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/Data/EntityMap/Graphics.hs

@@ -0,0 +1,28 @@
+{-# LANGUAGE ViewPatterns #-}
+--------------------------------------------------------------------------------
+module Xanthous.Data.EntityMap.Graphics where
+--------------------------------------------------------------------------------
+import Xanthous.Prelude
+--------------------------------------------------------------------------------
+import Xanthous.Util (takeWhileInclusive)
+import Xanthous.Data
+import Xanthous.Data.EntityMap
+import Xanthous.Entities
+import Xanthous.Util.Graphics (circle, line)
+--------------------------------------------------------------------------------
+
+-- | Given a point and a radius of vision, returns a list of all entities that
+-- are *visible* (eg, not blocked by an entity that obscures vision) from that
+-- point
+visibleEntities :: Position -> Word -> EntityMap SomeEntity -> EntityMap SomeEntity
+visibleEntities (view _Position -> pos) visionRadius em
+  = fromEIDsAndPositioned . fold . fold $ sightAdjustedLines
+  where
+    -- I love laziness!
+    radius = circle pos $ fromIntegral visionRadius
+    linesOfSight = radius <&> line pos
+    entitiesOnLines = linesOfSight <&> map getPositionedAt
+    sightAdjustedLines = entitiesOnLines <&> takeWhileInclusive (none $ blocksVision . snd)
+    getPositionedAt p =
+      let ppos = _Position # p
+      in atPositionWithIDs ppos em