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

From: SourceForge.net (noreply_at_[hidden])
Date: 2006-02-26 07:54:47

Patches item #1438991, was opened at 2006-02-26 07:54
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Category: None
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: David Benbennick (dbenbenn)
Assigned to: Nobody/Anonymous (nobody)
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?

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