Rand Documentation

Also see the main project readme.

Learning Rand

TODO. In the mean-time, we have some learning resources within the API documentation.

The following example programs may be of interest:


API documentation can be found:

Project policies

Open Participation

This project is open to contributions from anyone, with the main criteria of review being correctness, utility, project scope, and good documentation. Where correctness is less obvious (PRNGs and some type-conversion algorithms), additional criteria apply (see below).

Additionally we welcome feedback in the form of bug reports, feature requests (preferably with motivation and consideration for the scope of the project), code reviews, and input on current topics of discussion.

Since we must sometimes reject new features in order to limit the project's scope, you may wish to ask first before writing a new feature.

Stability and Portability

We try to follow semver rules regarding API-breaking changes and MAJOR.MINOR.PATCH versions:

  • New patch versions should not include API-breaking changes or major new features

  • Before 1.0, minor versions may include API breaking changes. After 1.0 they should not.

  • We may make pre-releases like 0.5.0-pre.0. In this case:

    • although these are public versions, they are not used by default unless opting into using a pre-release on the specific MAJOR.MINOR.PATCH version
    • pre-releases are considered semantically less than their final release (e.g. Cargo may automatically upgrade from 0.5.0-pre.0 to 0.5.0)
    • all pre-release versions are unstable and may make any change
    • we make no commitment to support users of pre-releases

Additionally, we must also consider value-breaking changes and portability. A function is value-stable if, given the same inputs:

  • it is portable (produces the same results on all platforms)
  • changing the output value for some input in a new library version is considered a breaking change

Note that some Rand functionality is supposed to be value stable, and some functionality is supposed to be non-deterministic (i.e. depend on something external). Some functionality may be deterministic but not value-stable.

A trait should define which of its functions are expected to be value-stable. An implementation of a trait must meet those stability requirements, unless the object for which the trait is implemented is explicitly not value-stable. As an example, SeedableRng::from_seed is required to be value-stable, but SeedableRng::from_rng is not. RNGs implementing the trait are value-stable when they guarantee SeedableRng::from_seed is value-stable, while SeedableRng::from_rng may receive optimisations.

Before 1.0, we allow any new minor version to break value-stability, though we do expect such changes to be mentioned in the changelog. Post 1.0 we have not yet determined exact stability rules.

Additionally, we expect patch versions not to change the output of any deterministic functions, even if not value-stable (this is not a hard requirement, but exceptions should be noted in the changelog).

Defining which parts of Rand are value-stable is still in progress. Many parts of rand_core have some documentation on value-stability.

Project Scope

The rand_core library has the following scope:

  • the core traits which RNGs may implement
  • tools for implementing these traits

The rand library has the following scope:

  • re-export all parts of rand_core applicable to end users
  • an interface to request entropy from an external source
  • hooks to provide entropy from several platform-specific sources
  • traits covering common RNG functionality
  • some PRNGs, notably StdRng and SmallRng
  • thread_rng auto-seeding source of randomness
  • conversion of random bits to common types and uses
  • shuffling and sampling from sequences
  • sampling from various random number distributions

Note: the scope of the project and above libraries may change. We are currently discussing moving PRNGs (#431) and distributions (#290) to other libraries or projects.

Code style

We do not currently have many policies on style other than:

  • neat and consistent
  • minimise changes which are purely stylistic, or move to a separate commit

Rand does make use of unsafe, both for performance and out of necessity. We consider this acceptable so long as correctness is easy to verify. In order to make this as simple as possible, we prefer that all parameters affecting safety of unsafe blocks are checked or prepared close to the unsafe code, and wherever possible within the same function (thus making the function safe).

New PRNG Algorithms

The Rand library includes several pseudo-random number generators, and we have received several requests to adopt new algorithms into the library. We must consider such requests in regards to several things:

  • whether the PRNG is cryptographically secure, and if so, how trustworthy such claims are
  • statistical quality of output
  • performance and features of the generator
  • scope of the project
  • reception and third-party review of the algorithm

In general, we expect the following, though we may make exceptions:

  • the author of the algorithm to publish an article of some type (e.g. a scientific article or web page) introducing the new algorithm and discussing its utility, strengths and weaknesses
  • review of statistical quality and any special features by third parties
  • good performance in automated test suites like PractRand and TestU01 (unless for some reason this is not expected, e.g. a mock generator)