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Diffstat (limited to 'vendor/rand/src/distributions/slice.rs')
-rw-r--r-- | vendor/rand/src/distributions/slice.rs | 117 |
1 files changed, 117 insertions, 0 deletions
diff --git a/vendor/rand/src/distributions/slice.rs b/vendor/rand/src/distributions/slice.rs new file mode 100644 index 000000000..3302deb2a --- /dev/null +++ b/vendor/rand/src/distributions/slice.rs @@ -0,0 +1,117 @@ +// Copyright 2021 Developers of the Rand project. +// +// 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. + +use crate::distributions::{Distribution, Uniform}; + +/// A distribution to sample items uniformly from a slice. +/// +/// [`Slice::new`] constructs a distribution referencing a slice and uniformly +/// samples references from the items in the slice. It may do extra work up +/// front to make sampling of multiple values faster; if only one sample from +/// the slice is required, [`SliceRandom::choose`] can be more efficient. +/// +/// Steps are taken to avoid bias which might be present in naive +/// implementations; for example `slice[rng.gen() % slice.len()]` samples from +/// the slice, but may be more likely to select numbers in the low range than +/// other values. +/// +/// This distribution samples with replacement; each sample is independent. +/// Sampling without replacement requires state to be retained, and therefore +/// cannot be handled by a distribution; you should instead consider methods +/// on [`SliceRandom`], such as [`SliceRandom::choose_multiple`]. +/// +/// # Example +/// +/// ``` +/// use rand::Rng; +/// use rand::distributions::Slice; +/// +/// let vowels = ['a', 'e', 'i', 'o', 'u']; +/// let vowels_dist = Slice::new(&vowels).unwrap(); +/// let rng = rand::thread_rng(); +/// +/// // build a string of 10 vowels +/// let vowel_string: String = rng +/// .sample_iter(&vowels_dist) +/// .take(10) +/// .collect(); +/// +/// println!("{}", vowel_string); +/// assert_eq!(vowel_string.len(), 10); +/// assert!(vowel_string.chars().all(|c| vowels.contains(&c))); +/// ``` +/// +/// For a single sample, [`SliceRandom::choose`][crate::seq::SliceRandom::choose] +/// may be preferred: +/// +/// ``` +/// use rand::seq::SliceRandom; +/// +/// let vowels = ['a', 'e', 'i', 'o', 'u']; +/// let mut rng = rand::thread_rng(); +/// +/// println!("{}", vowels.choose(&mut rng).unwrap()) +/// ``` +/// +/// [`SliceRandom`]: crate::seq::SliceRandom +/// [`SliceRandom::choose`]: crate::seq::SliceRandom::choose +/// [`SliceRandom::choose_multiple`]: crate::seq::SliceRandom::choose_multiple +#[derive(Debug, Clone, Copy)] +pub struct Slice<'a, T> { + slice: &'a [T], + range: Uniform<usize>, +} + +impl<'a, T> Slice<'a, T> { + /// Create a new `Slice` instance which samples uniformly from the slice. + /// Returns `Err` if the slice is empty. + pub fn new(slice: &'a [T]) -> Result<Self, EmptySlice> { + match slice.len() { + 0 => Err(EmptySlice), + len => Ok(Self { + slice, + range: Uniform::new(0, len), + }), + } + } +} + +impl<'a, T> Distribution<&'a T> for Slice<'a, T> { + fn sample<R: crate::Rng + ?Sized>(&self, rng: &mut R) -> &'a T { + let idx = self.range.sample(rng); + + debug_assert!( + idx < self.slice.len(), + "Uniform::new(0, {}) somehow returned {}", + self.slice.len(), + idx + ); + + // Safety: at construction time, it was ensured that the slice was + // non-empty, and that the `Uniform` range produces values in range + // for the slice + unsafe { self.slice.get_unchecked(idx) } + } +} + +/// Error type indicating that a [`Slice`] distribution was improperly +/// constructed with an empty slice. +#[derive(Debug, Clone, Copy)] +pub struct EmptySlice; + +impl core::fmt::Display for EmptySlice { + fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result { + write!( + f, + "Tried to create a `distributions::Slice` with an empty slice" + ) + } +} + +#[cfg(feature = "std")] +impl std::error::Error for EmptySlice {} |