Center the selected frame in the Stack module when possible

Closes #286 as it provides a better solution.

Author: cyrus-and

.gdbinit

@@ -1647,46 +1647,44 @@ Optionally list the frame arguments and locals too.'''
         # skip if the current thread is not stopped
         if not gdb.selected_thread().is_stopped():
             return []
-        # find the selected frame (i.e., the first to display)
-        selected_index = 0
-        frame = gdb.newest_frame()
-        while frame:
-            if frame == gdb.selected_frame():
-                break
-            frame = frame.older()
-            selected_index += 1
-        # format up to "limit" frames
-        frames = []
-        number = selected_index
+        # gather the frames
         more = False
-        while frame:
-            # the first is the selected one
-            selected = (len(frames) == 0)
-            # fetch frame info
-            style = R.style_selected_1 if selected else R.style_selected_2
-            frame_id = ansi(str(number), style)
-            info = Stack.get_pc_line(frame, style)
-            frame_lines = []
-            frame_lines.append('[{}] {}'.format(frame_id, info))
-            # add frame arguments and locals
-            variables = Variables.format_frame(
-                frame, self.show_arguments, self.show_locals, self.compact, self.align, self.sort)
-            frame_lines.extend(variables)
-            # add frame
-            frames.append(frame_lines)
-            # next
-            frame = frame.older()
-            number += 1
-            # check finished according to the limit
-            if self.limit and len(frames) == self.limit:
-                # more frames to show but limited
-                if frame:
-                    more = True
+        frames = [gdb.selected_frame()]
+        going_down = True
+        while True:
+            # stack frames limit reached
+            if len(frames) == self.limit:
+                more = True
                 break
+            # zigzag the frames starting from the selected one
+            if going_down:
+                frame = frames[-1].older()
+                if frame:
+                    frames.append(frame)
+                else:
+                    frame = frames[0].newer()
+                    if frame:
+                        frames.insert(0, frame)
+                    else:
+                        break
+            else:
+                frame = frames[0].newer()
+                if frame:
+                    frames.insert(0, frame)
+                else:
+                    frame = frames[-1].older()
+                    if frame:
+                        frames.append(frame)
+                    else:
+                        break
+            # switch direction
+            going_down = not going_down
         # format the output
         lines = []
-        for frame_lines in frames:
-            lines.extend(frame_lines)
+        for frame in frames:
+            number = frame.level()
+            selected = frame == gdb.selected_frame()
+            lines.extend(self.get_frame_lines(number, frame, selected))
         # add the placeholder
         if more:
             lines.append('[{}]'.format(ansi('+', R.style_selected_2)))
@@ -1729,6 +1727,19 @@ Optionally list the frame arguments and locals too.'''
             }
         }
 
+    def get_frame_lines(self, number, frame, selected=False):
+        # fetch frame info
+        style = R.style_selected_1 if selected else R.style_selected_2
+        frame_id = ansi(str(number), style)
+        info = Stack.get_pc_line(frame, style)
+        frame_lines = []
+        frame_lines.append('[{}] {}'.format(frame_id, info))
+        # add frame arguments and locals
+        variables = Variables.format_frame(
+            frame, self.show_arguments, self.show_locals, self.compact, self.align, self.sort)
+        frame_lines.extend(variables)
+        return frame_lines
+
     @staticmethod
     def format_line(prefix, line):
         prefix = ansi(prefix, R.style_low)