i wanted to spot out the big difference between rand and mt_rand when producing images using randomness as noise.
for example this is a comparation between rand and mt_rand on a 400x400 pixel png: http://oi43.tinypic.com/vwtppl.jpg
code:
<?php
header("Content-type: image/png");
$sizex=800;
$sizey=400;
$img = imagecreatetruecolor($sizex,$sizey);
$ink = imagecolorallocate($img,255,255,255);
for($i=0;$i<$sizex/2;$i++) {
for($j=0;$j<$sizey;$j++) {
imagesetpixel($img, rand(1,$sizex/2), rand(1,$sizey), $ink);
}
}
for($i=$sizex/2;$i<$sizex;$i++) {
for($j=0;$j<$sizey;$j++) {
imagesetpixel($img, mt_rand($sizex/2,$sizex), mt_rand(1,$sizey), $ink);
}
}
imagepng($img);
imagedestroy($img);
?>
the differences reduce when reducing the pixels of the image.. infact for a 100x100 pixel image the noise produced from the rand function is much more realistic than how it is for a 400x400 image: http://oi39.tinypic.com/5k0row.jpg
(rand is on the left, mt_rand on the right)
mt_rand
(PHP 4, PHP 5, PHP 7, PHP 8)
mt_rand — Génère une valeur aléatoire via le générateur de nombre aléatoire Mersenne Twister
Description
mt_rand
(
) : int
mt_rand
( int
$min
, int $max
) : intAttention
Cette fonction ne génère pas de valeurs sécurisées d'un point de vue cryptographique, et ne doit pas être utilisée dans un contexte de chiffrement. Si vous avez besoin d'une valeur sécurisée d'un point de vue cryptographique, utilisez plutôt random_int(), random_bytes(), ou openssl_random_pseudo_bytes().
De nombreux générateurs de nombres aléatoires provenant de vieilles bibliothèques libcs ont des comportements douteux et sont très lents. mt_rand() est une fonction de remplacement pour rand(). Elle utilise un générateur de nombres aléatoire de caractéristique connue, le " » Mersenne Twister " qui est 4 fois plus rapide que la fonction standard libc.
Appelée sans les arguments optionnels min et
max, mt_rand() retourne un nombre
pseudoaléatoire, entre 0 et mt_getrandmax().
Pour obtenir un nombre entre 5 et 15 inclus, il faut utiliser
mt_rand(5,15).
Liste de paramètres
-
min -
Valeur la plus basse qui peut être retournée (par défaut : 0)
-
max -
Valeur la plus haute qui peut être retournée (par défaut : mt_getrandmax()).
Valeurs de retour
Un entier aléatoire compris entre min (ou 0)
et max (ou mt_getrandmax(), inclusif),
ou false si le paramètre max est inférieur à
min.
Historique
| Version | Description |
|---|---|
| 7.2.0 | mt_rand() a reçu une correction de bogue pour un bug de polarisation modulo. Cela signifie que les séquences générées avec une valeur d'initialisation spécifique peuvent différer de php 7.1 sur les machines 64-bit. |
| 7.1.0 | rand() est devenu un alias de mt_rand(). |
| 7.1.0 |
mt_rand()
a été mis à jour
pour utiliser la version corrigée, correcte, de l'algorithme Twister
Mersenne. Pour revenir à l'ancien comportement, utilisez
mt_srand() avec MT_RAND_PHP
comme deuxième paramètre.
|
Exemples
Exemple #1 Exemple avec mt_rand()
<?php
echo mt_rand() . "\n";
echo mt_rand() . "\n";
echo mt_rand(5, 15);
?>
Résultat de l'exemple ci-dessus est similaire à :
1604716014 1478613278 6
Notes
Avertissement
La plage min max doit se situer
dans la plage mt_getrandmax(). i.e.
max - min) <=
mt_getrandmax() sinon, mt_rand() peut
retourner des nombres aléatoires plus pauvres qu'il ne le devrait.
Voir aussi
- mt_srand() - Initialise le générateur de nombres aléatoires Mersenne Twister
- mt_getrandmax() - La plus grande valeur aléatoire possible
- random_int() - Génère des nombres entiers pseudo-aléatoire cryptographiquement sécurisé
- random_bytes() - Génère des octets pseudo-aléatoire cryptographiquement sécurisé
- openssl_random_pseudo_bytes() - Génère une chaine pseudo-aléatoire d'octets
- rand() - Génère une valeur aléatoire
add a note
User Contributed Notes 24 notes
cyrax21 at gmail dot com ¶
9 years ago
syouseki ¶
5 years ago
mt_rand is not calculated by Mersenne Twister.
https://github.com/php/php-src/commit/a0724d30817600540946b41e40f4cfc2a0c30f80
demogracia at metropoliglobal dot com ¶
19 years ago
<?php
//
// Generates a random string with the specified length
// Chars are chosen from the provided [optional] list
//
function simpleRandString($length=16, $list="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"){
mt_srand((double)microtime()*1000000);
$newstring="";
if($length>0){
while(strlen($newstring)<$length){
$newstring.=$list[mt_rand(0, strlen($list)-1)];
}
}
return $newstring;
}
//
// Generates a random string with the specified length
// Includes: a-z, A-Z y 0-9
//
function randString($length=16) {
$newstring="";
if($length>0) {
while(strlen($newstring)<$length) {
$randnum = mt_rand(0,61);
if ($randnum < 10) {
$newstring.=chr($randnum+48);
} elseif ($randnum < 36) {
$newstring.=chr($randnum+55);
} else {
$newstring.=chr($randnum+61);
}
}
}
return $newstring;
}
?>
Jeff Cours ¶
6 years ago
With PHP 5.3.3, we're seeing odd behavior on 32 bit Linux.
This works fine on 64 bit Linux:
<?php
printf ("%08x\n", mt_rand (0, 0xFFFFFFFF));
?>
but on our 32 bit Linux development server, it's always yielding "00000000".
On that same machine, this:
<?php
printf ("%08x\n", mt_rand (0, 0xFFFFFFF0));
?>
seems to always yield either 00000000 or a number in the range fffffff2 to ffffffff. This:
<?php
printf ("%08x\n", mt_rand (0, 0xFFFFFF00));
?>
gives numbers where the last two digits vary, and so on through at least 0xF0000000.
However, this:
<?php
printf ("%08x\n", mt_rand (0, 0x7FFFFFFF));
?>
seems to be well-behaved.
The moral? On 32 bit systems, be careful about crossing the signed number boundary, 0x7FFFFFFF.
Anonymous ¶
2 years ago
The seed is the PID + LCG (https://github.com/php/php-src/search?q=GENERATE_SEED&unscoped_q=GENERATE_SEED)
nospamremove dot francois dot gannaz at silecs dot info ¶
4 years ago
rand() comes from the libc, and mt_rand() is internal to PHP. So the differences vary with their respective versions.
On a 64b Debian Stretch with PHP 5.6.21, there is no visible difference: http://oi64.tinypic.com/2nkqas6.jpg
This image compares the two functions. In the top half with random points, in the lower half with random intensity on each point. This is totally different from what was obtained 4 years ago in another note, with an unknown environment.
Here is the code for this visual comparison.
<?php
$sizex = 400;
$sizey = 400;
$img = imagecreatetruecolor(2 * $sizex, 2 * $sizey);
$white = imagecolorallocate($img, 255, 255, 255);
$inks = array_map(
function($r) use($img) { return imagecolorallocate($img, $r, $r, $r); },
range(0, 255)
);
for ($x = 0 ; $x < $sizex ; $x++) {
for ($y = 0 ; $y < $sizey ; $y++) {
// placing ($sizex x $sizey) white points at random in the top half
imagesetpixel($img, rand(0, $sizex - 1), rand(0, $sizey - 1), $white);
imagesetpixel($img, $sizex + mt_rand(0, $sizex - 1), mt_rand(0, $sizey - 1), $white);
// random intensity for each point in the lower half
imagesetpixel($img, $x, $sizey + $y, $inks[rand(0, 255)]);
imagesetpixel($img, $sizex + $x, $sizey + $y, $inks[mt_rand(0, 255)]);
}
}
header("Content-type: image/png");
imagepng($img);
?>
nilesh at itech7 dot com ¶
10 years ago
A class to generate 99.5% unqiue strings. I found that there is only one or two characters common between two subsequent strings.
<?php
class Local_RandomString {
protected $_length;
protected $_prevRand;
public function __construct($length = 15) {
$this->_length = $length;
}
public function getRand() {
$randStr = null;
$args[] = 'N' . $this->_length;
for($i = 0; $i < $this->_length; $i++) {
$args[] = mt_rand();
}
$randStr = substr(base64_encode((call_user_func_array('pack', $args))), 1, $this->_length);
$this->_prevRand = $randStr;
return $randStr;
}
public function setLength($l) {
$this->_length = (int) $l;
if($this->_length <= 0) {
throw new Exception('Invalid random string length');
}
}
public function getPrevRand() {
return $this->_prevRand;
}
}
?>
contact at sheyd dot fr ¶
9 years ago
To quickly build a human-readable random string for a captcha per example :
<?php
function random($length = 8)
{
$chars = 'bcdfghjklmnprstvwxzaeiou';
for ($p = 0; $p < $length; $p++)
{
$result .= ($p%2) ? $chars[mt_rand(19, 23)] : $chars[mt_rand(0, 18)];
}
return $result;
}
?>
Note that I have removed q and y from $chars to avoid readability problems.
Anonymous ¶
17 years ago
Here is a example of a very small, compact, quite random-random string generator. It will make a string with uppercase & lowercase letters, with numbers. You simply need to set $len in the for() structure, and then the string will be in $r. It has been designed for size, while it's still quite fast. Mind the wrapping, it should be 1 line.
<?php
for($len=8,$r='';strlen($r)<$len;$r.=chr(!mt_rand(0,2)?
mt_rand(48,57):(!mt_rand(0,1)?mt_rand(65,90):mt_rand
(97,122))));
?>
Armond Carroll
Pawe Krawczyk ¶
7 years ago
To reiterate the message about *not* using mt_rand() for anything security related, here's a new tool that has been just posted that recovers the seed value given a single mt_rand() output:
http://www.openwall.com/php_mt_seed/README
Hayley Watson ¶
8 years ago
Another graphical comparison of rand() and mt_rand(). It effectively draws a graph showing how the last generated number affects the next by plotting the numbers in consecutive pairs against each other.
<?php
header("Content-type: image/png");
$sizex=800;
$sizey=800;
$img = imagecreatetruecolor(2 * $sizex,$sizey);
$black = imagecolorallocate($img,0, 0, 0);
imagefilledrectangle($img, 0, 0, 2 * $sizex, $sizey, imagecolorallocate($img, 255, 255, 255));
$p = 0;
for($i=0; $i < 100000; $i++) {
$np = rand(0,$sizex);
imagesetpixel($img, $p, $np, $black);
$p = $np;
}
$p = 0;
for($i=0; $i < 100000; $i++) {
$np = mt_rand(0,$sizex);
imagesetpixel($img, $p + $sizex, $np, $black);
$p = $np;
}
imagepng($img);
imagedestroy($img);
?>
falkartis at gmail dot com ¶
6 years ago
I wrote another function to get a random float, if its not precise enougth jut add some '0' to the $mul parameter.
<?php
function f_rand($min=0,$max=1,$mul=1000000){
if ($min>$max) return false;
return mt_rand($min*$mul,$max*$mul)/$mul;
}
?>
I made following tests:
<?php
echo f_rand()."<br>"; //0.497153
echo f_rand(0.5)."<br>"; //0.857822
echo f_rand(0.5,0.6)."<br>"; //0.599956
echo f_rand(0,10)."<br>"; //5.801949
echo f_rand(0,2,2)."<br>"; //1.5
echo f_rand(0,2,10)."<br>"; //1.7
?>
geompse at yopmail dot com ¶
13 years ago
mt_rand() is not faster than rand() !
Tested over 100'000 iterations, with none/various/random arguments, mt_rand is always 3% slower than rand().
chagenbu at php dot net ¶
14 years ago
The algorithm used by mt_rand() changed in PHP 5.2.1. If you are relying on getting the same sequence from mt_rand() after calling mt_srand() with a known seed, upgrading to PHP 5.2.1 will break your code. See http://bugs.php.net/bug.php?id=40724 for something of an explanation; there is no workaround.
zolaar at nothanks dot com ¶
14 years ago
a better (and likely faster) way to generate a random 6-digit hex string:
<?php
$num = mt_rand ( 0, 0xffffff ); // trust the library, love the library...
$output = sprintf ( "%06x" , $num ); // muchas smoochas to you, PHP!
return $output;
?>
The mt_rand function won't give you a number outside the bounds you asked for -- no need to and-off the top bits -- and the sprintf function has params for length-padding & hexidecimal output. It's likely faster because most of the work is being done by the wicked fast C functions that PHP sits on top of, though YMMV in that dept.
nowhere at where dot net ¶
15 years ago
Allows characters 0-9, a-z
Weighted (and tested) ok.
<?php
function generate_string ($length = 20)
{
$nps = "";
for($i=0;$i<$length;$i++)
{
$nps .= chr( (mt_rand(1, 36) <= 26) ? mt_rand(97, 122) : mt_rand(48, 57 ));
}
return $nps;
}
?>
Miller ¶
7 years ago
Another generic random string function, but very small and fast.
<?php
function mt_rand_str ($l, $c = 'abcdefghijklmnopqrstuvwxyz1234567890') {
for ($s = '', $cl = strlen($c)-1, $i = 0; $i < $l; $s .= $c[mt_rand(0, $cl)], ++$i);
return $s;
}
echo mt_rand_str(8); // Something like mp2tmpsw
echo mt_rand_str(6, '0123456789ABCDEF'); // Something like B9CD0F
?>
Mark Seecof ¶
12 years ago
If you need some pseudorandom bits for security or cryptographic purposes (e.g.g., random IV for block cipher, random salt for password hash) mt_rand() is a poor source. On most Unix/Linux and/or MS-Windows platforms you can get a better grade of pseudorandom bits from the OS or system library, like this:
<?php
// get 128 pseudorandom bits in a string of 16 bytes
$pr_bits = '';
// Unix/Linux platform?
$fp = @fopen('/dev/urandom','rb');
if ($fp !== FALSE) {
$pr_bits .= @fread($fp,16);
@fclose($fp);
}
// MS-Windows platform?
if (@class_exists('COM')) {
// http://msdn.microsoft.com/en-us/library/aa388176(VS.85).aspx
try {
$CAPI_Util = new COM('CAPICOM.Utilities.1');
$pr_bits .= $CAPI_Util->GetRandom(16,0);
// if we ask for binary data PHP munges it, so we
// request base64 return value. We squeeze out the
// redundancy and useless ==CRLF by hashing...
if ($pr_bits) { $pr_bits = md5($pr_bits,TRUE); }
} catch (Exception $ex) {
// echo 'Exception: ' . $ex->getMessage();
}
}
if (strlen($pr_bits) < 16) {
// do something to warn system owner that
// pseudorandom generator is missing
}
?>
NB: it is generally safe to leave both the attempt to read /dev/urandom and the attempt to access CAPICOM in your code, though each will fail silently on the other's platform. Leave them both there so your code will be more portable.
mark omohundro, ajamyajax dot com ¶
12 years ago
just another example: both of these routines return a random decimal number between -1 and 1... since rand() only returns a max 'integer' value while mt_rand() return a max 'long' value -- at least on some platforms -- mt_rand() could be the better precision choice for some on any variation to this routine (but i don't think it matters here):
<?php
echo "-1 to 1 rand() value: ". (rand(getrandmax()*-1,getrandmax())/getrandmax()). "\n<br/>";
echo "-1 to 1 mt_rand() value: ". (mt_rand(mt_getrandmax()*-1,mt_getrandmax())/mt_getrandmax()). "\n";
?>
Nikhil S. ¶
1 year ago
A nifty function to generate pretty coupon codes. This will generate unique coupon codes and you don't have to do a database check whether the code already exists.
<?php
function giftCodes(){
$a='';
for($j=0; $j<=4; $j++){
$c = 4;
if($j == 1) { $a .= date("d"); $c=2; }
if($j == 3) { $a .= date("m"); $c=2; }
if($j == 4) { $a .= date("y"); $c=2; }
for($i=0; $i<= $c; $i++)
$a .= chr(!mt_rand(0,2)?mt_rand(48,57):mt_rand(65,90));
$a .='-';
}
return substr($a, 0, -1);
}
?>
rok dot kralj at gmail dot com ¶
13 years ago
mt_rand function returns just a whole numbers. If you want a random float, then here's an elegant way:
<?php
function random_float ($min,$max) {
return ($min+lcg_value()*(abs($max-$min)));
}
?>
Robin Leffmann ¶
10 years ago
Fast, pseudo-random binary data generation using mt_rand():
<?php
function rnd_bin( $length )
{
while( @$c++ * 16 < $length )
@$tmp .= md5( mt_rand(), true );
return substr( $tmp, 0, $length );
}
?>
j dot s dot shiuan at gmail dot com ¶
9 years ago
Another good way to get a random float is to divide the result of mt_rand.
Let's say we want a float between 0.75 and 1.25.
<?php
$i = mt_rand(75,125) / 100;
?>
fabiovh on gmail ¶
13 years ago
performance: for a repetitive task, it's much faster not to use the limit parameters, as shown below. just use the % operator.
$t=microtime(true);
for($i=0;$i<1000000;$i++)
mt_rand()%3;
echo microtime(true)-$t;
echo '|';
$t=microtime(true);
for($i=0;$i<1000000;$i++)
mt_rand(0,2);
echo microtime(true)-$t;
echo '|';
$t=microtime(true);
for($i=0;$i<1000000;$i++)
mt_rand();
echo microtime(true)-$t;
output: 0.316797971725|0.442242145538|0.253082036972
