Fix seeding logic in Arr::shuffle

[cviebrock]

The random comparator logic used is not really that random, as it's biased 2:1 in favour of not swapping the two elements.

values before	random comparator	result after
a, b	-1	a, b
a, b	0	a, b
a, b	1	b, a

This can be illustrated with the following code, which checks what the first element of a "randomized" array is when using the same logic as Arr::shuffle() when a seed is passed (assuming the seeds in this case are randomized as well).

$k = [ 'a', 'b', 'c', 'd', 'e' ];
$r = array_fill_keys($k, 0);

for ($i=0; $i<1000000; $i++) {
    $array = $k;

    usort($array, function () {
        return rand(-1, 1);
    });

    $r[ $array[0] ] ++;
}

print_r($r);

Results:

Array
(
    [a] => 564137
    [b] => 105554
    [c] => 281362
    [d] => 36514
    [e] => 12433
)

So, when that random logic runs, more than half the time the shuffled array still has the first element in the first position.

Changing the logic to use return rand(0,1) helps a bit, but is still biased:

Array
(
    [a] => 307937
    [b] => 204875
    [c] => 308190
    [d] => 116441
    [e] => 62557
)

I think the basic fix for this is to simply use PHP's shuffle() which returns a much more evenly randomized array. If the randomization needs to be seeded, we can just set that before calling shuffle() (and then set it again with a random seed, to prevent later logic in the code from being biased).

This PR does this.

[iainconnor]

Should this use mt_rand for pre-PHP7 installs?

[cviebrock]

This package's composer.json already has a "php": "^7.2" requirement, so I think using mt_rand() is safe.

[cviebrock]

Forgot that PRs are to be made to the framework. Closing this ticket; new PR is laravel/framework#27861