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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_