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https://issues.apache.org/jira/browse/MATH-1306?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15072260#comment-15072260
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Rostislav Krasny commented on MATH-1306:
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{quote}Since the new feature is about reusing the same array to fill different
regions of it at different times, why not also keep the small array for
reuse?{quote}
This is not about reusing the same array but about having a direct access to
the destination array. And reusing the same intermediate array is not always
possible or practical. But even without temporary array allocation in each loop
iteration the {{System.arraycopy()}} also takes time, although it is a native
method.
{quote}Could you please provide a use-case of such a simplification?{quote}
Cryptography. I had seen a cryptographic software that uses a method with a
signature similar to the proposed.
{quote}I only see that the signature is more complex; the purpose of Commons
Math is not to cover all hypothetical cases.{quote}
This is very common to have methods that works with regions of arrays or
collections. The mentioned here {{System.arraycopy()}} or methods of
{{java.util.Arrays}} class. And I'm sure this list could be much longer.
I just found the proposed nextByte() method signature already exists in
{{gnu.crypto.util.PRNG}} pseudo-random number generator.
[http://www.gnu.org/software/gnu-crypto/manual/api/gnu/crypto/util/PRNG.html]
Also there is a not exact but very similar method signature in
{{RandomData.OneShot}} of javacard
[https://docs.oracle.com/javacard/3.0.5/api/javacard/security/RandomData.OneShot.html]#nextBytes%28byte[],%20short,%20short%29
Apache Tomcat has a tricky method with similar functionality just because
{{java.lang.Random}} doesn't have the proposed method
[https://tomcat.apache.org/tomcat-7.0-doc/api/org/apache/catalina/tribes/util/UUIDGenerator.html]#nextBytes%28byte[],%20int,%20int,%20java.util.Random%29
So why not to add the proposed method signature into CM as well?
P.S. why URLs with # are not supported by Jira?
[https://docs.oracle.com/javacard/3.0.5/api/javacard/security/RandomData.OneShot.html#nextBytes%28byte[],%20short,%20short%29]
> Add public void nextBytes(byte[] bytes, int position, int length) method into
> the RandomGenerator interface
> -----------------------------------------------------------------------------------------------------------
>
> Key: MATH-1306
> URL: https://issues.apache.org/jira/browse/MATH-1306
> Project: Commons Math
> Issue Type: Improvement
> Affects Versions: 3.5
> Reporter: Rostislav Krasny
> Attachments: MersenneTwister1305.java, MersenneTwister1306.java
>
>
> I propose to add {{public void nextBytes(byte[] bytes, int position, int
> length)}} method into the {{RandomGenerator}} interface.
> Rationality: to improve performance and memory usage in cases when one needs
> to fill only a specified region of a byte array. Today to do that you need to
> use a temporary byte array and copy it by yourself into the specified region
> of the destination byte array.
> I propose the following code, based on the code of {{BitsStreamGenerator}}
> commited in MATH-1305
> {code:java}
> @Override
> public void nextBytes(byte[] bytes) {
> nextBytesFill(bytes, 0, bytes.length);
> }
> // TODO add this method into RandomGenerator interface
> //@Override
> public void nextBytes(byte[] bytes, int position, int length) {
> if (position < 0 || position > bytes.length - 1) {
> throw new
> OutOfRangeException(LocalizedFormats.OUT_OF_RANGE_SIMPLE, position, 0,
> bytes.length - 1);
> }
> if (length < 0 || length > bytes.length - position) {
> throw new
> OutOfRangeException(LocalizedFormats.OUT_OF_RANGE_SIMPLE, length, 0,
> bytes.length - position);
> }
> nextBytesFill(bytes, position, length);
> }
> private void nextBytesFill(byte[] bytes, int position, int length) {
> int index = position;
> // Position plus multiple 4 part of length (i.e. length with
> two least significant bits unset).
> final int indexLoopLimit = position + (length & 0x7ffffffc);
> // Start filling in the byte array, 4 bytes at a time.
> while (index < indexLoopLimit) {
> final int random = next(32);
> bytes[index++] = (byte) random;
> bytes[index++] = (byte) (random >>> 8);
> bytes[index++] = (byte) (random >>> 16);
> bytes[index++] = (byte) (random >>> 24);
> }
> final int indexLimit = position + length;
>
> // Fill in the remaining bytes.
> if (index < indexLimit) {
> int random = next(32);
> while (true) {
> bytes[index++] = (byte) random;
> if (index < indexLimit) {
> random >>>= 8;
> } else {
> break;
> }
> }
> }
> }
> {code}
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