Does it need to be pure mental ? In some cases yes, but if he has time to carefully write himself a note, he probably has time to roll dices or write number on pieces of paper, fold them, mix them, and draw one at random. Or take a random book and look at a random letter of a random page (using some correction algorithm to deal with the difference of letter frequency).
For all practical uses x’=(x*8+1) mod 49 is a simple PRNG that can be executed mentally easily. If you seed it with the next best number you see it gives suitably random numbers for every-day purposes (and when no dice are available). Note that this is taken from TAoCP by Knuth. I use it for fair choices and mental story telling.
It’s not hard to generate random numbers in your head in real life. Generate 5 or 6 “random” numbers from 0 to X-1, add them, and take the result mod X.
I don’t like things which use apparatuses because they introduce a dependency (and since this scheme is for use in extreme/unusual circumstances, it’s especially likely that Harry would not have leisure time or access to his pouch) and they make part of the process observable, hence, easier to realize the existence of & reverse-engineer.
A fully mental PRNG is doable under all circumstances in under a second and is unobservable except via Legilimency (which if it isn’t blocked, means one is screwed anyway since one can just be False-memory-charmed into remembering having done the verification*).
Does it need to be pure mental ? In some cases yes, but if he has time to carefully write himself a note, he probably has time to roll dices or write number on pieces of paper, fold them, mix them, and draw one at random. Or take a random book and look at a random letter of a random page (using some correction algorithm to deal with the difference of letter frequency).
For all practical uses x’=(x*8+1) mod 49 is a simple PRNG that can be executed mentally easily. If you seed it with the next best number you see it gives suitably random numbers for every-day purposes (and when no dice are available). Note that this is taken from TAoCP by Knuth. I use it for fair choices and mental story telling.
It’s not hard to generate random numbers in your head in real life. Generate 5 or 6 “random” numbers from 0 to X-1, add them, and take the result mod X.
I don’t like things which use apparatuses because they introduce a dependency (and since this scheme is for use in extreme/unusual circumstances, it’s especially likely that Harry would not have leisure time or access to his pouch) and they make part of the process observable, hence, easier to realize the existence of & reverse-engineer.
A fully mental PRNG is doable under all circumstances in under a second and is unobservable except via Legilimency (which if it isn’t blocked, means one is screwed anyway since one can just be False-memory-charmed into remembering having done the verification*).
* Kripkenstein would approve!