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Fix RAND_RANGE for mt_rand

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This commit is contained in:
Leigh 2016-07-05 12:13:38 +01:00
parent 7981a294bd
commit 1f5cfea087
1 changed files with 53 additions and 47 deletions
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100
ext/standard/mt_rand.c
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@ -167,6 +167,10 @@ PHPAPI uint32_t php_mt_rand(void)
register uint32_t s1;
if (UNEXPECTED(!BG(mt_rand_is_seeded))) {
php_mt_srand(GENERATE_SEED());
}
if (BG(left) == 0) {
php_mt_reload();
}
@ -196,31 +200,46 @@ PHP_FUNCTION(mt_srand)
}
/* }}} */
/*
* A bit of tricky math here. We want to avoid using a modulus because
* that simply tosses the high-order bits and might skew the distribution
* of random values over the range. Instead we map the range directly.
*
* We need to map the range from 0...M evenly to the range a...b
* Let n = the random number and n' = the mapped random number
*
* Then we have: n' = a + n(b-a)/M
*
* We have a problem here in that only n==M will get mapped to b which
# means the chances of getting b is much much less than getting any of
# the other values in the range. We can fix this by increasing our range
# artificially and using:
#
# n' = a + n(b-a+1)/M
*
# Now we only have a problem if n==M which would cause us to produce a
# number of b+1 which would be bad. So we bump M up by one to make sure
# this will never happen, and the final algorithm looks like this:
#
# n' = a + n(b-a+1)/(M+1)
*
* -RL
/* {{{ php_mt_rand_range
*/
PHPAPI zend_long php_mt_rand_range(zend_long min, zend_long max)
{
zend_ulong umax = max - min;
zend_ulong limit;
zend_ulong result;
#if ZEND_ULONG_MAX > UINT32_MAX
result = ((zend_ulong)php_mt_rand() << 32) | php_mt_rand();
#else
result = php_mt_rand();
#endif
/* Special case where no modulus is required */
if (UNEXPECTED(umax == ZEND_ULONG_MAX)) {
return (zend_long)result;
}
/* Increment the max so the range is inclusive of max */
umax++;
/* Powers of two are not biased */
if (EXPECTED((umax & (umax - 1)) != 0)) {
/* Ceiling under which ZEND_LONG_MAX % max == 0 */
limit = ZEND_ULONG_MAX - (ZEND_ULONG_MAX % umax) - 1;
/* Discard numbers over the limit to avoid modulo bias */
while (UNEXPECTED(result > limit)) {
#if ZEND_ULONG_MAX > UINT32_MAX
result = (result << 32) | php_mt_rand();
#else
result = php_mt_rand();
#endif
}
}
return (zend_long)((result % umax) + min);
}
/* }}} */
/* {{{ proto int mt_rand([int min, int max])
Returns a random number from Mersenne Twister */
@ -228,36 +247,23 @@ PHP_FUNCTION(mt_rand)
{
zend_long min;
zend_long max;
zend_long number;
int argc = ZEND_NUM_ARGS();
int argc = ZEND_NUM_ARGS();
if (argc != 0) {
if (zend_parse_parameters(argc, "ll", &min, &max) == FAILURE) {
return;
} else if (max < min) {
php_error_docref(NULL, E_WARNING, "max(" ZEND_LONG_FMT ") is smaller than min(" ZEND_LONG_FMT ")", max, min);
RETURN_FALSE;
}
if (argc == 0) {
// genrand_int31 in mt19937ar.c performs a right shift
RETURN_LONG(php_mt_rand() >> 1);
}
if (!BG(mt_rand_is_seeded)) {
php_mt_srand(GENERATE_SEED());
if (zend_parse_parameters(argc, "ll", &min, &max) == FAILURE) {
return;
}
/*
* Melo: hmms.. randomMT() returns 32 random bits...
* Yet, the previous php_rand only returns 31 at most.
* So I put a right shift to loose the lsb. It *seems*
* better than clearing the msb.
* Update:
* I talked with Cokus via email and it won't ruin the algorithm
*/
number = (zend_long) (php_mt_rand() >> 1);
if (argc == 2) {
RAND_RANGE(number, min, max, PHP_MT_RAND_MAX);
if (UNEXPECTED(max < min)) {
php_error_docref(NULL, E_WARNING, "max(" ZEND_LONG_FMT ") is smaller than min(" ZEND_LONG_FMT ")", max, min);
RETURN_FALSE;
}
RETURN_LONG(number);
RETURN_LONG(php_mt_rand_range(min, max));
}
/* }}} */