Traffic Jams

Results of trial run: Pathing/routing failure.

Details: This is where the borg got stuck:

 

The obvious problem is that our party is trying to walk North, and we've managed to pin an NPC between us and the stairway to the North. However, to really understand everything that's going on here, you're going to need to know more about how FF4 maps and mobile NPCs work.

Every map in FF4 that isn't an overworld map has a byte of layer information for each tile of the map. This layer info helps the game know which tiles you can walk on and when. To help visualize this, here's a picture of the save room in the Antlion's Den:

 

And here's an ASCII representation of the movement layers of that map:

                                 
   ###                           
  #,,,T#                         
 T,,__,_#                        
 #,,##_#                         
 #,##._.#                        
 #,.....T                        
  #.#.#.#                        
  #..S..#                        
  #.#.#.#                        
  #.....#                        
   ##.##                         
     +                           
                                 

• Pound signs (#) are layer 0 - obstacles. These are never passable under any circumstances. The capital Ts are actually layer 0 as well, but I've marked them differently because they contain treasures.
• Periods (.) and commas (,) are layers 1 and 2 respectively. Both can be walked on, but you can't step directly from layer 1 to layer 2 and vise versa. To get between them, you need to step onto layer 3, which is represented here by underscores (_). Layer 3 acts as a connecting layer - you can always step on and off of it.
• Save points (S) are layer 11, but they are essentially always accessible.
• Layers 17 and 19 represent transition squares - if you step onto one of these, you'll end up on a different map. I've chosen to mark these with plus signs (+) and greater than signs (>) respectively. Layer 17 is only accessible from layer 1. Layer 19 can be reached from anything.
• There's one other layer, layer 5, that acts as a bridge layer. You can always step on a bridge layer from layer 1 or 2, but when you leave the bridge you have to step off onto the same layer you entered on. This allows bridges that can be walked over or under, with different rules for where you can walk for each.

So that's how the movement rules work.  Except that that's only how they work for the player, which is where our movement bug comes in.

When the borg chooses a path to a destination and starts moving down it, at every step it checks for NPCs in the way. If someone's in the way, it checks to see if the NPC would get pinned in the players path if we took the next step. If so, it politely waits until the NPC moves out of the way before moving forward. Without this politeness, all too often the borg would get itself into situations like this:

Our destination is the square the NPC is standing on, they have no way to get out of the way, and the borg doesn't know how to solve the situation. The borg eventually gives up and panics.

The problem the borg ran into in this last run is that NPCs don't have the same movement rules as the player does. NPCs are restricted to either layer 1 or layer 2; they can't transition to any layer other than the one they started on, including layer 3. So when you have a map like the top floor of Summonville, whose movement layers look like this:

             ########                  
        #####........#                 
       #.......#####.##                
      T........#   #...#               
    ##..######>#   #...##              
   #...#     #.#######...#             
  #....#     #.......#...#             
  #...#       ######.###..##           
   #.#              T  #....#    
   #_#                 #....#    
   #.T####              ##.#           
   #......#####          #_#           
    #..........#     ####T.#           
    #..........#    #......#           
     ###....>..#   ##......#           
        #......####>#......#           
         ##.####....##.####            
          #_####....##_#               
          #.............#              
           ###.........#               
             #........#                
              ####T###              

...there's a few opportunities scattered about for the borg to pin an NPC in a place that it believes the NPC can escape.

Solution: The borg method that looks for a way for an NPC to escape the player's path, pathToAvoidPlayer(), now starts with all layer 3 tiles on the current map blacklisted. This gets us 99% of the truth for NPC mobility, and should help us avoid any more pinned NPCs.