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fastrand
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fastrand/src/lib.rs

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//! A simple and fast random number generator.
//!
//! The implementation uses [Wyrand](https://github.com/wangyi-fudan/wyhash), a simple and fast
//! generator but **not** cryptographically secure.
//!
//! # Examples
//!
//! Flip a coin:
//!
//! ```
//! if fastrand::bool() {
//! println!("heads");
//! } else {
//! println!("tails");
//! }
//! ```
//!
//! Generate a random `i32`:
//!
//! ```
//! let num = fastrand::i32(..);
//! ```
//!
//! Choose a random element in an array:
//!
//! ```
//! let v = vec![1, 2, 3, 4, 5];
//! let i = fastrand::usize(..v.len());
//! let elem = v[i];
//! ```
//!
//! Sample values from an array with `O(n)` complexity (`n` is the length of array):
//!
//! ```
//! fastrand::choose_multiple(vec![1, 4, 5].iter(), 2);
//! fastrand::choose_multiple(0..20, 12);
//! ```
//!
//!
//! Shuffle an array:
//!
//! ```
//! let mut v = vec![1, 2, 3, 4, 5];
//! fastrand::shuffle(&mut v);
//! ```
//!
//! Generate a random [`Vec`] or [`String`]:
//!
//! ```
//! use std::iter::repeat_with;
//!
//! let v: Vec<i32> = repeat_with(|| fastrand::i32(..)).take(10).collect();
//! let s: String = repeat_with(fastrand::alphanumeric).take(10).collect();
//! ```
//!
//! To get reproducible results on every run, initialize the generator with a seed:
//!
//! ```
//! // Pick an arbitrary number as seed.
//! fastrand::seed(7);
//!
//! // Now this prints the same number on every run:
//! println!("{}", fastrand::u32(..));
//! ```
//!
//! To be more efficient, create a new [`Rng`] instance instead of using the thread-local
//! generator:
//!
//! ```
//! use std::iter::repeat_with;
//!
//! let mut rng = fastrand::Rng::new();
//! let mut bytes: Vec<u8> = repeat_with(|| rng.u8(..)).take(10_000).collect();
//! ```
//!
//! This crate aims to expose a core set of useful randomness primitives. For more niche algorithms,
//! consider using the [`fastrand-contrib`] crate alongside this one.
//!
//! # Features
//!
//! - `std` (enabled by default): Enables the `std` library. This is required for the global
//! generator and global entropy. Without this feature, [`Rng`] can only be instantiated using
//! the [`with_seed`](Rng::with_seed) method.
//! - `js`: Assumes that WebAssembly targets are being run in a JavaScript environment. See the
//! [WebAssembly Notes](#webassembly-notes) section for more information.
//!
//! # WebAssembly Notes
//!
//! For non-WASI WASM targets, there is additional sublety to consider when utilizing the global RNG.
//! By default, `std` targets will use entropy sources in the standard library to seed the global RNG.
//! However, these sources are not available by default on WASM targets outside of WASI.
//!
//! If the `js` feature is enabled, this crate will assume that it is running in a JavaScript
//! environment. At this point, the [`getrandom`] crate will be used in order to access the available
//! entropy sources and seed the global RNG. If the `js` feature is not enabled, the global RNG will
//! use a predefined seed.
//!
//! [`fastrand-contrib`]: https://crates.io/crates/fastrand-contrib
//! [`getrandom`]: https://crates.io/crates/getrandom
#![no_std]
#![cfg_attr(docsrs, feature(doc_cfg))]
#![forbid(unsafe_code)]
#![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]
#![doc(
html_favicon_url = "https://raw.githubusercontent.com/smol-rs/smol/master/assets/images/logo_fullsize_transparent.png"
)]
#![doc(
html_logo_url = "https://raw.githubusercontent.com/smol-rs/smol/master/assets/images/logo_fullsize_transparent.png"
)]
#[cfg(feature = "alloc")]
extern crate alloc;
#[cfg(feature = "std")]
extern crate std;
use core::convert::{TryFrom, TryInto};
use core::ops::{Bound, RangeBounds};
#[cfg(feature = "alloc")]
use alloc::vec::Vec;
#[cfg(feature = "std")]
#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
mod global_rng;
#[cfg(feature = "std")]
pub use global_rng::*;
/// A random number generator.
#[derive(Debug, PartialEq, Eq)]
pub struct Rng(u64);
impl Clone for Rng {
/// Clones the generator by creating a new generator with the same seed.
fn clone(&self) -> Rng {
Rng::with_seed(self.0)
}
}
impl Rng {
/// Generates a random `u32`.
#[inline]
fn gen_u32(&mut self) -> u32 {
self.gen_u64() as u32
}
/// Generates a random `u64`.
#[inline]
fn gen_u64(&mut self) -> u64 {
// Constants for WyRand taken from: https://github.com/wangyi-fudan/wyhash/blob/master/wyhash.h#L151
// Updated for the final v4.2 implementation with improved constants for better entropy output.
const WY_CONST_0: u64 = 0x2d35_8dcc_aa6c_78a5;
const WY_CONST_1: u64 = 0x8bb8_4b93_962e_acc9;
let s = self.0.wrapping_add(WY_CONST_0);
self.0 = s;
let t = u128::from(s) * u128::from(s ^ WY_CONST_1);
(t as u64) ^ (t >> 64) as u64
}
/// Generates a random `u128`.
#[inline]
fn gen_u128(&mut self) -> u128 {
(u128::from(self.gen_u64()) << 64) | u128::from(self.gen_u64())
}
/// Generates a random `u32` in `0..n`.
#[inline]
fn gen_mod_u32(&mut self, n: u32) -> u32 {
// Adapted from: https://lemire.me/blog/2016/06/30/fast-random-shuffling/
let mut r = self.gen_u32();
let mut hi = mul_high_u32(r, n);
let mut lo = r.wrapping_mul(n);
if lo < n {
let t = n.wrapping_neg() % n;
while lo < t {
r = self.gen_u32();
hi = mul_high_u32(r, n);
lo = r.wrapping_mul(n);
}
}
hi
}
/// Generates a random `u64` in `0..n`.
#[inline]
fn gen_mod_u64(&mut self, n: u64) -> u64 {
// Adapted from: https://lemire.me/blog/2016/06/30/fast-random-shuffling/
let mut r = self.gen_u64();
let mut hi = mul_high_u64(r, n);
let mut lo = r.wrapping_mul(n);
if lo < n {
let t = n.wrapping_neg() % n;
while lo < t {
r = self.gen_u64();
hi = mul_high_u64(r, n);
lo = r.wrapping_mul(n);
}
}
hi
}
/// Generates a random `u128` in `0..n`.
#[inline]
fn gen_mod_u128(&mut self, n: u128) -> u128 {
// Adapted from: https://lemire.me/blog/2016/06/30/fast-random-shuffling/
let mut r = self.gen_u128();
let mut hi = mul_high_u128(r, n);
let mut lo = r.wrapping_mul(n);
if lo < n {
let t = n.wrapping_neg() % n;
while lo < t {
r = self.gen_u128();
hi = mul_high_u128(r, n);
lo = r.wrapping_mul(n);
}
}
hi
}
}
/// Computes `(a * b) >> 32`.
#[inline]
fn mul_high_u32(a: u32, b: u32) -> u32 {
(((a as u64) * (b as u64)) >> 32) as u32
}
/// Computes `(a * b) >> 64`.
#[inline]
fn mul_high_u64(a: u64, b: u64) -> u64 {
(((a as u128) * (b as u128)) >> 64) as u64
}
/// Computes `(a * b) >> 128`.
#[inline]
fn mul_high_u128(a: u128, b: u128) -> u128 {
// Adapted from: https://stackoverflow.com/a/28904636
let a_lo = a as u64 as u128;
let a_hi = (a >> 64) as u64 as u128;
let b_lo = b as u64 as u128;
let b_hi = (b >> 64) as u64 as u128;
let carry = (a_lo * b_lo) >> 64;
let carry = ((a_hi * b_lo) as u64 as u128 + (a_lo * b_hi) as u64 as u128 + carry) >> 64;
a_hi * b_hi + ((a_hi * b_lo) >> 64) + ((a_lo * b_hi) >> 64) + carry
}
macro_rules! rng_integer {
($t:tt, $unsigned_t:tt, $gen:tt, $mod:tt, $doc:tt) => {
#[doc = $doc]
///
/// Panics if the range is empty.
#[inline]
pub fn $t(&mut self, range: impl RangeBounds<$t>) -> $t {
let panic_empty_range = || {
panic!(
"empty range: {:?}..{:?}",
range.start_bound(),
range.end_bound()
)
};
let low = match range.start_bound() {
Bound::Unbounded => core::$t::MIN,
Bound::Included(&x) => x,
Bound::Excluded(&x) => x.checked_add(1).unwrap_or_else(panic_empty_range),
};
let high = match range.end_bound() {
Bound::Unbounded => core::$t::MAX,
Bound::Included(&x) => x,
Bound::Excluded(&x) => x.checked_sub(1).unwrap_or_else(panic_empty_range),
};
if low > high {
panic_empty_range();
}
if low == core::$t::MIN && high == core::$t::MAX {
self.$gen() as $t
} else {
let len = high.wrapping_sub(low).wrapping_add(1);
low.wrapping_add(self.$mod(len as $unsigned_t as _) as $t)
}
}
};
}
impl Rng {
/// Creates a new random number generator with the initial seed.
#[inline]
#[must_use = "this creates a new instance of `Rng`; if you want to initialize the thread-local generator, use `fastrand::seed()` instead"]
pub fn with_seed(seed: u64) -> Self {
Rng(seed)
}
/// Clones the generator by deterministically deriving a new generator based on the initial
/// seed.
///
/// This function can be used to create a new generator that is a "spinoff" of the old
/// generator. The new generator will not produce the same sequence of values as the
/// old generator.
///
/// # Example
///
/// ```
/// // Seed two generators equally, and clone both of them.
/// let mut base1 = fastrand::Rng::with_seed(0x4d595df4d0f33173);
/// base1.bool(); // Use the generator once.
///
/// let mut base2 = fastrand::Rng::with_seed(0x4d595df4d0f33173);
/// base2.bool(); // Use the generator once.
///
/// let mut rng1 = base1.fork();
/// let mut rng2 = base2.fork();
///
/// println!("rng1 returns {}", rng1.u32(..));
/// println!("rng2 returns {}", rng2.u32(..));
/// ```
#[inline]
#[must_use = "this creates a new instance of `Rng`"]
pub fn fork(&mut self) -> Self {
Rng::with_seed(self.gen_u64())
}
/// Generates a random `char` in ranges a-z and A-Z.
#[inline]
pub fn alphabetic(&mut self) -> char {
const CHARS: &[u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
*self.choice(CHARS).unwrap() as char
}
/// Generates a random `char` in ranges a-z, A-Z and 0-9.
#[inline]
pub fn alphanumeric(&mut self) -> char {
const CHARS: &[u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
*self.choice(CHARS).unwrap() as char
}
/// Generates a random `bool`.
#[inline]
pub fn bool(&mut self) -> bool {
self.u8(..) % 2 == 0
}
/// Generates a random digit in the given `base`.
///
/// Digits are represented by `char`s in ranges 0-9 and a-z.
///
/// Panics if the base is zero or greater than 36.
#[inline]
pub fn digit(&mut self, base: u32) -> char {
if base == 0 {
panic!("base cannot be zero");
}
if base > 36 {
panic!("base cannot be larger than 36");
}
let num = self.u8(..base as u8);
if num < 10 {
(b'0' + num) as char
} else {
(b'a' + num - 10) as char
}
}
/// Generates a random `f32` in range `0..1`.
pub fn f32(&mut self) -> f32 {
let b = 32;
let f = core::f32::MANTISSA_DIGITS - 1;
f32::from_bits((1 << (b - 2)) - (1 << f) + (self.u32(..) >> (b - f))) - 1.0
}
/// Generates a random `f64` in range `0..1`.
pub fn f64(&mut self) -> f64 {
let b = 64;
let f = core::f64::MANTISSA_DIGITS - 1;
f64::from_bits((1 << (b - 2)) - (1 << f) + (self.u64(..) >> (b - f))) - 1.0
}
/// Collects `amount` values at random from the iterator into a vector.
///
/// 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.
#[cfg(feature = "alloc")]
#[cfg_attr(docsrs, doc(cfg(feature = "alloc")))]
pub fn choose_multiple<T: Iterator>(&mut self, mut source: T, amount: usize) -> Vec<T::Item> {
// Adapted from: https://docs.rs/rand/latest/rand/seq/trait.IteratorRandom.html#method.choose_multiple
let mut reservoir = Vec::with_capacity(amount);
reservoir.extend(source.by_ref().take(amount));
// Continue unless the iterator was exhausted
//
// note: this prevents iterators that "restart" from causing problems.
// If the iterator stops once, then so do we.
if reservoir.len() == amount {
for (i, elem) in source.enumerate() {
let end = i + 1 + amount;
let k = self.usize(0..end);
if let Some(slot) = reservoir.get_mut(k) {
*slot = elem;
}
}
} else {
// If less than one third of the `Vec` was used, reallocate
// so that the unused space is not wasted. There is a corner
// case where `amount` was much less than `self.len()`.
if reservoir.capacity() > 3 * reservoir.len() {
reservoir.shrink_to_fit();
}
}
reservoir
}
rng_integer!(
i8,
u8,
gen_u32,
gen_mod_u32,
"Generates a random `i8` in the given range."
);
rng_integer!(
i16,
u16,
gen_u32,
gen_mod_u32,
"Generates a random `i16` in the given range."
);
rng_integer!(
i32,
u32,
gen_u32,
gen_mod_u32,
"Generates a random `i32` in the given range."
);
rng_integer!(
i64,
u64,
gen_u64,
gen_mod_u64,
"Generates a random `i64` in the given range."
);
rng_integer!(
i128,
u128,
gen_u128,
gen_mod_u128,
"Generates a random `i128` in the given range."
);
#[cfg(target_pointer_width = "16")]
rng_integer!(
isize,
usize,
gen_u32,
gen_mod_u32,
"Generates a random `isize` in the given range."
);
#[cfg(target_pointer_width = "32")]
rng_integer!(
isize,
usize,
gen_u32,
gen_mod_u32,
"Generates a random `isize` in the given range."
);
#[cfg(target_pointer_width = "64")]
rng_integer!(
isize,
usize,
gen_u64,
gen_mod_u64,
"Generates a random `isize` in the given range."
);
/// Generates a random `char` in range a-z.
#[inline]
pub fn lowercase(&mut self) -> char {
const CHARS: &[u8] = b"abcdefghijklmnopqrstuvwxyz";
*self.choice(CHARS).unwrap() as char
}
/// Initializes this generator with the given seed.
#[inline]
pub fn seed(&mut self, seed: u64) {
self.0 = seed;
}
/// Gives back **current** seed that is being held by this generator.
#[inline]
pub fn get_seed(&self) -> u64 {
self.0
}
/// Choose an item from an iterator at random.
///
/// This function may have an unexpected result if the `len()` property of the
/// iterator does not match the actual number of items in the iterator. If
/// the iterator is empty, this returns `None`.
#[inline]
pub fn choice<I>(&mut self, iter: I) -> Option<I::Item>
where
I: IntoIterator,
I::IntoIter: ExactSizeIterator,
{
let mut iter = iter.into_iter();
// Get the item at a random index.
let len = iter.len();
if len == 0 {
return None;
}
let index = self.usize(0..len);
iter.nth(index)
}
/// Shuffles a slice randomly.
#[inline]
pub fn shuffle<T>(&mut self, slice: &mut [T]) {
for i in 1..slice.len() {
slice.swap(i, self.usize(..=i));
}
}
/// Fill a byte slice with random data.
#[inline]
pub fn fill(&mut self, slice: &mut [u8]) {
// We fill the slice by chunks of 8 bytes, or one block of
// WyRand output per new state.
let mut chunks = slice.chunks_exact_mut(core::mem::size_of::<u64>());
for chunk in chunks.by_ref() {
let n = self.gen_u64().to_ne_bytes();
// Safe because the chunks are always 8 bytes exactly.
chunk.copy_from_slice(&n);
}
let remainder = chunks.into_remainder();
// Any remainder will always be less than 8 bytes.
if !remainder.is_empty() {
// Generate one last block of 8 bytes of entropy
let n = self.gen_u64().to_ne_bytes();
// Use the remaining length to copy from block
remainder.copy_from_slice(&n[..remainder.len()]);
}
}
rng_integer!(
u8,
u8,
gen_u32,
gen_mod_u32,
"Generates a random `u8` in the given range."
);
rng_integer!(
u16,
u16,
gen_u32,
gen_mod_u32,
"Generates a random `u16` in the given range."
);
rng_integer!(
u32,
u32,
gen_u32,
gen_mod_u32,
"Generates a random `u32` in the given range."
);
rng_integer!(
u64,
u64,
gen_u64,
gen_mod_u64,
"Generates a random `u64` in the given range."
);
rng_integer!(
u128,
u128,
gen_u128,
gen_mod_u128,
"Generates a random `u128` in the given range."
);
#[cfg(target_pointer_width = "16")]
rng_integer!(
usize,
usize,
gen_u32,
gen_mod_u32,
"Generates a random `usize` in the given range."
);
#[cfg(target_pointer_width = "32")]
rng_integer!(
usize,
usize,
gen_u32,
gen_mod_u32,
"Generates a random `usize` in the given range."
);
#[cfg(target_pointer_width = "64")]
rng_integer!(
usize,
usize,
gen_u64,
gen_mod_u64,
"Generates a random `usize` in the given range."
);
#[cfg(target_pointer_width = "128")]
rng_integer!(
usize,
usize,
gen_u128,
gen_mod_u128,
"Generates a random `usize` in the given range."
);
/// Generates a random `char` in range A-Z.
#[inline]
pub fn uppercase(&mut self) -> char {
const CHARS: &[u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZ";
*self.choice(CHARS).unwrap() as char
}
/// Generates a random `char` in the given range.
///
/// Panics if the range is empty.
#[inline]
pub fn char(&mut self, range: impl RangeBounds<char>) -> char {
let panic_empty_range = || {
panic!(
"empty range: {:?}..{:?}",
range.start_bound(),
range.end_bound()
)
};
let surrogate_start = 0xd800u32;
let surrogate_len = 0x800u32;
let low = match range.start_bound() {
Bound::Unbounded => 0u8 as char,
Bound::Included(&x) => x,
Bound::Excluded(&x) => {
let scalar = if x as u32 == surrogate_start - 1 {
surrogate_start + surrogate_len
} else {
x as u32 + 1
};
char::try_from(scalar).unwrap_or_else(|_| panic_empty_range())
}
};
let high = match range.end_bound() {
Bound::Unbounded => core::char::MAX,
Bound::Included(&x) => x,
Bound::Excluded(&x) => {
let scalar = if x as u32 == surrogate_start + surrogate_len {
surrogate_start - 1
} else {
(x as u32).wrapping_sub(1)
};
char::try_from(scalar).unwrap_or_else(|_| panic_empty_range())
}
};
if low > high {
panic_empty_range();
}
let gap = if (low as u32) < surrogate_start && (high as u32) >= surrogate_start {
surrogate_len
} else {
0
};
let range = high as u32 - low as u32 - gap;
let mut val = self.u32(0..=range) + low as u32;
if val >= surrogate_start {
val += gap;
}
val.try_into().unwrap()
}
}
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