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// 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
// option. This file may not be copied, modified, or distributed
// except according to those terms.
//! Distribution trait and associates
use crate::Rng;
use core::iter;
#[cfg(feature = "alloc")]
use alloc::string::String;
/// 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
/// 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::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 mut rng = thread_rng();
///
/// // Vec of 16 x f32:
/// let v: Vec<f32> = Standard.sample_iter(&mut rng).take(16).collect();
///
/// // String:
/// let s: String = Alphanumeric
/// .sample_iter(&mut 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(&mut 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,
}
}
/// Create a distribution of values of 'S' by mapping the output of `Self`
/// through the closure `F`
///
/// # Example
///
/// ```
/// use rand::thread_rng;
/// use rand::distributions::{Distribution, Uniform};
///
/// let mut rng = thread_rng();
///
/// let die = Uniform::new_inclusive(1, 6);
/// let even_number = die.map(|num| num % 2 == 0);
/// while !even_number.sample(&mut rng) {
/// println!("Still odd; rolling again!");
/// }
/// ```
fn map<F, S>(self, func: F) -> DistMap<Self, F, T, S>
where
F: Fn(T) -> S,
Self: Sized,
{
DistMap {
distr: self,
func,
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 distribution of values of type `S` derived from the distribution `D`
/// by mapping its output of type `T` through the closure `F`.
///
/// This `struct` is created by the [`Distribution::map`] method.
/// See its documentation for more.
#[derive(Debug)]
pub struct DistMap<D, F, T, S> {
distr: D,
func: F,
phantom: ::core::marker::PhantomData<fn(T) -> S>,
}
impl<D, F, T, S> Distribution<S> for DistMap<D, F, T, S>
where
D: Distribution<T>,
F: Fn(T) -> S,
{
fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> S {
(self.func)(self.distr.sample(rng))
}
}
/// `String` sampler
///
/// Sampling a `String` of random characters is not quite the same as collecting
/// a sequence of chars. This trait contains some helpers.
#[cfg(feature = "alloc")]
pub trait DistString {
/// Append `len` random chars to `string`
fn append_string<R: Rng + ?Sized>(&self, rng: &mut R, string: &mut String, len: usize);
/// Generate a `String` of `len` random chars
#[inline]
fn sample_string<R: Rng + ?Sized>(&self, rng: &mut R, len: usize) -> String {
let mut s = String::new();
self.append_string(rng, &mut s, len);
s
}
}
#[cfg(test)]
mod tests {
use crate::distributions::{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_distributions_map() {
let dist = Uniform::new_inclusive(0, 5).map(|val| val + 15);
let mut rng = crate::test::rng(212);
let val = dist.sample(&mut rng);
assert!((15..=20).contains(&val));
}
#[test]
fn test_make_an_iter() {
fn ten_dice_rolls_other_than_five<R: Rng>(
rng: &mut R,
) -> impl Iterator<Item = i32> + '_ {
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!((1..=6).contains(&val) && val != 5);
count += 1;
}
assert_eq!(count, 10);
}
#[test]
#[cfg(feature = "alloc")]
fn test_dist_string() {
use core::str;
use crate::distributions::{Alphanumeric, DistString, Standard};
let mut rng = crate::test::rng(213);
let s1 = Alphanumeric.sample_string(&mut rng, 20);
assert_eq!(s1.len(), 20);
assert_eq!(str::from_utf8(s1.as_bytes()), Ok(s1.as_str()));
let s2 = Standard.sample_string(&mut rng, 20);
assert_eq!(s2.chars().count(), 20);
assert_eq!(str::from_utf8(s2.as_bytes()), Ok(s2.as_str()));
}
}