Add MP4::File::strip() to remove meta atom from file

This changes the modifications from the last commit in order to
achieve the following behavior: MP4::File::save() works in the
same way as before, i.e. it will never shrink the file and will
make space from removed items available as padding in the form of
a "free" atom. To completely remove the "meta" atom from the file,
a new method strip() is introduced, which can be used in the same
way as its MPEG::File::strip() counterpart.

taglib/mp4/mp4file.cpp

@@ -175,6 +175,26 @@ MP4::File::save()
   return d->tag->save();
 }
 
+bool
+MP4::File::strip(int tags)
+{
+  if(readOnly()) {
+    debug("MP4::File::strip() - Cannot strip tags from a read only file.");
+    return false;
+  }
+
+  if(!isValid()) {
+    debug("MP4::File::strip() -- Cannot strip tags from an invalid file.");
+    return false;
+  }
+
+  if(tags & MP4) {
+    return d->tag->strip();
+  }
+
+  return true;
+}
+
 bool
 MP4::File::hasMP4Tag() const
 {

taglib/mp4/mp4file.h

@@ -48,6 +48,19 @@ namespace TagLib {
     class TAGLIB_EXPORT File : public TagLib::File
     {
     public:
+      /*!
+       * This set of flags is used for strip() and is suitable for
+       * being OR-ed together.
+       */
+      enum TagTypes {
+        //! Empty set.  Matches no tag types.
+        NoTags  = 0x0000,
+        //! Matches MP4 tags.
+        MP4     = 0x0001,
+        //! Matches all tag types.
+        AllTags = 0xffff
+      };
+
       /*!
        * Constructs an MP4 file from \a file.  If \a readProperties is true the
        * file's audio properties will also be read.
@@ -114,6 +127,15 @@ namespace TagLib {
        */
       bool save();
 
+      /*!
+       * This will strip the tags that match the OR-ed together TagTypes from the
+       * file.  By default it strips all tags.  It returns true if the tags are
+       * successfully stripped.
+       *
+       * \note This will update the file immediately.
+       */
+      bool strip(int tags = AllTags);
+
       /*!
        * Returns whether or not the file on disk actually has an MP4 tag, or the
        * file has a Metadata Item List (ilst) atom.

taglib/mp4/mp4tag.cpp

@@ -528,11 +528,7 @@ MP4::Tag::save()
       debug("MP4: Unknown item name \"" + name + "\"");
     }
   }
-  // Leave data empty if there are no items. This will ensure that no meta atom
-  // is saved.
-  if (!data.isEmpty()) {
-    data = renderAtom("ilst", data);
-  }
+  data = renderAtom("ilst", data);
 
   AtomList path = d->atoms->path("moov", "udta", "meta", "ilst");
   if(path.size() == 4) {
@@ -545,6 +541,19 @@ MP4::Tag::save()
   return true;
 }
 
+bool
+MP4::Tag::strip()
+{
+  d->items.clear();
+
+  AtomList path = d->atoms->path("moov", "udta", "meta", "ilst");
+  if(path.size() == 4) {
+    saveExisting(ByteVector(), path);
+  }
+
+  return true;
+}
+
 void
 MP4::Tag::updateParents(const AtomList &path, long delta, int ignore)
 {
@@ -647,9 +656,6 @@ MP4::Tag::updateOffsets(long delta, long offset)
 void
 MP4::Tag::saveNew(ByteVector data)
 {
-  if(data.isEmpty())
-    return;
-
   data = renderAtom("meta", ByteVector(4, '\0') +
                     renderAtom("hdlr", ByteVector(8, '\0') + ByteVector("mdirappl") +
                                ByteVector(9, '\0')) +
@@ -724,11 +730,11 @@ MP4::Tag::saveExisting(ByteVector data, const AtomList &path)
     }
   }
   else {
-    // Strip meta
+    // Strip meta if data is empty, only the case when called from strip().
     MP4::Atom *udta = *(--it);
     AtomList &udtaChildren = udta->children;
     AtomList::Iterator metaIt = udtaChildren.find(meta);
-    if (metaIt != udtaChildren.end()) {
+    if(metaIt != udtaChildren.end()) {
       offset = meta->offset;
       delta = - meta->length;
       udtaChildren.erase(metaIt);

taglib/mp4/mp4tag.h

@@ -102,6 +102,11 @@ namespace TagLib {
          */
         bool contains(const String &key) const;
 
+        /*!
+         * Saves the associated file with the tag stripped.
+         */
+        bool strip();
+
         PropertyMap properties() const;
         void removeUnsupportedProperties(const StringList& properties);
         PropertyMap setProperties(const PropertyMap &properties);

tests/test_mp4.cpp

@@ -673,10 +673,8 @@ public:
       MP4::File f(copy.fileName().c_str());
       CPPUNIT_ASSERT(f.isValid());
       CPPUNIT_ASSERT(f.hasMP4Tag());
-      MP4::Tag *tag = f.tag();
-      CPPUNIT_ASSERT(!tag->isEmpty());
-      tag->setTitle("");
-      f.save();
+      CPPUNIT_ASSERT(!f.tag()->isEmpty());
+      f.strip();
     }
     {
       MP4::File f(copy.fileName().c_str());