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# STC [crand](../include/stc/crand.h): Pseudo Random Number Generator
This features a *64-bit PRNG* named **crand64**, and can generate bounded uniform and normal
distributed random numbers.
See [random](https://en.cppreference.com/w/cpp/header/random) for similar c++ functionality.
## Description
**crand64** is a novel, very fast PRNG, suited for parallel usage. It features a
Weyl-sequence as part of its state. It is based on *sfc64*, but has a different output function
and state size.
**sfc64** is the fastest among *pcg*, *xoshiro`**`*, and *lehmer*. It is equally fast or faster than
*sfc64* on most platforms. *wyrand* is faster on platforms with fast 128-bit multiplication, and has
2^64 period (https://github.com/lemire/SwiftWyhash/issues/10). *wyrand* is not suited for massive
parallel usage due to its limited minimal period.
**crand64** does not require multiplication or 128-bit integer operations. It has 320 bit state,
where 64-bits are constant per prng instance created.
There is no *jump function*, but each odd number Weyl-increment (state[4]) starts a new
unique 2^64 *minimum* length period, i.e. virtually unlimitied number of unique threads.
**crand64** passes *PractRand* (tested up to 8TB output), Vigna's Hamming weight test, and simple
correlation tests, i.e. *n* interleaved streams with only one-bit differences in initial state.
Also 32-bit and 16-bit versions passes PractRand up to their size limits.
For more, see the PRNG shootout by Vigna: http://prng.di.unimi.it and a debate between the authors of
xoshiro and pcg (Vigna/O'Neill) PRNGs: https://www.pcg-random.org/posts/on-vignas-pcg-critique.html
## Header file
All crand definitions and prototypes are available by including a single header file.
```c
#include <stc/crand.h>
```
## Methods
```c
void csrand(uint64_t seed); // seed global crand64 prng
uint64_t crand(void); // global crand_u64(rng)
double crandf(void); // global crand_f64(rng)
crand_t crand_init(uint64_t seed);
uint64_t crand_u64(crand_t* rng); // range [0, 2^64 - 1]
double crand_f64(crand_t* rng); // range [0.0, 1.0)
crand_uniform_t crand_uniform_init(int64_t low, int64_t high); // uniform-distribution range
int64_t crand_uniform(crand_t* rng, crand_uniform_t* dist);
crand_normal_t crand_normal_init(double mean, double stddev); // normal-gauss distribution
double crand_normal(crand_t* rng, crand_normal_t* dist);
```
## Types
| Name | Type definition | Used to represent... |
|:-------------------|:------------------------------------------|:-----------------------------|
| `crand_t` | `struct {uint64_t state[4];}` | The PRNG engine type |
| `crand_uniform_t` | `struct {int64_t lower; uint64_t range;}` | Integer uniform distribution |
| `crand_normal_t` | `struct {double mean, stddev;}` | Normal distribution type |
## Example
```c
#include <time.h>
#include <stc/crand.h>
#define i_implement
#include <stc/cstr.h>
// Declare int -> int sorted map. Uses typetag 'i' for ints.
#define i_key int
#define i_val intptr_t
#define i_tag i
#include <stc/csmap.h>
int main(void)
{
enum {N = 10000000};
const double Mean = -12.0, StdDev = 6.0, Scale = 74;
printf("Demo of gaussian / normal distribution of %d random samples\n", N);
// Setup random engine with normal distribution.
uint64_t seed = time(NULL);
crand_t rng = crand_init(seed);
crand_normal_t dist = crand_normal_init(Mean, StdDev);
// Create histogram map
csmap_i mhist = {0};
c_forrange (N) {
int index = (int)round(crand_normal(&rng, &dist));
csmap_i_emplace(&mhist, index, 0).ref->second += 1;
}
// Print the gaussian bar chart
cstr bar = {0};
c_foreach (i, csmap_i, mhist) {
int n = (int)(i.ref->second * StdDev * Scale * 2.5 / N);
if (n > 0) {
cstr_resize(&bar, n, '*');
printf("%4d %s\n", i.ref->first, cstr_str(&bar));
}
}
// Cleanup
cstr_drop(&bar);
csmap_i_drop(&mhist);
}
```
Output:
```
Demo of gaussian / normal distribution of 10000000 random samples
-29 *
-28 **
-27 ***
-26 ****
-25 *******
-24 *********
-23 *************
-22 ******************
-21 ***********************
-20 ******************************
-19 *************************************
-18 ********************************************
-17 ****************************************************
-16 ***********************************************************
-15 *****************************************************************
-14 *********************************************************************
-13 ************************************************************************
-12 *************************************************************************
-11 ************************************************************************
-10 *********************************************************************
-9 *****************************************************************
-8 ***********************************************************
-7 ****************************************************
-6 ********************************************
-5 *************************************
-4 ******************************
-3 ***********************
-2 ******************
-1 *************
0 *********
1 *******
2 ****
3 ***
4 **
5 *
```
|
