Boost C++ Libraries: Ticket #6095: boost::icl::is_empty fails for certain open intervals

attachment set

Fri, 04 Nov 2011 14:29:10 GMT

attachment → interval.hpp.diff

patch to check for overflow in is_empty

Joachim Faulhaber — Sun, 06 Nov 2011 16:53:21 GMT

Hi Marvin,

thank you for your report and the attached patch. I agree that the behavior of icl::intervals at open borders and for limited domain types is kind of nasty. Nevertheless after examining your information I am not convinced that we can speak of a bug here and that a code change is needed.

In your test case you propose that

  BOOST_CHECK( is_empty(Interval::open(max, max)));

is supposed to succeed. But an open interval Interval::open(max, x) has an open lower bound of max, which means that values of its domain_type do not exist that can be elements of the interval. My interpretation of intervals of this kind is that they are ill formed rather than empty. So the user has to take care via program logic that these types of intervals are not constructed in the same way she has to take care that ++i is not called if i == numeric_limits<int>::max() already.

Best regards, Joachim

Mon, 07 Nov 2011 08:21:30 GMT

Hi Joachim,

as a mathematician I have to disagree: what distinguishes the interval Interval::open(max, max) from Interval::open(0, 0)? For both there exist no elements, that is the point of being empty. Even intervals like Interval::closed(17, 4) are allowed and deemed empty!

As a programmer I might be able to accept the responsibility to avoid such cases, but only if I were able to do so. I encountered the problem not while putting open intervals into the container but closed ones. And for those borders like min and max certainly make sense.

What I was trying to do was

    BOOST_AUTO_TEST_CASE(totalRangeTest)
    {
        typedef int                                                              Value;
        typedef boost::icl::interval<Value>                                      Interval;
        typedef std::numeric_limits<Value>                                       Limits;
        typedef boost::icl::interval_map<Value, int, boost::icl::total_enricher> Container;
        Value const min(Limits::min());
        Value const max(Limits::max());
        boost::icl::interval_map<Value, int, boost::icl::total_enricher> intervals;
        intervals += std::make_pair(Interval::closed(min, max), 0);
        intervals += std::make_pair(Interval::right_open(0, 10),  3);
        BOOST_CHECK_EQUAL(intervals.iterative_size(), 3);
   }

and this fails BOOST_ASSERT(this->_map.find(inter_val) == this->_map.end()); in interval_base_map.hpp, line 530 while trying to calculate the new third element of the container (Interval::closed(10, max)) when inserting the second interval.

Drilling down on that, I found is_empty and its problematic behaviour.

Thank you for your response!

Best regards,
Marvin

status, milestone changed; keywords set

Joachim Faulhaber — Mon, 07 Nov 2011 22:26:42 GMT

keywords icl open interval integer overflow added
status new → assigned
milestone To Be Determined → Boost 1.48.0

Hi Marvin,

I hope you forgive me my temporal {arrog|ignor}ance ;) Your use case shows, that the user can hardly avoid hitting the overflow trap of open intervals that are computed internally in the library in your test case. Also your fix is a nifty one. So thanks again for the report. I will apply your fix and put you on the credits list :) Send in more use cases if you find more bugs related to the [min,max]-preinitialized usage.

Cheers,
Joachim

Tue, 08 Nov 2011 08:12:12 GMT

Hi Joachim,

no problem :)

But I would like to propose a maybe even more nifty fix:
An open interval ]a, b[ is empty if b <= a || b <= a + 1.

This does not only avoid the second increment, it even avoids the first one in many cases due to the short circuit ||-operator and does never run into the overflow case:
If the first check fails, we know that a < b and hence that there is at least one element to increment to without overflowing.

So this should even work for bound types that are less forgiving than int when hitting their bounds, (e.g. throw exceptions).

Best regards,
Marvin

attachment set

Tue, 08 Nov 2011 08:13:44 GMT

attachment → interval.hpp.2.diff

b <= a

b <= a + 1 fix

Joachim Faulhaber — Tue, 08 Nov 2011 18:14:49 GMT

Hi Marvin,

yes, this is nice and simple. I changed the code accordingly.

Thanx again,
Joachim

status changed; resolution set

anonymous — Sat, 12 Nov 2011 00:53:38 GMT

status assigned → closed
resolution → fixed

type, component changed

anonymous — Fri, 03 Feb 2012 07:11:10 GMT

type Bugs → Patches
component ICL → Building Boost

type, component changed

Joachim Faulhaber — Sat, 04 Feb 2012 17:32:23 GMT

type Patches → Bugs
component Building Boost → ICL