(svn r27989) -Fix (r27985): VA2 optimisation failed in various special cases:

- nvar=0 is meant to return the calculated result.
  - Missing references resolve to NULL and got identified with the default result.
  - Missing 'break' broke overlapping cases.
  - Splitting into non-overlapping cases could result in more than 256 cases.

src/newgrf.cpp

@@ -4698,6 +4698,8 @@ static void NewSpriteGroup(ByteReader *buf)
 
 			group->default_group = GetGroupFromGroupID(setid, type, buf->ReadWord());
 			group->error_group = ranges.size() > 0 ? ranges[0].group : group->default_group;
+			/* nvar == 0 is a special case -- we turn our value into a callback result */
+			group->calculated_result = ranges.size() == 0;
 
 			/* Sort ranges ascending. When ranges overlap, this may required clamping or splitting them */
 			std::vector<uint32> bounds;
@@ -4711,10 +4713,11 @@ static void NewSpriteGroup(ByteReader *buf)
 			std::vector<const SpriteGroup *> target;
 			for (uint j = 0; j < bounds.size(); ++j) {
 				uint32 v = bounds[j];
-				const SpriteGroup *t = NULL;
+				const SpriteGroup *t = group->default_group;
 				for (uint i = 0; i < ranges.size(); i++) {
 					if (ranges[i].low <= v && v <= ranges[i].high) {
 						t = ranges[i].group;
+						break;
 					}
 				}
 				target.push_back(t);
@@ -4723,7 +4726,7 @@ static void NewSpriteGroup(ByteReader *buf)
 
 			std::vector<DeterministicSpriteGroupRange> optimised;
 			for (uint j = 0; j < bounds.size(); ) {
-				if (target[j]) {
+				if (target[j] != group->default_group) {
 					DeterministicSpriteGroupRange r;
 					r.group = target[j];
 					r.low = bounds[j];

src/newgrf_spritegroup.cpp

@@ -252,7 +252,7 @@ const SpriteGroup *DeterministicSpriteGroup::Resolve(ResolverObject &object) con
 
 	object.last_value = last_value;
 
-	if (this->num_ranges == 0) {
+	if (this->calculated_result) {
 		/* nvar == 0 is a special case -- we turn our value into a callback result */
 		if (value != CALLBACK_FAILED) value = GB(value, 0, 15);
 		static CallbackResultSpriteGroup nvarzero(0, true);

src/newgrf_spritegroup.h

@@ -174,7 +174,8 @@ struct DeterministicSpriteGroup : SpriteGroup {
 	VarSpriteGroupScope var_scope;
 	DeterministicSpriteGroupSize size;
 	uint num_adjusts;
-	byte num_ranges;
+	uint num_ranges;
+	bool calculated_result;
 	DeterministicSpriteGroupAdjust *adjusts;
 	DeterministicSpriteGroupRange *ranges; // Dynamically allocated