Implement the start of creature AI

Add a Brain class, which determines for an entity the set of moves it
makes every step of the game, and begin to implement that for gormlaks.
The idea here is that every step of the game, a gormlak will move
towards the furthest-away wall it can see.

src/Xanthous/Game/Lenses.hs

@@ -0,0 +1,72 @@
+{-# LANGUAGE RecordWildCards #-}
+--------------------------------------------------------------------------------
+module Xanthous.Game.Lenses
+  ( positionedCharacter
+  , character
+  , characterPosition
+  , updateCharacterVision
+  , getInitialState
+  ) where
+--------------------------------------------------------------------------------
+import           Xanthous.Prelude
+--------------------------------------------------------------------------------
+import           System.Random
+--------------------------------------------------------------------------------
+import           Xanthous.Game.State
+import           Xanthous.Data
+import qualified Xanthous.Data.EntityMap as EntityMap
+import           Xanthous.Data.EntityMap.Graphics (visiblePositions)
+import           Xanthous.Entities.Character (Character, mkCharacter)
+--------------------------------------------------------------------------------
+
+getInitialState :: IO GameState
+getInitialState = do
+  _randomGen <- getStdGen
+  let char = mkCharacter
+      (_characterEntityID, _entities)
+        = EntityMap.insertAtReturningID
+          (Position 0 0)
+          (SomeEntity char)
+          mempty
+      _messageHistory = NoMessageHistory
+      _revealedPositions = mempty
+      _promptState = NoPrompt
+  pure GameState {..}
+
+
+positionedCharacter :: Lens' GameState (Positioned Character)
+positionedCharacter = lens getPositionedCharacter setPositionedCharacter
+  where
+    setPositionedCharacter :: GameState -> Positioned Character -> GameState
+    setPositionedCharacter game char
+      = game
+      &  entities . at (game ^. characterEntityID)
+      ?~ fmap SomeEntity char
+
+    getPositionedCharacter :: GameState -> Positioned Character
+    getPositionedCharacter game
+      = over positioned
+        ( fromMaybe (error "Invariant error: Character was not a character!")
+        . downcastEntity
+        )
+      . fromMaybe (error "Invariant error: Character not found!")
+      $ EntityMap.lookupWithPosition
+        (game ^. characterEntityID)
+        (game ^. entities)
+
+
+character :: Lens' GameState Character
+character = positionedCharacter . positioned
+
+characterPosition :: Lens' GameState Position
+characterPosition = positionedCharacter . position
+
+visionRadius :: Word
+visionRadius = 12 -- TODO make this dynamic
+
+-- | Update the revealed entities at the character's position based on their vision
+updateCharacterVision :: GameState -> GameState
+updateCharacterVision game =
+  let charPos = game ^. characterPosition
+      visible = visiblePositions charPos visionRadius $ game ^. entities
+  in game & revealedPositions <>~ visible