| // Copyright 2018 Developers of the Rand project. |
| // Copyright 2013-2017 The Rust Project Developers. |
| // |
| // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or |
| // https://www.apache.org/licenses/LICENSE-2.0> or the MIT license |
| // <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your |
| // option. This file may not be copied, modified, or distributed |
| // except according to those terms. |
| |
| //! Generating random samples from probability distributions |
| //! |
| //! This module is the home of the [`Distribution`] trait and several of its |
| //! implementations. It is the workhorse behind some of the convenient |
| //! functionality of the [`Rng`] trait, e.g. [`Rng::gen`] and of course |
| //! [`Rng::sample`]. |
| //! |
| //! Abstractly, a [probability distribution] describes the probability of |
| //! occurrence of each value in its sample space. |
| //! |
| //! More concretely, an implementation of `Distribution<T>` for type `X` is an |
| //! algorithm for choosing values from the sample space (a subset of `T`) |
| //! according to the distribution `X` represents, using an external source of |
| //! randomness (an RNG supplied to the `sample` function). |
| //! |
| //! A type `X` may implement `Distribution<T>` for multiple types `T`. |
| //! Any type implementing [`Distribution`] is stateless (i.e. immutable), |
| //! but it may have internal parameters set at construction time (for example, |
| //! [`Uniform`] allows specification of its sample space as a range within `T`). |
| //! |
| //! |
| //! # The `Standard` distribution |
| //! |
| //! The [`Standard`] distribution is important to mention. This is the |
| //! distribution used by [`Rng::gen`] and represents the "default" way to |
| //! produce a random value for many different types, including most primitive |
| //! types, tuples, arrays, and a few derived types. See the documentation of |
| //! [`Standard`] for more details. |
| //! |
| //! Implementing `Distribution<T>` for [`Standard`] for user types `T` makes it |
| //! possible to generate type `T` with [`Rng::gen`], and by extension also |
| //! with the [`random`] function. |
| //! |
| //! ## Random characters |
| //! |
| //! [`Alphanumeric`] is a simple distribution to sample random letters and |
| //! numbers of the `char` type; in contrast [`Standard`] may sample any valid |
| //! `char`. |
| //! |
| //! |
| //! # Uniform numeric ranges |
| //! |
| //! The [`Uniform`] distribution is more flexible than [`Standard`], but also |
| //! more specialised: it supports fewer target types, but allows the sample |
| //! space to be specified as an arbitrary range within its target type `T`. |
| //! Both [`Standard`] and [`Uniform`] are in some sense uniform distributions. |
| //! |
| //! Values may be sampled from this distribution using [`Rng::sample(Range)`] or |
| //! by creating a distribution object with [`Uniform::new`], |
| //! [`Uniform::new_inclusive`] or `From<Range>`. When the range limits are not |
| //! known at compile time it is typically faster to reuse an existing |
| //! `Uniform` object than to call [`Rng::sample(Range)`]. |
| //! |
| //! User types `T` may also implement `Distribution<T>` for [`Uniform`], |
| //! although this is less straightforward than for [`Standard`] (see the |
| //! documentation in the [`uniform`] module). Doing so enables generation of |
| //! values of type `T` with [`Rng::sample(Range)`]. |
| //! |
| //! ## Open and half-open ranges |
| //! |
| //! There are surprisingly many ways to uniformly generate random floats. A |
| //! range between 0 and 1 is standard, but the exact bounds (open vs closed) |
| //! and accuracy differ. In addition to the [`Standard`] distribution Rand offers |
| //! [`Open01`] and [`OpenClosed01`]. See "Floating point implementation" section of |
| //! [`Standard`] documentation for more details. |
| //! |
| //! # Non-uniform sampling |
| //! |
| //! Sampling a simple true/false outcome with a given probability has a name: |
| //! the [`Bernoulli`] distribution (this is used by [`Rng::gen_bool`]). |
| //! |
| //! For weighted sampling from a sequence of discrete values, use the |
| //! [`WeightedIndex`] distribution. |
| //! |
| //! This crate no longer includes other non-uniform distributions; instead |
| //! it is recommended that you use either [`rand_distr`] or [`statrs`]. |
| //! |
| //! |
| //! [probability distribution]: https://en.wikipedia.org/wiki/Probability_distribution |
| //! [`rand_distr`]: https://crates.io/crates/rand_distr |
| //! [`statrs`]: https://crates.io/crates/statrs |
| |
| //! [`random`]: crate::random |
| //! [`rand_distr`]: https://crates.io/crates/rand_distr |
| //! [`statrs`]: https://crates.io/crates/statrs |
| |
| use crate::Rng; |
| use core::iter; |
| |
| pub use self::bernoulli::{Bernoulli, BernoulliError}; |
| pub use self::float::{Open01, OpenClosed01}; |
| pub use self::other::Alphanumeric; |
| #[doc(inline)] pub use self::uniform::Uniform; |
| |
| #[cfg(feature = "alloc")] |
| pub use self::weighted_index::{WeightedError, WeightedIndex}; |
| |
| mod bernoulli; |
| pub mod uniform; |
| |
| #[deprecated(since = "0.8.0", note = "use rand::distributions::{WeightedIndex, WeightedError} instead")] |
| #[cfg(feature = "alloc")] |
| #[cfg_attr(doc_cfg, doc(cfg(feature = "alloc")))] |
| pub mod weighted; |
| #[cfg(feature = "alloc")] mod weighted_index; |
| |
| #[cfg(feature = "serde1")] |
| use serde::{Serialize, Deserialize}; |
| |
| mod float; |
| #[doc(hidden)] |
| pub mod hidden_export { |
| pub use super::float::IntoFloat; // used by rand_distr |
| } |
| mod integer; |
| mod other; |
| mod utils; |
| |
| /// Types (distributions) that can be used to create a random instance of `T`. |
| /// |
| /// It is possible to sample from a distribution through both the |
| /// `Distribution` and [`Rng`] traits, via `distr.sample(&mut rng)` and |
| /// `rng.sample(distr)`. They also both offer the [`sample_iter`] method, which |
| /// produces an iterator that samples from the distribution. |
| /// |
| /// All implementations are expected to be immutable; this has the significant |
| /// advantage of not needing to consider thread safety, and for most |
| /// distributions efficient state-less sampling algorithms are available. |
| /// |
| /// Implementations are typically expected to be portable with reproducible |
| /// results when used with a PRNG with fixed seed; see the |
| /// [portability chapter](https://rust-random.github.io/book/portability.html) |
| /// of The Rust Rand Book. In some cases this does not apply, e.g. the `usize` |
| /// type requires different sampling on 32-bit and 64-bit machines. |
| /// |
| /// [`sample_iter`]: Distribution::method.sample_iter |
| pub trait Distribution<T> { |
| /// Generate a random value of `T`, using `rng` as the source of randomness. |
| fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> T; |
| |
| /// Create an iterator that generates random values of `T`, using `rng` as |
| /// the source of randomness. |
| /// |
| /// Note that this function takes `self` by value. This works since |
| /// `Distribution<T>` is impl'd for `&D` where `D: Distribution<T>`, |
| /// however borrowing is not automatic hence `distr.sample_iter(...)` may |
| /// need to be replaced with `(&distr).sample_iter(...)` to borrow or |
| /// `(&*distr).sample_iter(...)` to reborrow an existing reference. |
| /// |
| /// # Example |
| /// |
| /// ``` |
| /// use rand::thread_rng; |
| /// use rand::distributions::{Distribution, Alphanumeric, Uniform, Standard}; |
| /// |
| /// let rng = thread_rng(); |
| /// |
| /// // Vec of 16 x f32: |
| /// let v: Vec<f32> = Standard.sample_iter(rng).take(16).collect(); |
| /// |
| /// // String: |
| /// let s: String = Alphanumeric.sample_iter(rng).take(7).map(char::from).collect(); |
| /// |
| /// // Dice-rolling: |
| /// let die_range = Uniform::new_inclusive(1, 6); |
| /// let mut roll_die = die_range.sample_iter(rng); |
| /// while roll_die.next().unwrap() != 6 { |
| /// println!("Not a 6; rolling again!"); |
| /// } |
| /// ``` |
| fn sample_iter<R>(self, rng: R) -> DistIter<Self, R, T> |
| where |
| R: Rng, |
| Self: Sized, |
| { |
| DistIter { |
| distr: self, |
| rng, |
| phantom: ::core::marker::PhantomData, |
| } |
| } |
| } |
| |
| impl<'a, T, D: Distribution<T>> Distribution<T> for &'a D { |
| fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> T { |
| (*self).sample(rng) |
| } |
| } |
| |
| |
| /// An iterator that generates random values of `T` with distribution `D`, |
| /// using `R` as the source of randomness. |
| /// |
| /// This `struct` is created by the [`sample_iter`] method on [`Distribution`]. |
| /// See its documentation for more. |
| /// |
| /// [`sample_iter`]: Distribution::sample_iter |
| #[derive(Debug)] |
| pub struct DistIter<D, R, T> { |
| distr: D, |
| rng: R, |
| phantom: ::core::marker::PhantomData<T>, |
| } |
| |
| impl<D, R, T> Iterator for DistIter<D, R, T> |
| where |
| D: Distribution<T>, |
| R: Rng, |
| { |
| type Item = T; |
| |
| #[inline(always)] |
| fn next(&mut self) -> Option<T> { |
| // Here, self.rng may be a reference, but we must take &mut anyway. |
| // Even if sample could take an R: Rng by value, we would need to do this |
| // since Rng is not copyable and we cannot enforce that this is "reborrowable". |
| Some(self.distr.sample(&mut self.rng)) |
| } |
| |
| fn size_hint(&self) -> (usize, Option<usize>) { |
| (usize::max_value(), None) |
| } |
| } |
| |
| impl<D, R, T> iter::FusedIterator for DistIter<D, R, T> |
| where |
| D: Distribution<T>, |
| R: Rng, |
| { |
| } |
| |
| #[cfg(features = "nightly")] |
| impl<D, R, T> iter::TrustedLen for DistIter<D, R, T> |
| where |
| D: Distribution<T>, |
| R: Rng, |
| { |
| } |
| |
| |
| /// A generic random value distribution, implemented for many primitive types. |
| /// Usually generates values with a numerically uniform distribution, and with a |
| /// range appropriate to the type. |
| /// |
| /// ## Provided implementations |
| /// |
| /// Assuming the provided `Rng` is well-behaved, these implementations |
| /// generate values with the following ranges and distributions: |
| /// |
| /// * Integers (`i32`, `u32`, `isize`, `usize`, etc.): Uniformly distributed |
| /// over all values of the type. |
| /// * `char`: Uniformly distributed over all Unicode scalar values, i.e. all |
| /// code points in the range `0...0x10_FFFF`, except for the range |
| /// `0xD800...0xDFFF` (the surrogate code points). This includes |
| /// unassigned/reserved code points. |
| /// * `bool`: Generates `false` or `true`, each with probability 0.5. |
| /// * Floating point types (`f32` and `f64`): Uniformly distributed in the |
| /// half-open range `[0, 1)`. See notes below. |
| /// * Wrapping integers (`Wrapping<T>`), besides the type identical to their |
| /// normal integer variants. |
| /// |
| /// The `Standard` distribution also supports generation of the following |
| /// compound types where all component types are supported: |
| /// |
| /// * Tuples (up to 12 elements): each element is generated sequentially. |
| /// * Arrays (up to 32 elements): each element is generated sequentially; |
| /// see also [`Rng::fill`] which supports arbitrary array length for integer |
| /// types and tends to be faster for `u32` and smaller types. |
| /// * `Option<T>` first generates a `bool`, and if true generates and returns |
| /// `Some(value)` where `value: T`, otherwise returning `None`. |
| /// |
| /// ## Custom implementations |
| /// |
| /// The [`Standard`] distribution may be implemented for user types as follows: |
| /// |
| /// ``` |
| /// # #![allow(dead_code)] |
| /// use rand::Rng; |
| /// use rand::distributions::{Distribution, Standard}; |
| /// |
| /// struct MyF32 { |
| /// x: f32, |
| /// } |
| /// |
| /// impl Distribution<MyF32> for Standard { |
| /// fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> MyF32 { |
| /// MyF32 { x: rng.gen() } |
| /// } |
| /// } |
| /// ``` |
| /// |
| /// ## Example usage |
| /// ``` |
| /// use rand::prelude::*; |
| /// use rand::distributions::Standard; |
| /// |
| /// let val: f32 = StdRng::from_entropy().sample(Standard); |
| /// println!("f32 from [0, 1): {}", val); |
| /// ``` |
| /// |
| /// # Floating point implementation |
| /// The floating point implementations for `Standard` generate a random value in |
| /// the half-open interval `[0, 1)`, i.e. including 0 but not 1. |
| /// |
| /// All values that can be generated are of the form `n * ε/2`. For `f32` |
| /// the 24 most significant random bits of a `u32` are used and for `f64` the |
| /// 53 most significant bits of a `u64` are used. The conversion uses the |
| /// multiplicative method: `(rng.gen::<$uty>() >> N) as $ty * (ε/2)`. |
| /// |
| /// See also: [`Open01`] which samples from `(0, 1)`, [`OpenClosed01`] which |
| /// samples from `(0, 1]` and `Rng::gen_range(0..1)` which also samples from |
| /// `[0, 1)`. Note that `Open01` uses transmute-based methods which yield 1 bit |
| /// less precision but may perform faster on some architectures (on modern Intel |
| /// CPUs all methods have approximately equal performance). |
| /// |
| /// [`Uniform`]: uniform::Uniform |
| #[derive(Clone, Copy, Debug)] |
| #[cfg_attr(feature = "serde1", derive(Serialize, Deserialize))] |
| pub struct Standard; |
| |
| |
| #[cfg(test)] |
| mod tests { |
| use super::{Distribution, Uniform}; |
| use crate::Rng; |
| |
| #[test] |
| fn test_distributions_iter() { |
| use crate::distributions::Open01; |
| let mut rng = crate::test::rng(210); |
| let distr = Open01; |
| let mut iter = Distribution::<f32>::sample_iter(distr, &mut rng); |
| let mut sum: f32 = 0.; |
| for _ in 0..100 { |
| sum += iter.next().unwrap(); |
| } |
| assert!(0. < sum && sum < 100.); |
| } |
| |
| #[test] |
| fn test_make_an_iter() { |
| fn ten_dice_rolls_other_than_five<'a, R: Rng>( |
| rng: &'a mut R, |
| ) -> impl Iterator<Item = i32> + 'a { |
| Uniform::new_inclusive(1, 6) |
| .sample_iter(rng) |
| .filter(|x| *x != 5) |
| .take(10) |
| } |
| |
| let mut rng = crate::test::rng(211); |
| let mut count = 0; |
| for val in ten_dice_rolls_other_than_five(&mut rng) { |
| assert!(val >= 1 && val <= 6 && val != 5); |
| count += 1; |
| } |
| assert_eq!(count, 10); |
| } |
| } |