Performance optimization: use db_from_complex everywhere.

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

src/syncfinder.cc

@@ -391,7 +391,7 @@ SyncFinder::sync_fft (const WavData& wav_data, size_t index, size_t frame_count,
           /* computing db-magnitude is expensive, so we better do it here */
           for (int ch = 0; ch < wav_data.n_channels(); ch++)
             for (int i = Params::min_band; i <= Params::max_band; i++)
-              fft_out_db[out_pos++] = db_from_factor (abs (frame_result[ch][i]), min_db);
+              fft_out_db[out_pos++] = db_from_complex (frame_result[ch][i], min_db);
 
           have_frames[f] = 1;
         }

src/wmcommon.cc

@@ -49,30 +49,6 @@ std::string Params::output_label;
 using std::vector;
 using std::complex;
 
-/*
- * glibc log2f is a lot faster than glibc log10
- */
-inline double
-fast_log10 (double l)
-{
-  constexpr double log2_log10_factor = 0.3010299956639811952; // 1 / log2 (10)
-
-  return log2f (l) * log2_log10_factor;
-}
-
-double
-db_from_factor (double factor, double min_dB)
-{
-  if (factor > 0)
-    {
-      double dB = fast_log10 (factor); /* Bell */
-      dB *= 20;
-      return dB;
-    }
-  else
-    return min_dB;
-}
-
 FFTAnalyzer::FFTAnalyzer (int n_channels) :
   m_n_channels (n_channels),
   m_fft_processor (Params::frame_size)

src/wmcommon.hh

@@ -136,8 +136,6 @@ struct MixEntry
 
 std::vector<MixEntry> gen_mix_entries();
 
-double db_from_factor (double factor, double min_dB);
-
 size_t mark_data_frame_count();
 size_t mark_sync_frame_count();
 
@@ -185,6 +183,28 @@ window_hamming (double x) /* sharp (rectangle) cutoffs at boundaries */
   return 0.54 + 0.46 * cos (M_PI * x);
 }
 
+static inline float
+db_from_complex (float re, float im, float min_dB)
+{
+  float abs2 = re * re + im * im;
+
+  if (abs2 > 0)
+    {
+      constexpr float log2_log10_factor = 3.01029995663981; // 10 / log2 (10)
+
+      // glibc log2f is a lot faster than glibc log10
+      return log2f (abs2) * log2_log10_factor;
+    }
+  else
+    return min_dB;
+}
+
+static inline float
+db_from_complex (std::complex<float> f, float min_dB)
+{
+  return db_from_complex (f.real(), f.imag(), min_dB);
+}
+
 int add_stream_watermark (AudioInputStream *in_stream, AudioOutputStream *out_stream, const std::string& bits, size_t zero_frames);
 int add_watermark (const std::string& infile, const std::string& outfile, const std::string& bits);
 int get_watermark (const std::string& infile, const std::string& orig_pattern);

src/wmget.cc

@@ -83,8 +83,8 @@ mix_decode (vector<vector<complex<float>>>& fft_out, int n_channels)
               const int u = mix_entries[b].up;
               const int d = mix_entries[b].down;
 
-              umag += db_from_factor (abs (fft_out[index][u]), min_db);
-              dmag += db_from_factor (abs (fft_out[index][d]), min_db);
+              umag += db_from_complex (fft_out[index][u], min_db);
+              dmag += db_from_complex (fft_out[index][d], min_db);
             }
         }
       if ((f % Params::frames_per_bit) == (Params::frames_per_bit - 1))
@@ -116,10 +116,11 @@ linear_decode (vector<vector<complex<float>>>& fft_out, int n_channels)
 
           const double min_db = -96;
           for (auto u : up)
-            umag += db_from_factor (abs (fft_out[index][u]), min_db);
+            umag += db_from_complex (fft_out[index][u], min_db);
 
           for (auto d : down)
-            dmag += db_from_factor (abs (fft_out[index][d]), min_db);
+            dmag += db_from_complex (fft_out[index][d], min_db);
+
         }
       if ((f % Params::frames_per_bit) == (Params::frames_per_bit - 1))
         {

src/wmspeed.cc

@@ -166,22 +166,6 @@ public:
   }
 };
 
-static inline float
-db_from_complex (float re, float im, float min_dB)
-{
-  float abs2 = re * re + im * im;
-
-  if (abs2 > 0)
-    {
-      constexpr float log2_log10_factor = 3.01029995663981; // 10 / log2 (10)
-
-      // glibc log2f is a lot faster than glibc log10
-      return log2f (abs2) * log2_log10_factor;
-    }
-  else
-    return min_dB;
-}
-
 void
 SpeedSync::prepare_mags()
 {