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64 lines
2.1 KiB
64 lines
2.1 KiB
// Copyright (c) 2011 The LevelDB Authors. All rights reserved. |
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// Use of this source code is governed by a BSD-style license that can be |
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// found in the LICENSE file. See the AUTHORS file for names of contributors. |
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#ifndef STORAGE_LEVELDB_UTIL_RANDOM_H_ |
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#define STORAGE_LEVELDB_UTIL_RANDOM_H_ |
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#include <stdint.h> |
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namespace leveldb { |
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// A very simple random number generator. Not especially good at |
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// generating truly random bits, but good enough for our needs in this |
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// package. |
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class Random { |
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private: |
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uint32_t seed_; |
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public: |
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explicit Random(uint32_t s) : seed_(s & 0x7fffffffu) { |
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// Avoid bad seeds. |
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if (seed_ == 0 || seed_ == 2147483647L) { |
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seed_ = 1; |
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} |
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} |
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uint32_t Next() { |
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static const uint32_t M = 2147483647L; // 2^31-1 |
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static const uint64_t A = 16807; // bits 14, 8, 7, 5, 2, 1, 0 |
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// We are computing |
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// seed_ = (seed_ * A) % M, where M = 2^31-1 |
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// |
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// seed_ must not be zero or M, or else all subsequent computed values |
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// will be zero or M respectively. For all other values, seed_ will end |
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// up cycling through every number in [1,M-1] |
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uint64_t product = seed_ * A; |
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// Compute (product % M) using the fact that ((x << 31) % M) == x. |
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seed_ = static_cast<uint32_t>((product >> 31) + (product & M)); |
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// The first reduction may overflow by 1 bit, so we may need to |
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// repeat. mod == M is not possible; using > allows the faster |
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// sign-bit-based test. |
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if (seed_ > M) { |
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seed_ -= M; |
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} |
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return seed_; |
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} |
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// Returns a uniformly distributed value in the range [0..n-1] |
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// REQUIRES: n > 0 |
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uint32_t Uniform(int n) { return Next() % n; } |
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// Randomly returns true ~"1/n" of the time, and false otherwise. |
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// REQUIRES: n > 0 |
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bool OneIn(int n) { return (Next() % n) == 0; } |
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// Skewed: pick "base" uniformly from range [0,max_log] and then |
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// return "base" random bits. The effect is to pick a number in the |
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// range [0,2^max_log-1] with exponential bias towards smaller numbers. |
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uint32_t Skewed(int max_log) { |
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return Uniform(1 << Uniform(max_log + 1)); |
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} |
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}; |
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} // namespace leveldb |
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#endif // STORAGE_LEVELDB_UTIL_RANDOM_H_
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