(svn r27422) -Fix [FS#6341]: Aircraft picked the wrong airport entry point, if airports were rotated by 180 degree. (marcole)

2 files changed, 18 insertions, 4 deletions

diff --git a/src/aircraft_cmd.cpp b/src/aircraft_cmd.cpp
index 18dcbea27..edb10af32 100644
--- a/src/aircraft_cmd.cpp
+++ b/src/aircraft_cmd.cpp
@@ -809,7 +809,7 @@ static byte AircraftGetEntryPoint(const Aircraft *v, const AirportFTAClass *apc,
 		/* We are northwest or southeast of the airport */
 		dir = delta_y < 0 ? DIAGDIR_NW : DIAGDIR_SE;
 	}
-	dir = ChangeDiagDir(dir, (DiagDirDiff)ReverseDiagDir(DirToDiagDir(rotation)));
+	dir = ChangeDiagDir(dir, DiagDirDifference(DIAGDIR_NE, DirToDiagDir(rotation)));
 	return apc->entry_points[dir];
 }
 
diff --git a/src/direction_func.h b/src/direction_func.h
index 888948584..12aee5863 100644
--- a/src/direction_func.h
+++ b/src/direction_func.h
@@ -61,11 +61,11 @@ static inline Direction ReverseDir(Direction d)
 
 
 /**
- * Calculate the difference between to directions
+ * Calculate the difference between two directions
  *
  * @param d0 The first direction as the base
  * @param d1 The second direction as the offset from the base
- * @return The difference how the second directions drifts of the first one.
+ * @return The difference how the second direction drifts of the first one.
  */
 static inline DirDiff DirDifference(Direction d0, Direction d1)
 {
@@ -79,7 +79,7 @@ static inline DirDiff DirDifference(Direction d0, Direction d1)
 /**
  * Applies two differences together
  *
- * This function adds two differences together and return the resulting
+ * This function adds two differences together and returns the resulting
  * difference. So adding two DIRDIFF_REVERSE together results in the
  * DIRDIFF_SAME difference.
  *
@@ -123,6 +123,20 @@ static inline DiagDirection ReverseDiagDir(DiagDirection d)
 	return (DiagDirection)(2 ^ d);
 }
 
+/**
+ * Calculate the difference between two DiagDirection values
+ *
+ * @param d0 The first direction as the base
+ * @param d1 The second direction as the offset from the base
+ * @return The difference how the second direction drifts of the first one.
+ */
+static inline DiagDirDiff DiagDirDifference(DiagDirection d0, DiagDirection d1)
+{
+	assert(IsValidDiagDirection(d0));
+	assert(IsValidDiagDirection(d1));
+	/* Cast to uint so compiler can use bitmask. Result can never be negative. */
+	return (DiagDirDiff)((uint)(d0 - d1) % 4);
+}
 
 /**
  * Applies a difference on a DiagDirection