This document discusses randomization functions in PHP like shuffle() and str_shuffle(). It examines how the implementations have changed over time in PHP versions. Specifically, it finds that earlier versions of shuffle() and str_shuffle() did not produce truly random results due to issues with the random number generation. The current implementations in recent PHP versions do properly randomize the values, but the document proposes reimplementing shuffle() to use mt_rand() for improved consistency across environments.

1. PHP と
乱打舞図
2014/03/15
第5回 闇PHP勉強会
do_aki

2. ＠do_aki
http://do-aki.net/

3. title was born from typo
らんだまいず
乱打舞図
モテカルロ法っぽくない？

4. randomize
• 無秩序に並び替えること
• 規則性がないこと
• 偏りがないこと

5. http://nmi.jp/archives/541
(http://www.lauradhamilton.com/random-
lessons-online-poker-exploit)

7. randomize function in php
shuffle
str_shuffle

8. give it try (php 5.5.10)
function bench($name, $suffle_func) {
$result = [];
for ($i=0; $i<60000;++$i) {
$a = range(1, 3);
$suffle_func($a);
$r = implode('', $a);
$result[$r]=isset($result[$r])? $result[$r]+1 : 1;
}
print "{$name}¥n";
ksort($result);
print_r($result);
}

9. shuffle / str_shuffle
bench("shuffle", "shuffle");
bench("str_shuffle", function(&$ary) {
$str = str_shuffle(implode('', $ary));
$ary = str_split($str, 1);
});

10. result of shuffle
shuffle
Array
(
[123] => 9873
[132] => 10104
[213] => 9999
[231] => 9944
[312] => 9998
[321] => 10082
)

11. result of str_shuffle
str_shuffle
Array
(
[123] => 9956
[132] => 9928
[213] => 10080
[231] => 9969
[312] => 10149
[321] => 9918
)

12. No Problem
(in current
version of php)

13. 過去の shuffle は等分散ではなかった

14. implementation of shuffle(current version)
long n_elems, rnd_idx, n_left;
char temp;
/* The implementation is stolen from array_data_shuffle */
/* Thus the characteristics of the randomization are the same */
n_elems = len;
if (n_elems <= 1) { return; }
n_left = n_elems;
while (--n_left) {
rnd_idx = php_rand(TSRMLS_C);
RAND_RANGE(rnd_idx, 0, n_left, PHP_RAND_MAX);
if (rnd_idx != n_left) {
temp = str[n_left];
str[n_left] = str[rnd_idx];
str[rnd_idx] = temp;
}
}
ext/standard/string.c より抜粋 / shuffle の実装も同じ

15. implementation of shuffle (php < 4.3.0)
PHP_FUNCTION(shuffle){
zval *array;
if (zend_parse_parameters(1 TSRMLS_CC, "a", &array) == FAILURE){
RETURN_FALSE;
}
if (zend_hash_sort(Z_ARRVAL_PP(&array),
(sort_func_t)php_mergesort, array_data_shuffle, 1 TSRMLS_CC) ==
FAILURE) {
RETURN_FALSE;
}
RETURN_TRUE;
}
}
static int array_data_shuffle(const void *a, const void *b
TSRMLS_DC) {
return (php_rand(TSRMLS_C) % 2) ? 1 : -1;
}

16. ＿人人人人人人人人人人人人＿
＞ ランダムマージソート ＜
￣Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y￣

17. implementation of str_shuffle (php < 4.3.0)
PHP_FUNCTION(str_shuffle)
{
/* Note : by using current php_string_shuffle for string */
/* with 6 chars (6! permutations) about 2/3 of them are */
/* computed for 6! calls for the function. So it isn't so */
/* unique. The ratio is the same for other lengths. */
char *str;
int i, str_len;
i = 0;
if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "s", &str,
&str_len) == FAILURE) {
RETURN_FALSE;
}
zend_qsort((void *)str, str_len, sizeof(char), php_string_shuffle TSRMLS_CC);
RETURN_STRINGL(str, str_len, 1);
}

18. implementation of str_shuffle
(php < 4.3.0)
static int php_string_shuffle(const void *a,
const void *b TSRMLS_DC)
{
long rnd;
rnd = php_rand(TSRMLS_C);
if (rnd % 3)
return 1;
else if (rnd % 5)
return 0;
else
return -1;
}

19. ＿人人人人人人人人人人人人人人人＿
＞ ランダム(?) クイックソート ＜
￣Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y^Y￣

20. emulate old shuffle (php)
bench("old_shuffle", function(&$ary){
usort($ary, function($a, $b) {
return (rand() % 2) ? 1 : -1;
});
});
bench("old_str_shuffle", function(&$ary){
usort($ary, function($a, $b) {
$rnd = rand();
if ($rnd % 3) {
return 1;
} elseif ($rnd % 5) {
return 0;
} else {
return -1;
}
});
});

21. result
old_shuffle
Array
(
[123] => 22358
[132] => 7627
[213] => 3771
[231] => 3827
[312] => 7487
[321] => 14930
)
old_str_shuffle
Array
(
[123] => 30982
[132] => 4453
[213] => 5994
[231] => 2928
[312] => 8979
[321] => 6664
)

22. 現在の shuffle は、本当に問題ないのか？

23. php_rand(C) := rand (php)

24. random in php
rand
mt_rand

25. rand vs mt_rand
• rand
– libc の random or lrand48 or rand
– environment dependent
• mt_rand
– Mersenne Twister (MT19937)
– implementation dependent

26. re-imprement of shuffle
(environment independent)
function mt_shuffle(&$ary) {
$n = count($ary);
while(--$n) {
$rnd_idx = mt_rand(0, $n);
if ($rnd_idx != $n) {
$tmp = $ary[$n];
$ary[$n] = $ary[$rnd_idx];
$ary[$rnd_idx] = $tmp;
}
}
}

27. Conclusion
• 現在の shuffle / str_shuffle は、均等に分配
される
• 異なる環境での再現性が必要なら mt_rand
つかって再実装しよう
• php 4 はオワコン

28. End Of Slide
Let’s enjoy PHP hack life ;-)