 33;
55   return value;
56 }
57 
58 }  // namespace blink
59