README.md

Rand

A Rust library for random number generation, featuring:

• Easy random value generation and usage via the Rng, SliceRandom and IteratorRandom traits
• Secure seeding via the getrandom crate and fast, convenient generation via thread_rng
• A modular design built over rand_core (see the book)
• Fast implementations of the best-in-class cryptographic and non-cryptographic generators
• A flexible distributions module
• Samplers for a large number of random number distributions via our own rand_distr and via the statrs
• Portably reproducible output
• #[no_std] compatibility (partial)
• Many performance optimisations

It's also worth pointing out what rand is not:

• Small. Most low-level crates are small, but the higher-level rand and rand_distr each contain a lot of functionality.
• Simple (implementation). We have a strong focus on correctness, speed and flexibility, but not simplicity. If you prefer a small-and-simple library, there are alternatives including fastrand and oorandom.
• Slow. We take performance seriously, with considerations also for set-up time of new distributions, commonly-used parameters, and parameters of the current sampler.

Documentation:

• The Rust Rand Book
• API reference (master branch)
• API reference (docs.rs)

Usage

Add this to your Cargo.toml:

[dependencies]
rand = "0.8.5"

To get started using Rand, see The Book.

Versions

Rand is mature (suitable for general usage, with infrequent breaking releases which minimise breakage) but not yet at 1.0. We maintain compatibility with pinned versions of the Rust compiler (see below).

Current Rand versions are:

• Version 0.7 was released in June 2019, moving most non-uniform distributions to an external crate, moving from_entropy to SeedableRng, and many small changes and fixes.
• Version 0.8 was released in December 2020 with many small changes.

A detailed changelog is available for releases.

When upgrading to the next minor series (especially 0.4 → 0.5), we recommend reading the Upgrade Guide.

Rand has not yet reached 1.0 implying some breaking changes may arrive in the future (SemVer allows each 0.x.0 release to include breaking changes), but is considered mature: breaking changes are minimised and breaking releases are infrequent.

Rand libs have inter-dependencies and make use of the semver trick in order to make traits compatible across crate versions. (This is especially important for RngCore and SeedableRng.) A few crate releases are thus compatibility shims, depending on the next lib version (e.g. rand_core versions 0.2.2 and 0.3.1). This means, for example, that rand_core_0_4_0::SeedableRng and rand_core_0_3_0::SeedableRng are distinct, incompatible traits, which can cause build errors. Usually, running cargo update is enough to fix any issues.

Yanked versions

Some versions of Rand crates have been yanked ("unreleased"). Where this occurs, the crate's CHANGELOG should be updated with a rationale, and a search on the issue tracker with the keyword yank should uncover the motivation.

Rust version requirements

Since version 0.8, Rand requires Rustc version 1.36 or greater. Rand 0.7 requires Rustc 1.32 or greater while versions 0.5 require Rustc 1.22 or greater, and 0.4 and 0.3 (since approx. June 2017) require Rustc version 1.15 or greater. Subsets of the Rand code may work with older Rust versions, but this is not supported.

Continuous Integration (CI) will always test the minimum supported Rustc version (the MSRV). The current policy is that this can be updated in any Rand release if required, but the change must be noted in the changelog.

Crate Features

Rand is built with these features enabled by default:

• std enables functionality dependent on the std lib
• alloc (implied by std) enables functionality requiring an allocator
• getrandom (implied by std) is an optional dependency providing the code behind rngs::OsRng
• std_rng enables inclusion of StdRng, thread_rng and random (the latter two also require that std be enabled)

Optionally, the following dependencies can be enabled:

• log enables logging via the log crate

Additionally, these features configure Rand:

• small_rng enables inclusion of the SmallRng PRNG
• nightly includes some additions requiring nightly Rust
• simd_support (experimental) enables sampling of SIMD values (uniformly random SIMD integers and floats), requiring nightly Rust

Note that nightly features are not stable and therefore not all library and compiler versions will be compatible. This is especially true of Rand's experimental simd_support feature.

Rand supports limited functionality in no_std mode (enabled via default-features = false). In this case, OsRng and from_entropy are unavailable (unless getrandom is enabled), large parts of seq are unavailable (unless alloc is enabled), and thread_rng and random are unavailable.

WASM support

Seeding entropy from OS on WASM target wasm32-unknown-unknown is not automatically supported by rand or getrandom. If you are fine with seeding the generator manually, you can disable the getrandom feature and use the methods on the SeedableRng trait. To enable seeding from OS, either use a different target such as wasm32-wasi or add a direct dependency on getrandom with the js feature (if the target supports JavaScript). See getrandom#WebAssembly support.

License

Rand is distributed under the terms of both the MIT license and the Apache License (Version 2.0).

See LICENSE-APACHE and LICENSE-MIT, and COPYRIGHT for details.