tomFlowee
c63519fa47
The leveldb and univalue 3rd party libraries are not installed and not needed by anyone outside of the Hub. So move them there, making it easier for 3rd party usage.
65 lines
2.1 KiB
C++
65 lines
2.1 KiB
C++
// 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) {
|
|
// Avoid bad seeds.
|
|
if (seed_ == 0 || seed_ == 2147483647L) {
|
|
seed_ = 1;
|
|
}
|
|
}
|
|
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_
|