Engine: fix legacy pathfinder imprecision in some old games

This fixes at least one known case in "Maniac Mansion Mania 16: Meteor Family".
See #663

Solution hinted by @tag2015

Engine/ac/route_finder_impl_legacy.cpp

@@ -49,6 +49,19 @@ static int waspossible = 1;
 static int suggestx;
 static int suggesty;
 
+// Configuration for the pathfinder
+struct PathfinderConfig
+{
+    const int MaxGranularity = 3;
+
+    // Short sweep is performed in certain radius around requested destination,
+    // when searching for a nearest walkable area in the vicinity
+    const int ShortSweepRadius = 50;
+    int ShortSweepGranularity = 3; // variable, depending on loaded game version
+    // Full sweep is performed over a whole walkable area
+    const int FullSweepGranularity = 5;
+};
+
 void init_pathfinder()
 {
   pathbackx = (int *)malloc(sizeof(int) * MAXPATHBACK);
@@ -139,9 +152,8 @@ int find_nearest_walkable_area(Bitmap *tempw, int fromX, int fromY, int toX, int
   return 0;
 }
 
-#define MAX_GRANULARITY 3
 static int walk_area_granularity[MAX_WALK_AREAS];
-static int is_route_possible(int fromx, int fromy, int tox, int toy, Bitmap *wss)
+static int is_route_possible(int fromx, int fromy, int tox, int toy, Bitmap *wss, const PathfinderConfig &pfc)
 {
   wallscreen = wss;
   suggestx = -1;
@@ -205,7 +217,7 @@ static int is_route_possible(int fromx, int fromy, int tox, int toy, Bitmap *wss
   // find the average "width" of a path in this walkable area
   for (dd = 1; dd < MAX_WALK_AREAS; dd++) {
     if (walk_area_times[dd] == 0) {
-      walk_area_granularity[dd] = MAX_GRANULARITY;
+      walk_area_granularity[dd] = pfc.MaxGranularity;
       continue;
     }
 
@@ -215,26 +227,26 @@ static int is_route_possible(int fromx, int fromy, int tox, int toy, Bitmap *wss
     else if (walk_area_granularity[dd] <= 15)
       walk_area_granularity[dd] = 3;
     else
-      walk_area_granularity[dd] = MAX_GRANULARITY;
+      walk_area_granularity[dd] = pfc.MaxGranularity;
 
 #ifdef DEBUG_PATHFINDER
     AGS::Common::Debug::Printf("area %d: Gran %d", dd, walk_area_granularity[dd]);
 #endif
   }
-  walk_area_granularity[0] = MAX_GRANULARITY;
+  walk_area_granularity[0] = pfc.MaxGranularity;
 
   tempw->FloodFill(fromx, fromy, 232);
   if (tempw->GetPixel(tox, toy) != 232) 
   {
     // Destination pixel is not walkable
-    // Try the 100x100 square around the target first at 3-pixel granularity
-    int tryFirstX = tox - 50, tryToX = tox + 50;
-    int tryFirstY = toy - 50, tryToY = toy + 50;
+    // Try the N x N square around the target first at X-pixel granularity
+    int tryFirstX = tox - pfc.ShortSweepRadius, tryToX = tox + pfc.ShortSweepRadius;
+    int tryFirstY = toy - pfc.ShortSweepRadius, tryToY = toy + pfc.ShortSweepRadius;
 
-    if (!find_nearest_walkable_area(tempw, tryFirstX, tryFirstY, tryToX, tryToY, tox, toy, 3))
+    if (!find_nearest_walkable_area(tempw, tryFirstX, tryFirstY, tryToX, tryToY, tox, toy, pfc.ShortSweepGranularity))
     {
       // Nothing found, sweep the whole room at 5 pixel granularity
-      find_nearest_walkable_area(tempw, 0, 0, tempw->GetWidth(), tempw->GetHeight(), tox, toy, 5);
+      find_nearest_walkable_area(tempw, 0, 0, tempw->GetWidth(), tempw->GetHeight(), tox, toy, pfc.FullSweepGranularity);
     }
 
     delete tempw;
@@ -396,7 +408,7 @@ static void round_down_coords(int &tmpx, int &tmpy)
   }
 }
 
-static int find_route_dijkstra(int fromx, int fromy, int destx, int desty)
+static int find_route_dijkstra(int fromx, int fromy, int destx, int desty, const PathfinderConfig &pfc)
 {
   int i, j;
 
@@ -433,10 +445,10 @@ static int find_route_dijkstra(int fromx, int fromy, int destx, int desty)
 
   int granularity = 3, newx = -1, newy, foundAnswer = -1, numreplace;
   int changeiter, numfound, adjcount;
-  int destxlow = destx - MAX_GRANULARITY;
-  int destylow = desty - MAX_GRANULARITY;
-  int destxhi = destxlow + MAX_GRANULARITY * 2;
-  int destyhi = destylow + MAX_GRANULARITY * 2;
+  int destxlow = destx - pfc.MaxGranularity;
+  int destylow = desty - pfc.MaxGranularity;
+  int destxhi = destxlow + pfc.MaxGranularity * 2;
+  int destyhi = destylow + pfc.MaxGranularity * 2;
   int modifier = 0;
   int totalfound = 0;
   int DIRECTION_BONUS = 0;
@@ -503,10 +515,10 @@ static int find_route_dijkstra(int fromx, int fromy, int destx, int desty)
 
       // edges of screen pose a problem, so if current and dest are within
       // certain distance of the edge, say we've got it
-      if ((newx >= wallscreen->GetWidth() - MAX_GRANULARITY) && (destx >= wallscreen->GetWidth() - MAX_GRANULARITY))
+      if ((newx >= wallscreen->GetWidth() - pfc.MaxGranularity) && (destx >= wallscreen->GetWidth() - pfc.MaxGranularity))
         newx = destx;
 
-      if ((newy >= wallscreen->GetHeight() - MAX_GRANULARITY) && (desty >= wallscreen->GetHeight() - MAX_GRANULARITY))
+      if ((newy >= wallscreen->GetHeight() - pfc.MaxGranularity) && (desty >= wallscreen->GetHeight() - pfc.MaxGranularity))
         newy = desty;
 
       // Found the desination, abort loop
@@ -578,7 +590,7 @@ static int find_route_dijkstra(int fromx, int fromy, int destx, int desty)
   return 1;
 }
 
-static int __find_route(int srcx, int srcy, short *tox, short *toy, int noredx)
+static int __find_route(int srcx, int srcy, short *tox, short *toy, int noredx, const PathfinderConfig &pfc)
 {
   assert(wallscreen != nullptr);
   assert(beenhere != nullptr);
@@ -599,7 +611,7 @@ static int __find_route(int srcx, int srcy, short *tox, short *toy, int noredx)
       return 1;
     }
 
-    if ((waspossible = is_route_possible(srcx, srcy, tox[0], toy[0], wallscreen)) == 0) {
+    if ((waspossible = is_route_possible(srcx, srcy, tox[0], toy[0], wallscreen, pfc)) == 0) {
       if (suggestx >= 0) {
         tox[0] = suggestx;
         toy[0] = suggesty;
@@ -615,7 +627,7 @@ static int __find_route(int srcx, int srcy, short *tox, short *toy, int noredx)
   }
 
   // Try the new pathfinding algorithm
-  if (find_route_dijkstra(srcx, srcy, tox[0], toy[0])) {
+  if (find_route_dijkstra(srcx, srcy, tox[0], toy[0], pfc)) {
     return 1;
   }
 
@@ -731,6 +743,11 @@ int find_route(short srcx, short srcy, short xx, short yy, int move_speed_x, int
   assert(pathbackx != nullptr);
   assert(pathbacky != nullptr);
 
+  // Setup pathfinder configuration, depending on the loaded game version;
+  // sweep granularity has changed between 3.0.0 and 3.0.1; see issue #663
+  PathfinderConfig pfc;
+  pfc.ShortSweepGranularity = (loaded_game_file_version > kGameVersion_300) ? 3 : 1;
+
 #ifdef DEBUG_PATHFINDER
   // __wnormscreen();
 #endif
@@ -767,9 +784,9 @@ int find_route(short srcx, short srcy, short xx, short yy, int move_speed_x, int
     for (aaa = 1; aaa < wallscreen->GetHeight(); aaa++)
       beenhere[aaa] = beenhere[0] + aaa * (wallscreen->GetWidth());
 
-    if (__find_route(srcx, srcy, &xx, &yy, nocross) == 0) {
+    if (__find_route(srcx, srcy, &xx, &yy, nocross, pfc) == 0) {
       leftorright = 1;
-      if (__find_route(srcx, srcy, &xx, &yy, nocross) == 0)
+      if (__find_route(srcx, srcy, &xx, &yy, nocross, pfc) == 0)
         pathbackstage = -1;
     }
     free(beenhere[0]);