Design: Create an index class

This will be used for groups, both stored and temporary.

Signed-off-by: Bryan Schumaker <bjschuma@gmail.com>

design.txt

@@ -20,11 +20,13 @@ Files:
 		database.h
 		database.hpp
 		group.h
+		index.h
 		library.h
 		playlist.h
 	ocarina/lib/
 		database.cpp
 		group.cpp
+		index.cpp
 		library.cpp
 		playlist.cpp
 	ocarina/tests/
@@ -134,6 +136,47 @@ API:
 
 
 
+Index: (lib/index.cpp)
+	An inverted index allows me to map multiple values to a single key.
+
+Index:
+	class Index {
+	private:
+		map<string, set<int>>
+		string filename;
+	public:
+		Index::Index(filename);
+		void load();
+		void save();
+		void insert(key, int);
+		void delete(key, int);
+		const set<int> &operator[](string);
+	};
+
+	File << key << map[key].size() << endl;
+	File << int_0 << int_1 << ... << int_n << endl;
+
+API:
+	Index.Index(filename);
+		Initializes an index using ~/.ocarina{-debug}/filename
+	Index.load();
+		Reads data from a file.  Call after static initialization of
+		Ocarina to ensure idle tasks are configured
+	Index.save();
+		Saves data to file
+	Index.insert(key, int);
+		1) If key does not exist, create it.
+		2) Add int to the list
+		3) Index.save()
+	Index.delete(key, int);
+		1) Remove int from the set of values associated with key
+		2) Do not delete key if set is empty
+		3) Index.save()
+	Index.operator[](string key);
+		Return the set associated with key
+
+
+
 Library: (lib/library.cpp)
 	The library manages databases containing track information added by the
 	user.  Ocarina 6 splits the library into multiple database tables for
@@ -222,7 +265,8 @@ Updating algorithm:
 	2) For each file in the scan directory, check if (lib_id, track_path)
 	   exists in the known_tracks map.
 	   2a) If the file is in the map, do nothing.
-	   2b) Else, add track to the library.
+	   2b) Else, add track to the library and to the library group
+	3) Save all databases
 
 	The taglib library should be used for finding artist, album, etc. tags
 	for each track.