testhls: improved rounding for ffmpeg frame cut (start/duration)

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

src/testhls.cc

@@ -106,13 +106,14 @@ ff_encode (const WavData& wav_data, const string& filename, size_t start_pos, si
 
   double length_s = double (wav_data.n_values()) / wav_data.n_channels() / wav_data.sample_rate();
 
-  /* subtracting 0.001 from cut_start_s ensures that rounding performed by printf()
-   * doesn't accidentally cut one frame more than necessary
+  /* subtracting/adding 0.000001 from/to cut_start_s ensures that rounding
+   * performed by printf() doesn't accidentally cut one frame more or less than
+   * intended
    */
-  double cut_start_s = cut_start / double (wav_data.sample_rate()) - 0.001;
+  double cut_start_s = cut_start / double (wav_data.sample_rate());
   double cut_end_s = cut_end / double (wav_data.sample_rate());
-  cmd = string_printf ("ffmpeg -v error -y -i '%s' -ss %.3f -t %.3f -f mpegts -output_ts_offset %f -muxdelay 0 -muxpreload 0 -c copy '%s-tcpy'",
-                       filename.c_str(), cut_start_s, length_s - (cut_start_s + cut_end_s), pts_start, filename.c_str());
+  cmd = string_printf ("ffmpeg -v error -y -i '%s' -ss %.6f -t %.6f -f mpegts -output_ts_offset %f -muxdelay 0 -muxpreload 0 -c copy '%s-tcpy'",
+                       filename.c_str(), cut_start_s - 0.000001, length_s - (cut_start_s + cut_end_s) + 0.000001, pts_start, filename.c_str());
   err = xsystem (cmd);
   if (err)
     return err;