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Fix Orbits when called with a range as seeds #5057

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Sep 21, 2022
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Fix Orbits when called with a range as seeds #5057

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7 changes: 5 additions & 2 deletions lib/oprt.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -1664,7 +1664,7 @@ end );

InstallMethod( Orbits, "for arbitrary domains", true, OrbitsishReq, 0,
function( G, D, gens, acts, act )
local orbs, orb,sort,plist,pos,use,o,nc,ld,ld1;
local orbs, orb,sort,plist,pos,use,o,nc,ld,ld1,pc;

sort:=Length(D)>0 and CanEasilySortElements(D[1]);
plist:=IsPlistRep(D);
Expand All @@ -1691,7 +1691,10 @@ local orbs, orb,sort,plist,pos,use,o,nc,ld,ld1;
fi;
else
for o in orb do
use[PositionCanonical(D,o)]:=true;
pc:=PositionCanonical(D,o);
if pc <> fail then
use[pc]:=true;
fi;
od;
# not plist -- do not take difference as there may be special
# `PositionCanonical' method.
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7 changes: 7 additions & 0 deletions tst/testbugfix/2022-09-15-Orbits.tst
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@@ -0,0 +1,7 @@
# Verify that Orbits works when the given seeds are a range
# See https://github.com/gap-system/gap/issues/5056
gap> act := function(omega,g) return PowerModInt(omega,Int(g),15); end;;
gap> ugrp := Units(ZmodnZ(Phi(15)));
<group of size 4 with 2 generators>
gap> orbs := Orbits(ugrp,[1..5],act);
[ [ 1 ], [ 2, 8 ], [ 3, 12 ], [ 4 ], [ 5 ] ]