From 5b4ec0be6b7ca9740fa72d99cfa9d9392dd5d14d Mon Sep 17 00:00:00 2001
From: Andrew Alderwick <andrew@alderwick.co.uk>
Date: Wed, 25 Aug 2021 21:59:30 +0100
Subject: [PATCH] Added a second PRNG with longer period

---
 projects/examples/demos/prng.tal | 43 +++++++++++++++++++++++---------
 1 file changed, 31 insertions(+), 12 deletions(-)

diff --git a/projects/examples/demos/prng.tal b/projects/examples/demos/prng.tal
index 051a23e..2565811 100644
--- a/projects/examples/demos/prng.tal
+++ b/projects/examples/demos/prng.tal
@@ -1,7 +1,6 @@
-( pseudo-random number generator )
-
-( based on 16-bit xorshift algorithm discussed on
-  http://www.retroprogramming.com/2017/07/xorshift-pseudorandom-numbers-in-z80.html )
+( pseudo-random number generator,
+  based on two 16-bit xorshift algorithms by George Marsaglia
+  http://www.jstatsoft.org/v08/i14/paper )
 
 ( devices )
 
@@ -18,17 +17,22 @@
 |0100 ( -> )
 	( init )
 	;on-frame .Screen/vector DEO2
+
 	( seed prng (must be nonzero) )
-	.DateTime/month DEI .DateTime/day DEI
-	#00 .DateTime/second #a0 SFT2 EOR2
-	#00 .DateTime/minute #40 SFT2 EOR2
-	.DateTime/hour EOR
+	#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 
+	#0fe5 .System/r DEO2
+	#0fc5 .System/g DEO2
 	#0f25 .System/b DEO2
 	BRK
 
@@ -42,16 +46,17 @@
 	BRK
 
 @draw-pixel
-	,prng JSR
+	,prng2 JSR
 	#00 SWP .Screen/x DEO2
 	#00 SWP .Screen/y DEO2
 	#01 .Screen/pixel DEO
 	JMP2r
 
 @prng ( -- number* )
-	( returns the next number in the 65,535-long sequence,
+	( 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
@@ -61,3 +66,17 @@
 
 	&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
+