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From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Newsgroups: comp.lang.javascript
Subject: Re: Random numbers
Date: Sat, 28 Oct 2017 18:54:32 +0200
Organization: PointedEars Software (PES)
Lines: 109
Message-ID: <2010509.iZASKD2KPV@PointedEars.de>
References: <osa2ld$6t0$1@gioia.aioe.org> <ose4is$14l2$1@gioia.aioe.org> <19e8d664-5c4b-4118-99de-5133a2317841@googlegroups.com> <osipj9$lfm$1@gioia.aioe.org> <random-20171022231003@ram.dialup.fu-berlin.de> <ot1sdu$1qkk$1@gioia.aioe.org>
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emf wrote:
^^^
It is considered polite here to post using one’s real name.

> function random(top) {
>      var now '= new Date();

Syntax error, see below.

>      var tim = now.getTime();
>      var 4rhdm;

Unused variable.  Use JSHint: <https://en.wikipedia.org/wiki/JSHint> p.

>      if (top != 0) {
>          rndm = (tim % top) + 1;
>      } else {
>          rndm = 0;  // garbage in, garbage out
>      }
>      return rndm;
> }
> 
> […]
> I do not know how Math.random() gets its pseudorandom results,

Presumably (the equivalent of) /dev/random.

> but I am sure this function's results should be better.

You are wrong.  *Dead* wrong.  And the fact that you did not realize it does 
not bode well for your being able to create a good PRNG.

> What do you think about using centiseconds?

Even greater nonsense, see below.

Using the modulo operation (“%”) on a *predictably* increasing numeric value 
like the return value of “new Date().getTime()” anywhere in a generator will 
get you *predictably* consecutive results belonging to *neighboring* 
equivalence classes in a modulus ring.  So there is _nothing_ *random* about 
it.

For example:

| -> var input = Array.apply(null, {length: 10}).map(function (e, i) {
|   return i;
| });
| -> input
| <- [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
| 
| -> input.map(function (e) { return e % 3; })
| <- [0, 1, 2, 0, 1, 2, 0, 1, 2, 0]

Here the modulus ring is

     [0]
    /   \
  [2]---[1]

whereas [0] means the equivalence class to which all e’s belong for which

  e ≡ 0 (mod 3)

holds; obviously, e ∈ {3n | n ∈ ℤ}. ¹)

With your modification, it is just

| -> input.map(function (e) { return (e % 3) + 1; })
| <- [1, 2, 3, 1, 2, 3, 1, 2, 3, 1]

     [1]
    /   \
  [3]---[2]

instead.

Obviously, if the dividend of Euclidean division is much greater than the 
divisor (which is the modulus), or the divisor is not much less than the 
dividend, it gets only worse:

| -> Array.apply(null, {length: 10}).map(function () {
|   return new Date().getTime();
| }).map(function (e) { return (e % 3) + 1; })
| [2, 2, 3, 3, 3, 3, 3, 3, 3, 3]

And with centiseconds it is even worse:

| -> Array.apply(null, {length: 10}).map(function () {
|   return Math.round(new Date().getTime() / 10);
| }).map(function (e) { return (e % 3) + 1; })
| [3, 3, 3, 3, 3, 3, 3, 3, 3, 3]

Very random, indeed :->

<https://en.wikipedia.org/wiki/Modulo_operation>
<https://en.wikipedia.org/wiki/Congruence_relation>
 
> […]
> -- '

Your “'” key is borked.

_____
¹) BTW, Chrome Dev Tools appear to be able now to tell 0 and −0 apart.
   Enter “-0” or “-1 % 1”: the result should be “-0”, not “0”.
-- 
PointedEars
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