( pseudo-random number generator,
  based on two 16-bit xorshift algorithms by George Marsaglia
  http://www.jstatsoft.org/v08/i14/paper )

( devices )

|00 @System     [ &vector $2 &wst      $1 &rst    $1 &pad   $4 &r      $2 &g      $2 &b    $2 &debug  $1 &halt $1 ]
|20 @Screen     [ &vector $2 &width    $2 &height $2 &pad   $2 &x      $2 &y      $2 &addr $2 &pixel  $1 &sprite $1 ]
|b0 @DateTime   [ &year   $2 &month    $1 &day    $1 &hour  $1 &minute $1 &second $1 &dotw $1 &doty   $2 &isdst $1 ]

( variables )

|0000

( program )

|0100 ( -> )
	( init )
	;on-frame .Screen/vector DEO2

	( seed prng (must be nonzero) )
	#00 .DateTime/second DEI
	#00 .DateTime/minute DEI #60 SFT2 EOR2
	#00 .DateTime/hour   DEI #c0 SFT2 EOR2 ;prng2/x STA2
	#00 .DateTime/hour   DEI #04 SFT2
	#00 .DateTime/day    DEI #10 SFT2 EOR2
	#00 .DateTime/month  DEI #60 SFT2 EOR2
	#00 .DateTime/year   DEI #a0 SFT2 EOR2 ;prng2/y STA2
	;prng2/x LDA2 ;prng2/y LDA2 EOR2
	ORAk ,&non-zero JCN INC2 &non-zero
	;prng/seed STA2

	( theme )
	#0fe5 .System/r DEO2
	#0fc5 .System/g DEO2
	#0f25 .System/b DEO2
	BRK

@on-frame ( -> )
	#c0
	&loop
		,draw-pixel JSR
		INC
		DUP ,&loop JCN
	POP
	BRK

@draw-pixel
	,prng2 JSR
	#00 SWP .Screen/x DEO2
	#00 SWP .Screen/y DEO2
	#01 .Screen/pixel DEO
	JMP2r

@prng ( -- number* )
	( returns the next number in a 65,535-long sequence,
	  which is never zero but every other 16-bit number
	  appears once before the sequence repeats )
	( http://www.retroprogramming.com/2017/07/ xorshift-pseudorandom-numbers-in-z80.html )
	,&seed LDR2
	DUP2 #70 SFT2 EOR2
	DUP2 #09 SFT2 EOR2
	DUP2 #80 SFT2 EOR2
	,&seed STR2k POP
	JMP2r

	&seed $2

@prng2 ( -- number* )
	( returns the next number in a (2^32-1)-long sequence )
	( http://b2d-f9r.blogspot.com/2010/08/ 16-bit-xorshift-rng-now-with-more.html )
	,&x LDR2
	DUP2 #50 SFT2 EOR2
	DUP2 #03 SFT2 EOR2
	,&y LDR2 DUP2 ,&x STR2
	DUP2 #01 SFT2 EOR2 EOR2
	,&y STR2k POP
	JMP2r

	&x $2
	&y $2