Handle the case when MP4 file header has zero bitrate

This incorporates [6ca536b] (mp4 properties: handle the case when
mp4 file header has zero bitrate) from PR taglib#899 with a more accurate
bitrate calculation and a unit test.

taglib/mp4/mp4properties.cpp

@@ -234,7 +234,14 @@ MP4::Properties::read(File *file, Atoms *atoms)
           pos += 3;
         }
         pos += 10;
-        d->bitrate = static_cast<int>((data.toUInt(pos) + 500) / 1000.0 + 0.5);
+        const unsigned int bitrateValue = data.toUInt(pos);
+        if(bitrateValue != 0 || d->length <= 0) {
+          d->bitrate = static_cast<int>((bitrateValue + 500) / 1000.0 + 0.5);
+        }
+        else {
+          d->bitrate = static_cast<int>(
+                (calculateMdatLength(atoms->atoms) * 8) / d->length);
+        }
       }
     }
   }

tests/test_mp4.cpp

@@ -43,6 +43,7 @@ class TestMP4 : public CppUnit::TestFixture
 {
   CPPUNIT_TEST_SUITE(TestMP4);
   CPPUNIT_TEST(testPropertiesAAC);
+  CPPUNIT_TEST(testPropertiesAACWithoutBitrate);
   CPPUNIT_TEST(testPropertiesALAC);
   CPPUNIT_TEST(testPropertiesALACWithoutBitrate);
   CPPUNIT_TEST(testPropertiesM4V);
@@ -81,6 +82,28 @@ class TestMP4 : public CppUnit::TestFixture
     CPPUNIT_ASSERT_EQUAL(MP4::Properties::AAC, f.audioProperties()->codec());
   }
 
+  void testPropertiesAACWithoutBitrate()
+  {
+    ByteVector aacData = PlainFile(TEST_FILE_PATH_C("has-tags.m4a")).readAll();
+    CPPUNIT_ASSERT_GREATER(1960U, aacData.size());
+    CPPUNIT_ASSERT_EQUAL(ByteVector("mp4a"), aacData.mid(1890, 4));
+    // Set the bitrate to zero
+    for (int offset = 1956; offset < 1960; ++offset) {
+      aacData[offset] = 0;
+    }
+    ByteVectorStream aacStream(aacData);
+    MP4::File f(&aacStream);
+    CPPUNIT_ASSERT(f.audioProperties());
+    CPPUNIT_ASSERT_EQUAL(3, f.audioProperties()->lengthInSeconds());
+    CPPUNIT_ASSERT_EQUAL(3708, f.audioProperties()->lengthInMilliseconds());
+    CPPUNIT_ASSERT_EQUAL(3, f.audioProperties()->bitrate());
+    CPPUNIT_ASSERT_EQUAL(2, f.audioProperties()->channels());
+    CPPUNIT_ASSERT_EQUAL(44100, f.audioProperties()->sampleRate());
+    CPPUNIT_ASSERT_EQUAL(16, f.audioProperties()->bitsPerSample());
+    CPPUNIT_ASSERT_EQUAL(false, f.audioProperties()->isEncrypted());
+    CPPUNIT_ASSERT_EQUAL(MP4::Properties::AAC, f.audioProperties()->codec());
+  }
+
   void testPropertiesALAC()
   {
     MP4::File f(TEST_FILE_PATH_C("empty_alac.m4a"));