fast_rand.h

// Copyright (c) 2015 Baidu.com, Inc. All Rights Reserved
// Author: Ge,Jun (gejun@baidu.com)
// Date: Thu Dec 31 13:35:39 CST 2015

#ifndef PUBLIC_COMMON_BASE_FAST_RAND_H
#define PUBLIC_COMMON_BASE_FAST_RAND_H

#include <stdint.h>

namespace base {

// Generate random values fast without global contentions.
// All functions in this header are thread-safe.

struct FastRandSeed {
    uint64_t s[2];
};

// Initialize the seed.
void init_fast_rand_seed(FastRandSeed* seed);

// Generate an unsigned 64-bit random number from thread-local or given seed.
// Cost: ~5ns
uint64_t fast_rand();
uint64_t fast_rand(FastRandSeed*);

// Generate an unsigned 64-bit random number inside [0, range) from
// thread-local seed.
// Returns 0 when range is 0.
// Cost: ~30ns
// Note that this can be used as an adapter for std::random_shuffle():
//   std::random_shuffle(myvector.begin(), myvector.end(), base::fast_rand_less_than);
uint64_t fast_rand_less_than(uint64_t range);

// Generate a 64-bit random number inside [min, max] (inclusive!)
// from thread-local seed.
// NOTE: this function needs to be a template to be overloadable properly.
// Cost: ~30ns
template <typename T> T fast_rand_in(T min, T max) {
    extern int64_t fast_rand_in_64(int64_t min, int64_t max);
    extern uint64_t fast_rand_in_u64(uint64_t min, uint64_t max);
    if ((T)-1 < 0) {
        return fast_rand_in_64((int64_t)min, (int64_t)max);
    } else {
        return fast_rand_in_u64((uint64_t)min, (uint64_t)max);
    }
}

// Generate a random double in [0, 1) from thread-local seed.
// Cost: ~15ns
double fast_rand_double();

}

#endif  // PUBLIC_COMMON_BASE_FAST_RAND_H