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diff --git a/compiler/src/Files.hs b/compiler/src/Files.hs
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+-- ldgallery - A static generator which turns a collection of tagged
+--             pictures into a searchable web gallery.
+--
+-- Copyright (C) 2019-2020  Pacien TRAN-GIRARD
+--
+-- This program is free software: you can redistribute it and/or modify
+-- it under the terms of the GNU Affero General Public License as
+-- published by the Free Software Foundation, either version 3 of the
+-- License, or (at your option) any later version.
+--
+-- This program is distributed in the hope that it will be useful,
+-- but WITHOUT ANY WARRANTY; without even the implied warranty of
+-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+-- GNU Affero General Public License for more details.
+--
+-- You should have received a copy of the GNU Affero General Public License
+-- along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+module Files
+  ( FileName, LocalPath, WebPath, Path(..)
+  , (</>), (</), (/>), (<.>)
+  , fileName, subPaths, pathLength
+  , localPath, webPath
+  , FSNode(..), AnchoredFSNode(..)
+  , nodeName, isHidden, flattenDir, filterDir
+  , readDirectory, copyTo
+  , ensureParentDir, remove, isOutdated
+  ) where
+
+
+import Control.Monad (mapM)
+import Data.Bool (bool)
+import Data.List (isPrefixOf, length, subsequences)
+import Data.Function ((&))
+import Data.Text (pack)
+import Data.Aeson (ToJSON)
+import qualified Data.Aeson as JSON
+
+import System.Directory
+  ( doesDirectoryExist
+  , doesPathExist
+  , getModificationTime
+  , listDirectory
+  , createDirectoryIfMissing
+  , removePathForcibly
+  , copyFile )
+
+import qualified System.FilePath
+import qualified System.FilePath.Posix
+
+
+type FileName = String
+type LocalPath = String
+type WebPath = String
+
+ -- | Reversed path component list
+data Path = Path [FileName] deriving Show
+
+instance ToJSON Path where
+  toJSON = JSON.String . pack . webPath
+
+instance Eq Path where
+  (Path left) == (Path right) = left == right
+
+(</>) :: Path -> Path -> Path
+(Path l) </> (Path r) = Path (r ++ l)
+
+(</) :: Path -> FileName -> Path
+(Path path) </ file = Path (file:path)
+
+(/>) :: FileName -> Path -> Path
+file /> (Path path) = Path (path ++ [file])
+
+(<.>) :: Path -> String -> Path
+(Path (filename:pathto)) <.> ext =
+  Path $ System.FilePath.addExtension filename ext : pathto
+(Path _) <.> ext = Path [ext]
+
+fileName :: Path -> Maybe FileName
+fileName (Path (name:_)) = Just name
+fileName _ = Nothing
+
+subPaths :: Path -> [Path]
+subPaths (Path path) = map Path $ subsequences path
+
+pathLength :: Path -> Int
+pathLength (Path path) = Data.List.length path
+
+localPath :: Path -> LocalPath
+localPath (Path path) = System.FilePath.joinPath $ reverse path
+
+webPath :: Path -> WebPath
+webPath (Path path) = System.FilePath.Posix.joinPath $ reverse path
+
+
+data FSNode =
+    File { path :: Path }
+  | Dir { path :: Path, items :: [FSNode] }
+  deriving Show
+
+data AnchoredFSNode = AnchoredFSNode
+  { anchor :: LocalPath
+  , root :: FSNode }
+  deriving Show
+
+nodeName :: FSNode -> Maybe FileName
+nodeName = fileName . path
+
+isHidden :: FSNode -> Bool
+isHidden = hiddenName . nodeName
+  where
+    hiddenName :: Maybe FileName -> Bool
+    hiddenName Nothing = False
+    hiddenName (Just filename) = "." `isPrefixOf` filename && length filename > 1
+
+-- | DFS with intermediate dirs first.
+flattenDir :: FSNode -> [FSNode]
+flattenDir file@(File _) = [file]
+flattenDir dir@(Dir _ items) = dir:(concatMap flattenDir items)
+
+-- | Filters a dir tree. The root is always returned.
+filterDir :: (FSNode -> Bool) -> AnchoredFSNode -> AnchoredFSNode
+filterDir cond (AnchoredFSNode anchor root) =
+  AnchoredFSNode anchor (filterNode root)
+  where
+  filterNode :: FSNode -> FSNode
+  filterNode file@(File _) = file
+  filterNode (Dir path items) =
+    filter cond items & map filterNode & Dir path
+
+readDirectory :: LocalPath -> IO AnchoredFSNode
+readDirectory root = mkNode (Path []) >>= return . AnchoredFSNode root
+  where
+    mkNode :: Path -> IO FSNode
+    mkNode path =
+      (doesDirectoryExist $ localPath (root /> path))
+      >>= bool (mkFileNode path) (mkDirNode path)
+
+    mkFileNode :: Path -> IO FSNode
+    mkFileNode path = return $ File path
+
+    mkDirNode :: Path -> IO FSNode
+    mkDirNode path =
+      (listDirectory $ localPath (root /> path))
+      >>= mapM (mkNode . ((</) path))
+      >>= return . Dir path
+
+copyTo :: FilePath -> AnchoredFSNode -> IO ()
+copyTo target AnchoredFSNode{anchor, root} = copyNode root
+  where
+    copyNode :: FSNode -> IO ()
+    copyNode (File path) =
+      copyFile (localPath $ anchor /> path) (localPath $ target /> path)
+
+    copyNode (Dir path items) =
+      createDirectoryIfMissing True (localPath $ target /> path)
+      >> mapM_ copyNode items
+
+ensureParentDir :: (FileName -> a -> IO b) -> FileName -> a -> IO b
+ensureParentDir writer filePath a =
+  createDirectoryIfMissing True parentDir
+  >> writer filePath a
+  where
+    parentDir = System.FilePath.dropFileName filePath
+
+remove :: FileName -> IO ()
+remove path =
+  do
+    putStrLn $ "Removing:\t" ++ path
+    removePathForcibly path
+
+isOutdated :: Bool -> FilePath -> FilePath -> IO Bool
+isOutdated onMissingTarget ref target =
+  do
+    refExists <- doesPathExist ref
+    targetExists <- doesPathExist target
+    if refExists && targetExists then
+      do
+        refTime <- getModificationTime ref
+        targetTime <- getModificationTime target
+        return (targetTime < refTime)
+    else
+      return onMissingTarget