github.com/cellofellow/gopkg@v0.0.0-20140722061823-eec0544a62ad/database/leveldb.chai2010/include/util/random.h (about)

     1  // Copyright (c) 2011 The LevelDB Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style license that can be
     3  // found in the LICENSE file. See the AUTHORS file for names of contributors.
     4  
     5  #ifndef STORAGE_LEVELDB_UTIL_RANDOM_H_
     6  #define STORAGE_LEVELDB_UTIL_RANDOM_H_
     7  
     8  #include <stdint.h>
     9  
    10  namespace leveldb {
    11  
    12  // A very simple random number generator.  Not especially good at
    13  // generating truly random bits, but good enough for our needs in this
    14  // package.
    15  class Random {
    16   private:
    17    uint32_t seed_;
    18   public:
    19    explicit Random(uint32_t s) : seed_(s & 0x7fffffffu) {
    20      // Avoid bad seeds.
    21      if (seed_ == 0 || seed_ == 2147483647L) {
    22        seed_ = 1;
    23      }
    24    }
    25    uint32_t Next() {
    26      static const uint32_t M = 2147483647L;   // 2^31-1
    27      static const uint64_t A = 16807;  // bits 14, 8, 7, 5, 2, 1, 0
    28      // We are computing
    29      //       seed_ = (seed_ * A) % M,    where M = 2^31-1
    30      //
    31      // seed_ must not be zero or M, or else all subsequent computed values
    32      // will be zero or M respectively.  For all other values, seed_ will end
    33      // up cycling through every number in [1,M-1]
    34      uint64_t product = seed_ * A;
    35  
    36      // Compute (product % M) using the fact that ((x << 31) % M) == x.
    37      seed_ = static_cast<uint32_t>((product >> 31) + (product & M));
    38      // The first reduction may overflow by 1 bit, so we may need to
    39      // repeat.  mod == M is not possible; using > allows the faster
    40      // sign-bit-based test.
    41      if (seed_ > M) {
    42        seed_ -= M;
    43      }
    44      return seed_;
    45    }
    46    // Returns a uniformly distributed value in the range [0..n-1]
    47    // REQUIRES: n > 0
    48    uint32_t Uniform(int n) { return Next() % n; }
    49  
    50    // Randomly returns true ~"1/n" of the time, and false otherwise.
    51    // REQUIRES: n > 0
    52    bool OneIn(int n) { return (Next() % n) == 0; }
    53  
    54    // Skewed: pick "base" uniformly from range [0,max_log] and then
    55    // return "base" random bits.  The effect is to pick a number in the
    56    // range [0,2^max_log-1] with exponential bias towards smaller numbers.
    57    uint32_t Skewed(int max_log) {
    58      return Uniform(1 << Uniform(max_log + 1));
    59    }
    60  };
    61  
    62  }  // namespace leveldb
    63  
    64  #endif  // STORAGE_LEVELDB_UTIL_RANDOM_H_