diff -r 6e4a90ed8830 -r da75d1460a4b src/ai/api/ai_tile.hpp
--- a/src/ai/api/ai_tile.hpp	Tue Apr 15 15:34:02 2008 +0000
+++ b/src/ai/api/ai_tile.hpp	Tue Apr 15 16:11:29 2008 +0000
@@ -58,7 +58,9 @@
 		SLOPE_STEEP_W  = SLOPE_STEEP | SLOPE_NWS,               //!< A steep slope falling to east (from west)
 		SLOPE_STEEP_S  = SLOPE_STEEP | SLOPE_WSE,               //!< A steep slope falling to north (from south)
 		SLOPE_STEEP_E  = SLOPE_STEEP | SLOPE_SEN,               //!< A steep slope falling to west (from east)
-		SLOPE_STEEP_N  = SLOPE_STEEP | SLOPE_ENW                //!< A steep slope falling to south (from north)
+		SLOPE_STEEP_N  = SLOPE_STEEP | SLOPE_ENW,               //!< A steep slope falling to south (from north)
+
+		SLOPE_INVALID  = 0xFF,                                  //!< An invalid slope
 	};
 
 	/**
@@ -92,12 +94,39 @@
 	static bool IsWater(TileIndex tile);
 
 	/**
+	 * Check if a tile has a steep slope.
+	 * @param slope The slope to check on.
+	 * @pre slope != SLOPE_INVALID.
+	 * @return True if the slope is a steep slope.
+	 */
+	static bool IsSteepSlope(Slope slope);
+
+	/**
+	 * Check if a tile has a halftile slope.
+	 * @param slope The slope to check on.
+	 * @pre slope != SLOPE_INVALID.
+	 * @return True if the slope is a halftile slope.
+	 */
+	static bool IsHalftileSlope(Slope slope);
+
+	/**
 	 * Get the slope of a tile.
 	 * @param tile The tile to check on.
 	 * @pre AIMap::IsValidTile(tile).
 	 * @return 0 means flat, others indicate internal state of slope.
 	 */
-	static int32 GetSlope(TileIndex tile);
+	static Slope GetSlope(TileIndex tile);
+
+	/**
+	 * Get the complement of the slope.
+	 * @param slope The slope to get the complement of.
+	 * @pre slope != SLOPE_INVALID.
+	 * @pre !IsSteepSlope(slope).
+	 * @pre !IsHalftileSlope(slope).
+	 * @return The complement of a slope. This means that all corners that
+	 *  weren't raised, are raised, and visa versa.
+	 */
+	static Slope GetComplementSlope(Slope slope);
 
 	/**
 	 * Get the height of the tile.