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Replacing rand()

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Joined: 23 Apr 2016
Posts: 1

PostPosted: Mon Apr 25, 2016 23:26    Post subject: Replacing rand() Reply with quote

There have been a few efforts in the past to provide better pseudo-random number generation for scripts in nwnx; but, this requires changing and recompiling scripts to take advantage of the new random function. Also, new random functions can't replace the random numbers used in attack rolls and other core-engine functions. I decided to try to provide a replacement PRNG that does override the core dice rolling functionality.

Turns out (on the linux build at least), it's just making a call to the system prng rand (or std::rand in C++). This can easily be verified by declaring an LD_PRELOAD with
`int rand() {return 42;}` which will make all D20s come up `3`. After that, it's a simple matter of creating a rand function which, on its first invocation, will initialize a mersenne twister prng from the system time and using it to feed random numbers.

To avoid modulo bias, the highest number returned from rand() should be (n*dsize)-1, since rand needs to work perfectly for all standard dice sizes, I'm using 191999 (8*12*20*100 - 1), which satisfies n*d-1 for all D&D values of d. The extra factor of 20 wouldn't be necessary, but rand() should be capable of returning numbers higher than 32767.


// Licens: LGPL2
#include <random>
#include <cstdio>
#include <cstdlib>
#include <chrono>

std::mt19937 my_generator;
std::uniform_int_distribution<int> my_distribution(0,191999);

int initted=0;

typedef std::chrono::high_resolution_clock myclock;

int rand(){
    if (!initted){

        myclock::time_point my_time = myclock::now();
        auto seed = my_time.time_since_epoch().count();
        printf("Using mt19937 mersenne_twister_engine with seed: %Li\n", seed);

    int my_rn = my_distribution(my_generator);
    if (getenv( "DEBUG\0" )){

        printf("my rn: %i\n", my_rn);
    return my_rn;

Compiled with gcc-4.8.5 on gentoo


g++ -fPIC -m32 -mcpu=i386 -o rand.so rand.cpp --shared -std=c++11

If there's interest, I can provide a binary build, but if you're running it on linux you can probably just compile it, right Smile

To use it, simply add it to the LD_PRELOAD in the startup script,



Or for the SP client

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Joined: 25 Aug 2010
Posts: 75

PostPosted: Thu May 19, 2016 0:50    Post subject: Reply with quote

Cool stuff!
the awakening (PW Action)
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