; Xorshift is a class of pseudorandom number generators discovered
; by George Marsaglia and detailed in his 2003 paper, Xorshift RNGs.
;
; 16-bit xorshift pseudorandom number generator by John Metcalf
; returns   hl = pseudorandom number
; corrupts   a

; generates 16-bit pseudorandom numbers with a period of 65535
; using the xorshift method:

; hl ^= hl << 7
; hl ^= hl >> 9
; hl ^= hl << 8

; some alternative shift triplets which also perform well are:
; 6, 7, 13; 7, 9, 13; 9, 7, 13.

	psect	text

	GLOBAL	_xrnd, _xrndseed

_xrnd:
	ld	hl,1		; seed must not be 0
	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
	ld	(_xrnd+1),hl
	res	7,h		;make-it positive...
	ret

_xrndseed:
	ld	a,r
  	ld	l,a
	ld	a,r
	ld	h,a
  	or	l		; HL must be not NULL
  	jr	nz,1f
	inc	hl
1:
	ld	(_xrnd+1),hl
	ret