------------------------------------------------------------------------------
--- This library is a direct port of the Haskell library System.FilePath
--- of Neil Mitchell.
---
--- @author Bjoern Peemoeller
--- @version November 2011
--- @category general
------------------------------------------------------------------------------

--
-- Some short examples:
--
-- You are given a C file, you want to figure out the corresponding object (.o) file:
--
-- @'replaceExtension' file \"o\"@
--
-- Haskell module Main imports Test, you have the file named main:
--
-- @['replaceFileName' path_to_main \"Test\" '<.>' ext | ext <- [\"hs\",\"lhs\"] ]@
--
-- You want to download a file from the web and save it to disk:
--
-- @do let file = 'makeValid' url
--    System.IO.createDirectoryIfMissing True ('takeDirectory' file)@
--
-- You want to compile a Haskell file, but put the hi file under \"interface\"
--
-- @'takeDirectory' file '</>' \"interface\" '</>' ('takeFileName' file \`replaceExtension\` \"hi\"@)
--
-- The examples in code format descibed by each function are used to generate
-- tests, and should give clear semantics for the functions.
-----------------------------------------------------------------------------

module System.FilePath
  (
  -- * Separator predicates
  FilePath,
  pathSeparator, pathSeparators, isPathSeparator,
  searchPathSeparator, isSearchPathSeparator,
  extSeparator, isExtSeparator,

  -- * Path methods (environment $PATH)
  splitSearchPath, getSearchPath,

  -- * Extension methods
  splitExtension,
  takeExtension, replaceExtension, dropExtension, addExtension, hasExtension,
  (<.>), splitExtensions, dropExtensions, takeExtensions, isExtensionOf,

  -- * Drive methods
  splitDrive, joinDrive,
  takeDrive, hasDrive, dropDrive, isDrive,

  -- * Operations on a FilePath, as a list of directories
  splitFileName,
  takeFileName, replaceFileName, dropFileName,
  takeBaseName, replaceBaseName,
  takeDirectory, replaceDirectory,
  combine, (</>),
  splitPath, joinPath, splitDirectories,

  -- * Low level FilePath operators
  hasTrailingPathSeparator,
  addTrailingPathSeparator,
  dropTrailingPathSeparator,

  -- * File name manipulators
  normalise, equalFilePath,
  makeRelative,
  isRelative, isAbsolute,
  isValid, makeValid
  )
  where

import Data.Char          (toLower, toUpper)
import Data.List          (isSuffixOf, isPrefixOf, init, last, intersperse)
import Data.Maybe         (isJust, fromJust)
import System.Environment (getEnv, isPosix, isWindows)

infixr 7  <.>
infixr 5  </>

---------------------------------------------------------------------
-- The basic functions

-- | The character that separates directories. In the case where more than
--   one character is possible, 'pathSeparator' is the \'ideal\' one.
--
-- > Windows: pathSeparator == '\\'
-- > Posix:   pathSeparator ==  '/'
-- > isPathSeparator pathSeparator
pathSeparator :: Char
pathSeparator = if isWindows then '\\' else '/'


-- | The list of all possible separators.
--
-- > Windows: pathSeparators == ['\\', '/']
-- > Posix:   pathSeparators == ['/']
-- > pathSeparator `elem` pathSeparators
pathSeparators :: [Char]
pathSeparators = if isWindows then "\\/" else "/"


-- | Rather than using @(== 'pathSeparator')@, use this. Test if something
--   is a path separator.
--
-- > isPathSeparator a == (a `elem` pathSeparators)
isPathSeparator :: Char -> Bool
isPathSeparator = (`elem` pathSeparators)


-- | The character that is used to separate the entries in the $PATH
--   environment variable.
--
-- > Windows: searchPathSeparator == ';'
-- > Posix:   searchPathSeparator == ':'
searchPathSeparator :: Char
searchPathSeparator = if isWindows then ';' else ':'


-- | Is the character a file separator?
--
-- > isSearchPathSeparator a == (a == searchPathSeparator)
isSearchPathSeparator :: Char -> Bool
isSearchPathSeparator = (== searchPathSeparator)


-- | File extension character
--
-- > extSeparator == '.'
extSeparator :: Char
extSeparator = '.'


-- | Is the character an extension character?
--
-- > isExtSeparator a == (a == extSeparator)
isExtSeparator :: Char -> Bool
isExtSeparator = (== extSeparator)


---------------------------------------------------------------------
-- Path methods (environment $PATH)

-- | Take a string, split it on the 'searchPathSeparator' character.
--
--   Follows the recommendations in
--   <http://www.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html>
--
-- > Posix:   splitSearchPath "File1:File2:File3"  == ["File1","File2","File3"]
-- > Posix:   splitSearchPath "File1::File2:File3" == ["File1",".","File2","File3"]
-- > Windows: splitSearchPath "File1;File2;File3"  == ["File1","File2","File3"]
-- > Windows: splitSearchPath "File1;;File2;File3" == ["File1","File2","File3"]
splitSearchPath :: String -> [FilePath]
splitSearchPath = f
    where
    f xs = case break isSearchPathSeparator xs of
           (pre, []    ) -> g pre
           (pre, _:post) -> g pre ++ f post

    g []      = ["." | isPosix]
    g x@(_:_) = [x]


-- | Get a list of filepaths in the $PATH.
getSearchPath :: IO [FilePath]
getSearchPath = getEnv "PATH" >>= return . splitSearchPath


---------------------------------------------------------------------
-- Extension methods

-- | Split on the extension. 'addExtension' is the inverse.
--
-- > uncurry (++) (splitExtension x) == x
-- > uncurry addExtension (splitExtension x) == x
-- > splitExtension "file.txt" == ("file",".txt")
-- > splitExtension "file" == ("file","")
-- > splitExtension "file/file.txt" == ("file/file",".txt")
-- > splitExtension "file.txt/boris" == ("file.txt/boris","")
-- > splitExtension "file.txt/boris.ext" == ("file.txt/boris",".ext")
-- > splitExtension "file/path.txt.bob.fred" == ("file/path.txt.bob",".fred")
-- > splitExtension "file/path.txt/" == ("file/path.txt/","")
splitExtension :: FilePath -> (String, String)
splitExtension x = case d of
    []     -> (x,"")
    (y:ys) -> (a ++ reverse ys, y : reverse c)
  where
    (a,b) = splitFileName_ x
    (c,d) = break isExtSeparator $ reverse b


-- | Get the extension of a file, returns @\"\"@ for no extension, @.ext@ otherwise.
--
-- > takeExtension x == snd (splitExtension x)
-- > Valid x => takeExtension (addExtension x "ext") == ".ext"
-- > Valid x => takeExtension (replaceExtension x "ext") == ".ext"
takeExtension :: FilePath -> String
takeExtension = snd . splitExtension


-- | Set the extension of a file, overwriting one if already present.
--
-- > replaceExtension "file.txt" ".bob" == "file.bob"
-- > replaceExtension "file.txt" "bob" == "file.bob"
-- > replaceExtension "file" ".bob" == "file.bob"
-- > replaceExtension "file.txt" "" == "file"
-- > replaceExtension "file.fred.bob" "txt" == "file.fred.txt"
replaceExtension :: FilePath -> String -> FilePath
replaceExtension x y = dropExtension x <.> y


-- | Alias to 'addExtension', for people who like that sort of thing.
(<.>) :: FilePath -> String -> FilePath
(<.>) = addExtension


-- | Remove last extension, and the \".\" preceding it.
--
-- > dropExtension x == fst (splitExtension x)
dropExtension :: FilePath -> FilePath
dropExtension = fst . splitExtension


-- | Add an extension, even if there is already one there.
--   E.g. @addExtension \"foo.txt\" \"bat\" -> \"foo.txt.bat\"@.
--
-- > addExtension "file.txt" "bib" == "file.txt.bib"
-- > addExtension "file." ".bib" == "file..bib"
-- > addExtension "file" ".bib" == "file.bib"
-- > addExtension "/" "x" == "/.x"
-- > Valid x => takeFileName (addExtension (addTrailingPathSeparator x) "ext") == ".ext"
-- > Windows: addExtension "\\\\share" ".txt" == "\\\\share\\.txt"
addExtension :: FilePath -> String -> FilePath
addExtension file []       = file
addExtension file xs@(x:_) = joinDrive a res
    where
        res = if isExtSeparator x then b ++ xs
              else b ++ [extSeparator] ++ xs
        (a,b) = splitDrive file


-- | Does the given filename have an extension?
--
-- > null (takeExtension x) == not (hasExtension x)
hasExtension :: FilePath -> Bool
hasExtension = any isExtSeparator . takeFileName


-- | Split on all extensions
--
-- > uncurry (++) (splitExtensions x) == x
-- > uncurry addExtension (splitExtensions x) == x
-- > splitExtensions "file.tar.gz" == ("file",".tar.gz")
splitExtensions :: FilePath -> (FilePath, String)
splitExtensions x = (a ++ c, d)
    where
        (a,b) = splitFileName_ x
        (c,d) = break isExtSeparator b


-- | Drop all extensions
--
-- > not $ hasExtension (dropExtensions x)
dropExtensions :: FilePath -> FilePath
dropExtensions = fst . splitExtensions


-- | Get all extensions
--
-- > takeExtensions "file.tar.gz" == ".tar.gz"
takeExtensions :: FilePath -> String
takeExtensions = snd . splitExtensions

-- | Does the given filename have the specified extension?
--
-- > "png" `isExtensionOf` "/directory/file.png" == True
-- > ".png" `isExtensionOf` "/directory/file.png" == True
-- > ".tar.gz" `isExtensionOf` "bar/foo.tar.gz" == True
-- > "ar.gz" `isExtensionOf` "bar/foo.tar.gz" == False
-- > "png" `isExtensionOf` "/directory/file.png.jpg" == False
-- > "csv/table.csv" `isExtensionOf` "/data/csv/table.csv" == False
isExtensionOf :: String -> FilePath -> Bool
isExtensionOf extension path = case extension of
  ext@('.':_) -> isSuffixOf ext       $ takeExtensions path
  ext         -> isSuffixOf ('.':ext) $ takeExtensions path

---------------------------------------------------------------------
-- Drive methods

-- | Is the given character a valid drive letter?
-- only a-z and A-Z are letters, not isAlpha which is more unicodey
isLetter :: Char -> Bool
isLetter x = (x >= 'a' && x <= 'z') || (x >= 'A' && x <= 'Z')


-- | Split a path into a drive and a path.
--   On Unix, \/ is a Drive.
--
-- > uncurry (++) (splitDrive x) == x
-- > Windows: splitDrive "file" == ("","file")
-- > Windows: splitDrive "c:/file" == ("c:/","file")
-- > Windows: splitDrive "c:\\file" == ("c:\\","file")
-- > Windows: splitDrive "\\\\shared\\test" == ("\\\\shared\\","test")
-- > Windows: splitDrive "\\\\shared" == ("\\\\shared","")
-- > Windows: splitDrive "\\\\?\\UNC\\shared\\file" == ("\\\\?\\UNC\\shared\\","file")
-- > Windows: splitDrive "\\\\?\\UNCshared\\file" == ("\\\\?\\","UNCshared\\file")
-- > Windows: splitDrive "\\\\?\\d:\\file" == ("\\\\?\\d:\\","file")
-- > Windows: splitDrive "/d" == ("","/d")
-- > Posix:   splitDrive "/test" == ("/","test")
-- > Posix:   splitDrive "//test" == ("//","test")
-- > Posix:   splitDrive "test/file" == ("","test/file")
-- > Posix:   splitDrive "file" == ("","file")
splitDrive :: FilePath -> (FilePath, FilePath)
splitDrive x | isPosix    = span (== '/') x
             | isJust dl  = fromJust dl
             | isJust unc = fromJust unc
             | isJust shr = fromJust shr
             | otherwise  = ("",x)
    where dl  = readDriveLetter x
          unc = readDriveUNC x
          shr = readDriveShare x


addSlash :: FilePath -> FilePath -> (FilePath, FilePath)
addSlash a xs = (a++c,d)
    where (c,d) = span isPathSeparator xs


-- http://msdn.microsoft.com/library/default.asp?url=/library/en-us/fileio/fs/naming_a_file.asp
-- "\\?\D:\<path>" or "\\?\UNC\<server>\<share>"
-- a is "\\?\"
readDriveUNC :: FilePath -> Maybe (FilePath, FilePath)
readDriveUNC path = case path of
  (s1:s2:'?':s3:xs) -> if all isPathSeparator [s1,s2,s3]
      then let rdl = case readDriveLetter xs of
                 Just (a,b) -> Just (s1:s2:'?':s3:a,b)
                 Nothing -> Nothing
           in case map toUpper xs of
        ('U':'N':'C':s4:_) ->
          if isPathSeparator s4
              then
                let (a,b) = readDriveShareName (drop 4 xs)
                in Just (s1:s2:'?':s3:take 4 xs ++ a, b)
              else rdl
        _ -> rdl
      else Nothing
  _ -> Nothing


{- c:\ -}
readDriveLetter :: String -> Maybe (FilePath, FilePath)
readDriveLetter path = case path of
  (x:':':y:xs) -> if isLetter x && isPathSeparator y
                     then Just $ addSlash [x,':'] (y:xs)
                     else if isLetter x then Just ([x,':'], (y:xs))
                                        else Nothing
  (x:':':xs)   -> if isLetter x then Just ([x,':'], xs) else Nothing
  _ -> Nothing


{- \\sharename\ -}
readDriveShare :: String -> Maybe (FilePath, FilePath)
readDriveShare path = case path of
  (s1:s2:xs) -> if isPathSeparator s1 && isPathSeparator s2
                   then let (a,b) = readDriveShareName xs in Just (s1:s2:a,b)
                   else Nothing
  _          -> Nothing


{- assume you have already seen \\ -}
{- share\bob -> "share","\","bob" -}
readDriveShareName :: String -> (FilePath, FilePath)
readDriveShareName name = addSlash a b
    where (a,b) = break isPathSeparator name


-- | Join a drive and the rest of the path.
--
-- >          uncurry joinDrive (splitDrive x) == x
-- > Windows: joinDrive "C:" "foo" == "C:foo"
-- > Windows: joinDrive "C:\\" "bar" == "C:\\bar"
-- > Windows: joinDrive "\\\\share" "foo" == "\\\\share\\foo"
-- > Windows: joinDrive "/:" "foo" == "/:\\foo"
joinDrive :: FilePath -> FilePath -> FilePath
joinDrive a b | isPosix = a ++ b
              | null a = b
              | null b = a
              | isPathSeparator (last a) = a ++ b
              | otherwise = case a of
                             [a1,':'] -> if isLetter a1
                                          then a ++ b
                                          else a ++ [pathSeparator] ++ b
                             _ -> a ++ [pathSeparator] ++ b


-- | Get the drive from a filepath.
--
-- > takeDrive x == fst (splitDrive x)
takeDrive :: FilePath -> FilePath
takeDrive = fst . splitDrive


-- | Delete the drive, if it exists.
--
-- > dropDrive x == snd (splitDrive x)
dropDrive :: FilePath -> FilePath
dropDrive = snd . splitDrive


-- | Does a path have a drive.
--
-- > not (hasDrive x) == null (takeDrive x)
hasDrive :: FilePath -> Bool
hasDrive = not . null . takeDrive


-- | Is an element a drive
isDrive :: FilePath -> Bool
isDrive = null . dropDrive


---------------------------------------------------------------------
-- Operations on a filepath, as a list of directories

-- | Split a filename into directory and file. 'combine' is the inverse.
--
-- > Valid x => uncurry (</>) (splitFileName x) == x || fst (splitFileName x) == "./"
-- > Valid x => isValid (fst (splitFileName x))
-- > splitFileName "file/bob.txt" == ("file/", "bob.txt")
-- > splitFileName "file/" == ("file/", "")
-- > splitFileName "bob" == ("./", "bob")
-- > Posix:   splitFileName "/" == ("/","")
-- > Windows: splitFileName "c:" == ("c:","")
splitFileName :: FilePath -> (String, String)
splitFileName x = (if null dir then "./" else dir, name)
    where
        (dir, name) = splitFileName_ x


-- version of splitFileName where, if the FilePath has no directory
-- component, the returned directory is "" rather than "./".  This
-- is used in cases where we are going to combine the returned
-- directory to make a valid FilePath, and having a "./" appear would
-- look strange and upset simple equality properties.  See
-- e.g. replaceFileName.
splitFileName_ :: FilePath -> (String, String)
splitFileName_ x = (c ++ reverse b, reverse a)
    where
        (a,b) = break isPathSeparator $ reverse d
        (c,d) = splitDrive x


-- | Set the filename.
--
-- > Valid x => replaceFileName x (takeFileName x) == x
replaceFileName :: FilePath -> String -> FilePath
replaceFileName x y = a </> y where (a,_) = splitFileName_ x


-- | Drop the filename.
--
-- > dropFileName x == fst (splitFileName x)
dropFileName :: FilePath -> FilePath
dropFileName = fst . splitFileName


-- | Get the file name.
--
-- > takeFileName "test/" == ""
-- > takeFileName x `isSuffixOf` x
-- > takeFileName x == snd (splitFileName x)
-- > Valid x => takeFileName (replaceFileName x "fred") == "fred"
-- > Valid x => takeFileName (x </> "fred") == "fred"
-- > Valid x => isRelative (takeFileName x)
takeFileName :: FilePath -> FilePath
takeFileName = snd . splitFileName


-- | Get the base name, without an extension or path.
--
-- > takeBaseName "file/test.txt" == "test"
-- > takeBaseName "dave.ext" == "dave"
-- > takeBaseName "" == ""
-- > takeBaseName "test" == "test"
-- > takeBaseName (addTrailingPathSeparator x) == ""
-- > takeBaseName "file/file.tar.gz" == "file.tar"
takeBaseName :: FilePath -> String
takeBaseName = dropExtension . takeFileName


-- | Set the base name.
--
-- > replaceBaseName "file/test.txt" "bob" == "file/bob.txt"
-- > replaceBaseName "fred" "bill" == "bill"
-- > replaceBaseName "/dave/fred/bob.gz.tar" "new" == "/dave/fred/new.tar"
-- > Valid x => replaceBaseName x (takeBaseName x) == x
replaceBaseName :: FilePath -> String -> FilePath
replaceBaseName pth nam = combineAlways a (nam <.> ext)
    where
        (a,b) = splitFileName_ pth
        ext = takeExtension b


-- | Is an item either a directory or the last character a path separator?
--
-- > hasTrailingPathSeparator "test" == False
-- > hasTrailingPathSeparator "test/" == True
hasTrailingPathSeparator :: FilePath -> Bool
hasTrailingPathSeparator []      = False
hasTrailingPathSeparator x@(_:_) = isPathSeparator (last x)


-- | Add a trailing file path separator if one is not already present.
--
-- > hasTrailingPathSeparator (addTrailingPathSeparator x)
-- > hasTrailingPathSeparator x ==> addTrailingPathSeparator x == x
-- > Posix:    addTrailingPathSeparator "test/rest" == "test/rest/"
addTrailingPathSeparator :: FilePath -> FilePath
addTrailingPathSeparator x = if hasTrailingPathSeparator x then x else x ++ [pathSeparator]


-- | Remove any trailing path separators
--
-- > dropTrailingPathSeparator "file/test/" == "file/test"
-- > Posix:    not (hasTrailingPathSeparator (dropTrailingPathSeparator x)) || isDrive x
-- > Posix:    dropTrailingPathSeparator "/" == "/"
-- > Windows:  dropTrailingPathSeparator "\\" == "\\"
dropTrailingPathSeparator :: FilePath -> FilePath
dropTrailingPathSeparator x =
    if hasTrailingPathSeparator x && not (isDrive x)
    then let x' = reverse $ dropWhile isPathSeparator $ reverse x
         in if null x' then [pathSeparator] else x'
    else x


-- | Get the directory name, move up one level.
--
-- >           takeDirectory x `isPrefixOf` x || takeDirectory x == "."
-- >           takeDirectory "foo" == "."
-- >           takeDirectory "/foo/bar/baz" == "/foo/bar"
-- >           takeDirectory "/foo/bar/baz/" == "/foo/bar/baz"
-- >           takeDirectory "foo/bar/baz" == "foo/bar"
-- > Windows:  takeDirectory "foo\\bar" == "foo"
-- > Windows:  takeDirectory "foo\\bar\\\\" == "foo\\bar"
-- > Windows:  takeDirectory "C:\\" == "C:\\"
takeDirectory :: FilePath -> FilePath
takeDirectory x = if isDrive file then file
                  else if null res && not (null file) then file
                  else res
    where
        res = reverse $ dropWhile isPathSeparator $ reverse file
        file = dropFileName x
        _ = isPrefixOf x -- warning suppression


-- | Set the directory, keeping the filename the same.
--
-- > Valid x => replaceDirectory x (takeDirectory x) `equalFilePath` x
replaceDirectory :: FilePath -> String -> FilePath
replaceDirectory x dir = combineAlways dir (takeFileName x)


-- | Combine two paths, if the second path 'isAbsolute', then it returns the second.
--
-- > Valid x => combine (takeDirectory x) (takeFileName x) `equalFilePath` x
-- > Posix:   combine "/" "test" == "/test"
-- > Posix:   combine "home" "bob" == "home/bob"
-- > Windows: combine "home" "bob" == "home\\bob"
-- > Windows: combine "home" "/bob" == "/bob"
combine :: FilePath -> FilePath -> FilePath
combine a b | hasDrive b || (not (null b) && isPathSeparator (head b)) = b
            | otherwise = combineAlways a b


-- | Combine two paths, assuming rhs is NOT absolute.
combineAlways :: FilePath -> FilePath -> FilePath
combineAlways a b | null a = b
                  | null b = a
                  | isPathSeparator (last a) = a ++ b
                  | isDrive a = joinDrive a b
                  | otherwise = a ++ [pathSeparator] ++ b


-- | A nice alias for 'combine'.
(</>) :: FilePath -> FilePath -> FilePath
(</>) = combine


-- | Split a path by the directory separator.
--
-- > concat (splitPath x) == x
-- > splitPath "test//item/" == ["test//","item/"]
-- > splitPath "test/item/file" == ["test/","item/","file"]
-- > splitPath "" == []
-- > Windows: splitPath "c:\\test\\path" == ["c:\\","test\\","path"]
-- > Posix:   splitPath "/file/test" == ["/","file/","test"]
splitPath :: FilePath -> [FilePath]
splitPath x = [drive | drive /= ""] ++ f path
    where
        (drive,path) = splitDrive x
        f []      = []
        f y@(_:_) = (a ++ c) : f d
            where
                (a,b) = break isPathSeparator y
                (c,d) = break (not . isPathSeparator) b


-- | Just as 'splitPath', but don't add the trailing slashes to each element.
--
-- > splitDirectories "test/file" == ["test","file"]
-- > splitDirectories "/test/file" == ["/","test","file"]
-- > Valid x => joinPath (splitDirectories x) `equalFilePath` x
-- > splitDirectories "" == []
splitDirectories :: FilePath -> [FilePath]
splitDirectories path =
        if hasDrive path then head pathComponents : f (tail pathComponents)
        else f pathComponents
    where
        pathComponents = splitPath path
        f xs = map g xs
        g x = if null res then x else res
            where res = takeWhile (not . isPathSeparator) x


-- | Join path elements back together.
--
-- > Valid x => joinPath (splitPath x) == x
-- > joinPath [] == ""
-- > Posix: joinPath ["test","file","path"] == "test/file/path"

-- Note that this definition on c:\\c:\\, join then split will give c:\\.
joinPath :: [FilePath] -> FilePath
joinPath x = foldr combine "" x

---------------------------------------------------------------------
-- File name manipulators

-- | Equality of two 'FilePath's.
--   If you call @System.Directory.canonicalizePath@
--   first this has a much better chance of working.
--   Note that this doesn't follow symlinks or DOSNAM~1s.
--
-- >          x == y ==> equalFilePath x y
-- >          normalise x == normalise y ==> equalFilePath x y
-- > Posix:   equalFilePath "foo" "foo/"
-- > Posix:   not (equalFilePath "foo" "/foo")
-- > Posix:   not (equalFilePath "foo" "FOO")
-- > Windows: equalFilePath "foo" "FOO"
equalFilePath :: FilePath -> FilePath -> Bool
equalFilePath a b = f a == f b
    where
        f x | isWindows = dropTrailSlash $ map toLower $ normalise x
            | otherwise = dropTrailSlash $ normalise x
        dropTrailSlash x | length x >= 2 && isPathSeparator (last x) = init x
                         | otherwise = x


-- | Contract a filename, based on a relative path.
--
--   There is no corresponding @makeAbsolute@ function, instead use
--   @System.Directory.canonicalizePath@ which has the same effect.
--
-- >          Valid y => equalFilePath x y || (isRelative x && makeRelative y x == x) || equalFilePath (y </> makeRelative y x) x
-- >          makeRelative x x == "."
-- >          null y || equalFilePath (makeRelative x (x </> y)) y || null (takeFileName x)
-- > Windows: makeRelative "C:\\Home" "c:\\home\\bob" == "bob"
-- > Windows: makeRelative "C:\\Home" "c:/home/bob" == "bob"
-- > Windows: makeRelative "C:\\Home" "D:\\Home\\Bob" == "D:\\Home\\Bob"
-- > Windows: makeRelative "C:\\Home" "C:Home\\Bob" == "C:Home\\Bob"
-- > Windows: makeRelative "/Home" "/home/bob" == "bob"
-- > Posix:   makeRelative "/Home" "/home/bob" == "/home/bob"
-- > Posix:   makeRelative "/home/" "/home/bob/foo/bar" == "bob/foo/bar"
-- > Posix:   makeRelative "/fred" "bob" == "bob"
-- > Posix:   makeRelative "/file/test" "/file/test/fred" == "fred"
-- > Posix:   makeRelative "/file/test" "/file/test/fred/" == "fred/"
-- > Posix:   makeRelative "some/path" "some/path/a/b/c" == "a/b/c"
makeRelative :: FilePath -> FilePath -> FilePath
makeRelative root path
 | equalFilePath root path = "."
 | takeAbs root /= takeAbs path = path
 | otherwise = f (dropAbs root) (dropAbs path)
    where
        f []      y = dropWhile isPathSeparator y
        f x@(_:_) y = let (x1,x2) = g x
                          (y1,y2) = g y
                      in if equalFilePath x1 y1 then f x2 y2 else path
        g x = (dropWhile isPathSeparator a, dropWhile isPathSeparator b)
            where (a,b) = break isPathSeparator $ dropWhile isPathSeparator x

        -- on windows, need to drop '/' which is kind of absolute, but not a drive
        dropAbs [] = dropDrive []
        dropAbs (x:xs) | isPathSeparator x = xs
                       | otherwise         = dropDrive (x:xs)

        takeAbs [] = map (\y -> if isPathSeparator y then pathSeparator else toLower y) $ takeDrive []
        takeAbs xs@(x:_)
          | isPathSeparator x = [pathSeparator]
          | otherwise         = map (\y -> if isPathSeparator y then pathSeparator else toLower y) $ takeDrive xs


-- | Normalise a file
--
-- * \/\/ outside of the drive can be made blank
--
-- * \/ -> 'pathSeparator'
--
-- * .\/ -> \"\"
--
-- > Posix:   normalise "/file/\\test////" == "/file/\\test/"
-- > Posix:   normalise "/file/./test" == "/file/test"
-- > Posix:   normalise "/test/file/../bob/fred/" == "/test/file/../bob/fred/"
-- > Posix:   normalise "../bob/fred/" == "../bob/fred/"
-- > Posix:   normalise "./bob/fred/" == "bob/fred/"
-- > Windows: normalise "c:\\file/bob\\" == "C:\\file\\bob\\"
-- > Windows: normalise "c:\\" == "C:\\"
-- > Windows: normalise "\\\\server\\test" == "\\\\server\\test"
-- > Windows: normalise "c:/file" == "C:\\file"
-- >          normalise "." == "."
-- > Posix:   normalise "./" == "./"
-- > Posix:   normalise "./." == "./"
-- > Posix:   normalise "bob/fred/." == "bob/fred/"
normalise :: FilePath -> FilePath
normalise path = joinDrive (normaliseDrive drv) (f pth)
              ++ [pathSeparator | isDirPath pth]
  where
    (drv,pth) = splitDrive path
    isDirPath xs = lastSep xs
        || not (null xs) && last xs == '.' && lastSep (init xs)
    lastSep xs = not (null xs) && isPathSeparator (last xs)
    f = joinPath . dropDots . splitDirectories . propSep

    propSep []            = []
    propSep xs@[x]
      | isPathSeparator x = [pathSeparator]
      | otherwise         = xs
    propSep (x:y:xs)
      | isPathSeparator x && isPathSeparator y = propSep (x:xs)
      | isPathSeparator x                      = pathSeparator : propSep (y:xs)
      | otherwise                              = x : propSep (y:xs)

    dropDots xs | all (== ".") xs = ["."]
                | otherwise       = dropDots' [] xs

    dropDots' acc [] = reverse acc
    dropDots' acc (x:xs) | x == "."  = dropDots' acc xs
                         | otherwise = dropDots' (x:acc) xs

normaliseDrive :: FilePath -> FilePath
normaliseDrive drive
  | isPosix   = drive
  | otherwise = if isJust $ readDriveLetter x2
                then map toUpper x2
                else drive
    where
        x2 = map repSlash drive
        repSlash x = if isPathSeparator x then pathSeparator else x


-- information for validity functions on Windows
-- see http://msdn.microsoft.com/library/default.asp?url=/library/en-us/fileio/fs/naming_a_file.asp
badCharacters :: [Char]
badCharacters = ":*?><|\""
badElements :: [FilePath]
badElements = ["CON", "PRN", "AUX", "NUL", "COM1", "COM2", "COM3", "COM4", "COM5", "COM6", "COM7", "COM8", "COM9", "LPT1", "LPT2", "LPT3", "LPT4", "LPT5", "LPT6", "LPT7", "LPT8", "LPT9", "CLOCK$"]


-- | Is a FilePath valid, i.e. could you create a file like it?
--
-- >          isValid "" == False
-- > Posix:   isValid "/random_ path:*" == True
-- > Posix:   isValid x == not (null x)
-- > Windows: isValid "c:\\test" == True
-- > Windows: isValid "c:\\test:of_test" == False
-- > Windows: isValid "test*" == False
-- > Windows: isValid "c:\\test\\nul" == False
-- > Windows: isValid "c:\\test\\prn.txt" == False
-- > Windows: isValid "c:\\nul\\file" == False
-- > Windows: isValid "\\\\" == False
isValid :: FilePath -> Bool
isValid []    = False
isValid path@(_:_)
  | isPosix   = True
  | otherwise =    not (any (`elem` badCharacters) x2)
                && not (any f $ splitDirectories x2)
                && not (length path >= 2 && all isPathSeparator path)
  where
    x2 = dropDrive path
    f x = map toUpper (dropExtensions x) `elem` badElements


-- | Take a FilePath and make it valid; does not change already valid FilePaths.
--
-- > isValid (makeValid x)
-- > isValid x ==> makeValid x == x
-- > makeValid "" == "_"
-- > Windows: makeValid "c:\\test:of_test" == "c:\\test_of_test"
-- > Windows: makeValid "test*" == "test_"
-- > Windows: makeValid "c:\\test\\nul" == "c:\\test\\nul_"
-- > Windows: makeValid "c:\\test\\prn.txt" == "c:\\test\\prn_.txt"
-- > Windows: makeValid "c:\\test/prn.txt" == "c:\\test/prn_.txt"
-- > Windows: makeValid "c:\\nul\\file" == "c:\\nul_\\file"
makeValid :: FilePath -> FilePath
makeValid []                                     = "_"
makeValid path@(_:_)
  | isPosix                                      = path
  | length path >= 2 && all isPathSeparator path = take 2 path ++ "drive"
  | otherwise = joinDrive drv $ validElements $ validChars pth
 where
  (drv,pth) = splitDrive path
  validChars x = map f x
  f x | x `elem` badCharacters = '_'
      | otherwise = x
  validElements x = joinPath $ map g $ splitPath x
  g x = h (reverse b) ++ reverse a
      where (a,b) = span isPathSeparator $ reverse x
  h x = if map toUpper a `elem` badElements then a ++ "_" <.> b else x
      where (a,b) = splitExtensions x


-- | Is a path relative, or is it fixed to the root?
--
-- > Windows: isRelative "path\\test" == True
-- > Windows: isRelative "c:\\test" == False
-- > Windows: isRelative "c:test" == True
-- > Windows: isRelative "c:" == True
-- > Windows: isRelative "\\\\foo" == False
-- > Windows: isRelative "/foo" == True
-- > Posix:   isRelative "test/path" == True
-- > Posix:   isRelative "/test" == False
isRelative :: FilePath -> Bool
isRelative = isRelativeDrive . takeDrive


-- > isRelativeDrive "" == True
-- > Windows: isRelativeDrive "c:\\" == False
-- > Windows: isRelativeDrive "c:/" == False
-- > Windows: isRelativeDrive "c:" == True
-- > Windows: isRelativeDrive "\\\\foo" == False
-- > Posix:   isRelativeDrive "/" == False
isRelativeDrive :: String -> Bool
isRelativeDrive x = null x ||
    maybe False (not . isPathSeparator . last . fst) (readDriveLetter x)


-- | @not . 'isRelative'@
--
-- > isAbsolute x == not (isRelative x)
isAbsolute :: FilePath -> Bool
isAbsolute = not . isRelative