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Date: Sun, 11 Jan 2015 22:12:42 +0000
To: PHP Internals List <internals@lists.php.net>
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Subject: rand(), mt_rand() and limits
From: ajf@ajf.me (Andrea Faulds)

Hey internals,

As we=E2=80=99re moving to clean some things up in PHP 7, I think it =
might be worth looking at the rand()/mt_rand() situation.

Currently, we have two standard-library functions to obtain a random =
number: rand() and mt_rand(). rand() uses the C standard library =
function, which on many stdlibs is slow or has a limited range (e.g. =
Win32 rand()=E2=80=99s maximum return value is SHORT_MAX). As a result, =
a drop-in replacement was added, mt_rand(), which uses Mersenne =
Twister-based random number generator that=E2=80=99s probably better =
than what the C standard library provides.

Having two different random number functions is confusing to new users, =
and an oft-cited issue with PHP=E2=80=99s standard library. Intuitively, =
a user would expect rand() to be the function to get a random number, =
but actually they should be using mt_rand(), which is usually better.=20

Another issue is the fact we allow random numbers to be seeded with =
srand() and mt_srand(). We have automatically seeded the random number =
generator for a while now, so there is no need to do it manually. We =
also explicitly do not guarantee that the random number generator will =
always produce the same output given the same seed, and in fact we=E2=80=99=
ve changed mt_rand()'s behaviour in the past. This eliminates the =
primary use-case for manual seeding when there is automatic seeding: =
predictable output for procedural generation.

Finally, a third issue is that mt_rand() does not produce good quality =
numbers on 64-bit platforms. mt_rand() will always produce a 32-bit =
value internally, and scale it up or down to fit the user-specified =
range. Unfortunately, this means that values for the $max parameter =
beyond 2^31 - 1 produce numbers with poor granularity.

Given all these, I would suggest that for PHP 7, we:

  * Get rid of rand(), srand() and getrandmax()
  * Rename mt_rand(), mt_srand() and mt_getrandmax() to rand(), srand(), =
and getrandmax() but add mt_* aliases for backwards-compatibility
  * Make mt_srand() and srand() do nothing and produce a deprecation =
notice
  * Use a 64-bit random number generation algorithm on 64-bit platforms =
(or invoke the 32-bit generator twice)

The end result should be that PHP has just one random number generation =
function, rand(), which can produce the full range from PHP_INT_MIN to =
PHP_INT_MAX with no scaling. This would be far more intuitive, I think.

Would that sound good?

Thanks!
--
Andrea Faulds
http://ajf.me/