From fcd5de4c62de48cf74b67a58cf8310e135f099ae Mon Sep 17 00:00:00 2001
From: Max Horn <max@quendi.de>
Date: Mon, 6 May 2024 18:37:52 +0200
Subject: [PATCH] Ensure subgroup is tested at least once, (#5709)

in finding induced permutation representation of FpGroup,
even if max parameter was increased in between. This resolves #5697

Co-authored-by: Alexander Hulpke <hulpke@math.colostate.edu>
---
 lib/grpfp.gi                                         |  3 +++
 .../2024-05-06-IsomorphismPermGroupOrFailFpGroup.tst | 12 ++++++++++++
 2 files changed, 15 insertions(+)
 create mode 100644 tst/testbugfix/2024-05-06-IsomorphismPermGroupOrFailFpGroup.tst

diff --git a/lib/grpfp.gi b/lib/grpfp.gi
index 3466204b8b..b84ded14ab 100644
--- a/lib/grpfp.gi
+++ b/lib/grpfp.gi
@@ -4051,6 +4051,7 @@ local mappow, G, max, p, gens, rels, comb, i, l, m, H, t, gen, sz,
             RelatorsOfFpGroup(G),[gen],true,false:
               cyclic:=true,limit:=1+max,quiet:=true );
     fi;
+
     if t=fail then
       # we cannot get the size within the permitted limits -- give up
       return fail;
@@ -4098,6 +4099,8 @@ local mappow, G, max, p, gens, rels, comb, i, l, m, H, t, gen, sz,
     max:=10^3*sz;
   fi;
 
+  amax:=Maximum(amax,max+1);
+
   useind:=false;
   t1:=timerFunc();
   while max<amax do
diff --git a/tst/testbugfix/2024-05-06-IsomorphismPermGroupOrFailFpGroup.tst b/tst/testbugfix/2024-05-06-IsomorphismPermGroupOrFailFpGroup.tst
new file mode 100644
index 0000000000..d893f3c54a
--- /dev/null
+++ b/tst/testbugfix/2024-05-06-IsomorphismPermGroupOrFailFpGroup.tst
@@ -0,0 +1,12 @@
+# Verify regression in IsomorphismPermGroupOrFailFpGroup is
+# resolved, see <https://github.com/gap-system/gap/issues/5697>
+gap> F:= FreeGroup(2);
+<free group on the generators [ f1, f2 ]>
+gap> gens:= GeneratorsOfGroup( F );
+[ f1, f2 ]
+gap> x:= gens[1];; y:= gens[2];;
+gap> rels:= [ y*x^-1*y^-1*x*y^-1*x^-1, x^-1*y*x*y*x^-1*y^-2 ];;
+gap> G:= F / rels;
+<fp group on the generators [ f1, f2 ]>
+gap> IsomorphismPermGroupOrFailFpGroup(G, 100000) <> fail;
+true