Keep track of number of songs played. The other stats all count per-

player: if one person plays on Hard and one on Easy, then each will
be incremented.  That's useful, but it's also helpful to track the simple
number of songs played, irrespective of how many people were playing
at the time.

stepmania/src/Profile.h

@@ -122,10 +122,14 @@ public:
 	set<int> m_UnlockedSongs;
 	mutable CString m_sLastPlayedMachineGuid;	// mutable because we overwrite this on save, and I don't want to remove const from the whole save chain. -Chris
 	mutable DateTime m_LastPlayedDate;
+	/* These stats count twice in the machine profile if two players are playing;
+	 * that's the only approach that makes sense for ByDifficulty and ByMeter. */
 	int m_iNumSongsPlayedByPlayMode[NUM_PLAY_MODES];
 	map<StyleID,int> m_iNumSongsPlayedByStyle;
 	int m_iNumSongsPlayedByDifficulty[NUM_DIFFICULTIES];
 	int m_iNumSongsPlayedByMeter[MAX_METER+1];
+	/* This stat counts once per song, even if two players are active. */
+	int m_iNumSingleSongsPlayed;
 	int m_iNumStagesPassedByPlayMode[NUM_PLAY_MODES];
 	int m_iNumStagesPassedByGrade[NUM_GRADES];