module Storage.Merge (
    generations,
    ancestors,
    precedes,
) where

import qualified Data.ByteString.Char8 as BC
import Data.List
import Data.Maybe
import Data.Set (Set)
import qualified Data.Set as S

import Storage
import Storage.Internal

previous :: Storable a => Stored a -> [Stored a]
previous (Stored ref _) = case load ref of
    Rec items | Just (RecRef dref) <- lookup (BC.pack "SDATA") items
              , Rec ditems <- load dref ->
                    map wrappedLoad $ catMaybes $ map (\case RecRef r -> Just r; _ -> Nothing) $
                        map snd $ filter ((== BC.pack "SPREV") . fst) ditems

              | otherwise ->
                    map wrappedLoad $ catMaybes $ map (\case RecRef r -> Just r; _ -> Nothing) $
                        map snd $ filter ((== BC.pack "PREV") . fst) items
    _ -> []


generations :: Storable a => [Stored a] -> [Set (Stored a)]
generations = unfoldr gen . (,S.empty)
    where gen (hs, cur) = case filter (`S.notMember` cur) $ previous =<< hs of
              []    -> Nothing
              added -> let next = foldr S.insert cur added
                        in Just (next, (added, next))

ancestors :: Storable a => [Stored a] -> Set (Stored a)
ancestors = last . (S.empty:) . generations

precedes :: Storable a => Stored a -> Stored a -> Bool
precedes x y = x `S.member` ancestors [y]