firefox-main/third_party/rust/fastrand/src
Name Description Size Coverage
global_rng.rs A global, thread-local random number generator. 5686 89 %
lib.rs A simple and fast random number generator. The implementation uses [Wyrand](https://github.com/wangyi-fudan/wyhash), 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`](Rng::with_seed) method. - `js`: Assumes that WebAssembly targets are being run in a JavaScript environment. See the [WebAssembly Notes](#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. [`fastrand-contrib`]: https://crates.io/crates/fastrand-contrib [`getrandom`]: https://crates.io/crates/getrandom 20396 64 %