[boost] [Boost-bugs] [ boost-Patches-1438991 ] rational::operator<(rational) fails due to overflow

From: SourceForge.net (noreply_at_[hidden])
Date: 2006-03-08 13:52:02

Patches item #1438991, was opened at 2006-02-26 04:54
Message generated for change (Settings changed) made by mclow
You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=307586&aid=1438991&group_id=7586

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Category: None
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: David Benbennick (dbenbenn)
>Assigned to: Jonathan Turkanis (turkanis)
Summary: rational::operator<(rational) fails due to overflow

Initial Comment:
Consider boost::rational<long>.  Pick two objects of
that type at random (that's well defined, since there
are only finitely many objects of that type), and pick
them to both have the same sign.  Call them a and b. 
Then the expression "a < b" returns the wrong value 50%
of the time!  The problem is that the calculation in
rational.hpp can overflow, and that overflow actually
happens most of the time.

Here's a patch that implements operator< without
overflow.  It's somewhat less efficient (I haven't
benchmarked it) but it gives the right answer.

Possibly the new version of operator< has bugs.  I've
run a few hundred million random tests, with the right
answer every time (I've used bc to check the answers),
but there might be a subtle problem.  In particular,
the new code assumes that integer division always
truncates towards 0, even when dividing negative
integers.  Perhaps that is not true on some obscure
platforms/compilers?

----------------------------------------------------------------------

Comment By: David Benbennick (dbenbenn)
Date: 2006-03-07 13:08

Message:
Logged In: YES 
user_id=95581

Here's a new patch that uses std::numeric_limits to select
the more efficient, direct comparison method for unbounded
types, and only uses the overflow-avoiding method for types
that can overflow.

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