Re-enable want_frames optimization in sync finder.

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

src/audiowmark.cc

@@ -1289,11 +1289,32 @@ private:
   sync_fft (const WavData& wav_data, size_t index, size_t count, vector<float>& fft_out_db, const vector<int>& want_frames)
   {
     FFTAnalyzer fft_analyzer (wav_data.n_channels());
+    const vector<float>& samples = wav_data.samples();
 
     fft_out_db.clear();
 
     /* computing db-magnitude is expensive, so we better do it here */
-    vector<vector<complex<float>>> fft_out = fft_analyzer.fft_range (wav_data.samples(), index, count);
+    vector<vector<complex<float>>> fft_out;
+
+    for (size_t f = 0; f < count; f++)
+      {
+        const size_t input_start = (index + f * Params::frame_size) * wav_data.n_channels();
+        const size_t input_end   = input_start + Params::frame_size * wav_data.n_channels();
+
+        if (input_end > samples.size() || (want_frames.size() && !want_frames[f]))
+          {
+            for (int ch = 0; ch < wav_data.n_channels(); ch++)
+              fft_out.push_back ({});
+          }
+        else
+          {
+            vector<float> input (samples.begin() + input_start, samples.begin() + input_end);
+
+            vector<vector<complex<float>>> frame_result = fft_analyzer.run_fft (input);
+            for (auto& fr : frame_result)
+              fft_out.emplace_back (std::move (fr));
+          }
+      }
     for (size_t p = 0; p < fft_out.size(); p++)
       {
         const double min_db = -96;