twagraph: fix merge_edges() ignoring the first edge

This fixes #441, reported by Jérôme Dubois.

* tests/python/mergedge.py: New file.
* tests/Makefile.am: Add it.
* spot/twa/twagraph.cc (merge_edges): Fix initialization of second
loop.
* NEWS: Mention the bug.

NEWS

@@ -1,6 +1,9 @@
 New in spot 2.9.5.dev (not yet released)
 
-  Nothing yet.
+  Bugs fixed:
+
+  - twa_graph::merge_edges() could fail to merge two transitions if the
+    destination transition was the first of the automaton.  (Issue #441)
 
 New in spot 2.9.5 (2020-11-19)
 

spot/twa/twagraph.cc

@@ -195,7 +195,8 @@ namespace spot
       trans.resize(out);
 
     tend = out;
-    out = in = 2;
+    out = 1;
+    in = 2;
 
     // FIXME: We could should also merge edges when using
     // fin_acceptance, but the rule for Fin sets are different than

tests/Makefile.am

@@ -391,6 +391,7 @@ TESTS_python = \
   python/ltlparse.py \
   python/ltlsimple.py \
   python/merge.py \
+  python/mergedge.py \
   python/minato.py \
   python/misc-ec.py \
   python/optionmap.py \

tests/python/mergedge.py

@@ -0,0 +1,28 @@
+#!/usr/bin/python3
+# -*- mode: python; coding: utf-8 -*-
+# Copyright (C) 2020 Laboratoire de Recherche et Développement de
+# l'EPITA.
+#
+# This file is part of Spot, a model checking library.
+#
+# Spot is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# Spot is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+# or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
+# License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+
+import spot
+
+aut = spot.automaton("""HOA: v1 States: 1 Start: 0 AP: 1 "a"
+Acceptance: 1 Inf(0) --BODY-- State: 0 [0] 0 [0] 0 {0} --END--""")
+assert aut.num_edges() == 2
+aut.merge_edges()
+assert aut.num_edges() == 1