rndi.mac

		TITLE	rndi (integer):integer - 27 May 87 - MT
		SUBTTL	'Psudo random number generator'
;
		INCLUDE	SYMBOLS
		DEFB	'%%RNDI 3.1 - 18 Aug 91%%'
;
;-- Function to generate 16-bit psudo random numbers.
;
;   This program is free software: you can redistribute it and/or modify it
;   under  the terms of the GNU General Public License as published by  the
;   Free Software Foundation, either version 3 of the License, or (at  your
;   option) any later version.
;
;   This  program  is distributed in the hope that it will be  useful,  but
;   WITHOUT   ANY   WARRANTY;  without  even  the   implied   warranty   of
;   MERCHANTABILITY  or  FITNESS  FOR A PARTICULAR  PURPOSE.  See  the  GNU
;   General Public License for more details.
;
;   You should have received a copy of the GNU General Public License along
;   with this program.  If not, see <http://www.gnu.org/licenses/>.
;
;   27 May 87	3.1   - Initial version - MT
;   18 Aug 91	      - Modified  to use call by value rather than call  by
;                       referance - MT
;   25 Dec 20	4.0   - Tidied  up  the  code  to use  the  current  symbol
;                       definitions and updated the formatting - MT
;		4.1   - Replaced the existing psudo random number generator
;			with a more modern one that gives linger cycles and
;                       a more even distrobution. 
;
;-- Generate  a  psudo  random number  using  George  Marsaglia's  xorshift 
;   algorythm  to  generate  a 16-bit psudo random  number.  This  combines
;   three  xorshift  operations where a  number is exclusive-or'ed  with  a
;   shifted copy of itselfthe previous value as the seed.
;
;	unsigned xorshift(n)
;	{
;		n ^= n << 7;
;		n ^= n >> 9;
;		n ^= n << 8;
;		return n;
;	}
;
RNDI::		FUNCTION		; Return value in HL.
		LD	A,H	
		RRA
		LD	A,L
		RRA
		XOR	H
		LD	H,A
		LD	A,L
		RRA
		LD	A,H
		RRA
		XOR	L
		LD	L,A
		XOR	H
		LD	H,A
		RET
;
		END