[RocksDB] Include 64bit random number generator

Summary: As title.

Test Plan: make check;

Reviewers: chip, MarkCallaghan

CC: leveldb

Differential Revision: https://reviews.facebook.net/D11061

util/random.h

@@ -5,6 +5,7 @@
 #ifndef STORAGE_LEVELDB_UTIL_RANDOM_H_
 #define STORAGE_LEVELDB_UTIL_RANDOM_H_
 
+#include <random>
 #include <stdint.h>
 
 namespace leveldb {
@@ -54,6 +55,35 @@ class Random {
   }
 };
 
+// A simple 64bit random number generator based on std::mt19937_64
+class Random64 {
+ private:
+  std::mt19937_64 generator_;
+
+ public:
+  explicit Random64(uint64_t s) : generator_(s) { }
+
+  // Generates the next random number
+  uint64_t Next() { return generator_(); }
+
+  // Returns a uniformly distributed value in the range [0..n-1]
+  // REQUIRES: n > 0
+  uint64_t Uniform(uint64_t n) {
+    return std::uniform_int_distribution<uint64_t>(0, n - 1)(generator_);
+  }
+
+  // Randomly returns true ~"1/n" of the time, and false otherwise.
+  // REQUIRES: n > 0
+  bool OneIn(uint64_t n) { return Uniform(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.
+  uint64_t Skewed(int max_log) {
+    return Uniform(1 << Uniform(max_log + 1));
+  }
+};
+
 }  // namespace leveldb
 
 #endif  // STORAGE_LEVELDB_UTIL_RANDOM_H_