XorShift.cpp

/*
   Copyright (c) 2010-2013, Laurent Debacker
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#include "XorShift.h"

/*	From http://b2d-f9r.blogspot.com/2010/08/16-bit-xorshift-rng-now-with-more.html:

	So, in Marsaglia's paper, the basic xorshift rng the code is:
	y^=(y<<a); y^=(y>>b); return (y^=(y<<c));

	I looked at this in my last post, and had a brief look at the (a,b,c) triplets to use. However,
	this only yields a period length of the word size used. In my case, that's 2^16-1 -- not much.
	However, further on in the paper, section 3.1, a method is described for longer periods through
	additional state and promotion:
	t=(x^(x<<a)); x=y; return y=(y^(y>>c))^(t^(t>>b));

	This is listed as 2^64-1 using 32-bit words, but there's no reason we can't use 16-bit words.
	This reduces the period to 2^32-1, and requires new triplets. I scanned (1,1,1) through till
	(15,15,15), here's the list:

	(1,1,7) (1,1,12) (1,1,13) (2,5,8) (2,5,13) (2,13,15) (2,15,13) (3,7,6)
	(5,3,1)* (5,3,8) (5,3,13)* (5,7,4)* (6,3,8)* (7,1,6) (7,1,15) (7,2,1)
	(8,3,9)* (9,14,5) (11,8,5)* (13,12,3) (14,1,15) (15,10,1)

	Values marked with an asterisk score well on the majority of diehard tests. It's probably not
	a good idea to use the other values, even though they do have a complete period. Notably,
	all values fail binary matrices tests for 31x31 and 32x32 - I think this is expected,
	and mentioned in Marsaglia2003.

	So, the routine in C:
	uint16_t rnd_xorshift_32() {
	  static uint16_t x=1,y=1;
	  uint16_t t=(x^(x<<5)); 
	  x=y; 
	  return y=(y^(y>>1))^(t^(t>>3));
	}
*/

XorShiftClass XorShift;

XorShiftClass::XorShiftClass()
{
	x = 0;
	y = 1;
}

void XorShiftClass::seed(uint16_t value)
{
	if(!x & !(y ^= value))
		y = 1;
}

void XorShiftClass::seed(uint32_t value)
{
	if(!(x ^= value) & !(y ^= value >> 16))
		y = 1;
}

void XorShiftClass::random(uint16_t *r)
{
	uint16_t t = x^(x<<5);
	x = y;
	*r = y = (y^(y>>1))^(t^(t>>3));
}

void XorShiftClass::random(uint32_t *r)
{
	random((uint16_t*)r);
	random((uint16_t*)r + 1);
}