Don't render moving entities that aren't visible

When the character walks away from or around the corner from entities
that move such that they're no longer visible, stop rendering them.
Still render static entities like walls, doors, and items though. This
prevents entities walking into a "revealed position" after the
character's left being visible despite not being in a line of sight any
more.

1 files changed, 32 insertions, 21 deletions

diff --git a/src/Xanthous/Game/Draw.hs b/src/Xanthous/Game/Draw.hs
index d98b48c02742..8a86101d9915 100644
--- a/src/Xanthous/Game/Draw.hs
+++ b/src/Xanthous/Game/Draw.hs
@@ -20,6 +20,7 @@ import           Xanthous.Game
                  , entities
                  , revealedPositions
                  , characterPosition
+                 , characterVisiblePositions
                  , character
                  , MessageHistory(..)
                  , messageHistory
@@ -62,10 +63,12 @@ drawPromptState (WaitingPrompt msg (Prompt _ pt ps pri _)) =
 
 drawEntities
   :: (Position -> Bool)
-    -- ^ Can we render a given position?
+    -- ^ Is a given position directly visible to the character?
+  -> (Position -> Bool)
+    -- ^ Has a given position *ever* been seen by the character?
   -> EntityMap SomeEntity -- ^ all entities
   -> Widget Name
-drawEntities canRenderPos allEnts
+drawEntities isVisible isRevealed allEnts
   = vBox rows
   where
     entityPositions = EntityMap.positions allEnts
@@ -74,23 +77,27 @@ drawEntities canRenderPos allEnts
     rows = mkRow <$> [0..maxY]
     mkRow rowY = hBox $ renderEntityAt . flip Position rowY <$> [0..maxX]
     renderEntityAt pos
-      | canRenderPos pos
+      = let entitiesAtPosition = allEnts ^. atPosition pos
+            immobileEntitiesAtPosition =
+              filter (not . entityCanMove) entitiesAtPosition
+        in renderTopEntity pos
+           $ if | isVisible  pos -> entitiesAtPosition
+                | isRevealed pos -> immobileEntitiesAtPosition
+                | otherwise      -> mempty
+    renderTopEntity pos ents
       = let neighbors = EntityMap.neighbors pos allEnts
         in maybe (str " ") (drawWithNeighbors neighbors)
-           $ maximumByOf
-             (atPosition pos . folded)
-             (compare `on` drawPriority)
-             allEnts
-      | otherwise = str " "
+           $ maximumBy (compare `on` drawPriority)
+           <$> fromNullable ents
 
 drawMap :: GameState -> Widget Name
 drawMap game
   = viewport Resource.MapViewport Both
   . cursorPosition game
   $ drawEntities
-    (\pos ->
-         (game ^. debugState . allRevealed)
-       || (pos `member` (game ^. revealedPositions)))
+    (\pos -> (game ^. debugState . allRevealed)
+            || (pos `member` (game ^. revealedPositions)))
+    (`member` characterVisiblePositions game)
     -- FIXME: this will break down as soon as creatures can walk around on their
     -- own, since we don't want to render things walking around when the
     -- character can't see them
@@ -99,17 +106,11 @@ drawMap game
 bullet :: Char
 bullet = '•'
 
-drawPanel :: GameState -> Panel -> Widget Name
-drawPanel game panel
-  = border
-  . hLimit 35
-  . viewport (Resource.Panel panel) Vertical
-  $ case panel of
-      InventoryPanel ->
-        drawWielded (game ^. character . inventory . wielded)
-        <=> drawBackpack (game ^. character . inventory . backpack)
+drawInventoryPanel :: GameState -> Widget Name
+drawInventoryPanel game
+  =   drawWielded  (game ^. character . inventory . wielded)
+  <=> drawBackpack (game ^. character . inventory . backpack)
   where
-    drawWielded :: Wielded -> Widget Name
     drawWielded (Hands Nothing Nothing) = emptyWidget
     drawWielded (DoubleHanded i) =
       txtWrap $ "You are holding " <> description i <> " in both hands"
@@ -132,6 +133,16 @@ drawPanel game panel
               (txtWrap . ((bullet <| " ") <>) . description)
               backpackItems)
 
+
+drawPanel :: GameState -> Panel -> Widget Name
+drawPanel game panel
+  = border
+  . hLimit 35
+  . viewport (Resource.Panel panel) Vertical
+  . case panel of
+      InventoryPanel -> drawInventoryPanel
+  $ game
+
 drawCharacterInfo :: Character -> Widget Name
 drawCharacterInfo ch = txt " " <+> charName <+> charHitpoints
   where