rand/distr/
distribution.rs

1// Copyright 2018 Developers of the Rand project.
2// Copyright 2013-2017 The Rust Project Developers.
3//
4// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
5// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
6// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
7// option. This file may not be copied, modified, or distributed
8// except according to those terms.
9
10//! Distribution trait and associates
11
12use crate::Rng;
13#[cfg(feature = "alloc")]
14use alloc::string::String;
15use core::iter;
16
17/// Types (distributions) that can be used to create a random instance of `T`.
18///
19/// It is possible to sample from a distribution through both the
20/// `Distribution` and [`Rng`] traits, via `distr.sample(&mut rng)` and
21/// `rng.sample(distr)`. They also both offer the [`sample_iter`] method, which
22/// produces an iterator that samples from the distribution.
23///
24/// All implementations are expected to be immutable; this has the significant
25/// advantage of not needing to consider thread safety, and for most
26/// distributions efficient state-less sampling algorithms are available.
27///
28/// Implementations are typically expected to be portable with reproducible
29/// results when used with a PRNG with fixed seed; see the
30/// [portability chapter](https://rust-random.github.io/book/portability.html)
31/// of The Rust Rand Book. In some cases this does not apply, e.g. the `usize`
32/// type requires different sampling on 32-bit and 64-bit machines.
33///
34/// [`sample_iter`]: Distribution::sample_iter
35pub trait Distribution<T> {
36    /// Generate a random value of `T`, using `rng` as the source of randomness.
37    fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> T;
38
39    /// Create an iterator that generates random values of `T`, using `rng` as
40    /// the source of randomness.
41    ///
42    /// Note that this function takes `self` by value. This works since
43    /// `Distribution<T>` is impl'd for `&D` where `D: Distribution<T>`,
44    /// however borrowing is not automatic hence `distr.sample_iter(...)` may
45    /// need to be replaced with `(&distr).sample_iter(...)` to borrow or
46    /// `(&*distr).sample_iter(...)` to reborrow an existing reference.
47    ///
48    /// # Example
49    ///
50    /// ```
51    /// use rand::distr::{Distribution, Alphanumeric, Uniform, StandardUniform};
52    ///
53    /// let mut rng = rand::rng();
54    ///
55    /// // Vec of 16 x f32:
56    /// let v: Vec<f32> = StandardUniform.sample_iter(&mut rng).take(16).collect();
57    ///
58    /// // String:
59    /// let s: String = Alphanumeric
60    ///     .sample_iter(&mut rng)
61    ///     .take(7)
62    ///     .map(char::from)
63    ///     .collect();
64    ///
65    /// // Dice-rolling:
66    /// let die_range = Uniform::new_inclusive(1, 6).unwrap();
67    /// let mut roll_die = die_range.sample_iter(&mut rng);
68    /// while roll_die.next().unwrap() != 6 {
69    ///     println!("Not a 6; rolling again!");
70    /// }
71    /// ```
72    fn sample_iter<R>(self, rng: R) -> Iter<Self, R, T>
73    where
74        R: Rng,
75        Self: Sized,
76    {
77        Iter {
78            distr: self,
79            rng,
80            phantom: core::marker::PhantomData,
81        }
82    }
83
84    /// Map sampled values to type `S`
85    ///
86    /// # Example
87    ///
88    /// ```
89    /// use rand::distr::{Distribution, Uniform};
90    ///
91    /// let die = Uniform::new_inclusive(1, 6).unwrap();
92    /// let even_number = die.map(|num| num % 2 == 0);
93    /// while !even_number.sample(&mut rand::rng()) {
94    ///     println!("Still odd; rolling again!");
95    /// }
96    /// ```
97    fn map<F, S>(self, func: F) -> Map<Self, F, T, S>
98    where
99        F: Fn(T) -> S,
100        Self: Sized,
101    {
102        Map {
103            distr: self,
104            func,
105            phantom: core::marker::PhantomData,
106        }
107    }
108}
109
110impl<T, D: Distribution<T> + ?Sized> Distribution<T> for &D {
111    fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> T {
112        (*self).sample(rng)
113    }
114}
115
116/// An iterator over a [`Distribution`]
117///
118/// This iterator yields random values of type `T` with distribution `D`
119/// from a random generator of type `R`.
120///
121/// Construct this `struct` using [`Distribution::sample_iter`] or
122/// [`Rng::sample_iter`]. It is also used by [`Rng::random_iter`] and
123/// [`crate::random_iter`].
124#[derive(Debug)]
125pub struct Iter<D, R, T> {
126    distr: D,
127    rng: R,
128    phantom: core::marker::PhantomData<T>,
129}
130
131impl<D, R, T> Iterator for Iter<D, R, T>
132where
133    D: Distribution<T>,
134    R: Rng,
135{
136    type Item = T;
137
138    #[inline(always)]
139    fn next(&mut self) -> Option<T> {
140        // Here, self.rng may be a reference, but we must take &mut anyway.
141        // Even if sample could take an R: Rng by value, we would need to do this
142        // since Rng is not copyable and we cannot enforce that this is "reborrowable".
143        Some(self.distr.sample(&mut self.rng))
144    }
145
146    fn size_hint(&self) -> (usize, Option<usize>) {
147        (usize::MAX, None)
148    }
149}
150
151impl<D, R, T> iter::FusedIterator for Iter<D, R, T>
152where
153    D: Distribution<T>,
154    R: Rng,
155{
156}
157
158/// A [`Distribution`] which maps sampled values to type `S`
159///
160/// This `struct` is created by the [`Distribution::map`] method.
161/// See its documentation for more.
162#[derive(Debug)]
163pub struct Map<D, F, T, S> {
164    distr: D,
165    func: F,
166    phantom: core::marker::PhantomData<fn(T) -> S>,
167}
168
169impl<D, F, T, S> Distribution<S> for Map<D, F, T, S>
170where
171    D: Distribution<T>,
172    F: Fn(T) -> S,
173{
174    fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> S {
175        (self.func)(self.distr.sample(rng))
176    }
177}
178
179/// Sample or extend a [`String`]
180///
181/// Helper methods to extend a [`String`] or sample a new [`String`].
182#[cfg(feature = "alloc")]
183pub trait SampleString {
184    /// Append `len` random chars to `string`
185    ///
186    /// Note: implementations may leave `string` with excess capacity. If this
187    /// is undesirable, consider calling [`String::shrink_to_fit`] after this
188    /// method.
189    fn append_string<R: Rng + ?Sized>(&self, rng: &mut R, string: &mut String, len: usize);
190
191    /// Generate a [`String`] of `len` random chars
192    ///
193    /// Note: implementations may leave the string with excess capacity. If this
194    /// is undesirable, consider calling [`String::shrink_to_fit`] after this
195    /// method.
196    #[inline]
197    fn sample_string<R: Rng + ?Sized>(&self, rng: &mut R, len: usize) -> String {
198        let mut s = String::new();
199        self.append_string(rng, &mut s, len);
200        s
201    }
202}
203
204#[cfg(test)]
205mod tests {
206    use crate::distr::{Distribution, Uniform};
207    use crate::Rng;
208
209    #[test]
210    fn test_distributions_iter() {
211        use crate::distr::Open01;
212        let mut rng = crate::test::rng(210);
213        let distr = Open01;
214        let mut iter = Distribution::<f32>::sample_iter(distr, &mut rng);
215        let mut sum: f32 = 0.;
216        for _ in 0..100 {
217            sum += iter.next().unwrap();
218        }
219        assert!(0. < sum && sum < 100.);
220    }
221
222    #[test]
223    fn test_distributions_map() {
224        let dist = Uniform::new_inclusive(0, 5).unwrap().map(|val| val + 15);
225
226        let mut rng = crate::test::rng(212);
227        let val = dist.sample(&mut rng);
228        assert!((15..=20).contains(&val));
229    }
230
231    #[test]
232    fn test_make_an_iter() {
233        fn ten_dice_rolls_other_than_five<R: Rng>(rng: &mut R) -> impl Iterator<Item = i32> + '_ {
234            Uniform::new_inclusive(1, 6)
235                .unwrap()
236                .sample_iter(rng)
237                .filter(|x| *x != 5)
238                .take(10)
239        }
240
241        let mut rng = crate::test::rng(211);
242        let mut count = 0;
243        for val in ten_dice_rolls_other_than_five(&mut rng) {
244            assert!((1..=6).contains(&val) && val != 5);
245            count += 1;
246        }
247        assert_eq!(count, 10);
248    }
249
250    #[test]
251    #[cfg(feature = "alloc")]
252    fn test_dist_string() {
253        use crate::distr::{Alphanumeric, SampleString, StandardUniform};
254        use core::str;
255        let mut rng = crate::test::rng(213);
256
257        let s1 = Alphanumeric.sample_string(&mut rng, 20);
258        assert_eq!(s1.len(), 20);
259        assert_eq!(str::from_utf8(s1.as_bytes()), Ok(s1.as_str()));
260
261        let s2 = StandardUniform.sample_string(&mut rng, 20);
262        assert_eq!(s2.chars().count(), 20);
263        assert_eq!(str::from_utf8(s2.as_bytes()), Ok(s2.as_str()));
264    }
265}
rand/distr/distribution.rs

rand/distr/
distribution.rs
