[fpc-devel] Different results of random(int32) and random(int64) for negative limit value

[Thaddy de Koning]

Of course 64 and 32 bit are the sizes, not the platform! That may not be 
clear.


On 5/24/2017 9:35 AM, Thaddy de Koning wrote:
>
> Jonas, sorry for the late response:
>
> The implementation is _*not *_undefined for negative values,_unless 
> you say that you define it as undefined_.
>
> Because you seem to have implemented it or most of it.
>
> It renders a mathematical comparable distribution in the negative to 
> the positive values.
>
> In both Turbo Pascal as in Delphi and because they use a different 
> algorithm and made an implementation error as well, the negative 
> values are indeed not defined. But that's because of the algorithm and 
> because of the implementation by Borland (yes, it stems from Borland 
> times).
>
> The Mersenne Twister we use is also valid for negative values and if 
> you want I can send you the mathematical proof.
>
> I already made the LCG in Delphi compatible mode available on the wiki 
> and that implementation differs in so far as that it corrects the 
> "undefined for negative values" for that algorithm too. It is 100% 
> compatible for the Delphi documented range, btw.
>
> I am busy evaluating important Random implementions for different 
> languages, so an FPC library is available for data that is generated 
> in a different language and relies on a particular PRNG.
>
> Also note that the output of the current random is strictly valid for 
> 32 bit only.
>
> In my code I already added a 64 bit version.
>
> Regards,
>
> Thaddy
>
>
>
>
> On 5/20/2017 2:57 PM, Jonas Maebe wrote:
>> On 20/05/17 14:36, Martin Schreiber wrote:
>>> Is this intended? If not, which one is correct?
>>
>> random(x) is undefined for negative parameters. It should have had an 
>> unsigned parameter, like in Turbo Pascal (where it is word). Delphi 
>> defines it as always returning a positive value, but I don't know 
>> what happens if you pass a negative parameter there.
>>
>>
>> Jonas
>> _______________________________________________
>> fpc-devel maillist  - fpc-devel at lists.freepascal.org
>> http://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
>

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