leveldb源码学习--Random简单随机数生成

leveldb在random.h中实现一个非常简单的随机数生成器，说成简陋也可以。

源码分析

构造函数

explicit Random(uint32_t s) : seed_(s & 0x7fffffffu) {
    // Avoid bad seeds.
    if (seed_ == 0 || seed_ == 2147483647L) {
      seed_ = 1;
    }
  }

构造函数用来初始化随机种子数，注意到作者认为当seed_为0或者 M(2^31-1)时是一种不好的种子数。这是因为在后面的随机数生成方法中采用的是seed_ = (seed_ * A) % M,如果seed_为0或者M,那么以后产生的所有seed_都是0了(具体表现在Next()函数中)。

Next()

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_;
  }

在Next()函数中，开始我没看明白的一点是seed_ 如何由product得来的，其实注意到product是一个uint64_t,而seed_和M是uint32_t。如同注释中提示到的Compute (product % M) using the fact that ((x << 31) % M) == x.,将(product % M)分成高32位的结果和低32位的结果，然后相加即可。(不知道为何能分开求的可以去面壁了)

Uniform() && OneIn()

  // 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; }

非常简单的两个函数，注释也写的很明白，要是不明白。那也没办法了

Skewed()

// 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));
  }

这个函数随机返回[0, 2^(max_log-1)]区间中的一个2的指数值。

ice_city liuhiter@gmail.com