-- Ring as an ADT

module Ring (
   Ring, Elt,
   newRing,
   current, deleteCurrent,
   rotate, size) where

----------------------------------------
-- Interface
----------------------------------------

newRing       :: [Elt] -> Ring
current       :: Ring -> Elt
deleteCurrent :: Ring -> Ring   -- after deleting current is the
                                -- element following the deleted elt
rotate        :: Ring -> Int -> Ring
size          :: Ring -> Int

----------------------------------------
-- Implementation
----------------------------------------

----------------------
-- type implementation
----------------------
type Elt = Int
data Ring = Gandalf [Elt]
   deriving (Show)  -- just for debugging
--------------------------
-- Function implementation
--------------------------
newRing elts                         = (Gandalf elts)
current (Gandalf (first:_))          = first
deleteCurrent (Gandalf (first:rest)) = (Gandalf rest)

rotate (Gandalf elts) amount  = Gandalf (newFront++newRear)
   where
      modAmount = amount `mod` ringSize
      ringSize  = length elts
      newFront  = drop amount elts
      newRear   = take amount elts

size (Gandalf elts) = length elts