congpairs: fix for non-trivial classes

semigroups · Jun 15, 2020 · e5cd086 · e5cd086
1 parent d8ddceb
commit e5cd086
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diff --git a/gap/congruences/congpairs.gi b/gap/congruences/congpairs.gi
@@ -38,6 +38,11 @@ function(cong)
   S := Range(cong);
   gens := GeneratorsOfSemigroup(S);
   words_part := CONG_PAIRS_NONTRIVIAL_CLASSES(cong);
+  if words_part = fail then
+    # libsemigroups can't compute the non-trivial classes, and threw an
+    # exception.
+    return List(Filtered(EquivalenceClasses(cong), x -> Size(x) > 1), AsList);
+  fi;
   return List(words_part, c -> List(c, w -> Product(w, i -> gens[i])));
 end);
 

diff --git a/src/congpairs.cc b/src/congpairs.cc
@@ -414,6 +414,12 @@ gap_list_t CONG_PAIRS_NONTRIVIAL_CLASSES(Obj self, gap_cong_t o) {
   initRNams();
   Congruence* cong = cong_obj_get_cpp(o);
 
+  try {
+    cong->nr_non_trivial_classes();
+  } catch (libsemigroups::LibsemigroupsException& e) {
+    return Fail;
+  }
+
   // Initialise gap_lists
   gap_list_t gap_lists
       = NEW_PLIST_IMM(T_PLIST_TAB, cong->nr_non_trivial_classes());

diff --git a/tst/testinstall.tst b/tst/testinstall.tst
@@ -1747,6 +1747,12 @@ false
 gap> IsUniversalSemigroupCongruence(cong);
 false
 
+# Issues 676-677
+gap> S := FreeBand(2);;
+gap> cong := SemigroupCongruence(S, []);;
+gap> NonTrivialCongruenceClasses(cong);
+[  ]
+
 # SEMIGROUPS_UnbindVariables
 gap> Unbind(B);
 gap> Unbind(D);