add datastructure

src/swarm-scenario/Swarm/Game/Display.hs

@@ -84,6 +84,41 @@ data ChildInheritance
   | DefaultDisplay
   deriving (Eq, Ord, Show, Generic, Hashable)
 
+data Presence
+  = -- | present
+    X
+  | -- | absent
+    O
+
+data Neighbors a = Neighbors
+  { e :: a
+  , w :: a
+  , n :: a
+  , s :: a
+  }
+
+-- | For a center cell that itself is a boundary,
+-- what should it be rendered as, given certain
+-- neighbor combinations?
+getBoundaryDisplay :: Neighbors Presence -> Maybe Char
+getBoundaryDisplay = \case
+  Neighbors {e = O, w = O, n = O, s = O} -> Nothing
+  Neighbors {e = X, w = X, n = O, s = O} -> Just '─'
+  Neighbors {e = X, w = O, n = O, s = O} -> Just '─'
+  Neighbors {e = O, w = X, n = O, s = O} -> Just '─'
+  Neighbors {e = O, w = O, n = X, s = X} -> Just '│'
+  Neighbors {e = O, w = O, n = O, s = X} -> Just '│'
+  Neighbors {e = O, w = O, n = X, s = O} -> Just '│'
+  Neighbors {e = X, w = X, n = X, s = X} -> Just '┼'
+  Neighbors {e = O, w = X, n = O, s = X} -> Just '┐'
+  Neighbors {e = X, w = O, n = O, s = X} -> Just '┌'
+  Neighbors {e = O, w = X, n = X, s = O} -> Just '┘'
+  Neighbors {e = X, w = O, n = X, s = O} -> Just '└'
+  Neighbors {e = O, w = X, n = X, s = X} -> Just '┤'
+  Neighbors {e = X, w = O, n = X, s = X} -> Just '├'
+  Neighbors {e = X, w = X, n = X, s = O} -> Just '┴'
+  Neighbors {e = X, w = X, n = O, s = X} -> Just '┬'
+
 -- | A record explaining how to display an entity in the TUI.
 data Display = Display
   { _defaultChar :: Char