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Fix potential infinite loop or recursion when computing size of infinite cyclic groups #3580

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Jul 17, 2019
Merged

Fix potential infinite loop or recursion when computing size of infinite cyclic groups #3580

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2 changes: 1 addition & 1 deletion lib/grp.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -73,7 +73,7 @@ InstallMethod( IsCyclic,

InstallMethod( Size,
"for a cyclic group",
[ IsGroup and IsCyclic and HasGeneratorsOfGroup ],
[ IsGroup and IsCyclic and HasGeneratorsOfGroup and CanEasilyCompareElements ],
{} -> -RankFilter(HasGeneratorsOfGroup),
function(G)
local gens;
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10 changes: 10 additions & 0 deletions tst/testbugfix/2019-07-17-cyclic-size.tst
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@@ -0,0 +1,10 @@
# Fix a bug that only occurs if all packages are loaded
gap> F:=FreeGroup(2);;
gap> F:=F/[F.1/F.2];;
gap> H:=Group(F.1);;
gap> IsCyclic(H);
true
gap> HasIsFinite(H);
false
gap> Size(H); # should be instant, not infinite recursion / loop
infinity