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# Random number generators
This describes the API of module **crandom**. Contains *pcg32* and a extremely fast *64-bit PRNG* inspired by *sfc64*.
The RNG's can generate uniform and normal distributions.
## Types
| crandom | Type definition | Used to represent... |
|:----------------------|:-----------------------------------------------------|:-------------------------------------|
| `crand_rng32_t` | `struct {uint64_t state[2];}` | The crandom type |
| `crand_uniform_i32_t` | `struct {int32_t offset; uint32_t range;}` | The crandom element type |
| `crand_uniform_f32_t` | `struct {float offset, range;}` | crandom iterator |
| `crand_rng64_t` | `struct {uint64_t state[4];}` | |
| `crand_uniform_i64_t` | `struct {int64_t offset; uint64_t range;}` | |
| `crand_uniform_f64_t` | `struct {double offset, range;}` | |
| `crand_normal_f64_t` | `struct {double mean, stddev, next; bool has_next;}` | |
## Header file
All cstr definitions and prototypes may be included in your C source file by including a single header file.
```c
#include "stc/crandom.h"
```
## Methods
### Interface for 32-bits random number generators
```c
(1) crand_rng32_t crand_rng32_init(uint64_t seed);
(2) crand_rng32_t crand_rng32_with_seq(uint64_t seed, uint64_t seq);
(3) uint32_t crand_i32(crand_rng32_t* rng);
(4) float crand_f32(crand_rng32_t* rng);
(5) crand_uniform_i32_t crand_uniform_i32_init(int32_t low, int32_t high);
(6) int32_t crand_uniform_i32(crand_rng32_t* rng, crand_uniform_i32_t* dist);
(7) uint32_t crand_unbiased_i32(crand_rng32_t* rng, crand_uniform_i32_t* dist);
(8) crand_uniform_f32_t crand_uniform_f32_init(float low, float high); /* */
(9) float crand_uniform_f32(crand_rng32_t* rng, crand_uniform_f32_t* dist);
```
(1-2) RNG engine initializers. (3) Integer random number generator with range \[0, 2^32). (5) Integer generator with range \[low, high].
(7) Unbiased version, see https://github.com/lemire/fastrange. (8) 32-bit float random number in range \[low, high), 23 bits resolution numbers.
### Interface for 64-bits random number generators
```c
(1) crand_rng64_t crand_rng64_with_seq(uint64_t seed, uint64_t seq);
(2) crand_rng64_t crand_rng64_init(uint64_t seed);
(3) uint64_t crand_i64(crand_rng64_t* rng);
(4) double crand_f64(crand_rng64_t* rng);
(5) crand_uniform_i64_t crand_uniform_i64_init(int64_t low, int64_t high);
(6) int64_t crand_uniform_i64(crand_rng64_t* rng, crand_uniform_i64_t* dist);
(7) crand_uniform_f64_t crand_uniform_f64_init(double low, double high);
(8) double crand_uniform_f64(crand_rng64_t* rng, crand_uniform_f64_t* dist);
(9) crand_normal_f64_t crand_normal_f64_init(double mean, double stddev);
(10) double crand_normal_f64(crand_rng64_t* rng, crand_normal_f64_t* dist);
```
(1-2) RNG engine initializers. (3) Integer generator, range \[0, 2^64): PRNG copyright Tyge Løvset, NORCE Research, 2020.
(4) Double random number in range \[low, high), 52 bit resolution. (5-8) Initializers and generators of uniform random numbers.
Integer generator has range \[low, high]. (9-10) Initializer and generator for normal distributed random numbers.
(9-10) Initializer and generator for normal-distributed random numbers.
## Example
```c
#include <stdio.h>
#include <time.h>
#include <math.h>
#include "stc/crandom.h"
#include "stc/cstr.h"
#include "stc/cmap.h"
#include "stc/cvec.h"
// Declare unordered map: int -> int with typetag 'i'.
using_cmap(i, int, size_t);
// Comparison of map keys.
static int compare(cmap_i_entry_t *a, cmap_i_entry_t *b) {
return c_default_compare(&a->first, &b->first);
}
// Declare vector of map entries, with comparison function.
using_cvec(e, cmap_i_entry_t, c_default_del, compare);
int main()
{
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_rng64_t rng = crand_rng64_init(seed);
crand_normal_f64_t dist = crand_normal_f64_init(Mean, StdDev);
// Create histogram map
cmap_i mhist = cmap_i_init();
for (size_t i = 0; i < N; ++i) {
int index = (int) round( crand_normal_f64(&rng, &dist) );
cmap_i_emplace(&mhist, index, 0).first->second += 1;
}
// Transfer map to vec and sort it by map entry keys.
cvec_e vhist = cvec_e_init();
c_foreach (i, cmap_i, mhist)
cvec_e_push_back(&vhist, *i.val);
cvec_e_sort(&vhist);
// Print the gaussian bar chart
cstr_t bar = cstr_init();
c_foreach (i, cvec_e, vhist) {
size_t n = (size_t) (i.val->second * StdDev * Scale * 2.5 / N);
if (n > 0) {
cstr_resize(&bar, n, '*');
printf("%4d %s\n", i.val->first, bar.str);
}
}
// Cleanup
cstr_del(&bar);
cmap_i_del(&mhist);
cvec_e_del(&vhist);
}
```
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 *
```
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