1// Copyright 2018 Developers of the Rand project.
2//
3// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
4// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
5// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
6// option. This file may not be copied, modified, or distributed
7// except according to those terms.
8
9//! The implementations of the `StandardUniform` distribution for other built-in types.
10
11#[cfg(feature = "alloc")]
12use alloc::string::String;
13use core::char;
14use core::num::Wrapping;
15
16#[cfg(feature = "alloc")]
17use crate::distr::SampleString;
18use crate::distr::{Distribution, StandardUniform, Uniform};
19use crate::Rng;
20
21use core::mem::{self, MaybeUninit};
22#[cfg(feature = "simd_support")]
23use core::simd::prelude::*;
24#[cfg(feature = "simd_support")]
25use core::simd::{LaneCount, MaskElement, SupportedLaneCount};
26#[cfg(feature = "serde")]
27use serde::{Deserialize, Serialize};
28
29// ----- Sampling distributions -----
30
31/// Sample a `u8`, uniformly distributed over ASCII letters and numbers:
32/// a-z, A-Z and 0-9.
33///
34/// # Example
35///
36/// ```
37/// use rand::Rng;
38/// use rand::distr::Alphanumeric;
39///
40/// let mut rng = rand::rng();
41/// let chars: String = (0..7).map(|_| rng.sample(Alphanumeric) as char).collect();
42/// println!("Random chars: {}", chars);
43/// ```
44///
45/// The [`SampleString`] trait provides an easier method of generating
46/// a random [`String`], and offers more efficient allocation:
47/// ```
48/// use rand::distr::{Alphanumeric, SampleString};
49/// let string = Alphanumeric.sample_string(&mut rand::rng(), 16);
50/// println!("Random string: {}", string);
51/// ```
52///
53/// # Passwords
54///
55/// Users sometimes ask whether it is safe to use a string of random characters
56/// as a password. In principle, all RNGs in Rand implementing `CryptoRng` are
57/// suitable as a source of randomness for generating passwords (if they are
58/// properly seeded), but it is more conservative to only use randomness
59/// directly from the operating system via the `getrandom` crate, or the
60/// corresponding bindings of a crypto library.
61///
62/// When generating passwords or keys, it is important to consider the threat
63/// model and in some cases the memorability of the password. This is out of
64/// scope of the Rand project, and therefore we defer to the following
65/// references:
66///
67/// - [Wikipedia article on Password Strength](https://en.wikipedia.org/wiki/Password_strength)
68/// - [Diceware for generating memorable passwords](https://en.wikipedia.org/wiki/Diceware)
69#[derive(Debug, Clone, Copy, Default)]
70#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
71pub struct Alphanumeric;
72
73// ----- Implementations of distributions -----
74
75impl Distribution<char> for StandardUniform {
76 #[inline]
77 fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> char {
78 // A valid `char` is either in the interval `[0, 0xD800)` or
79 // `(0xDFFF, 0x11_0000)`. All `char`s must therefore be in
80 // `[0, 0x11_0000)` but not in the "gap" `[0xD800, 0xDFFF]` which is
81 // reserved for surrogates. This is the size of that gap.
82 const GAP_SIZE: u32 = 0xDFFF - 0xD800 + 1;
83
84 // Uniform::new(0, 0x11_0000 - GAP_SIZE) can also be used, but it
85 // seemed slower.
86 let range = Uniform::new(GAP_SIZE, high:0x11_0000).unwrap();
87
88 let mut n = range.sample(rng);
89 if n <= 0xDFFF {
90 n -= GAP_SIZE;
91 }
92 unsafe { char::from_u32_unchecked(n) }
93 }
94}
95
96#[cfg(feature = "alloc")]
97impl SampleString for StandardUniform {
98 fn append_string<R: Rng + ?Sized>(&self, rng: &mut R, s: &mut String, len: usize) {
99 // A char is encoded with at most four bytes, thus this reservation is
100 // guaranteed to be sufficient. We do not shrink_to_fit afterwards so
101 // that repeated usage on the same `String` buffer does not reallocate.
102 s.reserve(4 * len);
103 s.extend(Distribution::<char>::sample_iter(self, rng).take(len));
104 }
105}
106
107impl Distribution<u8> for Alphanumeric {
108 fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> u8 {
109 const RANGE: u32 = 26 + 26 + 10;
110 const GEN_ASCII_STR_CHARSET: &[u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZ\
111 abcdefghijklmnopqrstuvwxyz\
112 0123456789";
113 // We can pick from 62 characters. This is so close to a power of 2, 64,
114 // that we can do better than `Uniform`. Use a simple bitshift and
115 // rejection sampling. We do not use a bitmask, because for small RNGs
116 // the most significant bits are usually of higher quality.
117 loop {
118 let var: u32 = rng.next_u32() >> (32 - 6);
119 if var < RANGE {
120 return GEN_ASCII_STR_CHARSET[var as usize];
121 }
122 }
123 }
124}
125
126#[cfg(feature = "alloc")]
127impl SampleString for Alphanumeric {
128 fn append_string<R: Rng + ?Sized>(&self, rng: &mut R, string: &mut String, len: usize) {
129 unsafe {
130 let v = string.as_mut_vec();
131 v.extend(self.sample_iter(rng).take(len));
132 }
133 }
134}
135
136impl Distribution<bool> for StandardUniform {
137 #[inline]
138 fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> bool {
139 // We can compare against an arbitrary bit of an u32 to get a bool.
140 // Because the least significant bits of a lower quality RNG can have
141 // simple patterns, we compare against the most significant bit. This is
142 // easiest done using a sign test.
143 (rng.next_u32() as i32) < 0
144 }
145}
146
147/// Note that on some hardware like x86/64 mask operations like [`_mm_blendv_epi8`]
148/// only care about a single bit. This means that you could use uniform random bits
149/// directly:
150///
151/// ```ignore
152/// // this may be faster...
153/// let x = unsafe { _mm_blendv_epi8(a.into(), b.into(), rng.random::<__m128i>()) };
154///
155/// // ...than this
156/// let x = rng.random::<mask8x16>().select(b, a);
157/// ```
158///
159/// Since most bits are unused you could also generate only as many bits as you need, i.e.:
160/// ```
161/// #![feature(portable_simd)]
162/// use std::simd::prelude::*;
163/// use rand::prelude::*;
164/// let mut rng = rand::rng();
165///
166/// let x = u16x8::splat(rng.random::<u8>() as u16);
167/// let mask = u16x8::splat(1) << u16x8::from([0, 1, 2, 3, 4, 5, 6, 7]);
168/// let rand_mask = (x & mask).simd_eq(mask);
169/// ```
170///
171/// [`_mm_blendv_epi8`]: https://www.intel.com/content/www/us/en/docs/intrinsics-guide/index.html#text=_mm_blendv_epi8&ig_expand=514/
172/// [`simd_support`]: https://github.com/rust-random/rand#crate-features
173#[cfg(feature = "simd_support")]
174impl<T, const LANES: usize> Distribution<Mask<T, LANES>> for StandardUniform
175where
176 T: MaskElement + Default,
177 LaneCount<LANES>: SupportedLaneCount,
178 StandardUniform: Distribution<Simd<T, LANES>>,
179 Simd<T, LANES>: SimdPartialOrd<Mask = Mask<T, LANES>>,
180{
181 #[inline]
182 fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> Mask<T, LANES> {
183 // `MaskElement` must be a signed integer, so this is equivalent
184 // to the scalar `i32 < 0` method
185 let var = rng.random::<Simd<T, LANES>>();
186 var.simd_lt(Simd::default())
187 }
188}
189
190/// Implement `Distribution<(A, B, C, ...)> for StandardUniform`, using the list of
191/// identifiers
192macro_rules! tuple_impl {
193 ($($tyvar:ident)*) => {
194 impl< $($tyvar,)* > Distribution<($($tyvar,)*)> for StandardUniform
195 where $(
196 StandardUniform: Distribution< $tyvar >,
197 )*
198 {
199 #[inline]
200 fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> ( $($tyvar,)* ) {
201 let out = ($(
202 // use the $tyvar's to get the appropriate number of
203 // repeats (they're not actually needed)
204 rng.random::<$tyvar>()
205 ,)*);
206
207 // Suppress the unused variable warning for empty tuple
208 let _rng = rng;
209
210 out
211 }
212 }
213 }
214}
215
216/// Looping wrapper for `tuple_impl`. Given (A, B, C), it also generates
217/// implementations for (A, B) and (A,)
218macro_rules! tuple_impls {
219 ($($tyvar:ident)*) => {tuple_impls!{[] $($tyvar)*}};
220
221 ([$($prefix:ident)*] $head:ident $($tail:ident)*) => {
222 tuple_impl!{$($prefix)*}
223 tuple_impls!{[$($prefix)* $head] $($tail)*}
224 };
225
226
227 ([$($prefix:ident)*]) => {
228 tuple_impl!{$($prefix)*}
229 };
230
231}
232
233tuple_impls! {A B C D E F G H I J K L}
234
235impl<T, const N: usize> Distribution<[T; N]> for StandardUniform
236where
237 StandardUniform: Distribution<T>,
238{
239 #[inline]
240 fn sample<R: Rng + ?Sized>(&self, _rng: &mut R) -> [T; N] {
241 let mut buff: [MaybeUninit<T>; N] = unsafe { MaybeUninit::uninit().assume_init() };
242
243 for elem in &mut buff {
244 *elem = MaybeUninit::new(_rng.random());
245 }
246
247 unsafe { mem::transmute_copy::<_, _>(&buff) }
248 }
249}
250
251impl<T> Distribution<Wrapping<T>> for StandardUniform
252where
253 StandardUniform: Distribution<T>,
254{
255 #[inline]
256 fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> Wrapping<T> {
257 Wrapping(rng.random())
258 }
259}
260
261#[cfg(test)]
262mod tests {
263 use super::*;
264 use crate::RngCore;
265
266 #[test]
267 fn test_misc() {
268 let rng: &mut dyn RngCore = &mut crate::test::rng(820);
269
270 rng.sample::<char, _>(StandardUniform);
271 rng.sample::<bool, _>(StandardUniform);
272 }
273
274 #[cfg(feature = "alloc")]
275 #[test]
276 fn test_chars() {
277 use core::iter;
278 let mut rng = crate::test::rng(805);
279
280 // Test by generating a relatively large number of chars, so we also
281 // take the rejection sampling path.
282 let word: String = iter::repeat(())
283 .map(|()| rng.random::<char>())
284 .take(1000)
285 .collect();
286 assert!(!word.is_empty());
287 }
288
289 #[test]
290 fn test_alphanumeric() {
291 let mut rng = crate::test::rng(806);
292
293 // Test by generating a relatively large number of chars, so we also
294 // take the rejection sampling path.
295 let mut incorrect = false;
296 for _ in 0..100 {
297 let c: char = rng.sample(Alphanumeric).into();
298 incorrect |= !c.is_ascii_alphanumeric();
299 }
300 assert!(!incorrect);
301 }
302
303 #[test]
304 fn value_stability() {
305 fn test_samples<T: Copy + core::fmt::Debug + PartialEq, D: Distribution<T>>(
306 distr: &D,
307 zero: T,
308 expected: &[T],
309 ) {
310 let mut rng = crate::test::rng(807);
311 let mut buf = [zero; 5];
312 for x in &mut buf {
313 *x = rng.sample(distr);
314 }
315 assert_eq!(&buf, expected);
316 }
317
318 test_samples(
319 &StandardUniform,
320 'a',
321 &[
322 '\u{8cdac}',
323 '\u{a346a}',
324 '\u{80120}',
325 '\u{ed692}',
326 '\u{35888}',
327 ],
328 );
329 test_samples(&Alphanumeric, 0, &[104, 109, 101, 51, 77]);
330 test_samples(&StandardUniform, false, &[true, true, false, true, false]);
331 test_samples(
332 &StandardUniform,
333 Wrapping(0i32),
334 &[
335 Wrapping(-2074640887),
336 Wrapping(-1719949321),
337 Wrapping(2018088303),
338 Wrapping(-547181756),
339 Wrapping(838957336),
340 ],
341 );
342
343 // We test only sub-sets of tuple and array impls
344 test_samples(&StandardUniform, (), &[(), (), (), (), ()]);
345 test_samples(
346 &StandardUniform,
347 (false,),
348 &[(true,), (true,), (false,), (true,), (false,)],
349 );
350 test_samples(
351 &StandardUniform,
352 (false, false),
353 &[
354 (true, true),
355 (false, true),
356 (false, false),
357 (true, false),
358 (false, false),
359 ],
360 );
361
362 test_samples(&StandardUniform, [0u8; 0], &[[], [], [], [], []]);
363 test_samples(
364 &StandardUniform,
365 [0u8; 3],
366 &[
367 [9, 247, 111],
368 [68, 24, 13],
369 [174, 19, 194],
370 [172, 69, 213],
371 [149, 207, 29],
372 ],
373 );
374 }
375}
376