Optimize watermark synthesis loop.

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

src/audiowmark.cc

@@ -694,14 +694,19 @@ add_watermark (const string& infile, const string& outfile, const string& bits)
           /* mix watermark signal to output frame */
           vector<float> fft_delta_out = ifft (fft_delta_spect[f * wav_data.n_channels() + ch]);
 
-          int last_frame_start = (f - 1) * Params::frame_size;
-          for (int i = 0; i < int (synth_window.size()); i++)
+          for (int dframe = -1; dframe <= 1; dframe++)
             {
-              int pos = (last_frame_start + i) * wav_data.n_channels() + ch;
+              const int wstart = (dframe + 1) * Params::frame_size;
 
-              if (pos >= 0 && pos < int (out_signal.size()))
+              if (f + dframe > 0 && f + dframe < frame_count (wav_data))
                 {
-                  out_signal[pos] += fft_delta_out[i % Params::frame_size] * synth_window[i];
+                  int pos = (f + dframe) * Params::frame_size * wav_data.n_channels() + ch;
+
+                  for (size_t x = 0; x < Params::frame_size; x++)
+                    {
+                      out_signal[pos] += fft_delta_out[x] * synth_window[wstart + x];
+                      pos += wav_data.n_channels();
+                    }
                 }
             }
         }