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- // Copyright (c) 2011 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 "base/rand_util.h"
- #include <limits.h>
- #include <math.h>
- #include <stdint.h>
- #include <algorithm>
- #include <limits>
- #include "base/check_op.h"
- #include "base/strings/string_util.h"
- namespace base {
- uint64_t RandUint64() {
- uint64_t number;
- RandBytes(&number, sizeof(number));
- return number;
- }
- int RandInt(int min, int max) {
- DCHECK_LE(min, max);
- uint64_t range = static_cast<uint64_t>(max) - static_cast<uint64_t>(min) + 1;
- // |range| is at most UINT_MAX + 1, so the result of RandGenerator(range)
- // is at most UINT_MAX. Hence it's safe to cast it from uint64_t to int64_t.
- int result =
- static_cast<int>(min + static_cast<int64_t>(base::RandGenerator(range)));
- DCHECK_GE(result, min);
- DCHECK_LE(result, max);
- return result;
- }
- double RandDouble() {
- return BitsToOpenEndedUnitInterval(base::RandUint64());
- }
- double BitsToOpenEndedUnitInterval(uint64_t bits) {
- // We try to get maximum precision by masking out as many bits as will fit
- // in the target type's mantissa, and raising it to an appropriate power to
- // produce output in the range [0, 1). For IEEE 754 doubles, the mantissa
- // is expected to accommodate 53 bits.
- static_assert(std::numeric_limits<double>::radix == 2,
- "otherwise use scalbn");
- static const int kBits = std::numeric_limits<double>::digits;
- uint64_t random_bits = bits & ((UINT64_C(1) << kBits) - 1);
- double result = ldexp(static_cast<double>(random_bits), -1 * kBits);
- DCHECK_GE(result, 0.0);
- DCHECK_LT(result, 1.0);
- return result;
- }
- uint64_t RandGenerator(uint64_t range) {
- DCHECK_GT(range, 0u);
- // We must discard random results above this number, as they would
- // make the random generator non-uniform (consider e.g. if
- // MAX_UINT64 was 7 and |range| was 5, then a result of 1 would be twice
- // as likely as a result of 3 or 4).
- uint64_t max_acceptable_value =
- (std::numeric_limits<uint64_t>::max() / range) * range - 1;
- uint64_t value;
- do {
- value = base::RandUint64();
- } while (value > max_acceptable_value);
- return value % range;
- }
- std::string RandBytesAsString(size_t length) {
- DCHECK_GT(length, 0u);
- std::string result;
- RandBytes(WriteInto(&result, length + 1), length);
- return result;
- }
- InsecureRandomGenerator::InsecureRandomGenerator() {
- a_ = base::RandUint64();
- b_ = base::RandUint64();
- }
- void InsecureRandomGenerator::ReseedForTesting(uint64_t seed) {
- a_ = seed;
- b_ = seed;
- }
- uint64_t InsecureRandomGenerator::RandUint64() {
- // Using XorShift128+, which is simple and widely used. See
- // https://en.wikipedia.org/wiki/Xorshift#xorshift+ for details.
- uint64_t t = a_;
- const uint64_t s = b_;
- a_ = s;
- t ^= t << 23;
- t ^= t >> 17;
- t ^= s ^ (s >> 26);
- b_ = t;
- return t + s;
- }
- uint32_t InsecureRandomGenerator::RandUint32() {
- // The generator usually returns an uint64_t, truncate it.
- //
- // It is noted in this paper (https://arxiv.org/abs/1810.05313) that the
- // lowest 32 bits fail some statistical tests from the Big Crush
- // suite. Use the higher ones instead.
- return this->RandUint64() >> 32;
- }
- double InsecureRandomGenerator::RandDouble() {
- uint64_t x = RandUint64();
- // From https://vigna.di.unimi.it/xorshift/.
- // 53 bits of mantissa, hence the "hexadecimal exponent" 1p-53.
- return (x >> 11) * 0x1.0p-53;
- }
- MetricsSubSampler::MetricsSubSampler() = default;
- bool MetricsSubSampler::ShouldSample(double probability) {
- return generator_.RandDouble() < probability;
- }
- } // namespace base
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