randltl: do not reset the seed between formulas

Reported by Joachim Klein.

* src/bin/randltl.cc: Here.
* NEWS: Mention the fix.

NEWS

@@ -35,6 +35,10 @@ New in spot 1.2.4a (not yet released)
       this was actually hiding the formulas for people using a
       terminal with white background... This version displays formula
       in bright blue instead.
+    - 'randltl -n -1 --seed 0' and 'randltl -n -1 --seed 1' used to
+      generate nearly the same list of formulas, shifted by one,
+      because the PRNG write reset with an incremented seed between
+      each output formula.  The PRNG is now reset only once.
 
 New in spot 1.2.4 (2014-05-15)
 

src/bin/randltl.cc

@@ -379,9 +379,10 @@ main(int argc, char** argv)
     error(2, 0, "No atomic proposition supplied?   Run '%s --help' for usage.",
 	  program_name);
 
+  spot::srand(opt_seed);
+
   typedef Sgi::hash_set<const spot::ltl::formula*,
 			const spot::ptr_hash<const spot::ltl::formula> > fset_t;
-
   fset_t unique_set;
 
   spot::ltl::ltl_simplifier simpl(simplifier_options());
@@ -392,7 +393,6 @@ main(int argc, char** argv)
       unsigned trials = MAX_TRIALS;
       bool ignore;
       const spot::ltl::formula* f = 0;
-      spot::srand(opt_seed++);
       do
 	{
 	  ignore = false;