trihexaflexagon: clarify the formula in get_angle_in_plan_relative_to_hexagon()

Write the formula in a clearer way and explain the meaning of the
integer division.

diff --git a/src/flexagon/trihexaflexagon.py b/src/flexagon/trihexaflexagon.py
index e3aab5c..faf4d90 100755 (executable)
--- a/src/flexagon/trihexaflexagon.py
+++ b/src/flexagon/trihexaflexagon.py
@@ -44,7 +44,12 @@ class Triangle(object):
     def get_angle_in_plan_relative_to_hexagon(self):
         """"Get the angle of the triangle in the plan relative to the rotation
         of the same triangle in the hexagon."""
-        return ((self.index + 5) % 6 // 2) * pi * 2 / 3 - pi / 6
+        # The meaning of the formula regarding the index is the following:
+        #   - rotate the indices by 1
+        #   - group by 2 (because couples of triangles move together in the
+        #     plan)
+        #   - multiply the group by a rotation factor
+        return pi + pi / 6 + (((self.index + 1) % 6) // 2) * pi * 2 / 3
 
     def get_angle_in_hexagon(self):
         """Get the angle of the triangle in the hexagons.