Do not assume that enum are signed

Clang 3.5 issues warning on constructs like: abs(f1 - f2). The thing is that
f1 and f2 are enum types, and the range given (all positive) allows the
compiler to choose an unsigned type (efficiency being one reason to prefer
unsigned arithmetic). If f1 < f2 are unsigned, then f1 - f2 wraps around zero
and the abs() becomes a no-op. It's the reinterpretation of the unsigned
result (large value) as a signed int that happens to give the correct result,
thanks to 2's complement. This is all tricky and dangerous!

In the spirit of the standard, we assume nothing on the signedness of enums,
and simply calculate the rank and file distances as:
- rank_dist(r1, r2) = r1 < r2 ? r2 - r1 : r1 - r2
- file_dist(f1, f2) = f1 < f2 ? f2 - f1 : f1 - f2
this logic can in fact be applied to any enum we may use, so for better
generality and to avoid code duplication, we use a template function diff()
here.

No functional change.

Resolves #95

src/bitboard.h

@@ -119,11 +119,11 @@ inline int square_distance(Square s1, Square s2) {
 }
 
 inline int file_distance(Square s1, Square s2) {
-  return abs(file_of(s1) - file_of(s2));
+  return dist(file_of(s1), file_of(s2));
 }
 
 inline int rank_distance(Square s1, Square s2) {
-  return abs(rank_of(s1) - rank_of(s2));
+  return dist(rank_of(s1), rank_of(s2));
 }
 
 

src/endgame.cpp

@@ -479,7 +479,7 @@ ScaleFactor Endgame<KRPKR>::operator()(const Position& pos) const {
   if (   r == RANK_6
       && square_distance(bksq, queeningSq) <= 1
       && rank_of(wksq) + tempo <= RANK_6
-      && (rank_of(brsq) == RANK_1 || (!tempo && abs(file_of(brsq) - f) >= 3)))
+      && (rank_of(brsq) == RANK_1 || (!tempo && dist(file_of(brsq), f) >= 3)))
       return SCALE_FACTOR_DRAW;
 
   if (   r >= RANK_6
@@ -535,7 +535,7 @@ ScaleFactor Endgame<KRPKR>::operator()(const Position& pos) const {
   {
       if (file_of(bksq) == file_of(wpsq))
           return ScaleFactor(10);
-      if (   abs(file_of(bksq) - file_of(wpsq)) == 1
+      if (   dist(file_of(bksq), file_of(wpsq)) == 1
           && square_distance(wksq, bksq) > 2)
           return ScaleFactor(24 - 2 * square_distance(wksq, bksq));
   }
@@ -749,7 +749,7 @@ ScaleFactor Endgame<KBPPKB>::operator()(const Position& pos) const {
         && opposite_colors(ksq, wbsq)
         && (   bbsq == blockSq2
             || (pos.attacks_from<BISHOP>(blockSq2) & pos.pieces(weakSide, BISHOP))
-            || abs(r1 - r2) >= 2))
+            || dist(r1, r2) >= 2))
         return SCALE_FACTOR_DRAW;
 
     else if (   ksq == blockSq2

src/types.h

@@ -254,6 +254,7 @@ enum Rank {
   RANK_1, RANK_2, RANK_3, RANK_4, RANK_5, RANK_6, RANK_7, RANK_8, RANK_NB
 };
 
+template <typename T> inline T dist(T x, T y) { return x < y ? y - x : x - y; }
 
 /// The Score enum stores a middlegame and an endgame value in a single integer
 /// (enum). The least significant 16 bits are used to store the endgame value