nanorand 0.3.0

A tiny, fast, zero-dep library for random number generation.
Documentation 
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// Based off lemire's wyrand C++ code at https://github.com/lemire/testingRNG/blob/master/source/wyrand.h

use super::RNG;

/// An instance of the wyrand random number generator.  
/// Seeded from the system entropy generator when available.  
/// **This generator is _NOT_ cryptographically secure.**
pub struct WyRand {
	seed: u64,
}

impl WyRand {
	/// Create a new [`WyRand`] instance, seeding from the system's default source of entropy.
	#[cfg(feature = "std")]
	pub fn new() -> Self {
		let mut entropy: [u8; std::mem::size_of::<u64>()] = Default::default();
		entropy.copy_from_slice(&crate::entropy::entropy_from_system(
			std::mem::size_of::<u64>(),
		));
		Self {
			seed: u64::from_ne_bytes(entropy),
		}
	}

	/// Create a new [`WyRand`] instance, using a provided seed.
	pub fn new_seed(seed: u64) -> Self {
		Self { seed }
	}
}

#[cfg(feature = "std")]
impl Default for WyRand {
	/// Create a new [`WyRand`] instance, seeding from the system's default source of entropy.
	fn default() -> Self {
		let mut entropy: [u8; std::mem::size_of::<u64>()] = Default::default();
		entropy.copy_from_slice(&crate::entropy::entropy_from_system(
			std::mem::size_of::<u64>(),
		));
		Self {
			seed: u64::from_ne_bytes(entropy),
		}
	}
}

impl RNG for WyRand {
	type Output = [u8; 8];

	fn rand(&mut self) -> Self::Output {
		self.seed = self.seed.wrapping_add(0xa0761d6478bd642f);
		let t: u128 = (self.seed as u128).wrapping_mul((self.seed ^ 0xe7037ed1a0b428db) as u128);
		let ret = ((t >> 64) ^ t) as u64;
		ret.to_ne_bytes()
	}

	fn rand_with_seed(seed: &[u8]) -> Self::Output {
		let mut seed_bytes = [0u8; 8];
		seed_bytes.iter_mut().zip(seed).for_each(|(a, b)| *a = *b);
		let seed = u64::from_ne_bytes(seed_bytes);
		let seed = seed.wrapping_add(0xa0761d6478bd642f);
		let t: u128 = (seed as u128).wrapping_mul((seed ^ 0xe7037ed1a0b428db) as u128);
		let ret = ((t >> 64) ^ t) as u64;
		ret.to_ne_bytes()
	}

	fn reseed(&mut self, new_seed: &[u8]) {
		let mut seed = [0u8; 8];
		seed.iter_mut().zip(new_seed).for_each(|(a, b)| *a = *b);
		self.seed = u64::from_ne_bytes(seed)
	}
}

impl Clone for WyRand {
	fn clone(&self) -> Self {
		Self { seed: self.seed }
	}
}

#[cfg(feature = "std")]
impl std::fmt::Display for WyRand {
	fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
		write!(f, "WyRand ({:p})", self)
	}
}