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-- | A store for storing and retreiving items
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE ScopedTypeVariables       #-}
module Hakyll.Core.Store
    ( Store
    , Result (..)
    , toMaybe
    , new
    , set
    , get
    , isMember
    , delete
    , hash
    ) where


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import           Control.Exception    (IOException, handle)
import qualified Crypto.Hash.MD5      as MD5
import           Data.Binary          (Binary, decode, encodeFile)
import qualified Data.ByteString      as B
import qualified Data.ByteString.Lazy as BL
import qualified Data.Cache.LRU.IO    as Lru
import           Data.List            (intercalate)
import           Data.Maybe           (isJust)
import qualified Data.Text            as T
import qualified Data.Text.Encoding   as T
import           Data.Typeable        (TypeRep, Typeable, cast, typeOf)
import           Numeric              (showHex)
import           System.Directory     (createDirectoryIfMissing)
import           System.Directory     (doesFileExist, removeFile)
import           System.FilePath      ((</>))
import           System.IO            (IOMode (..), hClose, openFile)
import           Text.Printf          (printf)


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-- | Simple wrapper type
data Box = forall a. Typeable a => Box a


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data Store = Store
    { -- | All items are stored on the filesystem
      storeDirectory :: FilePath
    , -- | Optionally, items are also kept in-memory
      storeMap       :: Maybe (Lru.AtomicLRU FilePath Box)
    }


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instance Show Store where
    show _ = "<Store>"


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-- | Result of a store query
data Result a
    = Found a                    -- ^ Found, result
    | NotFound                   -- ^ Not found
    | WrongType TypeRep TypeRep  -- ^ Expected, true type
    deriving (Show, Eq)


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-- | Convert result to 'Maybe'
toMaybe :: Result a -> Maybe a
toMaybe (Found x) = Just x
toMaybe _         = Nothing


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-- | Initialize the store
new :: Bool      -- ^ Use in-memory caching
    -> FilePath  -- ^ Directory to use for hard disk storage
    -> IO Store  -- ^ Store
new inMemory directory = do
    createDirectoryIfMissing True directory
    ref <- if inMemory then Just <$> Lru.newAtomicLRU csize else return Nothing
    return Store
        { storeDirectory = directory
        , storeMap       = ref
        }
  where
    csize = Just 500


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-- | Auxiliary: add an item to the in-memory cache
cacheInsert :: Typeable a => Store -> String -> a -> IO ()
cacheInsert (Store _ Nothing)    _   _     = return ()
cacheInsert (Store _ (Just lru)) key x =
    Lru.insert key (Box x) lru


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-- | Auxiliary: get an item from the in-memory cache
cacheLookup :: forall a. Typeable a => Store -> String -> IO (Result a)
cacheLookup (Store _ Nothing)    _   = return NotFound
cacheLookup (Store _ (Just lru)) key = do
    res <- Lru.lookup key lru
    return $ case res of
        Nothing      -> NotFound
        Just (Box x) -> case cast x of
            Just x' -> Found x'
            Nothing -> WrongType (typeOf (undefined :: a)) (typeOf x)


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cacheIsMember :: Store -> String -> IO Bool
cacheIsMember (Store _ Nothing)    _   = return False
cacheIsMember (Store _ (Just lru)) key = isJust <$> Lru.lookup key lru


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-- | Auxiliary: delete an item from the in-memory cache
cacheDelete :: Store -> String -> IO ()
cacheDelete (Store _ Nothing)    _   = return ()
cacheDelete (Store _ (Just lru)) key = do
    _ <- Lru.delete key lru
    return ()


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-- | Store an item
set :: (Binary a, Typeable a) => Store -> [String] -> a -> IO ()
set store identifier value = do
    encodeFile (storeDirectory store </> key) value
    cacheInsert store key value
  where
    key = hash identifier


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-- | Load an item
get :: (Binary a, Typeable a) => Store -> [String] -> IO (Result a)
get store identifier = do
    -- First check the in-memory map
    ref <- cacheLookup store key
    case ref of
        -- Not found in the map, try the filesystem
        NotFound -> do
            exists <- doesFileExist path
            if not exists
                -- Not found in the filesystem either
                then return NotFound
                -- Found in the filesystem
                else do
                    v <- decodeClose
                    cacheInsert store key v
                    return $ Found v
        -- Found in the in-memory map (or wrong type), just return
        s -> return s
  where
    key  = hash identifier
    path = storeDirectory store </> key

    -- 'decodeFile' from Data.Binary which closes the file ASAP
    decodeClose = do
        h   <- openFile path ReadMode
        lbs <- BL.hGetContents h
        BL.length lbs `seq` hClose h
        return $ decode lbs


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-- | Strict function
isMember :: Store -> [String] -> IO Bool
isMember store identifier = do
    inCache <- cacheIsMember store key
    if inCache then return True else doesFileExist path
  where
    key  = hash identifier
    path = storeDirectory store </> key


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-- | Delete an item
delete :: Store -> [String] -> IO ()
delete store identifier = do
    cacheDelete store key
    deleteFile $ storeDirectory store </> key
  where
    key  = hash identifier


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-- | Delete a file unless it doesn't exist...
deleteFile :: FilePath -> IO ()
deleteFile = handle (\(_ :: IOException) -> return ()) . removeFile


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-- | Mostly meant for internal usage
hash :: [String] -> String
hash = toHex . B.unpack . MD5.hash . T.encodeUtf8 . T.pack . intercalate "/"
  where
    toHex [] = ""
    toHex (x : xs) | x < 16 = '0' : showHex x (toHex xs)
                   | otherwise = showHex x (toHex xs)