Place the chacracter in the level at startup time

Randomly select a position in the largest contiguous region of the
generated level in which to place the character at startup time.

src/Xanthous/Generators/LevelContents.hs

@@ -0,0 +1,26 @@
+--------------------------------------------------------------------------------
+module Xanthous.Generators.LevelContents
+  ( chooseCharacterPosition
+  ) where
+--------------------------------------------------------------------------------
+import Xanthous.Prelude
+--------------------------------------------------------------------------------
+import Control.Monad.Random
+import Data.Array.IArray (amap)
+--------------------------------------------------------------------------------
+import Xanthous.Generators.Util
+import Xanthous.Random
+--------------------------------------------------------------------------------
+
+chooseCharacterPosition :: MonadRandom m => Cells -> m (Word, Word)
+chooseCharacterPosition cells = choose $ impureNonNull candidates
+  where
+    -- cells ends up with true = wall, we want true = can put a character here
+    placeableCells = amap not cells
+
+    -- find the largest contiguous region of cells in the cave.
+    candidates
+      = maximumBy (compare `on` length)
+      $ fromMaybe (error "No regions generated! this should never happen.")
+      $ fromNullable
+      $ regions placeableCells