Rand

Rand is a set of crates supporting (pseudo-)random generators:

• Built over a standard RNG trait: rand_core::RngCore
• With fast implementations of both strong and small generators: rand::rngs, and more RNGs: rand_chacha, rand_xoshiro, rand_pcg, rngs repo
• rand::rng is an asymptotically-fast, automatically-seeded and reasonably strong generator available on all std targets
• Direct support for seeding generators from the getrandom crate

With broad support for random value generation and random processes:

• StandardUniform random value sampling, Uniform-ranged value sampling and more
• Samplers for a large number of non-uniform random number distributions via our own rand_distr and via the statrs
• Random processes (mostly choose and shuffle) via rand::seq traits

All with:

• Portably reproducible output
• #[no_std] compatibility (partial)
• Many performance optimisations thanks to contributions from the wide user-base

Rand is not:

• Small (LoC). 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.
• Primarily a cryptographic library. rand does provide some generators which aim to support unpredictable value generation under certain constraints; see SECURITY.md for details. Users are expected to determine for themselves whether rand's functionality meets their own security requirements.

Documentation:

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

Versions

Rand is mature (suitable for general usage, with infrequent breaking releases which minimise breakage) but not yet at 1.0. Current MAJOR.MINOR versions are:

• Version 0.9 was released in January 2025.

See the CHANGELOG or Upgrade Guide for more details.

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
• sys_rng (implied by std) enables rngs::SysRng, using the getrandom crate
• std_rng enables inclusion of StdRng, ThreadRng

Optionally, the following dependencies can be enabled:

• log enables logging via log

Additionally, these features configure Rand:

• simd_support (experimental) enables sampling of SIMD values (uniformly random SIMD integers and floats). Since std::simd is not yet stable this feature requires nightly Rust and may cause build failures.

• unbiased use unbiased sampling for algorithms supporting this option: Uniform distribution.

  (By default, bias affecting no more than one in 2^48 samples is accepted.)

  Note: enabling this option is expected to affect reproducibility of results.

Rand supports limited functionality in no_std mode (enabled via default-features = false). In this case, SysRng is unavailable (unless sys_rng is enabled), large parts of seq are unavailable (unless alloc is enabled), and ThreadRng is unavailable.

Portability

Reproducibility

Achieving reproducible results requires not only deterministic algorithms with fixed inputs but also a commitment to stability of algorithms and some platform-specific considerations. A subset of rand does aim to support reproducibility; read more about this in the book: Portability.

WebAssembly support

The WASI and Emscripten targets are directly supported. The wasm32-unknown-unknown target is not automatically supported. To enable support for this target, refer to the getrandom documentation for WebAssembly. Alternatively, the sys_rng feature may be disabled.

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.