summaryrefslogtreecommitdiffstats
path: root/src/rocksdb/util/random.h
diff options
context:
space:
mode:
Diffstat (limited to 'src/rocksdb/util/random.h')
-rw-r--r--src/rocksdb/util/random.h166
1 files changed, 166 insertions, 0 deletions
diff --git a/src/rocksdb/util/random.h b/src/rocksdb/util/random.h
new file mode 100644
index 000000000..f97b2126f
--- /dev/null
+++ b/src/rocksdb/util/random.h
@@ -0,0 +1,166 @@
+// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
+// This source code is licensed under both the GPLv2 (found in the
+// COPYING file in the root directory) and Apache 2.0 License
+// (found in the LICENSE.Apache file in the root directory).
+//
+// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file. See the AUTHORS file for names of contributors.
+
+#pragma once
+#include <stdint.h>
+#include <random>
+
+#include "rocksdb/rocksdb_namespace.h"
+
+namespace ROCKSDB_NAMESPACE {
+
+// A very simple random number generator. Not especially good at
+// generating truly random bits, but good enough for our needs in this
+// package.
+class Random {
+ private:
+ enum : uint32_t {
+ M = 2147483647L // 2^31-1
+ };
+ enum : uint64_t {
+ A = 16807 // bits 14, 8, 7, 5, 2, 1, 0
+ };
+
+ uint32_t seed_;
+
+ static uint32_t GoodSeed(uint32_t s) { return (s & M) != 0 ? (s & M) : 1; }
+
+ public:
+ // This is the largest value that can be returned from Next()
+ enum : uint32_t { kMaxNext = M };
+
+ explicit Random(uint32_t s) : seed_(GoodSeed(s)) {}
+
+ void Reset(uint32_t s) { seed_ = GoodSeed(s); }
+
+ uint32_t Next() {
+ // We are computing
+ // seed_ = (seed_ * A) % M, where M = 2^31-1
+ //
+ // seed_ must not be zero or M, or else all subsequent computed values
+ // will be zero or M respectively. For all other values, seed_ will end
+ // up cycling through every number in [1,M-1]
+ uint64_t product = seed_ * A;
+
+ // Compute (product % M) using the fact that ((x << 31) % M) == x.
+ seed_ = static_cast<uint32_t>((product >> 31) + (product & M));
+ // The first reduction may overflow by 1 bit, so we may need to
+ // repeat. mod == M is not possible; using > allows the faster
+ // sign-bit-based test.
+ if (seed_ > M) {
+ seed_ -= M;
+ }
+ return seed_;
+ }
+
+ // Returns a uniformly distributed value in the range [0..n-1]
+ // REQUIRES: n > 0
+ uint32_t Uniform(int n) { return Next() % n; }
+
+ // Randomly returns true ~"1/n" of the time, and false otherwise.
+ // REQUIRES: n > 0
+ bool OneIn(int n) { return Uniform(n) == 0; }
+
+ // "Optional" one-in-n, where 0 or negative always returns false
+ // (may or may not consume a random value)
+ bool OneInOpt(int n) { return n > 0 && OneIn(n); }
+
+ // Returns random bool that is true for the given percentage of
+ // calls on average. Zero or less is always false and 100 or more
+ // is always true (may or may not consume a random value)
+ bool PercentTrue(int percentage) {
+ return static_cast<int>(Uniform(100)) < percentage;
+ }
+
+ // Skewed: pick "base" uniformly from range [0,max_log] and then
+ // return "base" random bits. The effect is to pick a number in the
+ // range [0,2^max_log-1] with exponential bias towards smaller numbers.
+ uint32_t Skewed(int max_log) {
+ return Uniform(1 << Uniform(max_log + 1));
+ }
+
+ // Returns a Random instance for use by the current thread without
+ // additional locking
+ static Random* GetTLSInstance();
+};
+
+// A good 32-bit random number generator based on std::mt19937.
+// This exists in part to avoid compiler variance in warning about coercing
+// uint_fast32_t from mt19937 to uint32_t.
+class Random32 {
+ private:
+ std::mt19937 generator_;
+
+ public:
+ explicit Random32(uint32_t s) : generator_(s) {}
+
+ // Generates the next random number
+ uint32_t Next() { return static_cast<uint32_t>(generator_()); }
+
+ // Returns a uniformly distributed value in the range [0..n-1]
+ // REQUIRES: n > 0
+ uint32_t Uniform(uint32_t n) {
+ return static_cast<uint32_t>(
+ std::uniform_int_distribution<std::mt19937::result_type>(
+ 0, n - 1)(generator_));
+ }
+
+ // Returns an *almost* uniformly distributed value in the range [0..n-1].
+ // Much faster than Uniform().
+ // REQUIRES: n > 0
+ uint32_t Uniformish(uint32_t n) {
+ // fastrange (without the header)
+ return static_cast<uint32_t>((uint64_t(generator_()) * uint64_t(n)) >> 32);
+ }
+
+ // Randomly returns true ~"1/n" of the time, and false otherwise.
+ // REQUIRES: n > 0
+ bool OneIn(uint32_t n) { return Uniform(n) == 0; }
+
+ // Skewed: pick "base" uniformly from range [0,max_log] and then
+ // return "base" random bits. The effect is to pick a number in the
+ // range [0,2^max_log-1] with exponential bias towards smaller numbers.
+ uint32_t Skewed(int max_log) {
+ return Uniform(uint32_t{1} << Uniform(max_log + 1));
+ }
+
+ // Reset the seed of the generator to the given value
+ void Seed(uint32_t new_seed) { generator_.seed(new_seed); }
+};
+
+// A good 64-bit random number generator based on std::mt19937_64
+class Random64 {
+ private:
+ std::mt19937_64 generator_;
+
+ public:
+ explicit Random64(uint64_t s) : generator_(s) { }
+
+ // Generates the next random number
+ uint64_t Next() { return generator_(); }
+
+ // Returns a uniformly distributed value in the range [0..n-1]
+ // REQUIRES: n > 0
+ uint64_t Uniform(uint64_t n) {
+ return std::uniform_int_distribution<uint64_t>(0, n - 1)(generator_);
+ }
+
+ // Randomly returns true ~"1/n" of the time, and false otherwise.
+ // REQUIRES: n > 0
+ bool OneIn(uint64_t n) { return Uniform(n) == 0; }
+
+ // Skewed: pick "base" uniformly from range [0,max_log] and then
+ // return "base" random bits. The effect is to pick a number in the
+ // range [0,2^max_log-1] with exponential bias towards smaller numbers.
+ uint64_t Skewed(int max_log) {
+ return Uniform(uint64_t(1) << Uniform(max_log + 1));
+ }
+};
+
+} // namespace ROCKSDB_NAMESPACE