--- ../patch/_airport_entry/direction_func.h	2015-07-12 22:30:37 +0200
+++ src/direction_func.h	2015-07-12 22:47:13 +0200
@@ -59,6 +59,17 @@
 	return (Direction)(4 ^ d);
 }
 
+/**
+ * Return the reverse of a direction difference
+ *
+ * @param d The direction difference to get the reverse from
+ * @return The reverse direction difference
+ */
+static inline DirDiff ReverseDirDiff(DirDiff d)
+{
+	/* Cast to uint so compiler can use bitmask. Result can never be negative. */
+	return (DirDiff)((uint)(DIRDIFF_SAME - d) % 8);
+}
 
 /**
  * Calculate the difference between two directions
@@ -124,6 +135,18 @@
 }
 
 /**
+ * Return the reverse of a DiagDirDiff difference
+ *
+ * @param d The direction difference to get the reverse from
+ * @return The reverse direction difference
+ */
+static inline DiagDirDiff ReverseDiagDirDiff(DiagDirDiff d)
+{
+	/* Cast to uint so compiler can use bitmask. Result can never be negative. */
+	return (DiagDirDiff)((uint)(DIAGDIRDIFF_SAME - d) % 4);
+}
+
+/**
  * Calculate the difference between two DiagDirection values
  *
  * @param d0 The first direction as the base
@@ -139,6 +162,23 @@
 }
 
 /**
+ * Applies two DiagDirDiff differences together
+ *
+ * This function adds two differences together and returns the resulting
+ * difference. So adding two DIAGDIRDIFF_REVERSE together results in the
+ * DIAGDIRDIFF_SAME difference.
+ *
+ * @param d The first difference
+ * @param delta The second difference to add on
+ * @return The resulting difference
+ */
+static inline DiagDirDiff ChangeDiagDirDiff(DiagDirDiff d, DiagDirDiff delta)
+{
+	/* Cast to uint so compiler can use bitmask. Result can never be negative. */
+	return (DiagDirDiff)((uint)(d + delta) % 4);
+}
+
+/**
  * Applies a difference on a DiagDirection
  *
  * This function applies a difference on a DiagDirection and returns