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diff --git a/app/Store.hs b/app/Store.hs
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+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE TypeFamilyDependencies #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE TypeSynonymInstances #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE DeriveAnyClass #-}
+{-# LANGUAGE StrictData #-}
+{-# OPTIONS_GHC -Wno-orphans #-}
+
+-- |  Read-only on-disk key-value store
+
+module Store where
+
+import Control.DeepSeq (NFData)
+import Control.Monad.IO.Class (liftIO)
+import qualified Control.Monad.State.Strict as S
+import qualified Data.ByteString as BS
+import qualified Data.ByteString.Short as BSS
+import Data.Either (Either)
+import qualified Data.Map.Strict as M
+import Data.Serialize (Serialize, decode, encode, get, put)
+import GHC.Generics (Generic)
+import Prelude hiding (lookup)
+import System.Directory (doesFileExist)
+import System.FilePath ((</>))
+import System.IO (Handle, IOMode(..), hTell, withFile)
+import System.IO.MMap (mmapFileByteString)
+
+data Store = Store
+  { index :: M.Map BSS.ShortByteString Location
+  , values :: BS.ByteString
+  }
+  
+data Location = Location
+  { offset :: Int
+  , length :: Int
+  } deriving (Show, Eq, Ord, Generic, NFData)
+
+instance Serialize Location
+instance Serialize BSS.ShortByteString where
+  put = put . BSS.fromShort
+  get = BSS.toShort <$> get
+
+class StoreItem item where
+  toByteString :: item -> BS.ByteString
+  fromByteString :: BS.ByteString -> Either String item
+  type KeyArgs item = arg | arg -> item
+  itemKey :: KeyArgs item -> BSS.ShortByteString
+
+indexFileName :: FilePath
+indexFileName = "index"
+
+valuesFileName :: FilePath
+valuesFileName = "values"
+
+data ReadMode
+  = ReadEntireFile
+  | MemoryMapFile
+  deriving (Show, Eq)
+ 
+load :: FilePath -> ReadMode -> IO (Either String Store)
+load directoryPath readMode = do
+  let valuesFilePath = directoryPath </> valuesFileName
+      indexFilePath = directoryPath </> indexFileName
+  (valuesFileExists, indexFileExists) <-
+    (,) <$> doesFileExist indexFilePath <*> doesFileExist valuesFilePath
+  case (valuesFileExists, indexFileExists) of
+    (True, True) -> do
+      indexFile <- BS.readFile indexFilePath
+      valuesFile <-
+        case readMode of
+          ReadEntireFile -> BS.readFile valuesFilePath
+          MemoryMapFile -> mmapFileByteString valuesFilePath Nothing
+      let eitherIndex = decode @(M.Map BSS.ShortByteString Location) indexFile
+      case eitherIndex of
+        Right locMap -> return . Right $ Store {index = locMap, values = valuesFile}
+        Left err -> return . Left $ "Error while decoding index : " ++ err
+    (False, False) ->
+      return . Left $ "Cannot find index and values in " ++ directoryPath
+    (True, False) -> return . Left $ "Cannot find index in " ++ directoryPath
+    (False, True) -> return . Left $ "Cannot find values in " ++ directoryPath
+
+lookup :: (StoreItem item) => KeyArgs item -> Store -> Either String item
+lookup keyArgs = lookupByteString (itemKey keyArgs)
+      
+lookupByteString ::
+     (StoreItem item) => BSS.ShortByteString -> Store -> Either String item
+lookupByteString key store =
+  case M.lookup key (index store) of
+    Just (Location off len) ->
+      fromByteString . BS.take len . BS.drop off $ values store
+    Nothing -> Left $ "Cannot find item with key " ++ show key
+
+data State =
+  State (M.Map BSS.ShortByteString Location)
+        Handle
+
+add :: (StoreItem item) => item -> KeyArgs item -> S.StateT State IO ()
+add item keyArgs = do
+  let bs = toByteString item
+      len = BS.length bs
+  State locMap handle <- S.get
+  off <- liftIO $ fromIntegral <$> hTell handle
+  _ <- liftIO $ BS.hPut handle bs
+  S.put $ State (M.insert (itemKey keyArgs) (Location off len) locMap) handle
+
+createStore ::
+     FilePath -> (Handle -> IO (M.Map BSS.ShortByteString Location)) -> IO ()
+createStore directoryPath action =
+  withFile (directoryPath </> valuesFileName) WriteMode $ \valuesHandle -> do
+    locMap <- action valuesHandle
+    BS.writeFile (directoryPath </> indexFileName) (encode locMap)
+  
+writeValues ::
+     Handle
+  -> M.Map BSS.ShortByteString Location
+  -> S.StateT State IO ()
+  -> IO (M.Map BSS.ShortByteString Location)
+writeValues handle locMap action = do
+  State locMap' _ <- S.execStateT action (State locMap handle)
+  return locMap'