1//! A simple and fast random number generator.
2//!
3//! The implementation uses [Wyrand](https://github.com/wangyi-fudan/wyhash), a simple and fast
4//! generator but **not** cryptographically secure.
5//!
6//! # Examples
7//!
8//! Flip a coin:
9//!
10//! ```
11//! if fastrand::bool() {
12//! println!("heads");
13//! } else {
14//! println!("tails");
15//! }
16//! ```
17//!
18//! Generate a random `i32`:
19//!
20//! ```
21//! let num = fastrand::i32(..);
22//! ```
23//!
24//! Choose a random element in an array:
25//!
26//! ```
27//! let v = vec![1, 2, 3, 4, 5];
28//! let i = fastrand::usize(..v.len());
29//! let elem = v[i];
30//! ```
31//!
32//! Sample values from an array with `O(n)` complexity (`n` is the length of array):
33//!
34//! ```
35//! fastrand::choose_multiple([1, 4, 5], 2);
36//! fastrand::choose_multiple(0..20, 12);
37//! ```
38//!
39//!
40//! Shuffle an array:
41//!
42//! ```
43//! let mut v = vec![1, 2, 3, 4, 5];
44//! fastrand::shuffle(&mut v);
45//! ```
46//!
47//! Generate a random [`Vec`] or [`String`]:
48//!
49//! ```
50//! use std::iter::repeat_with;
51//!
52//! let v: Vec<i32> = repeat_with(|| fastrand::i32(..)).take(10).collect();
53//! let s: String = repeat_with(fastrand::alphanumeric).take(10).collect();
54//! ```
55//!
56//! To get reproducible results on every run, initialize the generator with a seed:
57//!
58//! ```
59//! // Pick an arbitrary number as seed.
60//! fastrand::seed(7);
61//!
62//! // Now this prints the same number on every run:
63//! println!("{}", fastrand::u32(..));
64//! ```
65//!
66//! To be more efficient, create a new [`Rng`] instance instead of using the thread-local
67//! generator:
68//!
69//! ```
70//! use std::iter::repeat_with;
71//!
72//! let mut rng = fastrand::Rng::new();
73//! let mut bytes: Vec<u8> = repeat_with(|| rng.u8(..)).take(10_000).collect();
74//! ```
75//!
76//! This crate aims to expose a core set of useful randomness primitives. For more niche algorithms,
77//! consider using the [`fastrand-contrib`] crate alongside this one.
78//!
79//! # Features
80//!
81//! - `std` (enabled by default): Enables the `std` library. This is required for the global
82//! generator and global entropy. Without this feature, [`Rng`] can only be instantiated using
83//! the [`with_seed`](Rng::with_seed) method.
84//! - `js`: Assumes that WebAssembly targets are being run in a JavaScript environment. See the
85//! [WebAssembly Notes](#webassembly-notes) section for more information.
86//!
87//! # WebAssembly Notes
88//!
89//! For non-WASI WASM targets, there is additional sublety to consider when utilizing the global RNG.
90//! By default, `std` targets will use entropy sources in the standard library to seed the global RNG.
91//! However, these sources are not available by default on WASM targets outside of WASI.
92//!
93//! If the `js` feature is enabled, this crate will assume that it is running in a JavaScript
94//! environment. At this point, the [`getrandom`] crate will be used in order to access the available
95//! entropy sources and seed the global RNG. If the `js` feature is not enabled, the global RNG will
96//! use a predefined seed.
97//!
98//! [`fastrand-contrib`]: https://crates.io/crates/fastrand-contrib
99//! [`getrandom`]: https://crates.io/crates/getrandom
100
101#![no_std]
102#![cfg_attr(docsrs, feature(doc_cfg))]
103#![forbid(unsafe_code)]
104#![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]
105#![doc(
106 html_favicon_url = "https://raw.githubusercontent.com/smol-rs/smol/master/assets/images/logo_fullsize_transparent.png"
107)]
108#![doc(
109 html_logo_url = "https://raw.githubusercontent.com/smol-rs/smol/master/assets/images/logo_fullsize_transparent.png"
110)]
111
112#[cfg(feature = "alloc")]
113extern crate alloc;
114#[cfg(feature = "std")]
115extern crate std;
116
117use core::convert::{TryFrom, TryInto};
118use core::ops::{Bound, RangeBounds};
119
120#[cfg(feature = "alloc")]
121use alloc::vec::Vec;
122
123#[cfg(feature = "std")]
124#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
125mod global_rng;
126
127#[cfg(feature = "std")]
128pub use global_rng::*;
129
130/// A random number generator.
131#[derive(Debug, PartialEq, Eq)]
132pub struct Rng(u64);
133
134impl Clone for Rng {
135 /// Clones the generator by creating a new generator with the same seed.
136 fn clone(&self) -> Rng {
137 Rng::with_seed(self.0)
138 }
139}
140
141impl Rng {
142 /// Generates a random `u32`.
143 #[inline]
144 fn gen_u32(&mut self) -> u32 {
145 self.gen_u64() as u32
146 }
147
148 /// Generates a random `u64`.
149 #[inline]
150 fn gen_u64(&mut self) -> u64 {
151 // Constants for WyRand taken from: https://github.com/wangyi-fudan/wyhash/blob/master/wyhash.h#L151
152 // Updated for the final v4.2 implementation with improved constants for better entropy output.
153 const WY_CONST_0: u64 = 0x2d35_8dcc_aa6c_78a5;
154 const WY_CONST_1: u64 = 0x8bb8_4b93_962e_acc9;
155
156 let s = self.0.wrapping_add(WY_CONST_0);
157 self.0 = s;
158 let t = u128::from(s) * u128::from(s ^ WY_CONST_1);
159 (t as u64) ^ (t >> 64) as u64
160 }
161
162 /// Generates a random `u128`.
163 #[inline]
164 fn gen_u128(&mut self) -> u128 {
165 (u128::from(self.gen_u64()) << 64) | u128::from(self.gen_u64())
166 }
167
168 /// Generates a random `u32` in `0..n`.
169 #[inline]
170 fn gen_mod_u32(&mut self, n: u32) -> u32 {
171 // Adapted from: https://lemire.me/blog/2016/06/30/fast-random-shuffling/
172 let mut r = self.gen_u32();
173 let mut hi = mul_high_u32(r, n);
174 let mut lo = r.wrapping_mul(n);
175 if lo < n {
176 let t = n.wrapping_neg() % n;
177 while lo < t {
178 r = self.gen_u32();
179 hi = mul_high_u32(r, n);
180 lo = r.wrapping_mul(n);
181 }
182 }
183 hi
184 }
185
186 /// Generates a random `u64` in `0..n`.
187 #[inline]
188 fn gen_mod_u64(&mut self, n: u64) -> u64 {
189 // Adapted from: https://lemire.me/blog/2016/06/30/fast-random-shuffling/
190 let mut r = self.gen_u64();
191 let mut hi = mul_high_u64(r, n);
192 let mut lo = r.wrapping_mul(n);
193 if lo < n {
194 let t = n.wrapping_neg() % n;
195 while lo < t {
196 r = self.gen_u64();
197 hi = mul_high_u64(r, n);
198 lo = r.wrapping_mul(n);
199 }
200 }
201 hi
202 }
203
204 /// Generates a random `u128` in `0..n`.
205 #[inline]
206 fn gen_mod_u128(&mut self, n: u128) -> u128 {
207 // Adapted from: https://lemire.me/blog/2016/06/30/fast-random-shuffling/
208 let mut r = self.gen_u128();
209 let mut hi = mul_high_u128(r, n);
210 let mut lo = r.wrapping_mul(n);
211 if lo < n {
212 let t = n.wrapping_neg() % n;
213 while lo < t {
214 r = self.gen_u128();
215 hi = mul_high_u128(r, n);
216 lo = r.wrapping_mul(n);
217 }
218 }
219 hi
220 }
221}
222
223/// Computes `(a * b) >> 32`.
224#[inline]
225fn mul_high_u32(a: u32, b: u32) -> u32 {
226 (((a as u64) * (b as u64)) >> 32) as u32
227}
228
229/// Computes `(a * b) >> 64`.
230#[inline]
231fn mul_high_u64(a: u64, b: u64) -> u64 {
232 (((a as u128) * (b as u128)) >> 64) as u64
233}
234
235/// Computes `(a * b) >> 128`.
236#[inline]
237fn mul_high_u128(a: u128, b: u128) -> u128 {
238 // Adapted from: https://stackoverflow.com/a/28904636
239 let a_lo: u128 = a as u64 as u128;
240 let a_hi: u128 = (a >> 64) as u64 as u128;
241 let b_lo: u128 = b as u64 as u128;
242 let b_hi: u128 = (b >> 64) as u64 as u128;
243 let carry: u128 = (a_lo * b_lo) >> 64;
244 let carry: u128 = ((a_hi * b_lo) as u64 as u128 + (a_lo * b_hi) as u64 as u128 + carry) >> 64;
245 a_hi * b_hi + ((a_hi * b_lo) >> 64) + ((a_lo * b_hi) >> 64) + carry
246}
247
248macro_rules! rng_integer {
249 ($t:tt, $unsigned_t:tt, $gen:tt, $mod:tt, $doc:tt) => {
250 #[doc = $doc]
251 ///
252 /// Panics if the range is empty.
253 #[inline]
254 pub fn $t(&mut self, range: impl RangeBounds<$t>) -> $t {
255 let panic_empty_range = || {
256 panic!(
257 "empty range: {:?}..{:?}",
258 range.start_bound(),
259 range.end_bound()
260 )
261 };
262
263 let low = match range.start_bound() {
264 Bound::Unbounded => core::$t::MIN,
265 Bound::Included(&x) => x,
266 Bound::Excluded(&x) => x.checked_add(1).unwrap_or_else(panic_empty_range),
267 };
268
269 let high = match range.end_bound() {
270 Bound::Unbounded => core::$t::MAX,
271 Bound::Included(&x) => x,
272 Bound::Excluded(&x) => x.checked_sub(1).unwrap_or_else(panic_empty_range),
273 };
274
275 if low > high {
276 panic_empty_range();
277 }
278
279 if low == core::$t::MIN && high == core::$t::MAX {
280 self.$gen() as $t
281 } else {
282 let len = high.wrapping_sub(low).wrapping_add(1);
283 low.wrapping_add(self.$mod(len as $unsigned_t as _) as $t)
284 }
285 }
286 };
287}
288
289impl Rng {
290 /// Creates a new random number generator with the initial seed.
291 #[inline]
292 #[must_use = "this creates a new instance of `Rng`; if you want to initialize the thread-local generator, use `fastrand::seed()` instead"]
293 pub fn with_seed(seed: u64) -> Self {
294 Rng(seed)
295 }
296
297 /// Clones the generator by deterministically deriving a new generator based on the initial
298 /// seed.
299 ///
300 /// This function can be used to create a new generator that is a "spinoff" of the old
301 /// generator. The new generator will not produce the same sequence of values as the
302 /// old generator.
303 ///
304 /// # Example
305 ///
306 /// ```
307 /// // Seed two generators equally, and clone both of them.
308 /// let mut base1 = fastrand::Rng::with_seed(0x4d595df4d0f33173);
309 /// base1.bool(); // Use the generator once.
310 ///
311 /// let mut base2 = fastrand::Rng::with_seed(0x4d595df4d0f33173);
312 /// base2.bool(); // Use the generator once.
313 ///
314 /// let mut rng1 = base1.fork();
315 /// let mut rng2 = base2.fork();
316 ///
317 /// println!("rng1 returns {}", rng1.u32(..));
318 /// println!("rng2 returns {}", rng2.u32(..));
319 /// ```
320 #[inline]
321 #[must_use = "this creates a new instance of `Rng`"]
322 pub fn fork(&mut self) -> Self {
323 Rng::with_seed(self.gen_u64())
324 }
325
326 /// Generates a random `char` in ranges a-z and A-Z.
327 #[inline]
328 pub fn alphabetic(&mut self) -> char {
329 const CHARS: &[u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
330 *self.choice(CHARS).unwrap() as char
331 }
332
333 /// Generates a random `char` in ranges a-z, A-Z and 0-9.
334 #[inline]
335 pub fn alphanumeric(&mut self) -> char {
336 const CHARS: &[u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
337 *self.choice(CHARS).unwrap() as char
338 }
339
340 /// Generates a random `bool`.
341 #[inline]
342 pub fn bool(&mut self) -> bool {
343 self.u8(..) % 2 == 0
344 }
345
346 /// Generates a random digit in the given `base`.
347 ///
348 /// Digits are represented by `char`s in ranges 0-9 and a-z.
349 ///
350 /// Panics if the base is zero or greater than 36.
351 #[inline]
352 pub fn digit(&mut self, base: u32) -> char {
353 if base == 0 {
354 panic!("base cannot be zero");
355 }
356 if base > 36 {
357 panic!("base cannot be larger than 36");
358 }
359 let num = self.u8(..base as u8);
360 if num < 10 {
361 (b'0' + num) as char
362 } else {
363 (b'a' + num - 10) as char
364 }
365 }
366
367 /// Generates a random `f32` in range `0..1`.
368 pub fn f32(&mut self) -> f32 {
369 let b = 32;
370 let f = core::f32::MANTISSA_DIGITS - 1;
371 f32::from_bits((1 << (b - 2)) - (1 << f) + (self.u32(..) >> (b - f))) - 1.0
372 }
373
374 /// Generates a random `f64` in range `0..1`.
375 pub fn f64(&mut self) -> f64 {
376 let b = 64;
377 let f = core::f64::MANTISSA_DIGITS - 1;
378 f64::from_bits((1 << (b - 2)) - (1 << f) + (self.u64(..) >> (b - f))) - 1.0
379 }
380
381 /// Collects `amount` values at random from the iterable into a vector.
382 ///
383 /// The length of the returned vector equals `amount` unless the iterable
384 /// contains insufficient elements, in which case it equals the number of
385 /// elements available.
386 ///
387 /// Complexity is `O(n)` where `n` is the length of the iterable.
388 #[cfg(feature = "alloc")]
389 #[cfg_attr(docsrs, doc(cfg(feature = "alloc")))]
390 pub fn choose_multiple<I: IntoIterator>(&mut self, source: I, amount: usize) -> Vec<I::Item> {
391 // Adapted from: https://docs.rs/rand/latest/rand/seq/trait.IteratorRandom.html#method.choose_multiple
392 let mut reservoir = Vec::with_capacity(amount);
393 let mut iter = source.into_iter();
394
395 reservoir.extend(iter.by_ref().take(amount));
396
397 // Continue unless the iterator was exhausted
398 //
399 // note: this prevents iterators that "restart" from causing problems.
400 // If the iterator stops once, then so do we.
401 if reservoir.len() == amount {
402 for (i, elem) in iter.enumerate() {
403 let end = i + 1 + amount;
404 let k = self.usize(0..end);
405 if let Some(slot) = reservoir.get_mut(k) {
406 *slot = elem;
407 }
408 }
409 } else {
410 // If less than one third of the `Vec` was used, reallocate
411 // so that the unused space is not wasted. There is a corner
412 // case where `amount` was much less than `self.len()`.
413 if reservoir.capacity() > 3 * reservoir.len() {
414 reservoir.shrink_to_fit();
415 }
416 }
417 reservoir
418 }
419
420 rng_integer!(
421 i8,
422 u8,
423 gen_u32,
424 gen_mod_u32,
425 "Generates a random `i8` in the given range."
426 );
427
428 rng_integer!(
429 i16,
430 u16,
431 gen_u32,
432 gen_mod_u32,
433 "Generates a random `i16` in the given range."
434 );
435
436 rng_integer!(
437 i32,
438 u32,
439 gen_u32,
440 gen_mod_u32,
441 "Generates a random `i32` in the given range."
442 );
443
444 rng_integer!(
445 i64,
446 u64,
447 gen_u64,
448 gen_mod_u64,
449 "Generates a random `i64` in the given range."
450 );
451
452 rng_integer!(
453 i128,
454 u128,
455 gen_u128,
456 gen_mod_u128,
457 "Generates a random `i128` in the given range."
458 );
459
460 #[cfg(target_pointer_width = "16")]
461 rng_integer!(
462 isize,
463 usize,
464 gen_u32,
465 gen_mod_u32,
466 "Generates a random `isize` in the given range."
467 );
468 #[cfg(target_pointer_width = "32")]
469 rng_integer!(
470 isize,
471 usize,
472 gen_u32,
473 gen_mod_u32,
474 "Generates a random `isize` in the given range."
475 );
476 #[cfg(target_pointer_width = "64")]
477 rng_integer!(
478 isize,
479 usize,
480 gen_u64,
481 gen_mod_u64,
482 "Generates a random `isize` in the given range."
483 );
484
485 /// Generates a random `char` in range a-z.
486 #[inline]
487 pub fn lowercase(&mut self) -> char {
488 const CHARS: &[u8] = b"abcdefghijklmnopqrstuvwxyz";
489 *self.choice(CHARS).unwrap() as char
490 }
491
492 /// Initializes this generator with the given seed.
493 #[inline]
494 pub fn seed(&mut self, seed: u64) {
495 self.0 = seed;
496 }
497
498 /// Gives back **current** seed that is being held by this generator.
499 #[inline]
500 pub fn get_seed(&self) -> u64 {
501 self.0
502 }
503
504 /// Choose an item from an iterator at random.
505 ///
506 /// This function may have an unexpected result if the `len()` property of the
507 /// iterator does not match the actual number of items in the iterator. If
508 /// the iterator is empty, this returns `None`.
509 #[inline]
510 pub fn choice<I>(&mut self, iter: I) -> Option<I::Item>
511 where
512 I: IntoIterator,
513 I::IntoIter: ExactSizeIterator,
514 {
515 let mut iter = iter.into_iter();
516
517 // Get the item at a random index.
518 let len = iter.len();
519 if len == 0 {
520 return None;
521 }
522 let index = self.usize(0..len);
523
524 iter.nth(index)
525 }
526
527 /// Shuffles a slice randomly.
528 #[inline]
529 pub fn shuffle<T>(&mut self, slice: &mut [T]) {
530 for i in 1..slice.len() {
531 slice.swap(i, self.usize(..=i));
532 }
533 }
534
535 /// Fill a byte slice with random data.
536 #[inline]
537 pub fn fill(&mut self, slice: &mut [u8]) {
538 // We fill the slice by chunks of 8 bytes, or one block of
539 // WyRand output per new state.
540 let mut chunks = slice.chunks_exact_mut(core::mem::size_of::<u64>());
541 for chunk in chunks.by_ref() {
542 let n = self.gen_u64().to_ne_bytes();
543 // Safe because the chunks are always 8 bytes exactly.
544 chunk.copy_from_slice(&n);
545 }
546
547 let remainder = chunks.into_remainder();
548
549 // Any remainder will always be less than 8 bytes.
550 if !remainder.is_empty() {
551 // Generate one last block of 8 bytes of entropy
552 let n = self.gen_u64().to_ne_bytes();
553
554 // Use the remaining length to copy from block
555 remainder.copy_from_slice(&n[..remainder.len()]);
556 }
557 }
558
559 rng_integer!(
560 u8,
561 u8,
562 gen_u32,
563 gen_mod_u32,
564 "Generates a random `u8` in the given range."
565 );
566
567 rng_integer!(
568 u16,
569 u16,
570 gen_u32,
571 gen_mod_u32,
572 "Generates a random `u16` in the given range."
573 );
574
575 rng_integer!(
576 u32,
577 u32,
578 gen_u32,
579 gen_mod_u32,
580 "Generates a random `u32` in the given range."
581 );
582
583 rng_integer!(
584 u64,
585 u64,
586 gen_u64,
587 gen_mod_u64,
588 "Generates a random `u64` in the given range."
589 );
590
591 rng_integer!(
592 u128,
593 u128,
594 gen_u128,
595 gen_mod_u128,
596 "Generates a random `u128` in the given range."
597 );
598
599 #[cfg(target_pointer_width = "16")]
600 rng_integer!(
601 usize,
602 usize,
603 gen_u32,
604 gen_mod_u32,
605 "Generates a random `usize` in the given range."
606 );
607 #[cfg(target_pointer_width = "32")]
608 rng_integer!(
609 usize,
610 usize,
611 gen_u32,
612 gen_mod_u32,
613 "Generates a random `usize` in the given range."
614 );
615 #[cfg(target_pointer_width = "64")]
616 rng_integer!(
617 usize,
618 usize,
619 gen_u64,
620 gen_mod_u64,
621 "Generates a random `usize` in the given range."
622 );
623
624 /// Generates a random `char` in range A-Z.
625 #[inline]
626 pub fn uppercase(&mut self) -> char {
627 const CHARS: &[u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZ";
628 *self.choice(CHARS).unwrap() as char
629 }
630
631 /// Generates a random `char` in the given range.
632 ///
633 /// Panics if the range is empty.
634 #[inline]
635 pub fn char(&mut self, range: impl RangeBounds<char>) -> char {
636 let panic_empty_range = || {
637 panic!(
638 "empty range: {:?}..{:?}",
639 range.start_bound(),
640 range.end_bound()
641 )
642 };
643
644 let surrogate_start = 0xd800u32;
645 let surrogate_len = 0x800u32;
646
647 let low = match range.start_bound() {
648 Bound::Unbounded => 0u8 as char,
649 Bound::Included(&x) => x,
650 Bound::Excluded(&x) => {
651 let scalar = if x as u32 == surrogate_start - 1 {
652 surrogate_start + surrogate_len
653 } else {
654 x as u32 + 1
655 };
656 char::try_from(scalar).unwrap_or_else(|_| panic_empty_range())
657 }
658 };
659
660 let high = match range.end_bound() {
661 Bound::Unbounded => core::char::MAX,
662 Bound::Included(&x) => x,
663 Bound::Excluded(&x) => {
664 let scalar = if x as u32 == surrogate_start + surrogate_len {
665 surrogate_start - 1
666 } else {
667 (x as u32).wrapping_sub(1)
668 };
669 char::try_from(scalar).unwrap_or_else(|_| panic_empty_range())
670 }
671 };
672
673 if low > high {
674 panic_empty_range();
675 }
676
677 let gap = if (low as u32) < surrogate_start && (high as u32) >= surrogate_start {
678 surrogate_len
679 } else {
680 0
681 };
682 let range = high as u32 - low as u32 - gap;
683 let mut val = self.u32(0..=range) + low as u32;
684 if val >= surrogate_start {
685 val += gap;
686 }
687 val.try_into().unwrap()
688 }
689}
690