Section: 29.5.4.3 [rand.eng.mers], 99 [tr.rand.eng.mers] Status: NAD Submitter: Stephan Tolksdorf Opened: 2008-02-18 Last modified: 2023-05-24
Priority: Not Prioritized
View all other issues in [rand.eng.mers].
View all issues with NAD status.
Discussion:
99 [tr.rand.eng.mers](10) requires that operator== for the mersenne_twister
returns true if and only if the states of two mersenne_twisters,
consisting each of n integers between 0 and 2w - 1, are completely
equal. This is a contradiction with 99 [tr.rand.req](3) because the given
definition of the state also includes the lower r bits of x(i-n), which
will never be used to generate a random number. If two mersenne_twisters
only differ in the lower bits of x(i-n) they will not compare equal,
although they will produce an identical sequence of random numbers.
29.5.4.3 [rand.eng.mers] in the latest C++ draft does not specify the behaviour
of operator== but uses a similar definition of the state and, just like
99 [tr.rand.eng.mers], requires the textual representation of a
mersenne_twister_engine to consist of Xi-n to Xi-1, including the
lower bits of Xi-n. This leads to two problems: First, the
unsuspecting implementer is likely to erroneously compare the lower r
bits of Xi-n in operator==. Second, if only the lower r bits differ,
two mersenne_twister_engines will compare equal (if correctly
implemented) but have different textual representations, which
conceptually is a bit ugly.
I propose that a paragraph or footnote is added to 29.5.4.3 [rand.eng.mers] which
clarifies that the lower r bits of Xi-n are not to be compared in
operator== and operator!=. It would only be consequent if furthermore
the specification for the textual respresentation was changed to
Xi-n bitand ((2w - 1) - (2r - 1)), Xi-(n-1), ..., Xi-1 or
something similar.
These changes would likely have no practical effect, but would allow an implementation that does the right thing to be standard-conformant.
[ Bellevue: ]
Fermi Lab has no objection to the proposed change. However it feels that more time is needed to check the details, which would suggest a change to REVIEW.
Bill feels that this is NAD, not enough practical importance to abandon the simple definition of equality, and someone would have to do a lot more study to ensure that all cases are covered for a very small payback. The submitter admits that "These changes would likely have no practical effect,", and according to Plum's razor this means that it is not worth the effort!
Revisted: Agree that the fact that there is no practical difference means that no change can be justified.
Proposed resolution:
In 29.5.4.3 [rand.eng.mers]:
Insert at the end of para 2.:
[Note: The lower
rbits ofXi-ndo not influence the state transition and hence should not be compared when comparing twomersenne_twister_engineobjects. -- end note]In para 5. change:
The textual representation of
xiconsists of the values ofXi-n bitand ((2w - 1) - (2r - 1)), Xi-(n-1), ..., Xi-1, in that order.