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// Copyright 2018 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "third_party/blink/renderer/platform/time_clamper.h"
#include "base/bit_cast.h"
#include "third_party/blink/renderer/platform/wtf/assertions.h"
#include "third_party/blink/renderer/platform/wtf/cryptographically_random_number.h"
#include <cmath>
namespace blink {
TimeClamper::TimeClamper() {
CryptographicallyRandomValues(&secret_, sizeof(secret_));
}
double TimeClamper::ClampTimeResolution(double time_seconds) const {
DCHECK_GE(time_seconds, 0);
double interval = floor(time_seconds / kResolutionSeconds);
double clamped_time = interval * kResolutionSeconds;
double tick_threshold = ThresholdFor(clamped_time);
if (time_seconds >= tick_threshold)
return (interval + 1) * kResolutionSeconds;
return clamped_time;
}
inline double TimeClamper::ThresholdFor(double clamped_time) const {
uint64_t time_hash = MurmurHash3(bit_cast<int64_t>(clamped_time) ^ secret_);
return clamped_time + kResolutionSeconds * ToDouble(time_hash);
}
// static
inline double TimeClamper::ToDouble(uint64_t value) {
// Exponent for double values for [1.0 .. 2.0]
static const uint64_t kExponentBits = uint64_t{0x3FF0000000000000};
static const uint64_t kMantissaMask = uint64_t{0x000FFFFFFFFFFFFF};
uint64_t random = (value & kMantissaMask) | kExponentBits;
return bit_cast<double>(random) - 1;
}
// static
inline uint64_t TimeClamper::MurmurHash3(uint64_t value) {
value ^= value >> 33;
value *= uint64_t{0xFF51AFD7ED558CCD};
value ^= value >> 33;
value *= uint64_t{0xC4CEB9FE1A85EC53};
value ^= value >> 33;
return value;
}
} // namespace blink
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