-/// squares_aligned() returns true if the squares s1, s2 and s3 are aligned
-/// either on a straight or on a diagonal line.
+/// distance() functions return the distance between x and y, defined as the
+/// number of steps for a king in x to reach y. Works with squares, ranks, files.
+
+template<typename T> inline int distance(T x, T y) { return x < y ? y - x : x - y; }
+template<> inline int distance<Square>(Square x, Square y) { return SquareDistance[x][y]; }
+
+template<typename T1, typename T2> inline int distance(T2 x, T2 y);
+template<> inline int distance<File>(Square x, Square y) { return distance(file_of(x), file_of(y)); }
+template<> inline int distance<Rank>(Square x, Square y) { return distance(rank_of(x), rank_of(y)); }