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Pseudo random sign flip
Posted: Fri Feb 20, 2015 7:25 am
by wilbert
I needed this for myself. Maybe someone else can use it as well.
What the procedure does is flip the sign of the value on a pseudo random basis.
So it flips, or it doesn't
Code: Select all
Procedure.i PRSF(value.i); Pseudo random sign flip
Static RN.l = $72486161
EnableASM
mov edx, RN
!lea eax, [edx + edx]
!sar edx, 31
!and edx, 0xc0000401
!xor edx, eax
mov RN, edx
!sar edx, 31
CompilerIf #PB_Compiler_Processor = #PB_Processor_x64
!movsx rdx, dx
!mov rax, [p.v_value]
!xor rax, rdx
!sub rax, rdx
CompilerElse
!mov eax, [p.v_value]
!xor eax, edx
!sub eax, edx
CompilerEndIf
DisableASM
ProcedureReturn
EndProcedure
Example
Code: Select all
L.l = 123
For i = 1 To 400
L = PRSF(L)
Debug L
Next
Re: Pseudo random sign flip
Posted: Fri Feb 20, 2015 7:35 am
by StarBootics
Nice but did you know that you can put the example on the same code as your procedure like this :
Code: Select all
CompilerIf #PB_Compiler_IsMainFile
L.l = 123
For i = 1 To 400
L = PRSF(L)
Debug L
Next
CompilerEndIf
Best regards
StarBootics
Re: Pseudo random sign flip
Posted: Fri Feb 20, 2015 11:25 am
by kvitaliy
Code: Select all
Procedure PRSF(value.i)
If Random(1,0)
ProcedureReturn -1 * value
Else
ProcedureReturn value
EndIf
EndProcedure
;Example
L.l = 123
For i = 1 To 400
L = PRSF(L)
Debug L
Next
Re: Pseudo random sign flip
Posted: Fri Feb 20, 2015 11:37 am
by wilbert
@StarBootics,
I have seen it before on the forum but usually don't use it myself.
Thanks for mentioning.
@kvitality,
The procedure was created for a situation where it would be called very often.
Your approach works fine and probably gives better random values but is 4-5 times slower on my computer.
Re: Pseudo random sign flip
Posted: Fri Feb 20, 2015 12:02 pm
by luis
@kvitality
Who would have thought of that, it's amazing !
But really. Who would have thought of that ? The multiplication is not needed. It's there just to wast CPU cycles I suppose.
Shame on you Wilbert for not thinking about that and writing the code in asm.
Re: Pseudo random sign flip
Posted: Fri Feb 20, 2015 12:04 pm
by kvitaliy
wilbert wrote: is 4-5 times slower on my computer.
If speed is important, you're right
Re: Pseudo random sign flip
Posted: Fri Feb 20, 2015 6:12 pm
by BasicallyPure
Perhaps a macro would be faster than a procedure but not as fast as Wilbert's code of course.
Code: Select all
Macro RandomSign(value)
value * (-1 + Random(1)<<1)
EndMacro
For n = 1 To 100
x = RandomSign(n)
Debug x
Select Sign(x)
Case 1 : pos.i + 1
Case -1 : neg.i + 1
EndSelect
Next n
Debug "positive results = " + pos
Debug "negative results = " + neg
Re: Pseudo random sign flip
Posted: Fri Feb 20, 2015 6:28 pm
by Little John
BasicallyPure wrote:if the multiplication is not there then how would the procedure return the negative of 'value'.
For instance
Re: Pseudo random sign flip
Posted: Fri Feb 20, 2015 6:33 pm
by luis
BasicallyPure wrote:
I disagree, if the multiplication is not there then how would the procedure return the negative of 'value'.
Reconsider it, quick !
I forgot the obligatory facial expression in this case:
BTW: even the "else" was not needed.
Re: Pseudo random sign flip
Posted: Fri Feb 20, 2015 6:47 pm
by BasicallyPure
I realized the answer after I posted but didn't complete the edit fast enough.
