xorshiro128+.rs

// Based on a C implementation written by David Blackman and Sebastiano
// Vigna (vigna@acm.org): http://xoroshiro.di.unimi.it/xoroshiro128plus.c
// Translated to Rust by Yuri Vishnevsky. Code kept close to the original.

/* This is the successor to xorshift128+. It is the fastest full-period
   generator passing BigCrush without systematic failures, but due to the
   relatively short period it is acceptable only for applications with a
   mild amount of parallelism; otherwise, use a xorshift1024* generator.

   Beside passing BigCrush, this generator passes the PractRand test suite
   up to (and included) 16TB, with the exception of binary rank tests,
   which fail due to the lowest bit being an LFSR; all other bits pass all
   tests. We suggest to use a sign test to extract a random Boolean value.
   
   Note that the generator uses a simulated rotate operation, which most C
   compilers will turn into a single instruction. In Java, you can use
   Long.rotateLeft(). In languages that do not make low-level rotation
   instructions accessible xorshift128+ could be faster.

   The state must be seeded so that it is not everywhere zero. If you have
   a 64-bit seed, we suggest to seed a splitmix64 generator and use its
   output to fill s. */

// Notes:
// - The Rust nursery wants to add an implementation:
// https://github.com/rust-lang-nursery/rand/commit/e161873c10b84369451b4af0b42282436ccaa31b
// - The comment below mentions a relatively short period; it is 2^128 - 1.

pub struct Xoroshiro128PlusRng {
	state: [u64; 2]
}
impl Xoroshiro128PlusRng {
	pub fn new(s0: u64, s1: u64) -> Xoroshiro128PlusRng {
		Xoroshiro128PlusRng { state: [s0, s1] }
	}

	#[inline(always)]
	pub fn next(&mut self) -> u64 {
		let s0 = self.state[0];
		let s1 = self.state[1];
		let result = s0.wrapping_add(s1);

		let s1 =  s1 ^ s0;
		self.state[0] = s0.rotate_left(55) ^ s1 ^ (s1 << 14); // a, b
		self.state[1] = s1.rotate_left(36); // c

		result
	}

	pub fn jump(&mut self) {
		const JUMP: [u64; 2] = [0xbeac0467eba5facb, 0xd86b048b86aa9922];

		let mut s0 = 0;
		let mut s1 = 0;
		for j in JUMP.iter() {
			for b in 0..64 {
				if j & (1u64 << b) != 0 {
					s0 ^= self.state[0];
					s1 ^= self.state[1];
				}
				self.next();
			}
		}

		self.state[0] = s0;
		self.state[1] = s1;
	}
}