X-Git-Url: https://git.ao2.it/flexagon-toolkit.git/blobdiff_plain/5f2e900515a64fe483569855b89f26abcc89a441..3143f02f27e92c9d3475485e706bce8ffd73b6b3:/src/flexagon/trihexaflexagon.py

diff --git a/src/flexagon/trihexaflexagon.py b/src/flexagon/trihexaflexagon.py
index e3aab5c..7bd4f72 100755
--- a/src/flexagon/trihexaflexagon.py
+++ b/src/flexagon/trihexaflexagon.py
@@ -44,7 +44,22 @@ 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 explicit formula for this angle would be:
+        #
+        #    pi + pi / 6 + (((self.index + 1) % 6) // 2) * pi * 2 / 3
+        #
+        # The meaning of the part 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
+        #
+        # The explicit formula shows clearly that triangles move in groups of
+        # 2 in the plan.
+        #
+        # However, use an implicit form for robustness, so that if the other
+        # angle functions change this one can be left untouched.
+        return self.get_angle_in_hexagon() - self.get_angle_in_plan()
 
     def get_angle_in_hexagon(self):
         """Get the angle of the triangle in the hexagons.