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Ali.Z



Joined: 08 Jan 2018
Posts: 687
Ali.Z 20 Feb 2023, 14:44
Code:
A = 41A7h ; for better statistics use BC8Fh
M = 7FFFFFFFh
Q = M / A
R = M mod A

pmrand:
mov eax,[s]
mov ecx,Q
cdq ;fixed -2 issue
idiv ecx
imul eax,R
imul edx,A
xchg eax,edx
sub eax,edx
jns @f
add eax,M
@@:
mov [s],eax
ret ;eax ret value

;...

s dd 1 ;must be non-zero and not equ to M
    

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Last edited by Ali.Z on 21 Feb 2023, 05:45; edited 3 times in total
Post 20 Feb 2023, 14:44
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 20218
Location: In your JS exploiting you and your system
revolution 21 Feb 2023, 03:58
A input value of -2 in [s] will generate zero. And the same for the (-2) shr 1 (= 0x7fffffff).

What do you mean by "better statistics"? Which statistics are you referring to?
Post 21 Feb 2023, 03:58
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Ali.Z



Joined: 08 Jan 2018
Posts: 687
Ali.Z 21 Feb 2023, 05:10
input value of 0x7FFFFFFF should not generate 0, this is silly mistake by me; I will fix it. (but there is no difference, even if I fix it, it would repeat 0x7FFFFFFF forever)
as for -2 generating 0, I really don't know; I will figure it out later.

better number distribution statistics (weighting).
note: it returns a value in open interval (0,M)

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Last edited by Ali.Z on 21 Feb 2023, 05:23; edited 2 times in total
Post 21 Feb 2023, 05:10
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bitRAKE



Joined: 21 Jul 2003
Posts: 3977
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bitRAKE 21 Feb 2023, 05:13
So, [s] range would be [1,0x7FFFFFFE], making 0x7FFFFFFF an invalid input?
Post 21 Feb 2023, 05:13
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Ali.Z



Joined: 08 Jan 2018
Posts: 687
Ali.Z 21 Feb 2023, 05:18
yes.

Ali.Z wrote:
I will fix it. (but there is no difference, even if I fix it, it would repeat 0x7FFFFFFF forever)


actually getting zero is good (if initial state is equ M), but if anyone don't want to get zero, then replace jns with jg. (in real world, it would never generate 0, the only way to get zero is by having initial state equ to M)

and fixed -2 issue.

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Post 21 Feb 2023, 05:18
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bitRAKE



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bitRAKE 21 Feb 2023, 15:37
The original Park-Miller article from 1988 can be found here:
http://www.cems.uwe.ac.uk/~irjohnso/coursenotes/ufeen8-15-m/p1192-parkmiller.pdf

... it says the A value was originally suggested in 1969.
Post 21 Feb 2023, 15:37
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Ali.Z



Joined: 08 Jan 2018
Posts: 687
Ali.Z 21 Feb 2023, 16:46

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Post 21 Feb 2023, 16:46
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bitRAKE



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bitRAKE 21 Feb 2023, 17:45
Nice, that links to the 1969 paper - which outlines the original testing method.
(Lehmer actually suggests 14^29 for the A, in 1954.)

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Post 21 Feb 2023, 17:45
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