Hello community. Kindly consider: | n i r | n := ( 2 ** 256 ) - 1 . "the largest 256-bit value" i := ( 0 to: n ) . "the interval 0 to 2**256-1" "Produce three sets of computed test results." 3 timesRepeat: [ Transcript show: ( r := i atRandom ) ; cr . Transcript show: ( r asByteArray hex ) ; cr . ] According to comments in Pharo library code, atRandom fails for integer magnitudes greater than 2**30 . A reason for this limitation is not given, and it's not clear to a novice such as myself why the computed result of atRandom is permitted to sometimes be in conflict with the semantics of atRandom . Can anyone provide a basic explanation of the rationale for a 'broken' atRandom ? Alternatively, is this a known bug? Thanks. dr
Try it in Squeak and pick the relevant methods. Open a bug report and commit the fix. Of course, original authorship will somehow be spoiled, so be kind and cite them in commit message. Le lun. 11 févr. 2019 à 07:59, David Richards < david.i.richards.iii@gmail.com> a écrit :
Hello community.
Kindly consider:
| n i r | n := ( 2 ** 256 ) - 1 . "the largest 256-bit value" i := ( 0 to: n ) . "the interval 0 to 2**256-1" "Produce three sets of computed test results." 3 timesRepeat: [ Transcript show: ( r := i atRandom ) ; cr . Transcript show: ( r asByteArray hex ) ; cr . ]
According to comments in Pharo library code, atRandom fails for integer magnitudes greater than 2**30 . A reason for this limitation is not given, and it's not clear to a novice such as myself why the computed result of atRandom is permitted to sometimes be in conflict with the semantics of atRandom .
Can anyone provide a basic explanation of the rationale for a 'broken' atRandom ?
Alternatively, is this a known bug?
Thanks. dr
Thanks Nicolas and Henry, For now, simple alternative expressions can easily generate the values I need for my project, such as: (String new: 32) collect: [ :each | '0123456789abcdef' atRandom ] Maybe in a few months (or years!) from now I will have acquired enough ambient familiarity with Pharo to attempt a fix. But, for the time being, I'll just avoid stepping on the landmine. In doing some research on this problem I spotted this, which seemed to possibly point a way toward a cutting edge RNG implementation for Pharo: http://xoshiro.di.unimi.it/#intro [image: image.png] David On Sun, 10 Feb 2019 at 23:21, Nicolas Cellier < nicolas.cellier.aka.nice@gmail.com> wrote:
Try it in Squeak and pick the relevant methods. Open a bug report and commit the fix. Of course, original authorship will somehow be spoiled, so be kind and cite them in commit message.
Le lun. 11 févr. 2019 à 07:59, David Richards < david.i.richards.iii@gmail.com> a écrit :
Hello community.
Kindly consider:
| n i r | n := ( 2 ** 256 ) - 1 . "the largest 256-bit value" i := ( 0 to: n ) . "the interval 0 to 2**256-1" "Produce three sets of computed test results." 3 timesRepeat: [ Transcript show: ( r := i atRandom ) ; cr . Transcript show: ( r asByteArray hex ) ; cr . ]
According to comments in Pharo library code, atRandom fails for integer magnitudes greater than 2**30 . A reason for this limitation is not given, and it's not clear to a novice such as myself why the computed result of atRandom is permitted to sometimes be in conflict with the semantics of atRandom .
Can anyone provide a basic explanation of the rationale for a 'broken' atRandom ?
Alternatively, is this a known bug?
Thanks. dr
Thanks for this link. Note that there are quite good classical PRNG already programmed for Pharo in package Polymath including a Mersenne twister and Marsaglia KISS as used in gfortran... (I did not check recently but there should be a catalog entry for Polymath). The one programmed in Squeak is a Mersenne Twister I think, carefully written for maximizing performance for 32bits VM (with 31bit SmallIntegers). IMO, the Park-Miller PRNG was a good default choice in the 70s when it was introduced in Smalltalk. But now, we have fast enough machines to not bother with its limitations. Le lun. 11 févr. 2019 à 15:58, David Richards < david.i.richards.iii@gmail.com> a écrit :
Thanks Nicolas and Henry,
For now, simple alternative expressions can easily generate the values I need for my project, such as:
(String new: 32) collect: [ :each | '0123456789abcdef' atRandom ]
Maybe in a few months (or years!) from now I will have acquired enough ambient familiarity with Pharo to attempt a fix. But, for the time being, I'll just avoid stepping on the landmine.
In doing some research on this problem I spotted this, which seemed to possibly point a way toward a cutting edge RNG implementation for Pharo:
http://xoshiro.di.unimi.it/#intro
[image: image.png]
David
On Sun, 10 Feb 2019 at 23:21, Nicolas Cellier < nicolas.cellier.aka.nice@gmail.com> wrote:
Try it in Squeak and pick the relevant methods. Open a bug report and commit the fix. Of course, original authorship will somehow be spoiled, so be kind and cite them in commit message.
Le lun. 11 févr. 2019 à 07:59, David Richards < david.i.richards.iii@gmail.com> a écrit :
Hello community.
Kindly consider:
| n i r | n := ( 2 ** 256 ) - 1 . "the largest 256-bit value" i := ( 0 to: n ) . "the interval 0 to 2**256-1" "Produce three sets of computed test results." 3 timesRepeat: [ Transcript show: ( r := i atRandom ) ; cr . Transcript show: ( r asByteArray hex ) ; cr . ]
According to comments in Pharo library code, atRandom fails for integer magnitudes greater than 2**30 . A reason for this limitation is not given, and it's not clear to a novice such as myself why the computed result of atRandom is permitted to sometimes be in conflict with the semantics of atRandom .
Can anyone provide a basic explanation of the rationale for a 'broken' atRandom ?
Alternatively, is this a known bug?
Thanks. dr
The "random" index for atRandom is, basically, computed prngOutput mod: selectionRange. For large selection ranges, it fails because the period of the PRNG used by atRandom is 2^30th, with the output an integer in the range 1 - 2^30th. (in other words, the prng is a repeating sequence of every number between 1 and 2^30th) That means: - There will be bias when the range you pick from approaches 2^30th. - There will be gaps (indexes that won't ever be picked) once you pass 2^30th. The reason is techical; the default RNG is a reasonable compromise between speed, memory, and usable range. If the intended use requires a larger range, an appropriate random number generator can be provided using atRandom: It certainly could give an error and/or select another RNG automatically if outside applicable range, but no one has been bothered enough to do either, yet. Cheers, Henry -- Sent from: http://forum.world.st/Pharo-Smalltalk-Developers-f1294837.html
participants (3)
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David Richards -
Henrik Sperre Johansen -
Nicolas Cellier