Add command line parameter to set the random seed.

Signed-off-by: Stefan Westerfeld <stefan@space.twc.de>

src/audiowmark.cc

@@ -28,6 +28,7 @@ namespace Params
   static double water_delta  = 0.015; // strength of the watermark
   static double pre_scale    = 0.95;  // rescale the signal to avoid clipping after watermark is added
   static bool   shuffle      = false;
+  static unsigned int seed   = 0;
 }
 
 void
@@ -145,6 +146,10 @@ parse_options (int   *argc_p,
 	{
           Params::shuffle = true;
 	}
+      else if (check_arg (argc, argv, &i, "--seed", &opt_arg))
+	{
+          Params::seed = atoi (opt_arg);
+	}
       else if (check_arg (argc, argv, &i, "--water-delta", &opt_arg))
 	{
           Params::water_delta = atof (opt_arg);
@@ -227,14 +232,22 @@ get_frame (WavData& wav_data, int f, int ch)
   return result;
 }
 
+std::mt19937_64
+init_rng (uint64_t seed)
+{
+  const uint64_t  prime = 3126986573;
+
+  std::mt19937_64 rng;
+  rng.seed (seed + prime * Params::seed);
+
+  return rng;
+}
+
 void
 get_up_down (int f, vector<int>& up, vector<int>& down)
 {
   vector<int> used (Params::frame_size / 2);
-  std::mt19937_64 rng;
-
-  // use per frame random seed, may want to have cryptographically secure algorithm
-  rng.seed (f);
+  std::mt19937_64 rng = init_rng (f); // use per frame random seed, may want to have cryptographically secure algorithm
 
   auto choose_bands = [&used, &rng] (vector<int>& bands) {
     while (bands.size() < Params::bands_per_frame)
@@ -256,9 +269,7 @@ gen_shuffle (size_t size, int seed)
 {
   vector<int> result (size);
 
-  std::mt19937_64 rng;
-  // should use cryptographically secure generator, properly seeded
-  rng.seed (seed);
+  std::mt19937_64 rng = init_rng (seed); // should use cryptographically secure generator, properly seeded
 
   for (size_t i = 0; i < result.size(); i++)
     result[i] = i;