#11950: Bug in topological_sort
----------------------------+-----------------------------------------------
Reporter: andrejv | Owner: jason, ncohen, rlm
Type: defect | Status: needs_review
Priority: major | Milestone: sage-4.7.3
Component: graph theory | Keywords:
Work_issues: | Upstream: N/A
Reviewer: | Author:
Merged: | Dependencies:
----------------------------+-----------------------------------------------
Changes (by ncohen):
* status: new => needs_review
Comment:
For the story, I stared at this algorithm for 50 minutes thinking it was
returning a wrong ordering (I must have misread the ordering when first
checking), to end up convinced that the orderings returned *HAD* to be
valid.
Only then I noticed that the problem lied in some vertices being returned
twice. The fix was easy : once you have said that a vertex needs to be
deactivated *DO NOT CONSIDER IT EVER AGAIN*. The fix lies in the lines :
{{{
# This vertex may have been deactivated since we added it.
if bitset_not_in(activated, u):
stack.pop(-1)
continue
}}}
But I modified the code in some other places to make it clearer, and added
some documentation. Sorry for that `:-)` !
Nathann
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/11950#comment:1>
Sage <http://www.sagemath.org>
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