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59 lignes
2.0 KiB
59 lignes
2.0 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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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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