#15836: BasisMatroid.circuits() returns a malformed SetSystem when called on the
empty matroid
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Reporter: Rudi | Owner: Rudi
Type: defect | Status: new
Priority: minor | Milestone: sage-6.2
Component: matroid theory | Resolution:
Keywords: | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
u/Rudi/ticket/15836 | 329566023aaef2d2f534bfe7cf1753052055b335
Dependencies: | Stopgaps:
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Changes (by Rudi):
* owner: => Rudi
* commit: => 329566023aaef2d2f534bfe7cf1753052055b335
Comment:
The problem here is essentially due to the fact that one cannot create a
bitset of length 0. To adjust for this, I worked with bitsets of length 1
in the exceptional case of a BasisExchangeMatroid (or SetSystem) with an
empty ground set. But then bitset_complement() does not precisely do what
it should do, and may create a set containing an element index pointing
outside the ground set.
So what I did now is catch the possibility of an empty ground set in
several enumerative functions. I will go on to have a look at the more
low-level functions. I hope that they simply work, because I would hate to
incur a speed penalty to repair a problem that arises only for the empty
matroid.
----
New commits:
||[http://git.sagemath.org/sage.git/commit/?id=329566023aaef2d2f534bfe7cf1753052055b335
3295660]||{{{Adjusted several enumerative functions BasisExchangeMatroid
to separately handle the fringe}}}||
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Ticket URL: <http://trac.sagemath.org/ticket/15836#comment:2>
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