A.D.A. Amiga Demoscene Archive

Amiga Demoscene Archive Forum / Coding / floating point random number generator

Author	Message
dalton Member	#1 - Posted: 20 Mar 2010 11:33 Reply Quote Hi, does anybody have a simple routine for generating floating point random numbers? Right now I use the an LFSR generator on the 68k, and move the result over to FPU and scale. It works but I think it's an ugly solution...
rload Member	#2 - Posted: 22 Mar 2010 17:14 Reply Quote Maybe you can just generate the mantissa bits of the float as ints with the usual rotate-subtract which is used in so many 4k intros and such to produce a float between 0 and 1. Then you can get the number in range by multiplying it up afterwards or use the fscale if you only need it in a po2 range. random move.l random_seed,d0 ror.l d0,d0 subq.l #1,d0 ; demoscene random move.l d0,d1 and.l #(1<<24)-1,d1 ; random mantissa bits in the 23 low bits or.l #exponent_bits_and_sign_bit,d1 fmove.s d1,fp0 ; load fp0 with d1's bits. not sure this is the syntax. no format conversion should happen fmul.s #range,fp0 move.l d0,random_seed rts random_seed dc.l 1243462 I'm sorry this is probably really bad code, but maybe a discussion starter for better suggestions. Also it is not tested or assembled or anything.
rload Member	#3 - Posted: 22 Mar 2010 17:17 Reply Quote And come to think of it.. This is probably just the thing you are describing in your post :)
dalton Member	#4 - Posted: 23 Mar 2010 17:29 Reply Quote thanks for the reply! it's almost the same, I do: move.l seed, d0 rol.r d0,d0 addq.l #7,d0 move.l d0, seed fmove.l d0, fp0 fmul.s #1/32768, fp0 ; to get random number in range [-1,1] I sort of like your idea of loading without format conversion though. I believe my code will have much poorer resolution in the floating point number. don't ask me about the difference in ror/rol and addq/subq. I have no idea =)

A.D.A. Amiga Demoscene Archive, Version 3.0