Circular.hs

module Data.List.PointedList.Circular
  ( -- Re-export many of the regular PointedList features
  module Data.List.PointedList

  -- And, of course, export the alternatives here
  , next
  , previous
  , delete
  , deleteLeft
  , deleteRight
  ) where

import Data.List.PointedList
  ( PointedList(..)
  , singleton
  , fromList
  , fromListEnd
  , focus
  , insert
  , insertLeft
  , insertRight
  , length
  , positions
  , contextMap
  , move
  , find
  , index
  )
import qualified Data.List.PointedList as PL

next :: PointedList a -> PointedList a
next pl@(PointedList [] b []) = pl
next    (PointedList a  b []) = let (x:xs) = reverse a in
                                  PointedList [] x (xs ++ [b])
next pl = PL.tryNext pl 

previous :: PointedList a -> PointedList a
previous pl@(PointedList [] b []) = pl
previous    (PointedList [] b c ) = let (x:xs) = reverse c in
                                      PointedList (xs ++ [b]) x []
previous pl = PL.tryPrevious pl

-- | An alias of 'deleteRight'.
delete :: PointedList a -> Maybe (PointedList a)
delete = deleteRight

-- | Possibly delete the element at the focus, then move the element on the
--   left to the focus. If no element is on the left, focus on the element to
--   the right. If the deletion will cause the list to be empty, return
--   @Nothing@.
deleteLeft :: PointedList a -> Maybe (PointedList a)
deleteLeft (PointedList []     _ []) = Nothing
deleteLeft (PointedList (l:ls) _ rs) = Just $ PointedList ls l rs
deleteLeft (PointedList []     _ rs) = let (x:xs) = reverse rs in
                                         Just $ PointedList xs x []

-- | Possibly delete the element at the focus, then move the element on the
--   right to the focus. If no element is on the right, focus on the element to
--   the left. If the deletion will cause the list to be empty, return
--   @Nothing@.
deleteRight :: PointedList a -> Maybe (PointedList a)
deleteRight (PointedList [] _ []    ) = Nothing
deleteRight (PointedList ls _ (r:rs)) = Just $ PointedList ls r rs
deleteRight (PointedList ls _ []    ) = let (x:xs) = reverse ls in
                                            Just $ PointedList [] x xs