Progressively reveal the map to the player

As the character walks around the map, progressively reveal the entities
on the map to them, using an algorithm based on well known
circle-rasterizing and line-rasterizing algorithms to calculate lines of
sight that are potentially obscured by walls.

src/Xanthous/Game/Draw.hs

@@ -17,6 +17,7 @@ import Xanthous.Entities
 import Xanthous.Game
   ( GameState(..)
   , entities
+  , revealedEntities
   , characterPosition
   , MessageHistory(..)
   , messageHistory
@@ -35,8 +36,11 @@ drawMessages (MessageHistory (lastMessage :| _) True) = txt lastMessage
 --   (MessageHistory _ False) -> padTop (Pad 2) $ str " "
 --   (MessageHistory (lastMessage :| _) True) -> txt lastMessage
 
-drawEntities :: EntityMap SomeEntity -> Widget Name
-drawEntities em
+drawEntities
+  :: EntityMap SomeEntity -- ^ visible entities
+  -> EntityMap SomeEntity -- ^ all entities
+  -> Widget Name
+drawEntities em allEnts
   = vBox rows
   where
     entityPositions = EntityMap.positions em
@@ -45,7 +49,7 @@ drawEntities em
     rows = mkRow <$> [0..maxY]
     mkRow rowY = hBox $ renderEntityAt . flip Position rowY <$> [0..maxX]
     renderEntityAt pos =
-      let neighbors = EntityMap.neighbors pos em
+      let neighbors = EntityMap.neighbors pos allEnts
       in maybe (str " ") (drawWithNeighbors neighbors)
          $ em ^? atPosition pos . folded
 
@@ -53,8 +57,9 @@ drawMap :: GameState -> Widget Name
 drawMap game
   = viewport MapViewport Both
   . showCursor Character (game ^. characterPosition . loc)
-  . drawEntities
-  $ game ^. entities
+  $ drawEntities
+    (game ^. revealedEntities)
+    (game ^. entities)
 
 drawGame :: GameState -> [Widget Name]
 drawGame game