blob: dd5a70d336c95caf65502daba8fd446363c24ca2 [file] [log] [blame] [edit]
// Copyright (c) 2013, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
#include "vm/random.h"
#include "vm/dart.h"
#include "vm/flags.h"
#include "vm/os.h"
namespace dart {
DEFINE_FLAG(uint64_t,
random_seed,
0,
"Override the random seed for debugging.");
Random::Random() {
uint64_t seed = FLAG_random_seed;
if (seed == 0) {
Dart_EntropySource callback = Dart::entropy_source_callback();
if (callback != nullptr) {
if (!callback(reinterpret_cast<uint8_t*>(&seed), sizeof(seed))) {
// Callback failed. Reset the seed to 0.
seed = 0;
}
}
}
if (seed == 0) {
// We did not get a seed so far. As a fallback we do use the current time.
seed = OS::GetCurrentTimeMicros();
}
Initialize(seed);
}
void Random::Initialize(uint64_t seed) {
ASSERT(seed != 0);
// Crank the next state a couple of times.
_state = seed;
NextState();
NextState();
NextState();
NextState();
}
Random::Random(uint64_t seed) {
Initialize(seed);
}
Random::~Random() {
// Nothing to be done here.
}
// The algorithm used here is Multiply with Carry (MWC) with a Base b = 2^32.
// http://en.wikipedia.org/wiki/Multiply-with-carry
// The constant A is selected from "Numerical Recipes 3rd Edition" p.348 B1.
uint64_t Random::NextState() {
const uint64_t MASK_32 = 0xffffffff;
const uint64_t A = 0xffffda61;
uint64_t old_state = _state;
while (true) {
const uint64_t state_lo = old_state & MASK_32;
const uint64_t state_hi = (old_state >> 32) & MASK_32;
const uint64_t new_state = (A * state_lo) + state_hi;
if (_state.compare_exchange_weak(old_state, new_state,
std::memory_order_relaxed,
std::memory_order_relaxed)) {
return new_state;
}
}
}
uint32_t Random::NextUInt32() {
const uint64_t MASK_32 = 0xffffffff;
return static_cast<uint32_t>(NextState() & MASK_32);
}
static Random* global_random = nullptr;
static Mutex* global_random_mutex = nullptr;
void Random::Init() {
ASSERT(global_random_mutex == nullptr);
global_random_mutex = new Mutex();
ASSERT(global_random == nullptr);
global_random = new Random();
}
void Random::Cleanup() {
delete global_random_mutex;
global_random_mutex = nullptr;
delete global_random;
global_random = nullptr;
}
uint64_t Random::GlobalNextUInt64() {
MutexLocker locker(global_random_mutex);
return global_random->NextUInt64();
}
double Random::NextDouble() {
uint64_t mantissa = NextUInt64() & 0xFFFFFFFFFFFFF;
// The exponent value 0 in biased form.
const uint64_t exp = 1023;
return bit_cast<double>(exp << 52 | mantissa) - 1.0;
}
} // namespace dart