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From: "Daniel Shahaf" <d.s@daniel.shahaf.name>
To: zsh-workers@zsh.org
Subject: Re: [PATCH] zsh/random module [UPDATED]
Date: Thu, 24 Nov 2022 00:17:57 +0000	[thread overview]
Message-ID: <41712a61-75ab-4825-bbbc-d223823229ea@app.fastmail.com> (raw)
In-Reply-To: <aec52e64-dcd2-4893-8fca-4128e590e12b@app.fastmail.com>

Daniel Shahaf wrote on Wed, 23 Nov 2022 23:54 +00:00:
> dana wrote on Wed, 23 Nov 2022 21:42 +00:00:
>> On Wed 23 Nov 2022, at 13:46, Daniel Shahaf wrote:
>>> Oh, and bump that 16 to something 3 or 4 times as big, because a 1/65536
>>> chance isn't really enough in a world where automated builds (CI,
>>> distros' QA, etc.) is a thing.
>>
>> I feel like it should be very nearly impossible for a test to fail just for
>> randomness reasons. Maybe it's over-kill but in my draft reply to the patch i
>> was going to suggest something like this:
>>
>>   () {
>>     repeat $(( 10 ** 5 )); do
>>       getrandom -L4 -U5 -c64 -a tmpa
>>       [[ $tmpa[(r)5] == 5 ]] && return 0
>>     done
>>     return 1
>>   }
>>
>
> No maybe about it :)
>
> With these parameters, the probability of a false positive is 2 to the
> power of minus the overall number of iterations, i.e., 2**(-6.4 million),
> which is 1/[a number that has 1.9M decimal digits].
>
> To be clear, it's not 1/1.9M, which is about the probability of a random
> Londoner being at 10 Downing Street right now.  It's 1/[10 ** 1.9M],
> which is about the probability of correctly guessing the genders of all
> Londoners.

To be clear, I don't mean guessing the gender /ratio/.  I mean filling
a spreadsheet with one row for each Londoner and having to guess each and
every row correctly, without any information about the particular Londoner.

> If you converted the entire Earth's mass to CPUs and ran «getrandom -L4
> -U5 -c64» on it repeatedly until Sol died, and the CPUs all operated at
> 4GHz, and there were no bugs in anything, the chance of getting a single
> run to not return a 5 would still be something like a billion to one
> (give or take several zeroes depending on CPU mass, the argument to -c,
> and so on).
>
> That's why in practice, if a single -c64 call ever doesn't return a 5,
> it's safe to assume there's a bug.
>
> Conversely, if you actually retain those 6.4 million iterations, what's
> the probability that the outer loop will return 0 on the first iteration
> and then a gamma ray will flip that to 0?


  reply	other threads:[~2022-11-24  0:18 UTC|newest]

Thread overview: 46+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-11-02 17:13 [PATCH] zsh/random module Clinton Bunch
2022-11-03 17:50 ` Bart Schaefer
2022-11-04  3:17 ` dana
2022-11-04  6:22   ` Clinton Bunch
2022-11-04  7:27     ` dana
2022-11-04 12:57       ` Clinton Bunch
2022-11-08  0:18         ` [PATCH] zsh/random module [UPDATED] Clinton Bunch
2022-11-18 14:30           ` Clinton Bunch
2022-11-19  6:42             ` Lawrence Velázquez
2022-11-18 16:23           ` Stephane Chazelas
2022-11-18 17:08             ` Clinton Bunch
2022-11-18 18:12               ` Stephane Chazelas
2022-11-18 18:38                 ` Clinton Bunch
2022-11-23 19:52                   ` Daniel Shahaf
2022-11-24 16:19                     ` Stephane Chazelas
2022-11-24 16:30                       ` Roman Perepelitsa
2022-11-24 22:39                         ` Clinton Bunch
2022-11-25  8:53                           ` Stephane Chazelas
2022-11-25  9:40                             ` Stephane Chazelas
2022-11-28 16:37                               ` further discussion of zsh/random (was [PATCH] zsh/random module [UPDATED]) Clinton Bunch
2022-11-21  1:07           ` [PATCH] zsh/random module [UPDATED] Matthew Martin
2022-11-21  1:59             ` Clinton Bunch
2022-11-21  2:21               ` Matthew Martin
2022-11-21  2:57                 ` Clinton Bunch
2022-11-21  3:14                   ` Lawrence Velázquez
2022-11-21  4:17                     ` Bart Schaefer
2022-11-21  5:05                       ` Clinton Bunch
2022-11-22 13:42                         ` dana
2022-11-23 19:49                         ` Daniel Shahaf
2022-11-22 17:44                       ` Oliver Kiddle
2022-11-22 19:48                         ` Clinton Bunch
2022-11-23  1:23                   ` Matthew Martin
2022-11-23  2:58                     ` Clinton Bunch
2022-11-23  4:14                       ` Matthew Martin
2022-11-23 13:41                         ` Clinton Bunch
2022-11-23 20:33                           ` Daniel Shahaf
2022-11-23 21:42                             ` dana
2022-11-23 23:54                               ` Daniel Shahaf
2022-11-24  0:17                                 ` Daniel Shahaf [this message]
2022-11-24  1:05                                 ` dana
2022-11-24 13:52                               ` Clinton Bunch
2022-11-23 19:46           ` Daniel Shahaf
2022-11-24  2:58             ` Clinton Bunch
2022-11-24 10:07               ` nimaje+zml
2022-11-24 13:19                 ` Clinton Bunch
2022-11-24 14:33             ` Clinton Bunch

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