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 </pre><pre class="rust"><code><span class="comment">// Copyright 2018 Developers of the Rand project.
 //
 // Licensed under the Apache License, Version 2.0 &lt;LICENSE-APACHE or
 // https://www.apache.org/licenses/LICENSE-2.0&gt; or the MIT license
 // &lt;LICENSE-MIT or https://opensource.org/licenses/MIT&gt;, at your
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.

 </span><span class="doccomment">//! Sequence-related functionality
 //!
 //! This module provides:
 //!
 //! *   [`SliceRandom`] slice sampling and mutation
 //! *   [`IteratorRandom`] iterator sampling
 //! *   [`index::sample`] low-level API to choose multiple indices from
 //!     `0..length`
 //!
 //! Also see:
 //!
 //! *   [`crate::distributions::WeightedIndex`] distribution which provides
 //!     weighted index sampling.
 //!
 //! In order to make results reproducible across 32-64 bit architectures, all
 //! `usize` indices are sampled as a `u32` where possible (also providing a
 //! small performance boost in some cases).


 </span><span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
 #[cfg_attr(doc_cfg, doc(cfg(feature = <span class="string">&quot;alloc&quot;</span>)))]
 </span><span class="kw">pub mod </span>index;

 <span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)] </span><span class="kw">use </span>core::ops::Index;

 <span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)] </span><span class="kw">use </span>alloc::vec::Vec;

 <span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
 </span><span class="kw">use </span><span class="kw">crate</span>::distributions::uniform::{SampleBorrow, SampleUniform};
 <span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)] </span><span class="kw">use </span><span class="kw">crate</span>::distributions::WeightedError;
 <span class="kw">use </span><span class="kw">crate</span>::Rng;

 <span class="doccomment">/// Extension trait on slices, providing random mutation and sampling methods.
 ///
 /// This trait is implemented on all `[T]` slice types, providing several
 /// methods for choosing and shuffling elements. You must `use` this trait:
 ///
 /// ```
 /// use rand::seq::SliceRandom;
 ///
 /// let mut rng = rand::thread_rng();
 /// let mut bytes = &quot;Hello, random!&quot;.to_string().into_bytes();
 /// bytes.shuffle(&amp;mut rng);
 /// let str = String::from_utf8(bytes).unwrap();
 /// println!(&quot;{}&quot;, str);
 /// ```
 /// Example output (non-deterministic):
 /// ```none
 /// l,nmroHado !le
 /// ```
 </span><span class="kw">pub trait </span>SliceRandom {
     <span class="doccomment">/// The element type.
     </span><span class="kw">type </span>Item;

     <span class="doccomment">/// Returns a reference to one random element of the slice, or `None` if the
     /// slice is empty.
     ///
     /// For slices, complexity is `O(1)`.
     ///
     /// # Example
     ///
     /// ```
     /// use rand::thread_rng;
     /// use rand::seq::SliceRandom;
     ///
     /// let choices = [1, 2, 4, 8, 16, 32];
     /// let mut rng = thread_rng();
     /// println!(&quot;{:?}&quot;, choices.choose(&amp;mut rng));
     /// assert_eq!(choices[..0].choose(&amp;mut rng), None);
     /// ```
     </span><span class="kw">fn </span>choose&lt;R&gt;(<span class="kw-2">&amp;</span><span class="self">self</span>, rng: <span class="kw-2">&amp;mut </span>R) -&gt; <span class="prelude-ty">Option</span>&lt;<span class="kw-2">&amp;</span><span class="self">Self</span>::Item&gt;
     <span class="kw">where </span>R: Rng + <span class="question-mark">?</span>Sized;

     <span class="doccomment">/// Returns a mutable reference to one random element of the slice, or
     /// `None` if the slice is empty.
     ///
     /// For slices, complexity is `O(1)`.
     </span><span class="kw">fn </span>choose_mut&lt;R&gt;(<span class="kw-2">&amp;mut </span><span class="self">self</span>, rng: <span class="kw-2">&amp;mut </span>R) -&gt; <span class="prelude-ty">Option</span>&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>::Item&gt;
     <span class="kw">where </span>R: Rng + <span class="question-mark">?</span>Sized;

     <span class="doccomment">/// Chooses `amount` elements from the slice at random, without repetition,
     /// and in random order. The returned iterator is appropriate both for
     /// collection into a `Vec` and filling an existing buffer (see example).
     ///
     /// In case this API is not sufficiently flexible, use [`index::sample`].
     ///
     /// For slices, complexity is the same as [`index::sample`].
     ///
     /// # Example
     /// ```
     /// use rand::seq::SliceRandom;
     ///
     /// let mut rng = &amp;mut rand::thread_rng();
     /// let sample = &quot;Hello, audience!&quot;.as_bytes();
     ///
     /// // collect the results into a vector:
     /// let v: Vec&lt;u8&gt; = sample.choose_multiple(&amp;mut rng, 3).cloned().collect();
     ///
     /// // store in a buffer:
     /// let mut buf = [0u8; 5];
     /// for (b, slot) in sample.choose_multiple(&amp;mut rng, buf.len()).zip(buf.iter_mut()) {
     ///     *slot = *b;
     /// }
     /// ```
     </span><span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
     #[cfg_attr(doc_cfg, doc(cfg(feature = <span class="string">&quot;alloc&quot;</span>)))]
     </span><span class="kw">fn </span>choose_multiple&lt;R&gt;(<span class="kw-2">&amp;</span><span class="self">self</span>, rng: <span class="kw-2">&amp;mut </span>R, amount: usize) -&gt; SliceChooseIter&lt;<span class="self">Self</span>, <span class="self">Self</span>::Item&gt;
     <span class="kw">where </span>R: Rng + <span class="question-mark">?</span>Sized;

     <span class="doccomment">/// Similar to [`choose`], but where the likelihood of each outcome may be
     /// specified.
     ///
     /// The specified function `weight` maps each item `x` to a relative
     /// likelihood `weight(x)`. The probability of each item being selected is
     /// therefore `weight(x) / s`, where `s` is the sum of all `weight(x)`.
     ///
     /// For slices of length `n`, complexity is `O(n)`.
     /// See also [`choose_weighted_mut`], [`distributions::weighted`].
     ///
     /// # Example
     ///
     /// ```
     /// use rand::prelude::*;
     ///
     /// let choices = [(&#39;a&#39;, 2), (&#39;b&#39;, 1), (&#39;c&#39;, 1)];
     /// let mut rng = thread_rng();
     /// // 50% chance to print &#39;a&#39;, 25% chance to print &#39;b&#39;, 25% chance to print &#39;c&#39;
     /// println!(&quot;{:?}&quot;, choices.choose_weighted(&amp;mut rng, |item| item.1).unwrap().0);
     /// ```
     /// [`choose`]: SliceRandom::choose
     /// [`choose_weighted_mut`]: SliceRandom::choose_weighted_mut
     /// [`distributions::weighted`]: crate::distributions::weighted
     </span><span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
     #[cfg_attr(doc_cfg, doc(cfg(feature = <span class="string">&quot;alloc&quot;</span>)))]
     </span><span class="kw">fn </span>choose_weighted&lt;R, F, B, X&gt;(
         <span class="kw-2">&amp;</span><span class="self">self</span>, rng: <span class="kw-2">&amp;mut </span>R, weight: F,
     ) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="kw-2">&amp;</span><span class="self">Self</span>::Item, WeightedError&gt;
     <span class="kw">where
         </span>R: Rng + <span class="question-mark">?</span>Sized,
         F: Fn(<span class="kw-2">&amp;</span><span class="self">Self</span>::Item) -&gt; B,
         B: SampleBorrow&lt;X&gt;,
         X: SampleUniform
             + <span class="kw">for</span>&lt;<span class="lifetime">&#39;a</span>&gt; ::core::ops::AddAssign&lt;<span class="kw-2">&amp;</span><span class="lifetime">&#39;a </span>X&gt;
             + ::core::cmp::PartialOrd&lt;X&gt;
             + Clone
             + Default;

     <span class="doccomment">/// Similar to [`choose_mut`], but where the likelihood of each outcome may
     /// be specified.
     ///
     /// The specified function `weight` maps each item `x` to a relative
     /// likelihood `weight(x)`. The probability of each item being selected is
     /// therefore `weight(x) / s`, where `s` is the sum of all `weight(x)`.
     ///
     /// For slices of length `n`, complexity is `O(n)`.
     /// See also [`choose_weighted`], [`distributions::weighted`].
     ///
     /// [`choose_mut`]: SliceRandom::choose_mut
     /// [`choose_weighted`]: SliceRandom::choose_weighted
     /// [`distributions::weighted`]: crate::distributions::weighted
     </span><span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
     #[cfg_attr(doc_cfg, doc(cfg(feature = <span class="string">&quot;alloc&quot;</span>)))]
     </span><span class="kw">fn </span>choose_weighted_mut&lt;R, F, B, X&gt;(
         <span class="kw-2">&amp;mut </span><span class="self">self</span>, rng: <span class="kw-2">&amp;mut </span>R, weight: F,
     ) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>::Item, WeightedError&gt;
     <span class="kw">where
         </span>R: Rng + <span class="question-mark">?</span>Sized,
         F: Fn(<span class="kw-2">&amp;</span><span class="self">Self</span>::Item) -&gt; B,
         B: SampleBorrow&lt;X&gt;,
         X: SampleUniform
             + <span class="kw">for</span>&lt;<span class="lifetime">&#39;a</span>&gt; ::core::ops::AddAssign&lt;<span class="kw-2">&amp;</span><span class="lifetime">&#39;a </span>X&gt;
             + ::core::cmp::PartialOrd&lt;X&gt;
             + Clone
             + Default;

     <span class="doccomment">/// Similar to [`choose_multiple`], but where the likelihood of each element&#39;s
     /// inclusion in the output may be specified. The elements are returned in an
     /// arbitrary, unspecified order.
     ///
     /// The specified function `weight` maps each item `x` to a relative
     /// likelihood `weight(x)`. The probability of each item being selected is
     /// therefore `weight(x) / s`, where `s` is the sum of all `weight(x)`.
     ///
     /// If all of the weights are equal, even if they are all zero, each element has
     /// an equal likelihood of being selected.
     ///
     /// The complexity of this method depends on the feature `partition_at_index`.
     /// If the feature is enabled, then for slices of length `n`, the complexity
     /// is `O(n)` space and `O(n)` time. Otherwise, the complexity is `O(n)` space and
     /// `O(n * log amount)` time.
     ///
     /// # Example
     ///
     /// ```
     /// use rand::prelude::*;
     ///
     /// let choices = [(&#39;a&#39;, 2), (&#39;b&#39;, 1), (&#39;c&#39;, 1)];
     /// let mut rng = thread_rng();
     /// // First Draw * Second Draw = total odds
     /// // -----------------------
     /// // (50% * 50%) + (25% * 67%) = 41.7% chance that the output is `[&#39;a&#39;, &#39;b&#39;]` in some order.
     /// // (50% * 50%) + (25% * 67%) = 41.7% chance that the output is `[&#39;a&#39;, &#39;c&#39;]` in some order.
     /// // (25% * 33%) + (25% * 33%) = 16.6% chance that the output is `[&#39;b&#39;, &#39;c&#39;]` in some order.
     /// println!(&quot;{:?}&quot;, choices.choose_multiple_weighted(&amp;mut rng, 2, |item| item.1).unwrap().collect::&lt;Vec&lt;_&gt;&gt;());
     /// ```
     /// [`choose_multiple`]: SliceRandom::choose_multiple
     </span><span class="comment">//
     // Note: this is feature-gated on std due to usage of f64::powf.
     // If necessary, we may use alloc+libm as an alternative (see PR #1089).
     </span><span class="attribute">#[cfg(feature = <span class="string">&quot;std&quot;</span>)]
     #[cfg_attr(doc_cfg, doc(cfg(feature = <span class="string">&quot;std&quot;</span>)))]
     </span><span class="kw">fn </span>choose_multiple_weighted&lt;R, F, X&gt;(
         <span class="kw-2">&amp;</span><span class="self">self</span>, rng: <span class="kw-2">&amp;mut </span>R, amount: usize, weight: F,
     ) -&gt; <span class="prelude-ty">Result</span>&lt;SliceChooseIter&lt;<span class="self">Self</span>, <span class="self">Self</span>::Item&gt;, WeightedError&gt;
     <span class="kw">where
         </span>R: Rng + <span class="question-mark">?</span>Sized,
         F: Fn(<span class="kw-2">&amp;</span><span class="self">Self</span>::Item) -&gt; X,
         X: Into&lt;f64&gt;;

     <span class="doccomment">/// Shuffle a mutable slice in place.
     ///
     /// For slices of length `n`, complexity is `O(n)`.
     ///
     /// # Example
     ///
     /// ```
     /// use rand::seq::SliceRandom;
     /// use rand::thread_rng;
     ///
     /// let mut rng = thread_rng();
     /// let mut y = [1, 2, 3, 4, 5];
     /// println!(&quot;Unshuffled: {:?}&quot;, y);
     /// y.shuffle(&amp;mut rng);
     /// println!(&quot;Shuffled:   {:?}&quot;, y);
     /// ```
     </span><span class="kw">fn </span>shuffle&lt;R&gt;(<span class="kw-2">&amp;mut </span><span class="self">self</span>, rng: <span class="kw-2">&amp;mut </span>R)
     <span class="kw">where </span>R: Rng + <span class="question-mark">?</span>Sized;

     <span class="doccomment">/// Shuffle a slice in place, but exit early.
     ///
     /// Returns two mutable slices from the source slice. The first contains
     /// `amount` elements randomly permuted. The second has the remaining
     /// elements that are not fully shuffled.
     ///
     /// This is an efficient method to select `amount` elements at random from
     /// the slice, provided the slice may be mutated.
     ///
     /// If you only need to choose elements randomly and `amount &gt; self.len()/2`
     /// then you may improve performance by taking
     /// `amount = values.len() - amount` and using only the second slice.
     ///
     /// If `amount` is greater than the number of elements in the slice, this
     /// will perform a full shuffle.
     ///
     /// For slices, complexity is `O(m)` where `m = amount`.
     </span><span class="kw">fn </span>partial_shuffle&lt;R&gt;(
         <span class="kw-2">&amp;mut </span><span class="self">self</span>, rng: <span class="kw-2">&amp;mut </span>R, amount: usize,
     ) -&gt; (<span class="kw-2">&amp;mut </span>[<span class="self">Self</span>::Item], <span class="kw-2">&amp;mut </span>[<span class="self">Self</span>::Item])
     <span class="kw">where </span>R: Rng + <span class="question-mark">?</span>Sized;
 }

 <span class="doccomment">/// Extension trait on iterators, providing random sampling methods.
 ///
 /// This trait is implemented on all iterators `I` where `I: Iterator + Sized`
 /// and provides methods for
 /// choosing one or more elements. You must `use` this trait:
 ///
 /// ```
 /// use rand::seq::IteratorRandom;
 ///
 /// let mut rng = rand::thread_rng();
 ///
 /// let faces = &quot;😀😎😐😕😠😢&quot;;
 /// println!(&quot;I am {}!&quot;, faces.chars().choose(&amp;mut rng).unwrap());
 /// ```
 /// Example output (non-deterministic):
 /// ```none
 /// I am 😀!
 /// ```
 </span><span class="kw">pub trait </span>IteratorRandom: Iterator + Sized {
     <span class="doccomment">/// Choose one element at random from the iterator.
     ///
     /// Returns `None` if and only if the iterator is empty.
     ///
     /// This method uses [`Iterator::size_hint`] for optimisation. With an
     /// accurate hint and where [`Iterator::nth`] is a constant-time operation
     /// this method can offer `O(1)` performance. Where no size hint is
     /// available, complexity is `O(n)` where `n` is the iterator length.
     /// Partial hints (where `lower &gt; 0`) also improve performance.
     ///
     /// Note that the output values and the number of RNG samples used
     /// depends on size hints. In particular, `Iterator` combinators that don&#39;t
     /// change the values yielded but change the size hints may result in
     /// `choose` returning different elements. If you want consistent results
     /// and RNG usage consider using [`IteratorRandom::choose_stable`].
     </span><span class="kw">fn </span>choose&lt;R&gt;(<span class="kw-2">mut </span><span class="self">self</span>, rng: <span class="kw-2">&amp;mut </span>R) -&gt; <span class="prelude-ty">Option</span>&lt;<span class="self">Self</span>::Item&gt;
     <span class="kw">where </span>R: Rng + <span class="question-mark">?</span>Sized {
         <span class="kw">let </span>(<span class="kw-2">mut </span>lower, <span class="kw-2">mut </span>upper) = <span class="self">self</span>.size_hint();
         <span class="kw">let </span><span class="kw-2">mut </span>consumed = <span class="number">0</span>;
         <span class="kw">let </span><span class="kw-2">mut </span>result = <span class="prelude-val">None</span>;

         <span class="comment">// Handling for this condition outside the loop allows the optimizer to eliminate the loop
         // when the Iterator is an ExactSizeIterator. This has a large performance impact on e.g.
         // seq_iter_choose_from_1000.
         </span><span class="kw">if </span>upper == <span class="prelude-val">Some</span>(lower) {
             <span class="kw">return if </span>lower == <span class="number">0 </span>{
                 <span class="prelude-val">None
             </span>} <span class="kw">else </span>{
                 <span class="self">self</span>.nth(gen_index(rng, lower))
             };
         }

         <span class="comment">// Continue until the iterator is exhausted
         </span><span class="kw">loop </span>{
             <span class="kw">if </span>lower &gt; <span class="number">1 </span>{
                 <span class="kw">let </span>ix = gen_index(rng, lower + consumed);
                 <span class="kw">let </span>skip = <span class="kw">if </span>ix &lt; lower {
                     result = <span class="self">self</span>.nth(ix);
                     lower - (ix + <span class="number">1</span>)
                 } <span class="kw">else </span>{
                     lower
                 };
                 <span class="kw">if </span>upper == <span class="prelude-val">Some</span>(lower) {
                     <span class="kw">return </span>result;
                 }
                 consumed += lower;
                 <span class="kw">if </span>skip &gt; <span class="number">0 </span>{
                     <span class="self">self</span>.nth(skip - <span class="number">1</span>);
                 }
             } <span class="kw">else </span>{
                 <span class="kw">let </span>elem = <span class="self">self</span>.next();
                 <span class="kw">if </span>elem.is_none() {
                     <span class="kw">return </span>result;
                 }
                 consumed += <span class="number">1</span>;
                 <span class="kw">if </span>gen_index(rng, consumed) == <span class="number">0 </span>{
                     result = elem;
                 }
             }

             <span class="kw">let </span>hint = <span class="self">self</span>.size_hint();
             lower = hint.<span class="number">0</span>;
             upper = hint.<span class="number">1</span>;
         }
     }

     <span class="doccomment">/// Choose one element at random from the iterator.
     ///
     /// Returns `None` if and only if the iterator is empty.
     ///
     /// This method is very similar to [`choose`] except that the result
     /// only depends on the length of the iterator and the values produced by
     /// `rng`. Notably for any iterator of a given length this will make the
     /// same requests to `rng` and if the same sequence of values are produced
     /// the same index will be selected from `self`. This may be useful if you
     /// need consistent results no matter what type of iterator you are working
     /// with. If you do not need this stability prefer [`choose`].
     ///
     /// Note that this method still uses [`Iterator::size_hint`] to skip
     /// constructing elements where possible, however the selection and `rng`
     /// calls are the same in the face of this optimization. If you want to
     /// force every element to be created regardless call `.inspect(|e| ())`.
     ///
     /// [`choose`]: IteratorRandom::choose
     </span><span class="kw">fn </span>choose_stable&lt;R&gt;(<span class="kw-2">mut </span><span class="self">self</span>, rng: <span class="kw-2">&amp;mut </span>R) -&gt; <span class="prelude-ty">Option</span>&lt;<span class="self">Self</span>::Item&gt;
     <span class="kw">where </span>R: Rng + <span class="question-mark">?</span>Sized {
         <span class="kw">let </span><span class="kw-2">mut </span>consumed = <span class="number">0</span>;
         <span class="kw">let </span><span class="kw-2">mut </span>result = <span class="prelude-val">None</span>;

         <span class="kw">loop </span>{
             <span class="comment">// Currently the only way to skip elements is `nth()`. So we need to
             // store what index to access next here.
             // This should be replaced by `advance_by()` once it is stable:
             // https://github.com/rust-lang/rust/issues/77404
             </span><span class="kw">let </span><span class="kw-2">mut </span>next = <span class="number">0</span>;

             <span class="kw">let </span>(lower, <span class="kw">_</span>) = <span class="self">self</span>.size_hint();
             <span class="kw">if </span>lower &gt;= <span class="number">2 </span>{
                 <span class="kw">let </span>highest_selected = (<span class="number">0</span>..lower)
                     .filter(|ix| gen_index(rng, consumed+ix+<span class="number">1</span>) == <span class="number">0</span>)
                     .last();

                 consumed += lower;
                 next = lower;

                 <span class="kw">if let </span><span class="prelude-val">Some</span>(ix) = highest_selected {
                     result = <span class="self">self</span>.nth(ix);
                     next -= ix + <span class="number">1</span>;
                     <span class="macro">debug_assert!</span>(result.is_some(), <span class="string">&quot;iterator shorter than size_hint().0&quot;</span>);
                 }
             }

             <span class="kw">let </span>elem = <span class="self">self</span>.nth(next);
             <span class="kw">if </span>elem.is_none() {
                 <span class="kw">return </span>result
             }

             <span class="kw">if </span>gen_index(rng, consumed+<span class="number">1</span>) == <span class="number">0 </span>{
                 result = elem;
             }
             consumed += <span class="number">1</span>;
         }
     }

     <span class="doccomment">/// Collects values at random from the iterator into a supplied buffer
     /// until that buffer is filled.
     ///
     /// Although the elements are selected randomly, the order of elements in
     /// the buffer is neither stable nor fully random. If random ordering is
     /// desired, shuffle the result.
     ///
     /// Returns the number of elements added to the buffer. This equals the length
     /// of the buffer unless the iterator contains insufficient elements, in which
     /// case this equals the number of elements available.
     ///
     /// Complexity is `O(n)` where `n` is the length of the iterator.
     /// For slices, prefer [`SliceRandom::choose_multiple`].
     </span><span class="kw">fn </span>choose_multiple_fill&lt;R&gt;(<span class="kw-2">mut </span><span class="self">self</span>, rng: <span class="kw-2">&amp;mut </span>R, buf: <span class="kw-2">&amp;mut </span>[<span class="self">Self</span>::Item]) -&gt; usize
     <span class="kw">where </span>R: Rng + <span class="question-mark">?</span>Sized {
         <span class="kw">let </span>amount = buf.len();
         <span class="kw">let </span><span class="kw-2">mut </span>len = <span class="number">0</span>;
         <span class="kw">while </span>len &lt; amount {
             <span class="kw">if let </span><span class="prelude-val">Some</span>(elem) = <span class="self">self</span>.next() {
                 buf[len] = elem;
                 len += <span class="number">1</span>;
             } <span class="kw">else </span>{
                 <span class="comment">// Iterator exhausted; stop early
                 </span><span class="kw">return </span>len;
             }
         }

         <span class="comment">// Continue, since the iterator was not exhausted
         </span><span class="kw">for </span>(i, elem) <span class="kw">in </span><span class="self">self</span>.enumerate() {
             <span class="kw">let </span>k = gen_index(rng, i + <span class="number">1 </span>+ amount);
             <span class="kw">if let </span><span class="prelude-val">Some</span>(slot) = buf.get_mut(k) {
                 <span class="kw-2">*</span>slot = elem;
             }
         }
         len
     }

     <span class="doccomment">/// Collects `amount` values at random from the iterator into a vector.
     ///
     /// This is equivalent to `choose_multiple_fill` except for the result type.
     ///
     /// Although the elements are selected randomly, the order of elements in
     /// the buffer is neither stable nor fully random. If random ordering is
     /// desired, shuffle the result.
     ///
     /// The length of the returned vector equals `amount` unless the iterator
     /// contains insufficient elements, in which case it equals the number of
     /// elements available.
     ///
     /// Complexity is `O(n)` where `n` is the length of the iterator.
     /// For slices, prefer [`SliceRandom::choose_multiple`].
     </span><span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
     #[cfg_attr(doc_cfg, doc(cfg(feature = <span class="string">&quot;alloc&quot;</span>)))]
     </span><span class="kw">fn </span>choose_multiple&lt;R&gt;(<span class="kw-2">mut </span><span class="self">self</span>, rng: <span class="kw-2">&amp;mut </span>R, amount: usize) -&gt; Vec&lt;<span class="self">Self</span>::Item&gt;
     <span class="kw">where </span>R: Rng + <span class="question-mark">?</span>Sized {
         <span class="kw">let </span><span class="kw-2">mut </span>reservoir = Vec::with_capacity(amount);
         reservoir.extend(<span class="self">self</span>.by_ref().take(amount));

         <span class="comment">// Continue unless the iterator was exhausted
         //
         // note: this prevents iterators that &quot;restart&quot; from causing problems.
         // If the iterator stops once, then so do we.
         </span><span class="kw">if </span>reservoir.len() == amount {
             <span class="kw">for </span>(i, elem) <span class="kw">in </span><span class="self">self</span>.enumerate() {
                 <span class="kw">let </span>k = gen_index(rng, i + <span class="number">1 </span>+ amount);
                 <span class="kw">if let </span><span class="prelude-val">Some</span>(slot) = reservoir.get_mut(k) {
                     <span class="kw-2">*</span>slot = elem;
                 }
             }
         } <span class="kw">else </span>{
             <span class="comment">// Don&#39;t hang onto extra memory. There is a corner case where
             // `amount` was much less than `self.len()`.
             </span>reservoir.shrink_to_fit();
         }
         reservoir
     }
 }


 <span class="kw">impl</span>&lt;T&gt; SliceRandom <span class="kw">for </span>[T] {
     <span class="kw">type </span>Item = T;

     <span class="kw">fn </span>choose&lt;R&gt;(<span class="kw-2">&amp;</span><span class="self">self</span>, rng: <span class="kw-2">&amp;mut </span>R) -&gt; <span class="prelude-ty">Option</span>&lt;<span class="kw-2">&amp;</span><span class="self">Self</span>::Item&gt;
     <span class="kw">where </span>R: Rng + <span class="question-mark">?</span>Sized {
         <span class="kw">if </span><span class="self">self</span>.is_empty() {
             <span class="prelude-val">None
         </span>} <span class="kw">else </span>{
             <span class="prelude-val">Some</span>(<span class="kw-2">&amp;</span><span class="self">self</span>[gen_index(rng, <span class="self">self</span>.len())])
         }
     }

     <span class="kw">fn </span>choose_mut&lt;R&gt;(<span class="kw-2">&amp;mut </span><span class="self">self</span>, rng: <span class="kw-2">&amp;mut </span>R) -&gt; <span class="prelude-ty">Option</span>&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>::Item&gt;
     <span class="kw">where </span>R: Rng + <span class="question-mark">?</span>Sized {
         <span class="kw">if </span><span class="self">self</span>.is_empty() {
             <span class="prelude-val">None
         </span>} <span class="kw">else </span>{
             <span class="kw">let </span>len = <span class="self">self</span>.len();
             <span class="prelude-val">Some</span>(<span class="kw-2">&amp;mut </span><span class="self">self</span>[gen_index(rng, len)])
         }
     }

     <span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
     </span><span class="kw">fn </span>choose_multiple&lt;R&gt;(<span class="kw-2">&amp;</span><span class="self">self</span>, rng: <span class="kw-2">&amp;mut </span>R, amount: usize) -&gt; SliceChooseIter&lt;<span class="self">Self</span>, <span class="self">Self</span>::Item&gt;
     <span class="kw">where </span>R: Rng + <span class="question-mark">?</span>Sized {
         <span class="kw">let </span>amount = ::core::cmp::min(amount, <span class="self">self</span>.len());
         SliceChooseIter {
             slice: <span class="self">self</span>,
             _phantom: Default::default(),
             indices: index::sample(rng, <span class="self">self</span>.len(), amount).into_iter(),
         }
     }

     <span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
     </span><span class="kw">fn </span>choose_weighted&lt;R, F, B, X&gt;(
         <span class="kw-2">&amp;</span><span class="self">self</span>, rng: <span class="kw-2">&amp;mut </span>R, weight: F,
     ) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="kw-2">&amp;</span><span class="self">Self</span>::Item, WeightedError&gt;
     <span class="kw">where
         </span>R: Rng + <span class="question-mark">?</span>Sized,
         F: Fn(<span class="kw-2">&amp;</span><span class="self">Self</span>::Item) -&gt; B,
         B: SampleBorrow&lt;X&gt;,
         X: SampleUniform
             + <span class="kw">for</span>&lt;<span class="lifetime">&#39;a</span>&gt; ::core::ops::AddAssign&lt;<span class="kw-2">&amp;</span><span class="lifetime">&#39;a </span>X&gt;
             + ::core::cmp::PartialOrd&lt;X&gt;
             + Clone
             + Default,
     {
         <span class="kw">use </span><span class="kw">crate</span>::distributions::{Distribution, WeightedIndex};
         <span class="kw">let </span>distr = WeightedIndex::new(<span class="self">self</span>.iter().map(weight))<span class="question-mark">?</span>;
         <span class="prelude-val">Ok</span>(<span class="kw-2">&amp;</span><span class="self">self</span>[distr.sample(rng)])
     }

     <span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
     </span><span class="kw">fn </span>choose_weighted_mut&lt;R, F, B, X&gt;(
         <span class="kw-2">&amp;mut </span><span class="self">self</span>, rng: <span class="kw-2">&amp;mut </span>R, weight: F,
     ) -&gt; <span class="prelude-ty">Result</span>&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>::Item, WeightedError&gt;
     <span class="kw">where
         </span>R: Rng + <span class="question-mark">?</span>Sized,
         F: Fn(<span class="kw-2">&amp;</span><span class="self">Self</span>::Item) -&gt; B,
         B: SampleBorrow&lt;X&gt;,
         X: SampleUniform
             + <span class="kw">for</span>&lt;<span class="lifetime">&#39;a</span>&gt; ::core::ops::AddAssign&lt;<span class="kw-2">&amp;</span><span class="lifetime">&#39;a </span>X&gt;
             + ::core::cmp::PartialOrd&lt;X&gt;
             + Clone
             + Default,
     {
         <span class="kw">use </span><span class="kw">crate</span>::distributions::{Distribution, WeightedIndex};
         <span class="kw">let </span>distr = WeightedIndex::new(<span class="self">self</span>.iter().map(weight))<span class="question-mark">?</span>;
         <span class="prelude-val">Ok</span>(<span class="kw-2">&amp;mut </span><span class="self">self</span>[distr.sample(rng)])
     }

     <span class="attribute">#[cfg(feature = <span class="string">&quot;std&quot;</span>)]
     </span><span class="kw">fn </span>choose_multiple_weighted&lt;R, F, X&gt;(
         <span class="kw-2">&amp;</span><span class="self">self</span>, rng: <span class="kw-2">&amp;mut </span>R, amount: usize, weight: F,
     ) -&gt; <span class="prelude-ty">Result</span>&lt;SliceChooseIter&lt;<span class="self">Self</span>, <span class="self">Self</span>::Item&gt;, WeightedError&gt;
     <span class="kw">where
         </span>R: Rng + <span class="question-mark">?</span>Sized,
         F: Fn(<span class="kw-2">&amp;</span><span class="self">Self</span>::Item) -&gt; X,
         X: Into&lt;f64&gt;,
     {
         <span class="kw">let </span>amount = ::core::cmp::min(amount, <span class="self">self</span>.len());
         <span class="prelude-val">Ok</span>(SliceChooseIter {
             slice: <span class="self">self</span>,
             _phantom: Default::default(),
             indices: index::sample_weighted(
                 rng,
                 <span class="self">self</span>.len(),
                 |idx| weight(<span class="kw-2">&amp;</span><span class="self">self</span>[idx]).into(),
                 amount,
             )<span class="question-mark">?
             </span>.into_iter(),
         })
     }

     <span class="kw">fn </span>shuffle&lt;R&gt;(<span class="kw-2">&amp;mut </span><span class="self">self</span>, rng: <span class="kw-2">&amp;mut </span>R)
     <span class="kw">where </span>R: Rng + <span class="question-mark">?</span>Sized {
         <span class="kw">for </span>i <span class="kw">in </span>(<span class="number">1</span>..<span class="self">self</span>.len()).rev() {
             <span class="comment">// invariant: elements with index &gt; i have been locked in place.
             </span><span class="self">self</span>.swap(i, gen_index(rng, i + <span class="number">1</span>));
         }
     }

     <span class="kw">fn </span>partial_shuffle&lt;R&gt;(
         <span class="kw-2">&amp;mut </span><span class="self">self</span>, rng: <span class="kw-2">&amp;mut </span>R, amount: usize,
     ) -&gt; (<span class="kw-2">&amp;mut </span>[<span class="self">Self</span>::Item], <span class="kw-2">&amp;mut </span>[<span class="self">Self</span>::Item])
     <span class="kw">where </span>R: Rng + <span class="question-mark">?</span>Sized {
         <span class="comment">// This applies Durstenfeld&#39;s algorithm for the
         // [Fisher–Yates shuffle](https://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle#The_modern_algorithm)
         // for an unbiased permutation, but exits early after choosing `amount`
         // elements.

         </span><span class="kw">let </span>len = <span class="self">self</span>.len();
         <span class="kw">let </span>end = <span class="kw">if </span>amount &gt;= len { <span class="number">0 </span>} <span class="kw">else </span>{ len - amount };

         <span class="kw">for </span>i <span class="kw">in </span>(end..len).rev() {
             <span class="comment">// invariant: elements with index &gt; i have been locked in place.
             </span><span class="self">self</span>.swap(i, gen_index(rng, i + <span class="number">1</span>));
         }
         <span class="kw">let </span>r = <span class="self">self</span>.split_at_mut(end);
         (r.<span class="number">1</span>, r.<span class="number">0</span>)
     }
 }

 <span class="kw">impl</span>&lt;I&gt; IteratorRandom <span class="kw">for </span>I <span class="kw">where </span>I: Iterator + Sized {}


 <span class="doccomment">/// An iterator over multiple slice elements.
 ///
 /// This struct is created by
 /// [`SliceRandom::choose_multiple`](trait.SliceRandom.html#tymethod.choose_multiple).
 </span><span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
 #[cfg_attr(doc_cfg, doc(cfg(feature = <span class="string">&quot;alloc&quot;</span>)))]
 #[derive(Debug)]
 </span><span class="kw">pub struct </span>SliceChooseIter&lt;<span class="lifetime">&#39;a</span>, S: <span class="question-mark">?</span>Sized + <span class="lifetime">&#39;a</span>, T: <span class="lifetime">&#39;a</span>&gt; {
     slice: <span class="kw-2">&amp;</span><span class="lifetime">&#39;a </span>S,
     _phantom: ::core::marker::PhantomData&lt;T&gt;,
     indices: index::IndexVecIntoIter,
 }

 <span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
 </span><span class="kw">impl</span>&lt;<span class="lifetime">&#39;a</span>, S: Index&lt;usize, Output = T&gt; + <span class="question-mark">?</span>Sized + <span class="lifetime">&#39;a</span>, T: <span class="lifetime">&#39;a</span>&gt; Iterator <span class="kw">for </span>SliceChooseIter&lt;<span class="lifetime">&#39;a</span>, S, T&gt; {
     <span class="kw">type </span>Item = <span class="kw-2">&amp;</span><span class="lifetime">&#39;a </span>T;

     <span class="kw">fn </span>next(<span class="kw-2">&amp;mut </span><span class="self">self</span>) -&gt; <span class="prelude-ty">Option</span>&lt;<span class="self">Self</span>::Item&gt; {
         <span class="comment">// TODO: investigate using SliceIndex::get_unchecked when stable
         </span><span class="self">self</span>.indices.next().map(|i| <span class="kw-2">&amp;</span><span class="self">self</span>.slice[i <span class="kw">as </span>usize])
     }

     <span class="kw">fn </span>size_hint(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; (usize, <span class="prelude-ty">Option</span>&lt;usize&gt;) {
         (<span class="self">self</span>.indices.len(), <span class="prelude-val">Some</span>(<span class="self">self</span>.indices.len()))
     }
 }

 <span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
 </span><span class="kw">impl</span>&lt;<span class="lifetime">&#39;a</span>, S: Index&lt;usize, Output = T&gt; + <span class="question-mark">?</span>Sized + <span class="lifetime">&#39;a</span>, T: <span class="lifetime">&#39;a</span>&gt; ExactSizeIterator
     <span class="kw">for </span>SliceChooseIter&lt;<span class="lifetime">&#39;a</span>, S, T&gt;
 {
     <span class="kw">fn </span>len(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; usize {
         <span class="self">self</span>.indices.len()
     }
 }


 <span class="comment">// Sample a number uniformly between 0 and `ubound`. Uses 32-bit sampling where
 // possible, primarily in order to produce the same output on 32-bit and 64-bit
 // platforms.
 </span><span class="attribute">#[inline]
 </span><span class="kw">fn </span>gen_index&lt;R: Rng + <span class="question-mark">?</span>Sized&gt;(rng: <span class="kw-2">&amp;mut </span>R, ubound: usize) -&gt; usize {
     <span class="kw">if </span>ubound &lt;= (core::u32::MAX <span class="kw">as </span>usize) {
         rng.gen_range(<span class="number">0</span>..ubound <span class="kw">as </span>u32) <span class="kw">as </span>usize
     } <span class="kw">else </span>{
         rng.gen_range(<span class="number">0</span>..ubound)
     }
 }


 <span class="attribute">#[cfg(test)]
 </span><span class="kw">mod </span>test {
     <span class="kw">use super</span>::<span class="kw-2">*</span>;
     <span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)] </span><span class="kw">use </span><span class="kw">crate</span>::Rng;
     <span class="attribute">#[cfg(all(feature = <span class="string">&quot;alloc&quot;</span>, not(feature = <span class="string">&quot;std&quot;</span>)))] </span><span class="kw">use </span>alloc::vec::Vec;

     <span class="attribute">#[test]
     </span><span class="kw">fn </span>test_slice_choose() {
         <span class="kw">let </span><span class="kw-2">mut </span>r = <span class="kw">crate</span>::test::rng(<span class="number">107</span>);
         <span class="kw">let </span>chars = [
             <span class="string">&#39;a&#39;</span>, <span class="string">&#39;b&#39;</span>, <span class="string">&#39;c&#39;</span>, <span class="string">&#39;d&#39;</span>, <span class="string">&#39;e&#39;</span>, <span class="string">&#39;f&#39;</span>, <span class="string">&#39;g&#39;</span>, <span class="string">&#39;h&#39;</span>, <span class="string">&#39;i&#39;</span>, <span class="string">&#39;j&#39;</span>, <span class="string">&#39;k&#39;</span>, <span class="string">&#39;l&#39;</span>, <span class="string">&#39;m&#39;</span>, <span class="string">&#39;n&#39;</span>,
         ];
         <span class="kw">let </span><span class="kw-2">mut </span>chosen = [<span class="number">0i32</span>; <span class="number">14</span>];
         <span class="comment">// The below all use a binomial distribution with n=1000, p=1/14.
         // binocdf(40, 1000, 1/14) ~= 2e-5; 1-binocdf(106, ..) ~= 2e-5
         </span><span class="kw">for _ in </span><span class="number">0</span>..<span class="number">1000 </span>{
             <span class="kw">let </span>picked = <span class="kw-2">*</span>chars.choose(<span class="kw-2">&amp;mut </span>r).unwrap();
             chosen[(picked <span class="kw">as </span>usize) - (<span class="string">&#39;a&#39; </span><span class="kw">as </span>usize)] += <span class="number">1</span>;
         }
         <span class="kw">for </span>count <span class="kw">in </span>chosen.iter() {
             <span class="macro">assert!</span>(<span class="number">40 </span>&lt; <span class="kw-2">*</span>count &amp;&amp; <span class="kw-2">*</span>count &lt; <span class="number">106</span>);
         }

         chosen.iter_mut().for_each(|x| <span class="kw-2">*</span>x = <span class="number">0</span>);
         <span class="kw">for _ in </span><span class="number">0</span>..<span class="number">1000 </span>{
             <span class="kw-2">*</span>chosen.choose_mut(<span class="kw-2">&amp;mut </span>r).unwrap() += <span class="number">1</span>;
         }
         <span class="kw">for </span>count <span class="kw">in </span>chosen.iter() {
             <span class="macro">assert!</span>(<span class="number">40 </span>&lt; <span class="kw-2">*</span>count &amp;&amp; <span class="kw-2">*</span>count &lt; <span class="number">106</span>);
         }

         <span class="kw">let </span><span class="kw-2">mut </span>v: [isize; <span class="number">0</span>] = [];
         <span class="macro">assert_eq!</span>(v.choose(<span class="kw-2">&amp;mut </span>r), <span class="prelude-val">None</span>);
         <span class="macro">assert_eq!</span>(v.choose_mut(<span class="kw-2">&amp;mut </span>r), <span class="prelude-val">None</span>);
     }

     <span class="attribute">#[test]
     </span><span class="kw">fn </span>value_stability_slice() {
         <span class="kw">let </span><span class="kw-2">mut </span>r = <span class="kw">crate</span>::test::rng(<span class="number">413</span>);
         <span class="kw">let </span>chars = [
             <span class="string">&#39;a&#39;</span>, <span class="string">&#39;b&#39;</span>, <span class="string">&#39;c&#39;</span>, <span class="string">&#39;d&#39;</span>, <span class="string">&#39;e&#39;</span>, <span class="string">&#39;f&#39;</span>, <span class="string">&#39;g&#39;</span>, <span class="string">&#39;h&#39;</span>, <span class="string">&#39;i&#39;</span>, <span class="string">&#39;j&#39;</span>, <span class="string">&#39;k&#39;</span>, <span class="string">&#39;l&#39;</span>, <span class="string">&#39;m&#39;</span>, <span class="string">&#39;n&#39;</span>,
         ];
         <span class="kw">let </span><span class="kw-2">mut </span>nums = [<span class="number">0</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>, <span class="number">7</span>, <span class="number">8</span>, <span class="number">9</span>, <span class="number">10</span>, <span class="number">11</span>, <span class="number">12</span>];

         <span class="macro">assert_eq!</span>(chars.choose(<span class="kw-2">&amp;mut </span>r), <span class="prelude-val">Some</span>(<span class="kw-2">&amp;</span><span class="string">&#39;l&#39;</span>));
         <span class="macro">assert_eq!</span>(nums.choose_mut(<span class="kw-2">&amp;mut </span>r), <span class="prelude-val">Some</span>(<span class="kw-2">&amp;mut </span><span class="number">10</span>));

         <span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
         </span><span class="macro">assert_eq!</span>(
             <span class="kw-2">&amp;</span>chars
                 .choose_multiple(<span class="kw-2">&amp;mut </span>r, <span class="number">8</span>)
                 .cloned()
                 .collect::&lt;Vec&lt;char&gt;&gt;(),
             <span class="kw-2">&amp;</span>[<span class="string">&#39;d&#39;</span>, <span class="string">&#39;m&#39;</span>, <span class="string">&#39;b&#39;</span>, <span class="string">&#39;n&#39;</span>, <span class="string">&#39;c&#39;</span>, <span class="string">&#39;k&#39;</span>, <span class="string">&#39;h&#39;</span>, <span class="string">&#39;e&#39;</span>]
         );

         <span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
         </span><span class="macro">assert_eq!</span>(chars.choose_weighted(<span class="kw-2">&amp;mut </span>r, |<span class="kw">_</span>| <span class="number">1</span>), <span class="prelude-val">Ok</span>(<span class="kw-2">&amp;</span><span class="string">&#39;f&#39;</span>));
         <span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
         </span><span class="macro">assert_eq!</span>(nums.choose_weighted_mut(<span class="kw-2">&amp;mut </span>r, |<span class="kw">_</span>| <span class="number">1</span>), <span class="prelude-val">Ok</span>(<span class="kw-2">&amp;mut </span><span class="number">5</span>));

         <span class="kw">let </span><span class="kw-2">mut </span>r = <span class="kw">crate</span>::test::rng(<span class="number">414</span>);
         nums.shuffle(<span class="kw-2">&amp;mut </span>r);
         <span class="macro">assert_eq!</span>(nums, [<span class="number">9</span>, <span class="number">5</span>, <span class="number">3</span>, <span class="number">10</span>, <span class="number">7</span>, <span class="number">12</span>, <span class="number">8</span>, <span class="number">11</span>, <span class="number">6</span>, <span class="number">4</span>, <span class="number">0</span>, <span class="number">2</span>, <span class="number">1</span>]);
         nums = [<span class="number">0</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>, <span class="number">7</span>, <span class="number">8</span>, <span class="number">9</span>, <span class="number">10</span>, <span class="number">11</span>, <span class="number">12</span>];
         <span class="kw">let </span>res = nums.partial_shuffle(<span class="kw-2">&amp;mut </span>r, <span class="number">6</span>);
         <span class="macro">assert_eq!</span>(res.<span class="number">0</span>, <span class="kw-2">&amp;mut </span>[<span class="number">7</span>, <span class="number">4</span>, <span class="number">8</span>, <span class="number">6</span>, <span class="number">9</span>, <span class="number">3</span>]);
         <span class="macro">assert_eq!</span>(res.<span class="number">1</span>, <span class="kw-2">&amp;mut </span>[<span class="number">0</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">12</span>, <span class="number">11</span>, <span class="number">5</span>, <span class="number">10</span>]);
     }

     <span class="attribute">#[derive(Clone)]
     </span><span class="kw">struct </span>UnhintedIterator&lt;I: Iterator + Clone&gt; {
         iter: I,
     }
     <span class="kw">impl</span>&lt;I: Iterator + Clone&gt; Iterator <span class="kw">for </span>UnhintedIterator&lt;I&gt; {
         <span class="kw">type </span>Item = I::Item;

         <span class="kw">fn </span>next(<span class="kw-2">&amp;mut </span><span class="self">self</span>) -&gt; <span class="prelude-ty">Option</span>&lt;<span class="self">Self</span>::Item&gt; {
             <span class="self">self</span>.iter.next()
         }
     }

     <span class="attribute">#[derive(Clone)]
     </span><span class="kw">struct </span>ChunkHintedIterator&lt;I: ExactSizeIterator + Iterator + Clone&gt; {
         iter: I,
         chunk_remaining: usize,
         chunk_size: usize,
         hint_total_size: bool,
     }
     <span class="kw">impl</span>&lt;I: ExactSizeIterator + Iterator + Clone&gt; Iterator <span class="kw">for </span>ChunkHintedIterator&lt;I&gt; {
         <span class="kw">type </span>Item = I::Item;

         <span class="kw">fn </span>next(<span class="kw-2">&amp;mut </span><span class="self">self</span>) -&gt; <span class="prelude-ty">Option</span>&lt;<span class="self">Self</span>::Item&gt; {
             <span class="kw">if </span><span class="self">self</span>.chunk_remaining == <span class="number">0 </span>{
                 <span class="self">self</span>.chunk_remaining = ::core::cmp::min(<span class="self">self</span>.chunk_size, <span class="self">self</span>.iter.len());
             }
             <span class="self">self</span>.chunk_remaining = <span class="self">self</span>.chunk_remaining.saturating_sub(<span class="number">1</span>);

             <span class="self">self</span>.iter.next()
         }

         <span class="kw">fn </span>size_hint(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; (usize, <span class="prelude-ty">Option</span>&lt;usize&gt;) {
             (
                 <span class="self">self</span>.chunk_remaining,
                 <span class="kw">if </span><span class="self">self</span>.hint_total_size {
                     <span class="prelude-val">Some</span>(<span class="self">self</span>.iter.len())
                 } <span class="kw">else </span>{
                     <span class="prelude-val">None
                 </span>},
             )
         }
     }

     <span class="attribute">#[derive(Clone)]
     </span><span class="kw">struct </span>WindowHintedIterator&lt;I: ExactSizeIterator + Iterator + Clone&gt; {
         iter: I,
         window_size: usize,
         hint_total_size: bool,
     }
     <span class="kw">impl</span>&lt;I: ExactSizeIterator + Iterator + Clone&gt; Iterator <span class="kw">for </span>WindowHintedIterator&lt;I&gt; {
         <span class="kw">type </span>Item = I::Item;

         <span class="kw">fn </span>next(<span class="kw-2">&amp;mut </span><span class="self">self</span>) -&gt; <span class="prelude-ty">Option</span>&lt;<span class="self">Self</span>::Item&gt; {
             <span class="self">self</span>.iter.next()
         }

         <span class="kw">fn </span>size_hint(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; (usize, <span class="prelude-ty">Option</span>&lt;usize&gt;) {
             (
                 ::core::cmp::min(<span class="self">self</span>.iter.len(), <span class="self">self</span>.window_size),
                 <span class="kw">if </span><span class="self">self</span>.hint_total_size {
                     <span class="prelude-val">Some</span>(<span class="self">self</span>.iter.len())
                 } <span class="kw">else </span>{
                     <span class="prelude-val">None
                 </span>},
             )
         }
     }

     <span class="attribute">#[test]
     #[cfg_attr(miri, ignore)] </span><span class="comment">// Miri is too slow
     </span><span class="kw">fn </span>test_iterator_choose() {
         <span class="kw">let </span>r = <span class="kw-2">&amp;mut </span><span class="kw">crate</span>::test::rng(<span class="number">109</span>);
         <span class="kw">fn </span>test_iter&lt;R: Rng + <span class="question-mark">?</span>Sized, Iter: Iterator&lt;Item = usize&gt; + Clone&gt;(r: <span class="kw-2">&amp;mut </span>R, iter: Iter) {
             <span class="kw">let </span><span class="kw-2">mut </span>chosen = [<span class="number">0i32</span>; <span class="number">9</span>];
             <span class="kw">for _ in </span><span class="number">0</span>..<span class="number">1000 </span>{
                 <span class="kw">let </span>picked = iter.clone().choose(r).unwrap();
                 chosen[picked] += <span class="number">1</span>;
             }
             <span class="kw">for </span>count <span class="kw">in </span>chosen.iter() {
                 <span class="comment">// Samples should follow Binomial(1000, 1/9)
                 // Octave: binopdf(x, 1000, 1/9) gives the prob of *count == x
                 // Note: have seen 153, which is unlikely but not impossible.
                 </span><span class="macro">assert!</span>(
                     <span class="number">72 </span>&lt; <span class="kw-2">*</span>count &amp;&amp; <span class="kw-2">*</span>count &lt; <span class="number">154</span>,
                     <span class="string">&quot;count not close to 1000/9: {}&quot;</span>,
                     count
                 );
             }
         }

         test_iter(r, <span class="number">0</span>..<span class="number">9</span>);
         test_iter(r, [<span class="number">0</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>, <span class="number">7</span>, <span class="number">8</span>].iter().cloned());
         <span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
         </span>test_iter(r, (<span class="number">0</span>..<span class="number">9</span>).collect::&lt;Vec&lt;<span class="kw">_</span>&gt;&gt;().into_iter());
         test_iter(r, UnhintedIterator { iter: <span class="number">0</span>..<span class="number">9 </span>});
         test_iter(r, ChunkHintedIterator {
             iter: <span class="number">0</span>..<span class="number">9</span>,
             chunk_size: <span class="number">4</span>,
             chunk_remaining: <span class="number">4</span>,
             hint_total_size: <span class="bool-val">false</span>,
         });
         test_iter(r, ChunkHintedIterator {
             iter: <span class="number">0</span>..<span class="number">9</span>,
             chunk_size: <span class="number">4</span>,
             chunk_remaining: <span class="number">4</span>,
             hint_total_size: <span class="bool-val">true</span>,
         });
         test_iter(r, WindowHintedIterator {
             iter: <span class="number">0</span>..<span class="number">9</span>,
             window_size: <span class="number">2</span>,
             hint_total_size: <span class="bool-val">false</span>,
         });
         test_iter(r, WindowHintedIterator {
             iter: <span class="number">0</span>..<span class="number">9</span>,
             window_size: <span class="number">2</span>,
             hint_total_size: <span class="bool-val">true</span>,
         });

         <span class="macro">assert_eq!</span>((<span class="number">0</span>..<span class="number">0</span>).choose(r), <span class="prelude-val">None</span>);
         <span class="macro">assert_eq!</span>(UnhintedIterator { iter: <span class="number">0</span>..<span class="number">0 </span>}.choose(r), <span class="prelude-val">None</span>);
     }

     <span class="attribute">#[test]
     #[cfg_attr(miri, ignore)] </span><span class="comment">// Miri is too slow
     </span><span class="kw">fn </span>test_iterator_choose_stable() {
         <span class="kw">let </span>r = <span class="kw-2">&amp;mut </span><span class="kw">crate</span>::test::rng(<span class="number">109</span>);
         <span class="kw">fn </span>test_iter&lt;R: Rng + <span class="question-mark">?</span>Sized, Iter: Iterator&lt;Item = usize&gt; + Clone&gt;(r: <span class="kw-2">&amp;mut </span>R, iter: Iter) {
             <span class="kw">let </span><span class="kw-2">mut </span>chosen = [<span class="number">0i32</span>; <span class="number">9</span>];
             <span class="kw">for _ in </span><span class="number">0</span>..<span class="number">1000 </span>{
                 <span class="kw">let </span>picked = iter.clone().choose_stable(r).unwrap();
                 chosen[picked] += <span class="number">1</span>;
             }
             <span class="kw">for </span>count <span class="kw">in </span>chosen.iter() {
                 <span class="comment">// Samples should follow Binomial(1000, 1/9)
                 // Octave: binopdf(x, 1000, 1/9) gives the prob of *count == x
                 // Note: have seen 153, which is unlikely but not impossible.
                 </span><span class="macro">assert!</span>(
                     <span class="number">72 </span>&lt; <span class="kw-2">*</span>count &amp;&amp; <span class="kw-2">*</span>count &lt; <span class="number">154</span>,
                     <span class="string">&quot;count not close to 1000/9: {}&quot;</span>,
                     count
                 );
             }
         }

         test_iter(r, <span class="number">0</span>..<span class="number">9</span>);
         test_iter(r, [<span class="number">0</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>, <span class="number">7</span>, <span class="number">8</span>].iter().cloned());
         <span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
         </span>test_iter(r, (<span class="number">0</span>..<span class="number">9</span>).collect::&lt;Vec&lt;<span class="kw">_</span>&gt;&gt;().into_iter());
         test_iter(r, UnhintedIterator { iter: <span class="number">0</span>..<span class="number">9 </span>});
         test_iter(r, ChunkHintedIterator {
             iter: <span class="number">0</span>..<span class="number">9</span>,
             chunk_size: <span class="number">4</span>,
             chunk_remaining: <span class="number">4</span>,
             hint_total_size: <span class="bool-val">false</span>,
         });
         test_iter(r, ChunkHintedIterator {
             iter: <span class="number">0</span>..<span class="number">9</span>,
             chunk_size: <span class="number">4</span>,
             chunk_remaining: <span class="number">4</span>,
             hint_total_size: <span class="bool-val">true</span>,
         });
         test_iter(r, WindowHintedIterator {
             iter: <span class="number">0</span>..<span class="number">9</span>,
             window_size: <span class="number">2</span>,
             hint_total_size: <span class="bool-val">false</span>,
         });
         test_iter(r, WindowHintedIterator {
             iter: <span class="number">0</span>..<span class="number">9</span>,
             window_size: <span class="number">2</span>,
             hint_total_size: <span class="bool-val">true</span>,
         });

         <span class="macro">assert_eq!</span>((<span class="number">0</span>..<span class="number">0</span>).choose(r), <span class="prelude-val">None</span>);
         <span class="macro">assert_eq!</span>(UnhintedIterator { iter: <span class="number">0</span>..<span class="number">0 </span>}.choose(r), <span class="prelude-val">None</span>);
     }

     <span class="attribute">#[test]
     #[cfg_attr(miri, ignore)] </span><span class="comment">// Miri is too slow
     </span><span class="kw">fn </span>test_iterator_choose_stable_stability() {
         <span class="kw">fn </span>test_iter(iter: <span class="kw">impl </span>Iterator&lt;Item = usize&gt; + Clone) -&gt; [i32; <span class="number">9</span>] {
             <span class="kw">let </span>r = <span class="kw-2">&amp;mut </span><span class="kw">crate</span>::test::rng(<span class="number">109</span>);
             <span class="kw">let </span><span class="kw-2">mut </span>chosen = [<span class="number">0i32</span>; <span class="number">9</span>];
             <span class="kw">for _ in </span><span class="number">0</span>..<span class="number">1000 </span>{
                 <span class="kw">let </span>picked = iter.clone().choose_stable(r).unwrap();
                 chosen[picked] += <span class="number">1</span>;
             }
             chosen
         }

         <span class="kw">let </span>reference = test_iter(<span class="number">0</span>..<span class="number">9</span>);
         <span class="macro">assert_eq!</span>(test_iter([<span class="number">0</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>, <span class="number">7</span>, <span class="number">8</span>].iter().cloned()), reference);

         <span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
         </span><span class="macro">assert_eq!</span>(test_iter((<span class="number">0</span>..<span class="number">9</span>).collect::&lt;Vec&lt;<span class="kw">_</span>&gt;&gt;().into_iter()), reference);
         <span class="macro">assert_eq!</span>(test_iter(UnhintedIterator { iter: <span class="number">0</span>..<span class="number">9 </span>}), reference);
         <span class="macro">assert_eq!</span>(test_iter(ChunkHintedIterator {
             iter: <span class="number">0</span>..<span class="number">9</span>,
             chunk_size: <span class="number">4</span>,
             chunk_remaining: <span class="number">4</span>,
             hint_total_size: <span class="bool-val">false</span>,
         }), reference);
         <span class="macro">assert_eq!</span>(test_iter(ChunkHintedIterator {
             iter: <span class="number">0</span>..<span class="number">9</span>,
             chunk_size: <span class="number">4</span>,
             chunk_remaining: <span class="number">4</span>,
             hint_total_size: <span class="bool-val">true</span>,
         }), reference);
         <span class="macro">assert_eq!</span>(test_iter(WindowHintedIterator {
             iter: <span class="number">0</span>..<span class="number">9</span>,
             window_size: <span class="number">2</span>,
             hint_total_size: <span class="bool-val">false</span>,
         }), reference);
         <span class="macro">assert_eq!</span>(test_iter(WindowHintedIterator {
             iter: <span class="number">0</span>..<span class="number">9</span>,
             window_size: <span class="number">2</span>,
             hint_total_size: <span class="bool-val">true</span>,
         }), reference);
     }

     <span class="attribute">#[test]
     #[cfg_attr(miri, ignore)] </span><span class="comment">// Miri is too slow
     </span><span class="kw">fn </span>test_shuffle() {
         <span class="kw">let </span><span class="kw-2">mut </span>r = <span class="kw">crate</span>::test::rng(<span class="number">108</span>);
         <span class="kw">let </span>empty: <span class="kw-2">&amp;mut </span>[isize] = <span class="kw-2">&amp;mut </span>[];
         empty.shuffle(<span class="kw-2">&amp;mut </span>r);
         <span class="kw">let </span><span class="kw-2">mut </span>one = [<span class="number">1</span>];
         one.shuffle(<span class="kw-2">&amp;mut </span>r);
         <span class="kw">let </span>b: <span class="kw-2">&amp;</span>[<span class="kw">_</span>] = <span class="kw-2">&amp;</span>[<span class="number">1</span>];
         <span class="macro">assert_eq!</span>(one, b);

         <span class="kw">let </span><span class="kw-2">mut </span>two = [<span class="number">1</span>, <span class="number">2</span>];
         two.shuffle(<span class="kw-2">&amp;mut </span>r);
         <span class="macro">assert!</span>(two == [<span class="number">1</span>, <span class="number">2</span>] || two == [<span class="number">2</span>, <span class="number">1</span>]);

         <span class="kw">fn </span>move_last(slice: <span class="kw-2">&amp;mut </span>[usize], pos: usize) {
             <span class="comment">// use slice[pos..].rotate_left(1); once we can use that
             </span><span class="kw">let </span>last_val = slice[pos];
             <span class="kw">for </span>i <span class="kw">in </span>pos..slice.len() - <span class="number">1 </span>{
                 slice[i] = slice[i + <span class="number">1</span>];
             }
             <span class="kw-2">*</span>slice.last_mut().unwrap() = last_val;
         }
         <span class="kw">let </span><span class="kw-2">mut </span>counts = [<span class="number">0i32</span>; <span class="number">24</span>];
         <span class="kw">for _ in </span><span class="number">0</span>..<span class="number">10000 </span>{
             <span class="kw">let </span><span class="kw-2">mut </span>arr: [usize; <span class="number">4</span>] = [<span class="number">0</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>];
             arr.shuffle(<span class="kw-2">&amp;mut </span>r);
             <span class="kw">let </span><span class="kw-2">mut </span>permutation = <span class="number">0usize</span>;
             <span class="kw">let </span><span class="kw-2">mut </span>pos_value = counts.len();
             <span class="kw">for </span>i <span class="kw">in </span><span class="number">0</span>..<span class="number">4 </span>{
                 pos_value /= <span class="number">4 </span>- i;
                 <span class="kw">let </span>pos = arr.iter().position(|<span class="kw-2">&amp;</span>x| x == i).unwrap();
                 <span class="macro">assert!</span>(pos &lt; (<span class="number">4 </span>- i));
                 permutation += pos * pos_value;
                 move_last(<span class="kw-2">&amp;mut </span>arr, pos);
                 <span class="macro">assert_eq!</span>(arr[<span class="number">3</span>], i);
             }
             <span class="kw">for </span>(i, <span class="kw-2">&amp;</span>a) <span class="kw">in </span>arr.iter().enumerate() {
                 <span class="macro">assert_eq!</span>(a, i);
             }
             counts[permutation] += <span class="number">1</span>;
         }
         <span class="kw">for </span>count <span class="kw">in </span>counts.iter() {
             <span class="comment">// Binomial(10000, 1/24) with average 416.667
             // Octave: binocdf(n, 10000, 1/24)
             // 99.9% chance samples lie within this range:
             </span><span class="macro">assert!</span>(<span class="number">352 </span>&lt;= <span class="kw-2">*</span>count &amp;&amp; <span class="kw-2">*</span>count &lt;= <span class="number">483</span>, <span class="string">&quot;count: {}&quot;</span>, count);
         }
     }

     <span class="attribute">#[test]
     </span><span class="kw">fn </span>test_partial_shuffle() {
         <span class="kw">let </span><span class="kw-2">mut </span>r = <span class="kw">crate</span>::test::rng(<span class="number">118</span>);

         <span class="kw">let </span><span class="kw-2">mut </span>empty: [u32; <span class="number">0</span>] = [];
         <span class="kw">let </span>res = empty.partial_shuffle(<span class="kw-2">&amp;mut </span>r, <span class="number">10</span>);
         <span class="macro">assert_eq!</span>((res.<span class="number">0</span>.len(), res.<span class="number">1</span>.len()), (<span class="number">0</span>, <span class="number">0</span>));

         <span class="kw">let </span><span class="kw-2">mut </span>v = [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>];
         <span class="kw">let </span>res = v.partial_shuffle(<span class="kw-2">&amp;mut </span>r, <span class="number">2</span>);
         <span class="macro">assert_eq!</span>((res.<span class="number">0</span>.len(), res.<span class="number">1</span>.len()), (<span class="number">2</span>, <span class="number">3</span>));
         <span class="macro">assert!</span>(res.<span class="number">0</span>[<span class="number">0</span>] != res.<span class="number">0</span>[<span class="number">1</span>]);
         <span class="comment">// First elements are only modified if selected, so at least one isn&#39;t modified:
         </span><span class="macro">assert!</span>(res.<span class="number">1</span>[<span class="number">0</span>] == <span class="number">1 </span>|| res.<span class="number">1</span>[<span class="number">1</span>] == <span class="number">2 </span>|| res.<span class="number">1</span>[<span class="number">2</span>] == <span class="number">3</span>);
     }

     <span class="attribute">#[test]
     #[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
     </span><span class="kw">fn </span>test_sample_iter() {
         <span class="kw">let </span>min_val = <span class="number">1</span>;
         <span class="kw">let </span>max_val = <span class="number">100</span>;

         <span class="kw">let </span><span class="kw-2">mut </span>r = <span class="kw">crate</span>::test::rng(<span class="number">401</span>);
         <span class="kw">let </span>vals = (min_val..max_val).collect::&lt;Vec&lt;i32&gt;&gt;();
         <span class="kw">let </span>small_sample = vals.iter().choose_multiple(<span class="kw-2">&amp;mut </span>r, <span class="number">5</span>);
         <span class="kw">let </span>large_sample = vals.iter().choose_multiple(<span class="kw-2">&amp;mut </span>r, vals.len() + <span class="number">5</span>);

         <span class="macro">assert_eq!</span>(small_sample.len(), <span class="number">5</span>);
         <span class="macro">assert_eq!</span>(large_sample.len(), vals.len());
         <span class="comment">// no randomization happens when amount &gt;= len
         </span><span class="macro">assert_eq!</span>(large_sample, vals.iter().collect::&lt;Vec&lt;<span class="kw">_</span>&gt;&gt;());

         <span class="macro">assert!</span>(small_sample
             .iter()
             .all(|e| { <span class="kw-2">**</span>e &gt;= min_val &amp;&amp; <span class="kw-2">**</span>e &lt;= max_val }));
     }

     <span class="attribute">#[test]
     #[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
     #[cfg_attr(miri, ignore)] </span><span class="comment">// Miri is too slow
     </span><span class="kw">fn </span>test_weighted() {
         <span class="kw">let </span><span class="kw-2">mut </span>r = <span class="kw">crate</span>::test::rng(<span class="number">406</span>);
         <span class="kw">const </span>N_REPS: u32 = <span class="number">3000</span>;
         <span class="kw">let </span>weights = [<span class="number">1u32</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">0</span>, <span class="number">5</span>, <span class="number">6</span>, <span class="number">7</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>, <span class="number">7</span>];
         <span class="kw">let </span>total_weight = weights.iter().sum::&lt;u32&gt;() <span class="kw">as </span>f32;

         <span class="kw">let </span>verify = |result: [i32; <span class="number">14</span>]| {
             <span class="kw">for </span>(i, count) <span class="kw">in </span>result.iter().enumerate() {
                 <span class="kw">let </span>exp = (weights[i] * N_REPS) <span class="kw">as </span>f32 / total_weight;
                 <span class="kw">let </span><span class="kw-2">mut </span>err = (<span class="kw-2">*</span>count <span class="kw">as </span>f32 - exp).abs();
                 <span class="kw">if </span>err != <span class="number">0.0 </span>{
                     err /= exp;
                 }
                 <span class="macro">assert!</span>(err &lt;= <span class="number">0.25</span>);
             }
         };

         <span class="comment">// choose_weighted
         </span><span class="kw">fn </span>get_weight&lt;T&gt;(item: <span class="kw-2">&amp;</span>(u32, T)) -&gt; u32 {
             item.<span class="number">0
         </span>}
         <span class="kw">let </span><span class="kw-2">mut </span>chosen = [<span class="number">0i32</span>; <span class="number">14</span>];
         <span class="kw">let </span><span class="kw-2">mut </span>items = [(<span class="number">0u32</span>, <span class="number">0usize</span>); <span class="number">14</span>]; <span class="comment">// (weight, index)
         </span><span class="kw">for </span>(i, item) <span class="kw">in </span>items.iter_mut().enumerate() {
             <span class="kw-2">*</span>item = (weights[i], i);
         }
         <span class="kw">for _ in </span><span class="number">0</span>..N_REPS {
             <span class="kw">let </span>item = items.choose_weighted(<span class="kw-2">&amp;mut </span>r, get_weight).unwrap();
             chosen[item.<span class="number">1</span>] += <span class="number">1</span>;
         }
         verify(chosen);

         <span class="comment">// choose_weighted_mut
         </span><span class="kw">let </span><span class="kw-2">mut </span>items = [(<span class="number">0u32</span>, <span class="number">0i32</span>); <span class="number">14</span>]; <span class="comment">// (weight, count)
         </span><span class="kw">for </span>(i, item) <span class="kw">in </span>items.iter_mut().enumerate() {
             <span class="kw-2">*</span>item = (weights[i], <span class="number">0</span>);
         }
         <span class="kw">for _ in </span><span class="number">0</span>..N_REPS {
             items.choose_weighted_mut(<span class="kw-2">&amp;mut </span>r, get_weight).unwrap().<span class="number">1 </span>+= <span class="number">1</span>;
         }
         <span class="kw">for </span>(ch, item) <span class="kw">in </span>chosen.iter_mut().zip(items.iter()) {
             <span class="kw-2">*</span>ch = item.<span class="number">1</span>;
         }
         verify(chosen);

         <span class="comment">// Check error cases
         </span><span class="kw">let </span>empty_slice = <span class="kw-2">&amp;mut </span>[<span class="number">10</span>][<span class="number">0</span>..<span class="number">0</span>];
         <span class="macro">assert_eq!</span>(
             empty_slice.choose_weighted(<span class="kw-2">&amp;mut </span>r, |<span class="kw">_</span>| <span class="number">1</span>),
             <span class="prelude-val">Err</span>(WeightedError::NoItem)
         );
         <span class="macro">assert_eq!</span>(
             empty_slice.choose_weighted_mut(<span class="kw-2">&amp;mut </span>r, |<span class="kw">_</span>| <span class="number">1</span>),
             <span class="prelude-val">Err</span>(WeightedError::NoItem)
         );
         <span class="macro">assert_eq!</span>(
             [<span class="string">&#39;x&#39;</span>].choose_weighted_mut(<span class="kw-2">&amp;mut </span>r, |<span class="kw">_</span>| <span class="number">0</span>),
             <span class="prelude-val">Err</span>(WeightedError::AllWeightsZero)
         );
         <span class="macro">assert_eq!</span>(
             [<span class="number">0</span>, -<span class="number">1</span>].choose_weighted_mut(<span class="kw-2">&amp;mut </span>r, |x| <span class="kw-2">*</span>x),
             <span class="prelude-val">Err</span>(WeightedError::InvalidWeight)
         );
         <span class="macro">assert_eq!</span>(
             [-<span class="number">1</span>, <span class="number">0</span>].choose_weighted_mut(<span class="kw-2">&amp;mut </span>r, |x| <span class="kw-2">*</span>x),
             <span class="prelude-val">Err</span>(WeightedError::InvalidWeight)
         );
     }

     <span class="attribute">#[test]
     </span><span class="kw">fn </span>value_stability_choose() {
         <span class="kw">fn </span>choose&lt;I: Iterator&lt;Item = u32&gt;&gt;(iter: I) -&gt; <span class="prelude-ty">Option</span>&lt;u32&gt; {
             <span class="kw">let </span><span class="kw-2">mut </span>rng = <span class="kw">crate</span>::test::rng(<span class="number">411</span>);
             iter.choose(<span class="kw-2">&amp;mut </span>rng)
         }

         <span class="macro">assert_eq!</span>(choose([].iter().cloned()), <span class="prelude-val">None</span>);
         <span class="macro">assert_eq!</span>(choose(<span class="number">0</span>..<span class="number">100</span>), <span class="prelude-val">Some</span>(<span class="number">33</span>));
         <span class="macro">assert_eq!</span>(choose(UnhintedIterator { iter: <span class="number">0</span>..<span class="number">100 </span>}), <span class="prelude-val">Some</span>(<span class="number">40</span>));
         <span class="macro">assert_eq!</span>(
             choose(ChunkHintedIterator {
                 iter: <span class="number">0</span>..<span class="number">100</span>,
                 chunk_size: <span class="number">32</span>,
                 chunk_remaining: <span class="number">32</span>,
                 hint_total_size: <span class="bool-val">false</span>,
             }),
             <span class="prelude-val">Some</span>(<span class="number">39</span>)
         );
         <span class="macro">assert_eq!</span>(
             choose(ChunkHintedIterator {
                 iter: <span class="number">0</span>..<span class="number">100</span>,
                 chunk_size: <span class="number">32</span>,
                 chunk_remaining: <span class="number">32</span>,
                 hint_total_size: <span class="bool-val">true</span>,
             }),
             <span class="prelude-val">Some</span>(<span class="number">39</span>)
         );
         <span class="macro">assert_eq!</span>(
             choose(WindowHintedIterator {
                 iter: <span class="number">0</span>..<span class="number">100</span>,
                 window_size: <span class="number">32</span>,
                 hint_total_size: <span class="bool-val">false</span>,
             }),
             <span class="prelude-val">Some</span>(<span class="number">90</span>)
         );
         <span class="macro">assert_eq!</span>(
             choose(WindowHintedIterator {
                 iter: <span class="number">0</span>..<span class="number">100</span>,
                 window_size: <span class="number">32</span>,
                 hint_total_size: <span class="bool-val">true</span>,
             }),
             <span class="prelude-val">Some</span>(<span class="number">90</span>)
         );
     }

     <span class="attribute">#[test]
     </span><span class="kw">fn </span>value_stability_choose_stable() {
         <span class="kw">fn </span>choose&lt;I: Iterator&lt;Item = u32&gt;&gt;(iter: I) -&gt; <span class="prelude-ty">Option</span>&lt;u32&gt; {
             <span class="kw">let </span><span class="kw-2">mut </span>rng = <span class="kw">crate</span>::test::rng(<span class="number">411</span>);
             iter.choose_stable(<span class="kw-2">&amp;mut </span>rng)
         }

         <span class="macro">assert_eq!</span>(choose([].iter().cloned()), <span class="prelude-val">None</span>);
         <span class="macro">assert_eq!</span>(choose(<span class="number">0</span>..<span class="number">100</span>), <span class="prelude-val">Some</span>(<span class="number">40</span>));
         <span class="macro">assert_eq!</span>(choose(UnhintedIterator { iter: <span class="number">0</span>..<span class="number">100 </span>}), <span class="prelude-val">Some</span>(<span class="number">40</span>));
         <span class="macro">assert_eq!</span>(
             choose(ChunkHintedIterator {
                 iter: <span class="number">0</span>..<span class="number">100</span>,
                 chunk_size: <span class="number">32</span>,
                 chunk_remaining: <span class="number">32</span>,
                 hint_total_size: <span class="bool-val">false</span>,
             }),
             <span class="prelude-val">Some</span>(<span class="number">40</span>)
         );
         <span class="macro">assert_eq!</span>(
             choose(ChunkHintedIterator {
                 iter: <span class="number">0</span>..<span class="number">100</span>,
                 chunk_size: <span class="number">32</span>,
                 chunk_remaining: <span class="number">32</span>,
                 hint_total_size: <span class="bool-val">true</span>,
             }),
             <span class="prelude-val">Some</span>(<span class="number">40</span>)
         );
         <span class="macro">assert_eq!</span>(
             choose(WindowHintedIterator {
                 iter: <span class="number">0</span>..<span class="number">100</span>,
                 window_size: <span class="number">32</span>,
                 hint_total_size: <span class="bool-val">false</span>,
             }),
             <span class="prelude-val">Some</span>(<span class="number">40</span>)
         );
         <span class="macro">assert_eq!</span>(
             choose(WindowHintedIterator {
                 iter: <span class="number">0</span>..<span class="number">100</span>,
                 window_size: <span class="number">32</span>,
                 hint_total_size: <span class="bool-val">true</span>,
             }),
             <span class="prelude-val">Some</span>(<span class="number">40</span>)
         );
     }

     <span class="attribute">#[test]
     </span><span class="kw">fn </span>value_stability_choose_multiple() {
         <span class="kw">fn </span>do_test&lt;I: Iterator&lt;Item = u32&gt;&gt;(iter: I, v: <span class="kw-2">&amp;</span>[u32]) {
             <span class="kw">let </span><span class="kw-2">mut </span>rng = <span class="kw">crate</span>::test::rng(<span class="number">412</span>);
             <span class="kw">let </span><span class="kw-2">mut </span>buf = [<span class="number">0u32</span>; <span class="number">8</span>];
             <span class="macro">assert_eq!</span>(iter.choose_multiple_fill(<span class="kw-2">&amp;mut </span>rng, <span class="kw-2">&amp;mut </span>buf), v.len());
             <span class="macro">assert_eq!</span>(<span class="kw-2">&amp;</span>buf[<span class="number">0</span>..v.len()], v);
         }

         do_test(<span class="number">0</span>..<span class="number">4</span>, <span class="kw-2">&amp;</span>[<span class="number">0</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>]);
         do_test(<span class="number">0</span>..<span class="number">8</span>, <span class="kw-2">&amp;</span>[<span class="number">0</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>, <span class="number">7</span>]);
         do_test(<span class="number">0</span>..<span class="number">100</span>, <span class="kw-2">&amp;</span>[<span class="number">58</span>, <span class="number">78</span>, <span class="number">80</span>, <span class="number">92</span>, <span class="number">43</span>, <span class="number">8</span>, <span class="number">96</span>, <span class="number">7</span>]);

         <span class="attribute">#[cfg(feature = <span class="string">&quot;alloc&quot;</span>)]
         </span>{
             <span class="kw">fn </span>do_test&lt;I: Iterator&lt;Item = u32&gt;&gt;(iter: I, v: <span class="kw-2">&amp;</span>[u32]) {
                 <span class="kw">let </span><span class="kw-2">mut </span>rng = <span class="kw">crate</span>::test::rng(<span class="number">412</span>);
                 <span class="macro">assert_eq!</span>(iter.choose_multiple(<span class="kw-2">&amp;mut </span>rng, v.len()), v);
             }

             do_test(<span class="number">0</span>..<span class="number">4</span>, <span class="kw-2">&amp;</span>[<span class="number">0</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>]);
             do_test(<span class="number">0</span>..<span class="number">8</span>, <span class="kw-2">&amp;</span>[<span class="number">0</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>, <span class="number">6</span>, <span class="number">7</span>]);
             do_test(<span class="number">0</span>..<span class="number">100</span>, <span class="kw-2">&amp;</span>[<span class="number">58</span>, <span class="number">78</span>, <span class="number">80</span>, <span class="number">92</span>, <span class="number">43</span>, <span class="number">8</span>, <span class="number">96</span>, <span class="number">7</span>]);
         }
     }

     <span class="attribute">#[test]
     #[cfg(feature = <span class="string">&quot;std&quot;</span>)]
     </span><span class="kw">fn </span>test_multiple_weighted_edge_cases() {
         <span class="kw">use super</span>::<span class="kw-2">*</span>;

         <span class="kw">let </span><span class="kw-2">mut </span>rng = <span class="kw">crate</span>::test::rng(<span class="number">413</span>);

         <span class="comment">// Case 1: One of the weights is 0
         </span><span class="kw">let </span>choices = [(<span class="string">&#39;a&#39;</span>, <span class="number">2</span>), (<span class="string">&#39;b&#39;</span>, <span class="number">1</span>), (<span class="string">&#39;c&#39;</span>, <span class="number">0</span>)];
         <span class="kw">for _ in </span><span class="number">0</span>..<span class="number">100 </span>{
             <span class="kw">let </span>result = choices
                 .choose_multiple_weighted(<span class="kw-2">&amp;mut </span>rng, <span class="number">2</span>, |item| item.<span class="number">1</span>)
                 .unwrap()
                 .collect::&lt;Vec&lt;<span class="kw">_</span>&gt;&gt;();

             <span class="macro">assert_eq!</span>(result.len(), <span class="number">2</span>);
             <span class="macro">assert!</span>(!result.iter().any(|val| val.<span class="number">0 </span>== <span class="string">&#39;c&#39;</span>));
         }

         <span class="comment">// Case 2: All of the weights are 0
         </span><span class="kw">let </span>choices = [(<span class="string">&#39;a&#39;</span>, <span class="number">0</span>), (<span class="string">&#39;b&#39;</span>, <span class="number">0</span>), (<span class="string">&#39;c&#39;</span>, <span class="number">0</span>)];

         <span class="macro">assert_eq!</span>(choices
             .choose_multiple_weighted(<span class="kw-2">&amp;mut </span>rng, <span class="number">2</span>, |item| item.<span class="number">1</span>)
             .unwrap().count(), <span class="number">2</span>);

         <span class="comment">// Case 3: Negative weights
         </span><span class="kw">let </span>choices = [(<span class="string">&#39;a&#39;</span>, -<span class="number">1</span>), (<span class="string">&#39;b&#39;</span>, <span class="number">1</span>), (<span class="string">&#39;c&#39;</span>, <span class="number">1</span>)];
         <span class="macro">assert_eq!</span>(
             choices
                 .choose_multiple_weighted(<span class="kw-2">&amp;mut </span>rng, <span class="number">2</span>, |item| item.<span class="number">1</span>)
                 .unwrap_err(),
             WeightedError::InvalidWeight
         );

         <span class="comment">// Case 4: Empty list
         </span><span class="kw">let </span>choices = [];
         <span class="macro">assert_eq!</span>(choices
             .choose_multiple_weighted(<span class="kw-2">&amp;mut </span>rng, <span class="number">0</span>, |<span class="kw">_</span>: <span class="kw-2">&amp;</span>()| <span class="number">0</span>)
             .unwrap().count(), <span class="number">0</span>);

         <span class="comment">// Case 5: NaN weights
         </span><span class="kw">let </span>choices = [(<span class="string">&#39;a&#39;</span>, core::f64::NAN), (<span class="string">&#39;b&#39;</span>, <span class="number">1.0</span>), (<span class="string">&#39;c&#39;</span>, <span class="number">1.0</span>)];
         <span class="macro">assert_eq!</span>(
             choices
                 .choose_multiple_weighted(<span class="kw-2">&amp;mut </span>rng, <span class="number">2</span>, |item| item.<span class="number">1</span>)
                 .unwrap_err(),
             WeightedError::InvalidWeight
         );

         <span class="comment">// Case 6: +infinity weights
         </span><span class="kw">let </span>choices = [(<span class="string">&#39;a&#39;</span>, core::f64::INFINITY), (<span class="string">&#39;b&#39;</span>, <span class="number">1.0</span>), (<span class="string">&#39;c&#39;</span>, <span class="number">1.0</span>)];
         <span class="kw">for _ in </span><span class="number">0</span>..<span class="number">100 </span>{
             <span class="kw">let </span>result = choices
                 .choose_multiple_weighted(<span class="kw-2">&amp;mut </span>rng, <span class="number">2</span>, |item| item.<span class="number">1</span>)
                 .unwrap()
                 .collect::&lt;Vec&lt;<span class="kw">_</span>&gt;&gt;();
             <span class="macro">assert_eq!</span>(result.len(), <span class="number">2</span>);
             <span class="macro">assert!</span>(result.iter().any(|val| val.<span class="number">0 </span>== <span class="string">&#39;a&#39;</span>));
         }

         <span class="comment">// Case 7: -infinity weights
         </span><span class="kw">let </span>choices = [(<span class="string">&#39;a&#39;</span>, core::f64::NEG_INFINITY), (<span class="string">&#39;b&#39;</span>, <span class="number">1.0</span>), (<span class="string">&#39;c&#39;</span>, <span class="number">1.0</span>)];
         <span class="macro">assert_eq!</span>(
             choices
                 .choose_multiple_weighted(<span class="kw-2">&amp;mut </span>rng, <span class="number">2</span>, |item| item.<span class="number">1</span>)
                 .unwrap_err(),
             WeightedError::InvalidWeight
         );

         <span class="comment">// Case 8: -0 weights
         </span><span class="kw">let </span>choices = [(<span class="string">&#39;a&#39;</span>, -<span class="number">0.0</span>), (<span class="string">&#39;b&#39;</span>, <span class="number">1.0</span>), (<span class="string">&#39;c&#39;</span>, <span class="number">1.0</span>)];
         <span class="macro">assert!</span>(choices
             .choose_multiple_weighted(<span class="kw-2">&amp;mut </span>rng, <span class="number">2</span>, |item| item.<span class="number">1</span>)
             .is_ok());
     }

     <span class="attribute">#[test]
     #[cfg(feature = <span class="string">&quot;std&quot;</span>)]
     </span><span class="kw">fn </span>test_multiple_weighted_distributions() {
         <span class="kw">use super</span>::<span class="kw-2">*</span>;

         <span class="comment">// The theoretical probabilities of the different outcomes are:
         // AB: 0.5  * 0.5  = 0.250
         // AC: 0.5  * 0.5  = 0.250
         // BA: 0.25 * 0.67 = 0.167
         // BC: 0.25 * 0.33 = 0.082
         // CA: 0.25 * 0.67 = 0.167
         // CB: 0.25 * 0.33 = 0.082
         </span><span class="kw">let </span>choices = [(<span class="string">&#39;a&#39;</span>, <span class="number">2</span>), (<span class="string">&#39;b&#39;</span>, <span class="number">1</span>), (<span class="string">&#39;c&#39;</span>, <span class="number">1</span>)];
         <span class="kw">let </span><span class="kw-2">mut </span>rng = <span class="kw">crate</span>::test::rng(<span class="number">414</span>);

         <span class="kw">let </span><span class="kw-2">mut </span>results = [<span class="number">0i32</span>; <span class="number">3</span>];
         <span class="kw">let </span>expected_results = [<span class="number">4167</span>, <span class="number">4167</span>, <span class="number">1666</span>];
         <span class="kw">for _ in </span><span class="number">0</span>..<span class="number">10000 </span>{
             <span class="kw">let </span>result = choices
                 .choose_multiple_weighted(<span class="kw-2">&amp;mut </span>rng, <span class="number">2</span>, |item| item.<span class="number">1</span>)
                 .unwrap()
                 .collect::&lt;Vec&lt;<span class="kw">_</span>&gt;&gt;();

             <span class="macro">assert_eq!</span>(result.len(), <span class="number">2</span>);

             <span class="kw">match </span>(result[<span class="number">0</span>].<span class="number">0</span>, result[<span class="number">1</span>].<span class="number">0</span>) {
                 (<span class="string">&#39;a&#39;</span>, <span class="string">&#39;b&#39;</span>) | (<span class="string">&#39;b&#39;</span>, <span class="string">&#39;a&#39;</span>) =&gt; {
                     results[<span class="number">0</span>] += <span class="number">1</span>;
                 }
                 (<span class="string">&#39;a&#39;</span>, <span class="string">&#39;c&#39;</span>) | (<span class="string">&#39;c&#39;</span>, <span class="string">&#39;a&#39;</span>) =&gt; {
                     results[<span class="number">1</span>] += <span class="number">1</span>;
                 }
                 (<span class="string">&#39;b&#39;</span>, <span class="string">&#39;c&#39;</span>) | (<span class="string">&#39;c&#39;</span>, <span class="string">&#39;b&#39;</span>) =&gt; {
                     results[<span class="number">2</span>] += <span class="number">1</span>;
                 }
                 (<span class="kw">_</span>, <span class="kw">_</span>) =&gt; <span class="macro">panic!</span>(<span class="string">&quot;unexpected result&quot;</span>),
             }
         }

         <span class="kw">let </span><span class="kw-2">mut </span>diffs = results
             .iter()
             .zip(<span class="kw-2">&amp;</span>expected_results)
             .map(|(a, b)| (a - b).abs());
         <span class="macro">assert!</span>(!diffs.any(|deviation| deviation &gt; <span class="number">100</span>));
     }
 }
 </code></pre></div>
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