{-# LANGUAGE UndecidableInstances, OverlappingInstances, FlexibleInstances,
             MultiParamTypeClasses, TemplateHaskell, PolymorphicComponents,
             DeriveDataTypeable,ExistentialQuantification #-}

{- |

This module defines typable indices and convenience functions. Should
be probably considered private to 'Happstack.Data.IxSet'.

module Happstack.Data.IxSet.Ix 
    ( Ix(..)
    , insert
    , delete
    , insertList
    , deleteList
    , union
    , intersection

import Data.Generics hiding (GT)
import           Data.List  (foldl')
import           Data.Map   (Map)
import qualified Data.Map   as Map
import           Data.Set   (Set)
import qualified Data.Set   as Set
import qualified Data.Generics.SYB.WithClass.Basics as SYBWC

-- the core datatypes

-- | 'Ix' is a 'Map' from some 'Typeable' key to a 'Set' of values for
-- that key.  'Ix' carries type information inside.
data Ix a = forall key . (Typeable key, Ord key) => 
            Ix (Map key (Set a)) (a -> [key])
    deriving Typeable

 -- minimal hacky instance
instance Data a => Data (Ix a) where
    toConstr (Ix _ _) = con_Ix_Data
    gunfold _ _     = error "gunfold"
    dataTypeOf _    = ixType_Data

con_Ix_Data :: Constr
con_Ix_Data = mkConstr ixType_Data "Ix" [] Prefix
ixType_Data :: DataType
ixType_Data = mkDataType "Happstack.Data.IxSet.Ix" [con_Ix_Data]

ixConstr :: SYBWC.Constr
ixConstr = SYBWC.mkConstr ixDataType "Ix" [] SYBWC.Prefix
ixDataType :: SYBWC.DataType
ixDataType = SYBWC.mkDataType "Ix" [ixConstr]

instance (SYBWC.Data ctx a, SYBWC.Sat (ctx (Ix a)))
       => SYBWC.Data ctx (Ix a) where
    gfoldl = error "gfoldl Ix"
    toConstr _ (Ix _ _)    = ixConstr
    gunfold = error "gunfold Ix"
    dataTypeOf _ _ = ixDataType

-- modification operations

-- | Convenience function for inserting into 'Map's of 'Set's as in
-- the case of an 'Ix'.  If they key did not already exist in the
-- 'Map', then a new 'Set' is added transparently.
insert :: (Ord a, Ord k)
       => k -> a -> Map k (Set a) -> Map k (Set a)
insert k v index = Map.insertWith' Set.union k (Set.singleton v) index

-- | Helper function to 'insert' a list of elements into a set.
insertList :: (Ord a, Ord k)
           => [(k,a)] -> Map k (Set a) -> Map k (Set a)
insertList xs index = foldl' (\m (k,v)-> insert k v m) index xs

-- | Convenience function for deleting from 'Map's of 'Set's. If the
-- resulting 'Set' is empty, then the entry is removed from the 'Map'.
delete :: (Ord a, Ord k)
       => k -> a -> Map k (Set a) -> Map k (Set a)
delete k v index = Map.update remove k index
    remove set = let set' = Set.delete v set
                 in if Set.null set' then Nothing else Just set'

-- | Helper function to 'delete' a list of elements from a set.
deleteList :: (Ord a, Ord k)
           => [(k,a)] -> Map k (Set a) -> Map k (Set a)
deleteList xs index = foldl' (\m (k,v) -> delete k v m) index xs

-- | Take union of two sets.
union :: (Ord a, Ord k)
       => Map k (Set a) -> Map k (Set a) -> Map k (Set a)
union index1 index2 = Map.unionWith Set.union index1 index2

-- | Take intersection of two sets
intersection :: (Ord a, Ord k)
             => Map k (Set a) -> Map k (Set a) -> Map k (Set a)
intersection index1 index2 = Map.filter (not . Set.null) $ 
                             Map.intersectionWith Set.intersection index1 index2