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https://issues.apache.org/jira/browse/MATH-1300?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15065416#comment-15065416
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Gilles commented on MATH-1300:
------------------------------
Thanks for pointing out that problem.
bq. Sequential calls to \[...\] nextBytes must generate the same sequence of
bytes, no matter by chunks of which size it was divided.
Attempting to figure out how general the claim is, I've implemented a more
general unit test based on your suggestion:
{code}
private void checkNexBytesChunks(int chunkSize,
int numChunks) {
final RandomGenerator rg = makeGenerator();
final long seed = 1234567L;
final byte[] b1 = new byte[chunkSize * numChunks];
final byte[] b2 = new byte[chunkSize];
// Generate the chunks in a single call.
rg.setSeed(seed);
rg.nextBytes(b1);
// Reset.
rg.setSeed(seed);
// Generate the chunks in consecutive calls.
for (int i = 0; i < numChunks; i++) {
rg.nextBytes(b2);
}
// Store last 128 bytes chunk of b1 into b3.
final byte[] b3 = new byte[chunkSize];
System.arraycopy(b1, b1.length - b3.length, b3, 0, b3.length);
// Sequence of calls must be the same.
Assert.assertArrayEquals("chunkSize=" + chunkSize + " numChunks=" +
numChunks,
b2, b3);
}
{code}
The original CM code always fails it, as you observed.
In your example test case (chunkSize=128 and numChunks=8), your fix makes the
test pass.
However, it fails whenever the size of the array (argument to "nextBytes") is
not a multiple of 4.
That is, the "chunkSize" does matter.
And "nextBytes" in the JDK's {{Random}} class also fails the test when the
array's size is not a multiple of 4.
So there are several issues:
# Do you have a reference that the behaviour _must_ be as your described?
# Is the requested behaviour supposed to work only when the array's size is a
multiple of 4? If so, should we add some note about it in the documentation?
# Is there a way to implement "nextBytes" in {{BitsStreamGenerator}} so that
the property holds for any size? And if so, should we consider making the
change (given that {{Random}} does not work that way)?
> BitsStreamGenerator#nextBytes(byte[]) is wrong
> ----------------------------------------------
>
> Key: MATH-1300
> URL: https://issues.apache.org/jira/browse/MATH-1300
> Project: Commons Math
> Issue Type: Bug
> Affects Versions: 3.5
> Reporter: Rostislav Krasny
> Attachments: MersenneTwister2.java, TestMersenneTwister.java
>
>
> Sequential calls to the BitsStreamGenerator#nextBytes(byte[]) must generate
> the same sequence of bytes, no matter by chunks of which size it was divided.
> This is also how java.util.Random#nextBytes(byte[]) works.
> When nextBytes(byte[]) is called with a bytes array of length multiple of 4
> it makes one unneeded call to next(int) method. This is wrong and produces an
> inconsistent behavior of classes like MersenneTwister.
> I made a new implementation of the BitsStreamGenerator#nextBytes(byte[]) see
> attached code.
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