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--- cw4:4rpl_tools [2025/06/30 20:12] – [Pseudo Random Number Generator, based on sinus] kalli
+++ cw4:4rpl_tools [2025/08/18 16:56] (current) – [Pseudo RNG based on Linear congruential generator (LCG)] kalli
@@ Line 3972: / Line 3972: @@
 	# abs <-power pow <-add add sin <-sinMul mul dup floor sub
 .05 pow 99419 sub sin 619 mul dup floor sub
+</code>
+</hidden>
+----
+===== Pseudo RNG based on Linear congruential generator (LCG) =====
+The downside of the above sinus based functions, is that they have few resulting digits. They are more than adequate to pick a random cell on a map (max 512 possibilities), but unsuited for generating random numbers larger than a few thousand.
+The below LCG functions should work for up to 6 digits = 10^6, but I've only done a test up to 10^5. Rolling 1 million random integers between 0 and 10^5 with @lcgRandInt, missed only 8 possible results. The same test with @sinRandInt missed 42458 possible results.
+<hidden click here for source code>
+<code LinearCongruentialGenerator.4rpl>
+# Quick Example.
+10000 @lcgRandInt
+:lcgRandInt # INPUT: integer first randInt + integer last randInt. OUTPUT: an integer in between the first and last randInt, excluding the last randInt.
+->last
+->first
+@indexLcgRand01 <-last <-first sub mul <-first add asint
+:lcgRand01 # INPUT: none.
+@indexLcgRand01
+:indexLcgRandInt # INPUT: the index of the seed sequence + integer first randInt + integer last randInt. OUTPUT: an integer in between the first and last randInt, excluding the last randInt.
+->last
+->first
+	@indexLcgRand01 <-last <-first sub mul <-first add asint
+:indexLcgRand01 # INPUT: the "index" of the seed sequence. OUTPUT: the next seed from that sequence.
+# The random numbers will be generated with the previous seed that is stored under that index.
+->i
+	<-prevSeedList[<-i] eq0 if
+		<-i <-mapConstant add 13 mul @seededLcgRand01 dup 10000000 mul 1 add asint ->prevSeedList[<-i]
+->seedCountList[<-i]
+	else
+		<-prevSeedList[<-i] @seededLcgRand01 dup 10000000 mul <-seedCountList[<-i] 1 add dup ->seedCountList[<-i] add asint ->prevSeedList[<-i]
+	endif
+:clearIndexSinLcgRand
+	<-seedCountList clearlist
+	<-prevSeedList clearlist
+:spikedLcgRandInt # INPUT: integer first randInt + integer last randInt. OUTPUT: an integer in between the first and last randInt, excluding the last randInt.
+->last
+->first
+	elapsedtime dup 10 log ceil 7 sub neg 10 swap pow mul <-first <-last @seededLcgRandInt
+:spikedLcgRand01
+	elapsedtime dup 10 log ceil 7 sub neg 10 swap pow mul @seededLcgRand01
+:seededLcgRandInt # INPUT: integer seed + integer first randInt + integer last randInt. OUTPUT: an integer in between the first and last randInt, excluding the last randInt.
+->last
+->first
+	@seededLcgRand01 <-last <-first sub mul <-first add asint
+:seededLcgRand01
+	asint 1103515245 mul 12345 add abs asfloat 2147483647 div 10 mul dup floor sub
+:once
+# Creating lists and a starting mapconstant for the seed sequences.
+	createlist ->prevSeedList
+	createlist ->seedCountList
+	getmapsize 2 div swap 2 div swap dup2 getterrain 1 add dup 99999 floodfillterrain getlistcount ->mapConstant
+	# <-SEED ->mapconstant # Alternative for map constant.
 </code>

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