The GUID->url map is because the metadatabase is keyed off URL. When you
want to retrieve the metadata, or at least the filename of a given song,
given the song GUID, you use that map to get the filename. The GUID is
unique, in that any files which share the same GUID are duplicates of each
other, perhaps the same track on multiple albums. However, they sound the
same, so it is irrelevant which one is retrieved when, for instance,
inserting into a playlist a list of songs, as identified by the song GUID.
-Sean
> As my plan for world domiation though efficient string caching unfolds,
the
> next step is rewrite the guid -> url multimap for GetFilename.
>
> The MetaData does not know the url. It knows the guid, which a keys to
> one (or more, it is a multimap) of urls.
>
> I see the Metdata is further wrapped into the PlaylistItem which
> holds:
>
> private:
> MetaData m_metadata;
> string m_url;
> PlaylistItemState m_state;
> };
>
> And the MusicCatalog has the methods:
>
> void WriteMetaDataToDatabase(const char *url, const MetaData metadata,
> MetadataStorageType type = kTypeTrack);
> MetaData *ReadMetaDataFromDatabase(const char *url);
> string GetFilename(const string &strGUID);
>
> ..and various Add*(url) and Remove*(url) operations.
>
> And it is the MusicCatalog that keeps track of guid->url multimap.
>
> Here is where a design document would help. What is the GUID, and
> what is its purpose in life? Answer: it is what the music brainz library
> generates into char guid[17].
>
> It's not a unique id since two different songs may have the same GUID
> but different urls. Thus the multimap. But GetFilename simply
> presents the first match returned. The only routines that notice
> multiple values with same GUID key are the Remove operations, which
> erase all the matches.
>
> So why rely on the guid->url mapping? Since PlayListItem simply
> combines metadata with url, why not put the url into the metadata?
> The url is the key into the database, and the url to metadata mapping
> is given by ReadMetaDataFromDatabase. With playlistitems, the one to
> one mapping of url and metdata is clear. But the meta->url mapping
> is clouded.
>
>
> Given the metadata, you have the kludge of Getfilename. What is
> the consequence of GetFilename shadowing some urls by only returning
> the first hit?
>
> Again, why is the sky blue? why is the grass green? why is url not
> in MetaData?
>
> I want to grok the design before I replace the map of strings with a
> map on char*. I do not want to redesign where the url is stored, but
> I do want to understand how it got to be the way it is.
>
> --
> Chris
> _______________________________________________
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>
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