Implement simple shuffle algorithm.

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

src/audiowmark.cc

@@ -27,6 +27,7 @@ namespace Params
   static int min_band        = 20;
   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;
 }
 
 void
@@ -140,6 +141,10 @@ parse_options (int   *argc_p,
 	{
           Params::block = atoi (opt_arg);
 	}
+      else if (check_arg (argc, argv, &i, "--shuffle"))
+	{
+          Params::shuffle = true;
+	}
       else if (check_arg (argc, argv, &i, "--water-delta", &opt_arg))
 	{
           Params::water_delta = atof (opt_arg);
@@ -246,6 +251,27 @@ get_up_down (int f, vector<int>& up, vector<int>& down)
   choose_bands (down);
 }
 
+vector<int>
+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);
+
+  for (size_t i = 0; i < result.size(); i++)
+    result[i] = i;
+
+  // Fisher–Yates shuffle
+  for (size_t i = 0; i < result.size() - 1; i++)
+    {
+      size_t j = i + rng() % (result.size() - i);
+      std::swap (result[i], result[j]);
+    }
+  return result;
+}
+
 static unsigned char
 from_hex_nibble (char c)
 {
@@ -408,7 +434,11 @@ add_watermark (const string& infile, const string& outfile, const string& bits)
     {
       for (int ch = 0; ch < wav_data.n_channels(); ch++)
         {
-          vector<float> frame = get_frame (wav_data, f, ch);
+          const int     f_offset = f - f % (Params::block * Params::frames_per_bit);
+          vector<int>   shuffle = gen_shuffle (Params::block * Params::frames_per_bit, f / (Params::block * Params::frames_per_bit));
+          const int     f_shuffle = Params::shuffle ? f_offset + shuffle[f % (Params::block * Params::frames_per_bit)] : f;
+
+          vector<float> frame = get_frame (wav_data, f_shuffle, ch);
           vector<float> new_frame;
 
           if (frame.size() == Params::frame_size)
@@ -422,7 +452,7 @@ add_watermark (const string& infile, const string& outfile, const string& bits)
               new_frame = frame;
             }
           for (size_t i = 0; i < new_frame.size(); i++)
-            out_signal[(f * Params::frame_size + i) * wav_data.n_channels() + ch] = new_frame[i] * Params::pre_scale;
+            out_signal[(f_shuffle * Params::frame_size + i) * wav_data.n_channels() + ch] = new_frame[i] * Params::pre_scale;
         }
     }
 
@@ -461,7 +491,9 @@ get_watermark (const string& infile, const string& orig_pattern)
     {
       for (int ch = 0; ch < wav_data.n_channels(); ch++)
         {
-          vector<float> frame = get_frame (wav_data, f, ch);
+          const int     f_offset = f - f % (Params::block * Params::frames_per_bit);
+          vector<int>   shuffle = gen_shuffle (Params::block * Params::frames_per_bit, f / (Params::block * Params::frames_per_bit));
+          vector<float> frame = get_frame (wav_data, Params::shuffle ? f_offset + shuffle[f % (Params::block * Params::frames_per_bit)] : f, ch);
           if (frame.size() == Params::frame_size)
             {
               /* windowing */