Crate fastrand

Expand description

A simple and fast random number generator.

The implementation uses Wyrand, a simple and fast generator but not cryptographically secure.

§Examples

Flip a coin:

if fastrand::bool() {
    println!("heads");
} else {
    println!("tails");
}

Generate a random i32:

let num = fastrand::i32(..);

Choose a random element in an array:

let v = vec![1, 2, 3, 4, 5];
let i = fastrand::usize(..v.len());
let elem = v[i];

Sample values from an array with O(n) complexity (n is the length of array):

fastrand::choose_multiple(vec![1, 4, 5].iter(), 2);
fastrand::choose_multiple(0..20, 12);

Shuffle an array:

let mut v = vec![1, 2, 3, 4, 5];
fastrand::shuffle(&mut v);

Generate a random Vec or [String]:

use std::iter::repeat_with;

let v: Vec<i32> = repeat_with(|| fastrand::i32(..)).take(10).collect();
let s: String = repeat_with(fastrand::alphanumeric).take(10).collect();

To get reproducible results on every run, initialize the generator with a seed:

// Pick an arbitrary number as seed.
fastrand::seed(7);

// Now this prints the same number on every run:
println!("{}", fastrand::u32(..));

To be more efficient, create a new Rng instance instead of using the thread-local generator:

use std::iter::repeat_with;

let mut rng = fastrand::Rng::new();
let mut bytes: Vec<u8> = repeat_with(|| rng.u8(..)).take(10_000).collect();

This crate aims to expose a core set of useful randomness primitives. For more niche algorithms, consider using the fastrand-contrib crate alongside this one.

§Features

std (enabled by default): Enables the std library. This is required for the global generator and global entropy. Without this feature, Rng can only be instantiated using the with_seed method.
js: Assumes that WebAssembly targets are being run in a JavaScript environment. See the WebAssembly Notes section for more information.

§WebAssembly Notes

For non-WASI WASM targets, there is additional sublety to consider when utilizing the global RNG. By default, std targets will use entropy sources in the standard library to seed the global RNG. However, these sources are not available by default on WASM targets outside of WASI.

If the js feature is enabled, this crate will assume that it is running in a JavaScript environment. At this point, the getrandom crate will be used in order to access the available entropy sources and seed the global RNG. If the js feature is not enabled, the global RNG will use a predefined seed.

Structs§

Rng
A random number generator.

Functions§