module Darcs.Patch.Merge
    ( Merge(..), mergeFL )
    where

import Darcs.Patch.Commute ( Commute )

import Darcs.Witnesses.Ordered ( (:\/:)(..), (:/\:)(..),
                                 FL(..), RL,
                                 reverseFL, reverseRL
                               )

import Data.Maybe ( fromJust )

#include "gadts.h"

-- | Things that can always be merged
class Commute p => Merge p where
    merge :: (p :\/: p) C(x y) -> (p :/\: p) C(x y)

instance Merge p => Merge (FL p) where
    merge (NilFL :\/: x) = x :/\: NilFL
    merge (x :\/: NilFL) = NilFL :/\: x
    merge ((x:>:xs) :\/: ys) = fromJust $ do ys' :/\: x' <- return $ mergeFL (x :\/: ys)
                                             xs' :/\: ys'' <- return $ merge (ys' :\/: xs)
                                             return (ys'' :/\: (x' :>: xs'))

mergeFL :: Merge p => (p :\/: FL p) C(x y) -> (FL p :/\: p) C(x y)
mergeFL (p :\/: NilFL) = NilFL :/\: p
mergeFL (p :\/: (x :>: xs)) = fromJust $ do x' :/\: p' <- return $ merge (p :\/: x)
                                            xs' :/\: p'' <- return $ mergeFL (p' :\/: xs)
                                            return ((x' :>: xs') :/\: p'')

instance Merge p => Merge (RL p) where
    merge (x :\/: y) = case merge (reverseRL x :\/: reverseRL y) of
                       (ry' :/\: rx') -> reverseFL ry' :/\: reverseFL rx'