You are viewing a plain text version of this content. The canonical link for it is here.
Posted to issues@impala.apache.org by "Jim Apple (JIRA)" <ji...@apache.org> on 2017/08/02 22:27:00 UTC
[jira] [Created] (IMPALA-5754) rand() algorithm is very non-random
Jim Apple created IMPALA-5754:
---------------------------------
Summary: rand() algorithm is very non-random
Key: IMPALA-5754
URL: https://issues.apache.org/jira/browse/IMPALA-5754
Project: IMPALA
Issue Type: Bug
Components: Backend
Reporter: Jim Apple
{{MathFunctions::Rand}} includes the line {{*seed = rand_r(seed);}}. I think this is incorrect use of {{rand_r}}, which sets its seed during the call, and so doesn't seed to set it in an assignment. This produces very unrandom output; the following program which simulates this typically loops after less than 20k distinct items, while a good PRNG would produce somewhere in the neighborhood of {{RAND_MAX/2}} items before looping.
{noformat}
#include <cstdlib>
#include <unordered_set>
#include <iostream>
using namespace std;
int main() {
unsigned int seed;
while(cin >> seed) {
unordered_set<int> history;
while (history.find(seed) == history.end()) {
history.insert(seed);
seed = rand_r(&seed);
if (0 == (history.size() & (history.size() - 1))) {
cout << history.size() << endl;
}
}
cout << history.size() << endl;
}
}
{noformat}
In any case, we should drop the use of {{rand_r}}; see IMPALA-4954.
--
This message was sent by Atlassian JIRA
(v6.4.14#64029)