chromium/base/rand_util.h

// Copyright 2012 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_RAND_UTIL_H_
#define BASE_RAND_UTIL_H_

#include <stddef.h>
#include <stdint.h>

#include <algorithm>
#include <string>
#include <vector>

#include "base/base_export.h"
#include "base/compiler_specific.h"
#include "base/containers/span.h"
#include "base/gtest_prod_util.h"
#include "build/build_config.h"

#if !BUILDFLAG(IS_NACL)
#include "third_party/boringssl/src/include/openssl/rand.h"
#endif

namespace memory_simulator {
class MemoryHolder;
}

namespace base {

class TimeDelta;

namespace internal {

#if BUILDFLAG(IS_ANDROID)
// Sets the implementation of RandBytes according to the corresponding
// base::Feature. Thread safe: allows to switch while RandBytes() is in use.
void ConfigureRandBytesFieldTrial();
#endif

#if !BUILDFLAG(IS_NACL)
void ConfigureBoringSSLBackedRandBytesFieldTrial();
#endif

// Returns a random double in range [0, 1). For use in allocator shim to avoid
// infinite recursion. Thread-safe.
BASE_EXPORT double RandDoubleAvoidAllocation();

}  // namespace internal

// Returns a random number in range [0, UINT64_MAX]. Thread-safe.
BASE_EXPORT uint64_t RandUint64();

// Returns a random number between min and max (inclusive). Thread-safe.
//
// TODO(crbug.com/40283703): Change from fully-closed to half-closed (i.e.
// exclude `max`) to parallel other APIs here.
BASE_EXPORT int RandInt(int min, int max);

// Returns a random number in range [0, range).  Thread-safe.
BASE_EXPORT uint64_t RandGenerator(uint64_t range);

// Returns a random double in range [0, 1). Thread-safe.
BASE_EXPORT double RandDouble();

// Returns a random float in range [0, 1). Thread-safe.
BASE_EXPORT float RandFloat();

// Returns a random duration in [`start`, `limit`). Thread-safe.
//
// REQUIRES: `start` < `limit`
BASE_EXPORT TimeDelta RandTimeDelta(TimeDelta start, TimeDelta limit);

// Returns a random duration in [`TimeDelta()`, `limit`). Thread-safe.
//
// REQUIRES: `limit.is_positive()`
BASE_EXPORT TimeDelta RandTimeDeltaUpTo(TimeDelta limit);

// Given input |bits|, convert with maximum precision to a double in
// the range [0, 1). Thread-safe.
BASE_EXPORT double BitsToOpenEndedUnitInterval(uint64_t bits);

// Given input `bits`, convert with maximum precision to a float in the range
// [0, 1). Thread-safe.
BASE_EXPORT float BitsToOpenEndedUnitIntervalF(uint64_t bits);

// Fills `output` with cryptographically secure random data. Thread-safe.
//
// Although implementations are required to use a cryptographically secure
// random number source, code outside of base/ that relies on this should use
// crypto::RandBytes instead to ensure the requirement is easily discoverable.
BASE_EXPORT void RandBytes(span<uint8_t> output);

// Creates a vector of `length` bytes, fills it with random data, and returns
// it. Thread-safe.
//
// Although implementations are required to use a cryptographically secure
// random number source, code outside of base/ that relies on this should use
// crypto::RandBytes instead to ensure the requirement is easily discoverable.
BASE_EXPORT std::vector<uint8_t> RandBytesAsVector(size_t length);

// DEPRECATED. Prefer RandBytesAsVector() above.
// Fills a string of length |length| with random data and returns it.
// Thread-safe.
//
// Note that this is a variation of |RandBytes| with a different return type.
// The returned string is likely not ASCII/UTF-8. Use with care.
//
// Although implementations are required to use a cryptographically secure
// random number source, code outside of base/ that relies on this should use
// crypto::RandBytes instead to ensure the requirement is easily discoverable.
BASE_EXPORT std::string RandBytesAsString(size_t length);

// An STL UniformRandomBitGenerator backed by RandUint64.
class RandomBitGenerator { … };

#if !BUILDFLAG(IS_NACL)
class NonAllocatingRandomBitGenerator { … };
#endif

// Shuffles [first, last) randomly. Thread-safe.
template <typename Itr>
void RandomShuffle(Itr first, Itr last) { … }

#if BUILDFLAG(IS_POSIX)
BASE_EXPORT int GetUrandomFD();
#endif

class MetricsSubSampler;

// Fast, insecure pseudo-random number generator.
//
// WARNING: This is not the generator you are looking for. This has significant
// caveats:
//   - It is non-cryptographic, so easy to miuse
//   - It is neither fork() nor clone()-safe.
//   - Synchronization is up to the client.
//
// Always prefer base::Rand*() above, unless you have a use case where its
// overhead is too high, or system calls are disallowed.
//
// Performance: As of 2021, rough overhead on Linux on a desktop machine of
// base::RandUint64() is ~800ns per call (it performs a system call). On Windows
// it is lower. On the same machine, this generator's cost is ~2ns per call,
// regardless of platform.
//
// This is different from |Rand*()| above as it is guaranteed to never make a
// system call to generate a new number, except to seed it.  This should *never*
// be used for cryptographic applications, and is not thread-safe.
//
// It is seeded using base::RandUint64() in the constructor, meaning that it
// doesn't need to be seeded. It can be re-seeded though, with
// ReseedForTesting(). Its period is long enough that it should not need to be
// re-seeded during use.
//
// Uses the XorShift128+ generator under the hood.
class BASE_EXPORT InsecureRandomGenerator { … };

class BASE_EXPORT MetricsSubSampler { … };

}  // namespace base

#endif  // BASE_RAND_UTIL_H_