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https://issues.apache.org/jira/browse/MATH-1322?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15139112#comment-15139112
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Morikawa Joe commented on MATH-1322:
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My patch is for setSeed(int seed), there is no problem in setSeed(long seed).
Actually, I compared apache MersenneTwiter with C++ mt19937 which is included
in random.
And apache MersenneTwister generated different random value from C++ mt19937
only if seed is negative.
The reason for the difference is that longMT is 0xffffffffffffffff.
{code:title=MersenneTwister.java|borderStyle=solid}
public void setSeed(int seed) {
// we use a long masked by 0xffffffffL as a poor man unsigned int
long longMT = seed;
// longMT is 0xffffffffffffffff, but it is 0x00000000ffffffff in C++ or other
language implementation.
// NB: unlike original C code, we are working with java longs, the cast
below makes masking unnecessary
mt[0]= (int) longMT;
for (mti = 1; mti < N; ++mti) {
// See Knuth TAOCP Vol2. 3rd Ed. P.106 for multiplier.
// initializer from the 2002-01-09 C version by Makoto Matsumoto
longMT = (1812433253l * (longMT ^ (longMT >> 30)) + mti) &
0xffffffffL;
mt[mti]= (int) longMT;
}
clear(); // Clear normal deviate cache
}
{code}
Followings are the samples.
{code:title=Sample.java|borderStyle=solid}
MersenneTwister rng = new MersenneTwister();
rng.setSeed((int) -1);
// Without patch, "-931718226" is output.
// After applying patch, "419326371" is output.
System.out.println(rng.nextInt());
{code}
{code:title=CppSample.cpp|borderStyle=solid}
#include <iostream>
#include <random>
int main()
{
std::mt19937 mt(0xFFFFFFFF);
// 419326371 is displayed
std::cout << mt() << std::endl;
std::mt19937 mt1(-1);
// 419326371 is displayed
std::cout << mt() << std::endl;
}
{code}
> Negative integer seed for MersenneTwister is not working.
> ---------------------------------------------------------
>
> Key: MATH-1322
> URL: https://issues.apache.org/jira/browse/MATH-1322
> Project: Commons Math
> Issue Type: Bug
> Affects Versions: 3.6
> Reporter: Morikawa Joe
> Labels: patch
> Attachments: MersenneTwister.patch
>
>
> MersenneTwister.setSeed(int seed) has a bug.
> If seed is negative, longMT also becomes negative.
> First 32 bits are filled by 0xFFFFFFFF.
> But it should be positive. First 32 bits should be filled by 0x00000000.
> Ex) Integer -1 is 0xffffffff.
> Long -1 is 0xffffffffffffffff.
> Long 0xffffffff is 4294967295.
> I created simple patch.
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