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{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE Rank2Types                 #-}
module Hakyll.Core.Rules.Internal
    ( RulesRead (..)
    , RuleSet (..)
    , RulesState (..)
    , emptyRulesState
    , Rules (..)
    , runRules
    ) where


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import           Control.Applicative            (Applicative, (<$>))
import           Control.Monad.Reader           (ask)
import           Control.Monad.RWS              (RWST, runRWST)
import           Control.Monad.Trans            (liftIO)
import           Data.Monoid                    (Monoid, mappend, mempty)
import           Data.Set                       (Set)
import qualified Data.Set                       as S


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import           Hakyll.Core.Compiler.Internal
import           Hakyll.Core.Identifier
import           Hakyll.Core.Identifier.Pattern
import           Hakyll.Core.Item.SomeItem
import           Hakyll.Core.Metadata
import           Hakyll.Core.Provider
import           Hakyll.Core.Routes


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data RulesRead = RulesRead
    { rulesProvider :: Provider
    , rulesMatches  :: [Identifier]
    , rulesVersion  :: Maybe String
    }


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data RuleSet = RuleSet
    { -- | Accumulated routes
      rulesRoutes    :: Routes
    , -- | Accumulated compilers
      rulesCompilers :: [(Identifier, Compiler SomeItem)]
    , -- | A set of the actually used files
      rulesResources :: Set Identifier
    }


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instance Monoid RuleSet where
    mempty = RuleSet mempty mempty mempty
    mappend (RuleSet r1 c1 s1) (RuleSet r2 c2 s2) =
        RuleSet (mappend r1 r2) (mappend c1 c2) (mappend s1 s2)


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data RulesState = RulesState
    { rulesRoute    :: Maybe Routes
    , rulesCompiler :: Maybe (Compiler SomeItem)
    }


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emptyRulesState :: RulesState
emptyRulesState = RulesState Nothing Nothing


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-- | The monad used to compose rules
newtype Rules a = Rules
    { unRules :: RWST RulesRead RuleSet RulesState IO a
    } deriving (Monad, Functor, Applicative)


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instance MonadMetadata Rules where
    getMetadata identifier = Rules $ do
        provider <- rulesProvider <$> ask
        liftIO $ resourceMetadata provider identifier

    getMatches pattern = Rules $ do
        provider <- rulesProvider <$> ask
        return $ filterMatches pattern $ resourceList provider


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-- | Run a Rules monad, resulting in a 'RuleSet'
runRules :: Rules a -> Provider -> IO RuleSet
runRules rules provider = do
    (_, _, ruleSet) <- runRWST (unRules rules) env emptyRulesState
    case findDuplicate (map fst $ rulesCompilers ruleSet) of
        Nothing  -> return ruleSet
        Just id' -> error $
            "Hakyll.Core.Rules.Internal: two different rules for " ++
            show id' ++ " exist, bailing out"

  where
    env = RulesRead
        { rulesProvider = provider
        , rulesMatches  = []
        , rulesVersion  = Nothing
        }


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findDuplicate :: Ord a => [a] -> Maybe a
findDuplicate = go S.empty
  where
    go _ []              = Nothing
    go s (x : xs)
        | x `S.member` s = Just x
        | otherwise      = go (S.insert x s) xs