Code refactoring

- Code refactoring mainly for API
- Delete a useless option in `setting.py`
- Change some constants in Link Play mode

latest version/core/song.py

@@ -9,6 +9,12 @@ class Chart:
         self.set_chart(song_id, difficulty)
         self.defnum: int = None
 
+    def to_dict(self) -> dict:
+        return {
+            'difficulty': self.difficulty,
+            'chart_const': self.chart_const
+        }
+
     @property
     def chart_const(self) -> float:
         return self.defnum / 10 if self.defnum else -1
@@ -37,6 +43,37 @@ class Song:
         self.name: str = None
         self.charts: dict = None
 
+    def to_dict(self) -> dict:
+        return {
+            'song_id': self.song_id,
+            'name': self.name,
+            'charts': [chart.to_dict() for chart in self.charts]
+        }
+
+    def from_list(self, x: list) -> 'Song':
+        if self.song_id is None:
+            self.song_id = x[0]
+        self.name = x[1]
+        self.charts = [Chart(self.c, self.song_id, 0), Chart(self.c, self.song_id, 1), Chart(
+            self.c, self.song_id, 2), Chart(self.c, self.song_id, 3)]
+        self.charts[0].defnum = x[2]
+        self.charts[1].defnum = x[3]
+        self.charts[2].defnum = x[4]
+        self.charts[3].defnum = x[5]
+        return self
+
     def insert(self) -> None:
         self.c.execute(
             '''insert into chart values (?,?,?,?,?,?)''', (self.song_id, self.name, self.charts[0].defnum, self.charts[1].defnum, self.charts[2].defnum, self.charts[3].defnum))
+
+    def select(self, song_id: str = None) -> 'Song':
+        if song_id is not None:
+            self.song_id = song_id
+
+        self.c.execute('''select * from chart where song_id=:a''', {
+                       'a': self.song_id})
+        x = self.c.fetchone()
+        if x is None:
+            raise NoData('The song `%s` does not exist.' % self.song_id)
+
+        return self.from_list(x)