Fix HighestWeightModule for Lie algebras in certain cases #2617
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This is a regression in GAP 4.9 compared to GAP 4.8. As far as I can tell, it is caused by the new sort functions; for some reason I don't quite understand, the new sort ends up comparing an element to itself. But the custom `lexord` sorting function used by `HighestWeightModule` was not prepared to deal with two equal (much less identical) inputs. Fixes gap-system#2491
fingolfin
added
kind: bug
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@ChrisJefferson I really wonder why the sorting function ends up comparing an element to itself (the input is free of duplicates; the comparison function really receives the same (identical) object twice as input. |
Codecov Report
@@ Coverage Diff @@
## master #2617 +/- ##
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+ Coverage 74.8% 74.81% +<.01%
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Files 479 479
Lines 242251 242193 -58
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- Hits 181206 181185 -21
+ Misses 61045 61008 -37
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Note that PR #2621 also fixes this issue, but I really want this one in, too, as arguably the comparator is wrong otherwise (we could have to equal but non-identical entries in a list being sorted, after all... OK, maybe not in the code being modified here, but...) |
sebasguts
approved these changes
Jul 6, 2018
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It worked before because the comparator never was called with two inputs that happened to be equal. But this changed when we changed our sorting algo. |
fingolfin
added
the
release notes: added
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This is a regression in GAP 4.9 compared to GAP 4.8. As far as I can tell, it
is caused by the new sort functions; for some reason I don't quite
understand, the new sort ends up comparing an element to itself. But the
custom
lexordsorting function used byHighestWeightModulewas notprepared to deal with two equal (much less identical) inputs.
Fixes #2491
I suggest to backport this to stable-4.9 -- it can wait for GAP 4.9.3, though, if adding it now is inconvenient.