dot: quote identifiers containing a minus

* spot/twaalgos/dot.cc: Quote identifiers containing a minus.
* tests/core/alternating.test: Add test case.
* NEWS: Mention the bug.

NEWS

@@ -120,6 +120,10 @@ New in spot 2.10.6.dev  (not yet released)
     into the universal edge vector, since the later can be
     reallocated during that process.
 
+  - Printing an alternating automaton with print_dot() using 'u' to
+    hide true state could produce some incorrect GraphViz output if
+    the automaton as a true state as part of a universal group.
+
 New in spot 2.10.6  (2022-05-18)
 
   Bugs fixed:

spot/twaalgos/dot.cc

@@ -578,10 +578,27 @@ namespace spot
         return tmp_dst.str();
       }
 
-      template<typename U, typename V>
-      void print_true_state(U to, V from) const
+      void print_hidden_true_name(unsigned to, unsigned from) const
       {
-        os_ << "  T" << to << 'T' << from << " [label=\"\", style=invis, ";
+        os_ << 'T' << to << 'T' << from;
+      }
+
+      void print_hidden_true_name(unsigned to, const std::string& from) const
+      {
+        bool neg = from[0] == '-';
+        if (neg)
+          os_ << '"';
+        os_ << 'T' << to << 'T' << from;
+        if (neg)
+          os_ << '"';
+      }
+
+      template <typename F>
+      void print_true_state(unsigned to, F from) const
+      {
+        os_ << "  ";
+        print_hidden_true_name(to, from);
+        os_ << " [label=\"\", style=invis, ";
         os_ << (opt_vertical_ ? "height=0]\n" : "width=0]\n");
       }
 
@@ -606,7 +623,7 @@ namespace spot
                   print_true_state(d, dest);
                 os_ << "  " << dest << " -> ";
                 if (dst_is_hidden_true_state)
-                  os_ << 'T' << d << 'T' << dest;
+                  print_hidden_true_name(d, dest);
                 else
                   os_ << d;
                 if ((style && *style) || opt_id_)

tests/core/alternating.test

@@ -1,6 +1,6 @@
 #!/bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2016-2018, 2020-2021 Laboratoire de Recherche et
+# Copyright (C) 2016-2018, 2020-2022 Laboratoire de Recherche et
 # Développement de l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
@@ -1009,3 +1009,41 @@ test '2_0_1_1_1_1_3_3' = "`autfilt --stats=$stats in`"
 
 autfilt --stats='%[x]U' in 2>stderr && exit2
 grep '%\[x\]U' stderr
+
+cat >in <<EOF
+HOA: v1
+States: 2
+Start: 1
+AP: 1 "a"
+acc-name: co-Buchi
+Acceptance: 1 Fin(0)
+properties: trans-labels explicit-labels trans-acc complete univ-branch
+--BODY--
+State: 0
+[t] 0
+State: 1
+[t] 0&1 {0}
+[0] 0
+--END--
+EOF
+autfilt -du in >out.dot
+# T0T-1 is not a valid name for GraphViz, it has to be quoted.
+cat >exp.dot <<EOF
+digraph "" {
+  rankdir=LR
+  label="Fin(0)\n[co-Büchi]"
+  labelloc="t"
+  node [shape="circle"]
+  I [label="", style=invis, width=0]
+  I -> 1
+  1 [label="1"]
+  1 -> -1 [label="1\n{0}", arrowhead=onormal]
+  -1 [label=<>,shape=point,width=0.05,height=0.05]
+  "T0T-1" [label="", style=invis, width=0]
+  -1 -> "T0T-1"
+  -1 -> 1
+  T0T1 [label="", style=invis, width=0]
+  1 -> T0T1 [label="a"]
+}
+EOF
+diff out.dot exp.dot