Fix Orbits with a range as seeds

gap-system · Oct 14, 2022 · 0a20c1b · 0a20c1b
1 parent 32d42ef
commit 0a20c1b
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Showing 2 changed files with 12 additions and 2 deletions.
diff --git a/lib/oprt.gi b/lib/oprt.gi
@@ -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);
@@ -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.

diff --git a/tst/testbugfix/2022-09-15-Orbits.tst b/tst/testbugfix/2022-09-15-Orbits.tst
@@ -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 ] ]