Calculate bitrate for ALAC files without it (taglib#961)

If the "alac" atom of an MP4 file has a zero bitrate, it is calculated.

taglib/mp4/mp4properties.cpp

@@ -31,6 +31,27 @@
 
 using namespace TagLib;
 
+namespace
+{
+  // Calculate the total bytes used by audio data, used to calculate the bitrate
+  long long calculateMdatLength(const MP4::AtomList &list)
+  {
+    long long totalLength = 0;
+    for(MP4::AtomList::ConstIterator it = list.begin(); it != list.end(); ++it) {
+      long length = (*it)->length;
+      if(length == 0)
+        return 0; // for safety, see checkValid() in mp4file.cpp
+
+      if((*it)->name == "mdat")
+        totalLength += length;
+
+      totalLength += calculateMdatLength((*it)->children);
+    }
+
+    return totalLength;
+  }
+}
+
 class MP4::Properties::PropertiesPrivate
 {
 public:
@@ -224,6 +245,13 @@ MP4::Properties::read(File *file, Atoms *atoms)
       d->channels      = data.at(73);
       d->bitrate       = static_cast<int>(data.toUInt(80U) / 1000.0 + 0.5);
       d->sampleRate    = data.toUInt(84U);
+
+      if(d->bitrate == 0 && d->length > 0) {
+        // There are files which do not contain a nominal bitrate, e.g. those
+        // generated by refalac64.exe. Calculate the bitrate from the audio
+        // data size (mdat atoms) and the duration.
+        d->bitrate = (calculateMdatLength(atoms->atoms) * 8) / d->length;
+      }
     }
   }