小组成员:10215300402-朱维清 & 10222140408 谷杰

63 行
2.1 KiB

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