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+// 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.
+
+#ifndef STORAGE_LEVELDB_UTIL_RANDOM_H_
+#define STORAGE_LEVELDB_UTIL_RANDOM_H_
+
+#include <stdint.h>
+
+namespace leveldb {
+
+// 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:
+ uint32_t seed_;
+ public:
+ explicit Random(uint32_t s) : seed_(s & 0x7fffffffu) { }
+ uint32_t Next() {
+ static const uint32_t M = 2147483647L; // 2^31-1
+ static const uint64_t A = 16807; // bits 14, 8, 7, 5, 2, 1, 0
+ // 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 (Next() % 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(1 << Uniform(max_log + 1));
+ }
+};
+
+} // namespace leveldb
+
+#endif // STORAGE_LEVELDB_UTIL_RANDOM_H_