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Fix regression in IsomorphismGroups leading to unexpected break loops #5711

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May 6, 2024
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Fix regression in IsomorphismGroups leading to unexpected break loops #5711

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6 changes: 6 additions & 0 deletions lib/morpheus.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -535,6 +535,12 @@ local hom, gens, c, ran, r, cen, img, u, orbs,
while Size(u)<Size(g) do
if Size(u)>1 then SortBy(c,Size);fi; # once an outside class is known, sort
of:=First(c,x->not Representative(x) in u);

if of=fail then
# rarely reps are in u
of:=First(c,
x->ForAny(GeneratorsOfGroup(g),y->not Representative(x)^y in u));
fi;
if MemoryUsage(Representative(of))*Size(of)
# if a single class only requires
<10000
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7 changes: 7 additions & 0 deletions tst/testbugfix/2024-05-06-Isom.tst
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@@ -0,0 +1,7 @@
# Fix #5708
gap> G:=Group([ (2,11,20)(3,10,19)(4,9,12)(5,8,13)(6,15,17)(7,14,18),
> (2,20)(3,4)(5,7)(8,18)(9,19)(10,12)(13,14)(15,17),
> (1,2)(3,20)(6,18)(7,19)(8,12)(9,11)(13,15)(16,17) ]);;
gap> H:=Group([ (1,2,3,4,5), (3,4,5), (6,7) ]);;
gap> IsomorphismGroups(G,H)<>fail;
true
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