{-# LANGUAGE GeneralizedNewtypeDeriving #-}
-- | Maintaining a list of favorites of some partially ordered type.
-- Only the best elements are kept.
--
-- To avoid name clashes, import this module qualified, as in
-- @
-- import Agda.Utils.Favorites (Favorites)
-- import qualified Agda.Utils.Favorites as Fav
-- @
module Agda.Utils.Favorites where
import Prelude hiding ( null )
import Data.Foldable (Foldable)
import Data.Functor
import Data.Semigroup
import qualified Data.List as List
import qualified Data.Set as Set
import Agda.Utils.Null
import Agda.Utils.PartialOrd
import Agda.Utils.Singleton
import Agda.Utils.Tuple
-- | A list of incomparable favorites.
newtype Favorites a = Favorites { toList :: [a] }
deriving (Foldable, Show, Null, Singleton a)
-- | Equality checking is a bit expensive, since we need to sort!
-- Maybe use a 'Set' of favorites in the first place?
instance Ord a => Eq (Favorites a) where
as == bs = Set.fromList (toList as) == Set.fromList (toList bs)
-- | Result of comparing a candidate with the current favorites.
data CompareResult a
= Dominates { dominated :: [a], notDominated :: [a] }
-- ^ Great, you are dominating a possibly (empty list of favorites)
-- but there is also a rest that is not dominated.
-- If @null dominated@, then @notDominated@ is necessarily the
-- complete list of favorites.
| IsDominated { dominator :: a }
-- ^ Sorry, but you are dominated by that favorite.
-- | Gosh, got some pretty @a@ here, compare with my current favorites!
-- Discard it if there is already one that is better or equal.
-- (Skewed conservatively: faithful to the old favorites.)
-- If there is no match for it, add it, and
-- dispose of all that are worse than @a@.
--
-- We require a partial ordering. Less is better! (Maybe paradoxically.)
compareWithFavorites :: PartialOrd a => a -> Favorites a -> CompareResult a
compareWithFavorites a favs = loop $ toList favs where
loop [] = Dominates [] []
loop as@(b : bs) = case comparable a b of
POLT -> dominates b $ loop bs -- @a@ is a new favorite, bye-bye, @b@
POLE -> dominates b $ loop bs -- ditto
POEQ -> IsDominated b -- @b@ is as least as good as @a@, bye-bye, @a@
POGE -> IsDominated b -- ditto
POGT -> IsDominated b -- ditto
POAny -> doesnotd b $ loop bs -- don't know, compare with my other favorites
-- add an outperformed favorite
dominates b (Dominates bs as) = Dominates (b : bs) as
dominates b r@IsDominated{} = r
-- add an uncomparable favorite
doesnotd b (Dominates as bs) = Dominates as (b : bs)
doesnotd b r@IsDominated{} = r
-- | Compare a new set of favorites to an old one and discard
-- the new favorites that are dominated by the old ones
-- and vice verse.
-- (Skewed conservatively: faithful to the old favorites.)
--
-- @compareFavorites new old = (new', old')@
compareFavorites :: PartialOrd a => Favorites a -> Favorites a ->
(Favorites a, Favorites a)
compareFavorites new old = mapFst Favorites $ loop (toList new) old where
loop [] old = ([], old)
loop (a : new) old = case compareWithFavorites a old of
-- Better: Discard all @old@ ones that @a@ dominates and keep @a@
Dominates _ old -> mapFst (a:) $ loop new (Favorites old)
-- Not better: Discard @a@
IsDominated{} -> loop new old
unionCompared :: (Favorites a, Favorites a) -> Favorites a
unionCompared (Favorites new, Favorites old) = Favorites $ new ++ old
-- | After comparing, do the actual insertion.
insertCompared :: a -> Favorites a -> CompareResult a -> Favorites a
insertCompared a _ (Dominates _ as) = Favorites (a : as)
insertCompared _ l IsDominated{} = l
-- | Compare, then insert accordingly.
-- @insert a l = insertCompared a l (compareWithFavorites a l)@
insert :: PartialOrd a => a -> Favorites a -> Favorites a
insert a l = insertCompared a l (compareWithFavorites a l)
-- | Insert all the favorites from the first list into the second.
union :: PartialOrd a => Favorites a -> Favorites a -> Favorites a
union (Favorites as) bs = List.foldr insert bs as
-- | Construct favorites from elements of a partial order.
-- The result depends on the order of the list if it
-- contains equal elements, since earlier seen elements
-- are favored over later seen equals.
-- The first element of the list is seen first.
fromList :: PartialOrd a => [a] -> Favorites a
fromList = List.foldl' (flip insert) empty
-- | 'Favorites' forms a 'Monoid' under 'empty' and 'union.
instance PartialOrd a => Semigroup (Favorites a) where
(<>) = union
instance PartialOrd a => Monoid (Favorites a) where
mempty = empty
mappend = (<>)