+class Random;
+
+/**
+ * A PRNG with one instance per thread. This PRNG uses a mersenne twister random
+ * number generator and is seeded from /dev/urandom. It should not be used for
+ * anything which requires security, only for statistical randomness.
+ *
+ * An instance of this class represents the current threads PRNG. This means
+ * copying an instance of this class across threads will result in corruption
+ *
+ * Most users will use the Random class which implicitly creates this class.
+ * However, if you are worried about performance, you can memoize the TLS
+ * lookups that get the per thread state by manually using this class:
+ *
+ * ThreadLocalPRNG rng = Random::threadLocalPRNG()
+ * for (...) {
+ * Random::rand32(rng);
+ * }
+ */
+class ThreadLocalPRNG {
+ public:
+ typedef uint32_t result_type;
+
+ uint32_t operator()() {
+ // Using a static method allows the compiler to avoid allocating stack space
+ // for this class.
+ return getImpl(local_);
+ }
+
+ static constexpr result_type min() {
+ return std::numeric_limits<result_type>::min();
+ }
+ static constexpr result_type max() {
+ return std::numeric_limits<result_type>::max();
+ }
+ friend class Random;
+
+ ThreadLocalPRNG() {
+ local_ = localInstance.get();
+ if (!local_) {
+ local_ = initLocal();
+ }
+ }
+
+ private:
+ class LocalInstancePRNG;
+ static LocalInstancePRNG* initLocal();
+ static folly::ThreadLocalPtr<ThreadLocalPRNG::LocalInstancePRNG>
+ localInstance;
+
+ static result_type getImpl(LocalInstancePRNG* local);
+ LocalInstancePRNG* local_;
+};
+
+
+
+class Random {
+
+ private:
+ template<class RNG>
+ using ValidRNG = typename std::enable_if<
+ std::is_unsigned<typename std::result_of<RNG&()>::type>::value,
+ RNG>::type;
+
+ public:
+
+ /**
+ * Returns a random uint32_t
+ */
+ template<class RNG = ThreadLocalPRNG>
+ static uint32_t rand32(ValidRNG<RNG> rrng = RNG()) {
+ uint32_t r = rrng.operator()();
+ return r;
+ }
+
+ /**
+ * Returns a random uint32_t in [0, max). If max == 0, returns 0.
+ */
+ template<class RNG = ThreadLocalPRNG>
+ static uint32_t rand32(uint32_t max, ValidRNG<RNG> rng = RNG()) {
+ if (max == 0) {
+ return 0;
+ }
+
+ return std::uniform_int_distribution<uint32_t>(0, max - 1)(rng);
+ }
+
+ /**
+ * Returns a random uint64_t
+ */
+ template<class RNG = ThreadLocalPRNG>
+ static uint64_t rand64(ValidRNG<RNG> rng = RNG()) {
+ return ((uint64_t) rng() << 32) | rng();
+ }
+
+ /**
+ * Returns a random uint64_t in [0, max). If max == 0, returns 0.
+ */
+ template<class RNG = ThreadLocalPRNG>
+ static uint64_t rand64(uint64_t max, ValidRNG<RNG> rng = RNG()) {
+ if (max == 0) {
+ return 0;
+ }
+
+ return std::uniform_int_distribution<uint64_t>(0, max - 1)(rng);
+ }
+
+ /**
+ * Returns true 1/n of the time. If n == 0, always returns false
+ */
+ template<class RNG = ThreadLocalPRNG>
+ static bool oneIn(uint32_t n, ValidRNG<RNG> rng = RNG()) {
+ if (n == 0) {
+ return false;
+ }
+
+ return rand32(n, rng) == 0;
+ }
+
+ /**
+ * Returns a double in [0, 1)
+ */
+ template<class RNG = ThreadLocalPRNG>
+ static double randDouble01(ValidRNG<RNG> rng = RNG()) {
+ return std::generate_canonical<double, std::numeric_limits<double>::digits>
+ (rng);
+ }
+
+};
+