    -- (c) The University of Glasgow 2006

    {-# LANGUAGE CPP #-}

    -- | Highly random utility functions
    --
    module Util (
            -- * Flags dependent on the compiler build
            ghciSupported, debugIsOn, ncgDebugIsOn,
            ghciTablesNextToCode,
            isWindowsHost, isDarwinHost,

            -- * General list processing
            zipEqual, zipWithEqual, zipWith3Equal, zipWith4Equal,
            zipLazy, stretchZipWith, zipWithAndUnzip,

            filterByList,

            unzipWith,

            mapFst, mapSnd, chkAppend,
            mapAndUnzip, mapAndUnzip3, mapAccumL2,
            nOfThem, filterOut, partitionWith, splitEithers,

            dropWhileEndLE,

            foldl1', foldl2, count, all2,

            lengthExceeds, lengthIs, lengthAtLeast,
            listLengthCmp, atLength,
            equalLength, compareLength, leLength,

            isSingleton, only, singleton,
            notNull, snocView,

            isIn, isn'tIn,

            -- * Tuples
            fstOf3, sndOf3, thirdOf3,
            firstM, first3M,
            third3,
            uncurry3,

            -- * List operations controlled by another list
            takeList, dropList, splitAtList, split,
            dropTail,

            -- * For loop
            nTimes,

            -- * Sorting
            sortWith, minWith, nubSort,

            -- * Comparisons
            isEqual, eqListBy, eqMaybeBy,
            thenCmp, cmpList,
            removeSpaces,
            (<&&>), (<||>),

            -- * Edit distance
            fuzzyMatch, fuzzyLookup,

            -- * Transitive closures
            transitiveClosure,

            -- * Strictness
            seqList,

            -- * Module names
            looksLikeModuleName,

            -- * Argument processing
            getCmd, toCmdArgs, toArgs,

            -- * Floating point
            readRational,

            -- * read helpers
            maybeRead, maybeReadFuzzy,

            -- * IO-ish utilities
            doesDirNameExist,
            getModificationUTCTime,
            modificationTimeIfExists,

            global, consIORef, globalM,

            -- * Filenames and paths
            Suffix,
            splitLongestPrefix,
            escapeSpaces,
            Direction(..), reslash,
            makeRelativeTo,

            -- * Utils for defining Data instances
            abstractConstr, abstractDataType, mkNoRepType,

            -- * Utils for printing C code
            charToC,

            -- * Hashing
            hashString,
        ) where

    #include "HsVersions.h"

    import Exception
    import Panic

    import Data.Data
    import Data.IORef       ( IORef, newIORef, atomicModifyIORef )
    import System.IO.Unsafe ( unsafePerformIO )
    import Data.List        hiding (group)

    #ifdef DEBUG
    import FastTypes
    #endif

    #if __GLASGOW_HASKELL__ < 709
    import Control.Applicative (Applicative)
    #endif
    import Control.Applicative ( liftA2 )
    import Control.Monad    ( liftM )
    import System.IO.Error as IO ( isDoesNotExistError )
    import System.Directory ( doesDirectoryExist, getModificationTime )
    import System.FilePath

    import Data.Char        ( isUpper, isAlphaNum, isSpace, chr, ord, isDigit )
    import Data.Int
    import Data.Ratio       ( (%) )
    import Data.Ord         ( comparing )
    import Data.Bits
    import Data.Word
    import qualified Data.IntMap as IM
    import qualified Data.Set as Set

    import Data.Time

    infixr 9 `thenCmp`

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Is DEBUG on, are we on Windows, etc?}
    *                                                                      *
    ************************************************************************

    These booleans are global constants, set by CPP flags.  They allow us to
    recompile a single module (this one) to change whether or not debug output
    appears. They sometimes let us avoid even running CPP elsewhere.

    It's important that the flags are literal constants (True/False). Then,
    with -0, tests of the flags in other modules will simplify to the correct
    branch of the conditional, thereby dropping debug code altogether when
    the flags are off.
    -}

    ghciSupported :: Bool
    #ifdef GHCI
    ghciSupported = True
    #else
    ghciSupported = False
    #endif

    debugIsOn :: Bool
    #ifdef DEBUG
    debugIsOn = True
    #else
    debugIsOn = False
    #endif

    ncgDebugIsOn :: Bool
    #ifdef NCG_DEBUG
    ncgDebugIsOn = True
    #else
    ncgDebugIsOn = False
    #endif

    ghciTablesNextToCode :: Bool
    #ifdef GHCI_TABLES_NEXT_TO_CODE
    ghciTablesNextToCode = True
    #else
    ghciTablesNextToCode = False
    #endif

    isWindowsHost :: Bool
    #ifdef mingw32_HOST_OS
    isWindowsHost = True
    #else
    isWindowsHost = False
    #endif

    isDarwinHost :: Bool
    #ifdef darwin_HOST_OS
    isDarwinHost = True
    #else
    isDarwinHost = False
    #endif

    {-
    ************************************************************************
    *                                                                      *
    \subsection{A for loop}
    *                                                                      *
    ************************************************************************
    -}

    -- | Compose a function with itself n times.  (nth rather than twice)
    nTimes :: Int -> (a -> a) -> (a -> a)
    nTimes 0 _ = id
    nTimes 1 f = f
    nTimes n f = f . nTimes (n-1) f

    fstOf3   :: (a,b,c) -> a
    sndOf3   :: (a,b,c) -> b
    thirdOf3 :: (a,b,c) -> c
    fstOf3      (a,_,_) =  a
    sndOf3      (_,b,_) =  b
    thirdOf3    (_,_,c) =  c

    third3 :: (c -> d) -> (a, b, c) -> (a, b, d)
    third3 f (a, b, c) = (a, b, f c)

    uncurry3 :: (a -> b -> c -> d) -> (a, b, c) -> d
    uncurry3 f (a, b, c) = f a b c

    firstM :: Monad m => (a -> m c) -> (a, b) -> m (c, b)
    firstM f (x, y) = liftM (\x' -> (x', y)) (f x)

    first3M :: Monad m => (a -> m d) -> (a, b, c) -> m (d, b, c)
    first3M f (x, y, z) = liftM (\x' -> (x', y, z)) (f x)

    {-
    ************************************************************************
    *                                                                      *
    \subsection[Utils-lists]{General list processing}
    *                                                                      *
    ************************************************************************
    -}

    filterOut :: (a->Bool) -> [a] -> [a]
    -- ^ Like filter, only it reverses the sense of the test
    filterOut _ [] = []
    filterOut p (x:xs) | p x       = filterOut p xs
                       | otherwise = x : filterOut p xs

    partitionWith :: (a -> Either b c) -> [a] -> ([b], [c])
    -- ^ Uses a function to determine which of two output lists an input element should join
    partitionWith _ [] = ([],[])
    partitionWith f (x:xs) = case f x of
                             Left  b -> (b:bs, cs)
                             Right c -> (bs, c:cs)
        where (bs,cs) = partitionWith f xs

    splitEithers :: [Either a b] -> ([a], [b])
    -- ^ Teases a list of 'Either's apart into two lists
    splitEithers [] = ([],[])
    splitEithers (e : es) = case e of
                            Left x -> (x:xs, ys)
                            Right y -> (xs, y:ys)
        where (xs,ys) = splitEithers es

    chkAppend :: [a] -> [a] -> [a]
    -- Checks for the second arguemnt being empty
    -- Used in situations where that situation is common
    chkAppend xs ys
      | null ys   = xs
      | otherwise = xs ++ ys

    {-
    A paranoid @zip@ (and some @zipWith@ friends) that checks the lists
    are of equal length.  Alastair Reid thinks this should only happen if
    DEBUGging on; hey, why not?
    -}

    zipEqual        :: String -> [a] -> [b] -> [(a,b)]
    zipWithEqual    :: String -> (a->b->c) -> [a]->[b]->[c]
    zipWith3Equal   :: String -> (a->b->c->d) -> [a]->[b]->[c]->[d]
    zipWith4Equal   :: String -> (a->b->c->d->e) -> [a]->[b]->[c]->[d]->[e]

    #ifndef DEBUG
    zipEqual      _ = zip
    zipWithEqual  _ = zipWith
    zipWith3Equal _ = zipWith3
    zipWith4Equal _ = zipWith4
    #else
    zipEqual _   []     []     = []
    zipEqual msg (a:as) (b:bs) = (a,b) : zipEqual msg as bs
    zipEqual msg _      _      = panic ("zipEqual: unequal lists:"++msg)

    zipWithEqual msg z (a:as) (b:bs)=  z a b : zipWithEqual msg z as bs
    zipWithEqual _   _ [] []        =  []
    zipWithEqual msg _ _ _          =  panic ("zipWithEqual: unequal lists:"++msg)

    zipWith3Equal msg z (a:as) (b:bs) (c:cs)
                                    =  z a b c : zipWith3Equal msg z as bs cs
    zipWith3Equal _   _ [] []  []   =  []
    zipWith3Equal msg _ _  _   _    =  panic ("zipWith3Equal: unequal lists:"++msg)

    zipWith4Equal msg z (a:as) (b:bs) (c:cs) (d:ds)
                                    =  z a b c d : zipWith4Equal msg z as bs cs ds
    zipWith4Equal _   _ [] [] [] [] =  []
    zipWith4Equal msg _ _  _  _  _  =  panic ("zipWith4Equal: unequal lists:"++msg)
    #endif

    -- | 'zipLazy' is a kind of 'zip' that is lazy in the second list (observe the ~)
    zipLazy :: [a] -> [b] -> [(a,b)]
    zipLazy []     _       = []
    zipLazy (x:xs) ~(y:ys) = (x,y) : zipLazy xs ys

    -- | 'filterByList' takes a list of Bools and a list of some elements and
    -- filters out these elements for which the corresponding value in the list of
    -- Bools is False. This function does not check whether the lists have equal
    -- length.
    filterByList :: [Bool] -> [a] -> [a]
    filterByList (True:bs)  (x:xs) = x : filterByList bs xs
    filterByList (False:bs) (_:xs) =     filterByList bs xs
    filterByList _          _      = []

    stretchZipWith :: (a -> Bool) -> b -> (a->b->c) -> [a] -> [b] -> [c]
    -- ^ @stretchZipWith p z f xs ys@ stretches @ys@ by inserting @z@ in
    -- the places where @p@ returns @True@

    stretchZipWith _ _ _ []     _ = []
    stretchZipWith p z f (x:xs) ys
      | p x       = f x z : stretchZipWith p z f xs ys
      | otherwise = case ys of
                    []     -> []
                    (y:ys) -> f x y : stretchZipWith p z f xs ys

    mapFst :: (a->c) -> [(a,b)] -> [(c,b)]
    mapSnd :: (b->c) -> [(a,b)] -> [(a,c)]

    mapFst f xys = [(f x, y) | (x,y) <- xys]
    mapSnd f xys = [(x, f y) | (x,y) <- xys]

    mapAndUnzip :: (a -> (b, c)) -> [a] -> ([b], [c])

    mapAndUnzip _ [] = ([], [])
    mapAndUnzip f (x:xs)
      = let (r1,  r2)  = f x
            (rs1, rs2) = mapAndUnzip f xs
        in
        (r1:rs1, r2:rs2)

    mapAndUnzip3 :: (a -> (b, c, d)) -> [a] -> ([b], [c], [d])

    mapAndUnzip3 _ [] = ([], [], [])
    mapAndUnzip3 f (x:xs)
      = let (r1,  r2,  r3)  = f x
            (rs1, rs2, rs3) = mapAndUnzip3 f xs
        in
        (r1:rs1, r2:rs2, r3:rs3)

    zipWithAndUnzip :: (a -> b -> (c,d)) -> [a] -> [b] -> ([c],[d])
    zipWithAndUnzip f (a:as) (b:bs)
      = let (r1,  r2)  = f a b
            (rs1, rs2) = zipWithAndUnzip f as bs
        in
        (r1:rs1, r2:rs2)
    zipWithAndUnzip _ _ _ = ([],[])

    mapAccumL2 :: (s1 -> s2 -> a -> (s1, s2, b)) -> s1 -> s2 -> [a] -> (s1, s2, [b])
    mapAccumL2 f s1 s2 xs = (s1', s2', ys)
      where ((s1', s2'), ys) = mapAccumL (\(s1, s2) x -> case f s1 s2 x of
                                                           (s1', s2', y) -> ((s1', s2'), y))
                                         (s1, s2) xs

    nOfThem :: Int -> a -> [a]
    nOfThem n thing = replicate n thing

    -- | @atLength atLen atEnd ls n@ unravels list @ls@ to position @n@. Precisely:
    --
    -- @
    --  atLength atLenPred atEndPred ls n
    --   | n < 0         = atLenPred n
    --   | length ls < n = atEndPred (n - length ls)
    --   | otherwise     = atLenPred (drop n ls)
    -- @
    atLength :: ([a] -> b)
             -> (Int -> b)
             -> [a]
             -> Int
             -> b
    atLength atLenPred atEndPred ls n
      | n < 0     = atEndPred n
      | otherwise = go n ls
      where
        go n [] = atEndPred n
        go 0 ls = atLenPred ls
        go n (_:xs) = go (n-1) xs

    -- Some special cases of atLength:

    lengthExceeds :: [a] -> Int -> Bool
    -- ^ > (lengthExceeds xs n) = (length xs > n)
    lengthExceeds = atLength notNull (const False)

    lengthAtLeast :: [a] -> Int -> Bool
    lengthAtLeast = atLength notNull (== 0)

    lengthIs :: [a] -> Int -> Bool
    lengthIs = atLength null (==0)

    listLengthCmp :: [a] -> Int -> Ordering
    listLengthCmp = atLength atLen atEnd
     where
      atEnd 0      = EQ
      atEnd x
       | x > 0     = LT -- not yet seen 'n' elts, so list length is < n.
       | otherwise = GT

      atLen []     = EQ
      atLen _      = GT

    equalLength :: [a] -> [b] -> Bool
    equalLength []     []     = True
    equalLength (_:xs) (_:ys) = equalLength xs ys
    equalLength _      _      = False

    compareLength :: [a] -> [b] -> Ordering
    compareLength []     []     = EQ
    compareLength (_:xs) (_:ys) = compareLength xs ys
    compareLength []     _      = LT
    compareLength _      []     = GT

    leLength :: [a] -> [b] -> Bool
    -- ^ True if length xs <= length ys
    leLength xs ys = case compareLength xs ys of
                       LT -> True
                       EQ -> True
                       GT -> False

    ----------------------------
    singleton :: a -> [a]
    singleton x = [x]

    isSingleton :: [a] -> Bool
    isSingleton [_] = True
    isSingleton _   = False

    notNull :: [a] -> Bool
    notNull [] = False
    notNull _  = True

    only :: [a] -> a
    #ifdef DEBUG
    only [a] = a
    #else
    only (a:_) = a
    #endif
    only _ = panic "Util: only"

    -- Debugging/specialising versions of \tr{elem} and \tr{notElem}

    isIn, isn'tIn :: Eq a => String -> a -> [a] -> Bool

    # ifndef DEBUG
    isIn    _msg x ys = x `elem` ys
    isn'tIn _msg x ys = x `notElem` ys

    # else /* DEBUG */
    isIn msg x ys
      = elem100 (_ILIT(0)) x ys
      where
        elem100 _ _ []        = False
        elem100 i x (y:ys)
          | i ># _ILIT(100) = trace ("Over-long elem in " ++ msg)
                                    (x `elem` (y:ys))
          | otherwise       = x == y || elem100 (i +# _ILIT(1)) x ys

    isn'tIn msg x ys
      = notElem100 (_ILIT(0)) x ys
      where
        notElem100 _ _ [] =  True
        notElem100 i x (y:ys)
          | i ># _ILIT(100) = trace ("Over-long notElem in " ++ msg)
                                    (x `notElem` (y:ys))
          | otherwise      =  x /= y && notElem100 (i +# _ILIT(1)) x ys
    # endif /* DEBUG */

    {-
    ************************************************************************
    *                                                                      *
    \subsubsection{Sort utils}
    *                                                                      *
    ************************************************************************
    -}

    sortWith :: Ord b => (a->b) -> [a] -> [a]
    sortWith get_key xs = sortBy (comparing get_key) xs

    minWith :: Ord b => (a -> b) -> [a] -> a
    minWith get_key xs = ASSERT( not (null xs) )
                         head (sortWith get_key xs)

    nubSort :: Ord a => [a] -> [a]
    nubSort = Set.toAscList . Set.fromList

    {-
    ************************************************************************
    *                                                                      *
    \subsection[Utils-transitive-closure]{Transitive closure}
    *                                                                      *
    ************************************************************************

    This algorithm for transitive closure is straightforward, albeit quadratic.
    -}

    transitiveClosure :: (a -> [a])         -- Successor function
                      -> (a -> a -> Bool)   -- Equality predicate
                      -> [a]
                      -> [a]                -- The transitive closure

    transitiveClosure succ eq xs
     = go [] xs
     where
       go done []                      = done
       go done (x:xs) | x `is_in` done = go done xs
                      | otherwise      = go (x:done) (succ x ++ xs)

       _ `is_in` []                 = False
       x `is_in` (y:ys) | eq x y    = True
                        | otherwise = x `is_in` ys

    {-
    ************************************************************************
    *                                                                      *
    \subsection[Utils-accum]{Accumulating}
    *                                                                      *
    ************************************************************************

    A combination of foldl with zip.  It works with equal length lists.
    -}

    foldl2 :: (acc -> a -> b -> acc) -> acc -> [a] -> [b] -> acc
    foldl2 _ z [] [] = z
    foldl2 k z (a:as) (b:bs) = foldl2 k (k z a b) as bs
    foldl2 _ _ _      _      = panic "Util: foldl2"

    all2 :: (a -> b -> Bool) -> [a] -> [b] -> Bool
    -- True if the lists are the same length, and
    -- all corresponding elements satisfy the predicate
    all2 _ []     []     = True
    all2 p (x:xs) (y:ys) = p x y && all2 p xs ys
    all2 _ _      _      = False

    -- Count the number of times a predicate is true

    count :: (a -> Bool) -> [a] -> Int
    count _ [] = 0
    count p (x:xs) | p x       = 1 + count p xs
                   | otherwise = count p xs

    {-
    @splitAt@, @take@, and @drop@ but with length of another
    list giving the break-off point:
    -}

    takeList :: [b] -> [a] -> [a]
    takeList [] _ = []
    takeList (_:xs) ls =
       case ls of
         [] -> []
         (y:ys) -> y : takeList xs ys

    dropList :: [b] -> [a] -> [a]
    dropList [] xs    = xs
    dropList _  xs@[] = xs
    dropList (_:xs) (_:ys) = dropList xs ys

    splitAtList :: [b] -> [a] -> ([a], [a])
    splitAtList [] xs     = ([], xs)
    splitAtList _ xs@[]   = (xs, xs)
    splitAtList (_:xs) (y:ys) = (y:ys', ys'')
        where
          (ys', ys'') = splitAtList xs ys

    -- drop from the end of a list
    dropTail :: Int -> [a] -> [a]
    -- Specification: dropTail n = reverse . drop n . reverse
    -- Better implemention due to Joachim Breitner
    -- http://www.joachim-breitner.de/blog/archives/600-On-taking-the-last-n-elements-of-a-list.html
    dropTail n xs
      = go (drop n xs) xs
      where
        go (_:ys) (x:xs) = x : go ys xs
        go _      _      = []  -- Stop when ys runs out
                               -- It'll always run out before xs does

    -- dropWhile from the end of a list. This is similar to Data.List.dropWhileEnd,
    -- but is lazy in the elements and strict in the spine. For reasonably short lists,
    -- such as path names and typical lines of text, dropWhileEndLE is generally
    -- faster than dropWhileEnd. Its advantage is magnified when the predicate is
    -- expensive--using dropWhileEndLE isSpace to strip the space off a line of text
    -- is generally much faster than using dropWhileEnd isSpace for that purpose.
    -- Specification: dropWhileEndLE p = reverse . dropWhile p . reverse
    -- Pay attention to the short-circuit (&&)! The order of its arguments is the only
    -- difference between dropWhileEnd and dropWhileEndLE.
    dropWhileEndLE :: (a -> Bool) -> [a] -> [a]
    dropWhileEndLE p = foldr (\x r -> if null r && p x then [] else x:r) []

    snocView :: [a] -> Maybe ([a],a)
            -- Split off the last element
    snocView [] = Nothing
    snocView xs = go [] xs
                where
                    -- Invariant: second arg is non-empty
                  go acc [x]    = Just (reverse acc, x)
                  go acc (x:xs) = go (x:acc) xs
                  go _   []     = panic "Util: snocView"

    split :: Char -> String -> [String]
    split c s = case rest of
                    []     -> [chunk]
                    _:rest -> chunk : split c rest
      where (chunk, rest) = break (==c) s

    {-
    ************************************************************************
    *                                                                      *
    \subsection[Utils-comparison]{Comparisons}
    *                                                                      *
    ************************************************************************
    -}

    isEqual :: Ordering -> Bool
    -- Often used in (isEqual (a `compare` b))
    isEqual GT = False
    isEqual EQ = True
    isEqual LT = False

    thenCmp :: Ordering -> Ordering -> Ordering
    {-# INLINE thenCmp #-}
    thenCmp EQ       ordering = ordering
    thenCmp ordering _        = ordering

    eqListBy :: (a->a->Bool) -> [a] -> [a] -> Bool
    eqListBy _  []     []     = True
    eqListBy eq (x:xs) (y:ys) = eq x y && eqListBy eq xs ys
    eqListBy _  _      _      = False

    eqMaybeBy :: (a ->a->Bool) -> Maybe a -> Maybe a -> Bool
    eqMaybeBy _  Nothing  Nothing  = True
    eqMaybeBy eq (Just x) (Just y) = eq x y
    eqMaybeBy _  _        _        = False

    cmpList :: (a -> a -> Ordering) -> [a] -> [a] -> Ordering
        -- `cmpList' uses a user-specified comparer

    cmpList _   []     [] = EQ
    cmpList _   []     _  = LT
    cmpList _   _      [] = GT
    cmpList cmp (a:as) (b:bs)
      = case cmp a b of { EQ -> cmpList cmp as bs; xxx -> xxx }

    removeSpaces :: String -> String
    removeSpaces = dropWhileEndLE isSpace . dropWhile isSpace

    -- Boolean operators lifted to Applicative
    (<&&>) :: Applicative f => f Bool -> f Bool -> f Bool
    (<&&>) = liftA2 (&&)
    infixr 3 <&&> -- same as (&&)

    (<||>) :: Applicative f => f Bool -> f Bool -> f Bool
    (<||>) = liftA2 (||)
    infixr 2 <||> -- same as (||)

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Edit distance}
    *                                                                      *
    ************************************************************************
    -}

    -- | Find the "restricted" Damerau-Levenshtein edit distance between two strings.
    -- See: <http://en.wikipedia.org/wiki/Damerau-Levenshtein_distance>.
    -- Based on the algorithm presented in "A Bit-Vector Algorithm for Computing
    -- Levenshtein and Damerau Edit Distances" in PSC'02 (Heikki Hyyro).
    -- See http://www.cs.uta.fi/~helmu/pubs/psc02.pdf and
    --     http://www.cs.uta.fi/~helmu/pubs/PSCerr.html for an explanation
    restrictedDamerauLevenshteinDistance :: String -> String -> Int
    restrictedDamerauLevenshteinDistance str1 str2
      = restrictedDamerauLevenshteinDistanceWithLengths m n str1 str2
      where
        m = length str1
        n = length str2

    restrictedDamerauLevenshteinDistanceWithLengths
      :: Int -> Int -> String -> String -> Int
    restrictedDamerauLevenshteinDistanceWithLengths m n str1 str2
      | m <= n
      = if n <= 32 -- n must be larger so this check is sufficient
        then restrictedDamerauLevenshteinDistance' (undefined :: Word32) m n str1 str2
        else restrictedDamerauLevenshteinDistance' (undefined :: Integer) m n str1 str2

      | otherwise
      = if m <= 32 -- m must be larger so this check is sufficient
        then restrictedDamerauLevenshteinDistance' (undefined :: Word32) n m str2 str1
        else restrictedDamerauLevenshteinDistance' (undefined :: Integer) n m str2 str1

    restrictedDamerauLevenshteinDistance'
      :: (Bits bv, Num bv) => bv -> Int -> Int -> String -> String -> Int
    restrictedDamerauLevenshteinDistance' _bv_dummy m n str1 str2
      | [] <- str1 = n
      | otherwise  = extractAnswer $
                     foldl' (restrictedDamerauLevenshteinDistanceWorker
                                 (matchVectors str1) top_bit_mask vector_mask)
                            (0, 0, m_ones, 0, m) str2
      where
        m_ones@vector_mask = (2 ^ m) - 1
        top_bit_mask = (1 `shiftL` (m - 1)) `asTypeOf` _bv_dummy
        extractAnswer (_, _, _, _, distance) = distance

    restrictedDamerauLevenshteinDistanceWorker
          :: (Bits bv, Num bv) => IM.IntMap bv -> bv -> bv
          -> (bv, bv, bv, bv, Int) -> Char -> (bv, bv, bv, bv, Int)
    restrictedDamerauLevenshteinDistanceWorker str1_mvs top_bit_mask vector_mask
                                               (pm, d0, vp, vn, distance) char2
      = seq str1_mvs $ seq top_bit_mask $ seq vector_mask $
        seq pm' $ seq d0' $ seq vp' $ seq vn' $
        seq distance'' $ seq char2 $
        (pm', d0', vp', vn', distance'')
      where
        pm' = IM.findWithDefault 0 (ord char2) str1_mvs

        d0' = ((((sizedComplement vector_mask d0) .&. pm') `shiftL` 1) .&. pm)
          .|. ((((pm' .&. vp) + vp) .&. vector_mask) `xor` vp) .|. pm' .|. vn
              -- No need to mask the shiftL because of the restricted range of pm

        hp' = vn .|. sizedComplement vector_mask (d0' .|. vp)
        hn' = d0' .&. vp

        hp'_shift = ((hp' `shiftL` 1) .|. 1) .&. vector_mask
        hn'_shift = (hn' `shiftL` 1) .&. vector_mask
        vp' = hn'_shift .|. sizedComplement vector_mask (d0' .|. hp'_shift)
        vn' = d0' .&. hp'_shift

        distance' = if hp' .&. top_bit_mask /= 0 then distance + 1 else distance
        distance'' = if hn' .&. top_bit_mask /= 0 then distance' - 1 else distance'

    sizedComplement :: Bits bv => bv -> bv -> bv
    sizedComplement vector_mask vect = vector_mask `xor` vect

    matchVectors :: (Bits bv, Num bv) => String -> IM.IntMap bv
    matchVectors = snd . foldl' go (0 :: Int, IM.empty)
      where
        go (ix, im) char = let ix' = ix + 1
                               im' = IM.insertWith (.|.) (ord char) (2 ^ ix) im
                           in seq ix' $ seq im' $ (ix', im')

    {-# SPECIALIZE INLINE restrictedDamerauLevenshteinDistance'
                          :: Word32 -> Int -> Int -> String -> String -> Int #-}
    {-# SPECIALIZE INLINE restrictedDamerauLevenshteinDistance'
                          :: Integer -> Int -> Int -> String -> String -> Int #-}

    {-# SPECIALIZE restrictedDamerauLevenshteinDistanceWorker
                   :: IM.IntMap Word32 -> Word32 -> Word32
                   -> (Word32, Word32, Word32, Word32, Int)
                   -> Char -> (Word32, Word32, Word32, Word32, Int) #-}
    {-# SPECIALIZE restrictedDamerauLevenshteinDistanceWorker
                   :: IM.IntMap Integer -> Integer -> Integer
                   -> (Integer, Integer, Integer, Integer, Int)
                   -> Char -> (Integer, Integer, Integer, Integer, Int) #-}

    {-# SPECIALIZE INLINE sizedComplement :: Word32 -> Word32 -> Word32 #-}
    {-# SPECIALIZE INLINE sizedComplement :: Integer -> Integer -> Integer #-}

    {-# SPECIALIZE matchVectors :: String -> IM.IntMap Word32 #-}
    {-# SPECIALIZE matchVectors :: String -> IM.IntMap Integer #-}

    fuzzyMatch :: String -> [String] -> [String]
    fuzzyMatch key vals = fuzzyLookup key [(v,v) | v <- vals]

    -- | Search for possible matches to the users input in the given list,
    -- returning a small number of ranked results
    fuzzyLookup :: String -> [(String,a)] -> [a]
    fuzzyLookup user_entered possibilites
      = map fst $ take mAX_RESULTS $ sortBy (comparing snd)
        [ (poss_val, distance) | (poss_str, poss_val) <- possibilites
                           , let distance = restrictedDamerauLevenshteinDistance
                                                poss_str user_entered
                           , distance <= fuzzy_threshold ]
      where
        -- Work out an approriate match threshold:
        -- We report a candidate if its edit distance is <= the threshold,
        -- The threshhold is set to about a quarter of the # of characters the user entered
        --   Length    Threshold
        --     1         0          -- Don't suggest *any* candidates
        --     2         1          -- for single-char identifiers
        --     3         1
        --     4         1
        --     5         1
        --     6         2
        --
        fuzzy_threshold = truncate $ fromIntegral (length user_entered + 2) / (4 :: Rational)
        mAX_RESULTS = 3

    {-
    ************************************************************************
    *                                                                      *
    \subsection[Utils-pairs]{Pairs}
    *                                                                      *
    ************************************************************************
    -}

    unzipWith :: (a -> b -> c) -> [(a, b)] -> [c]
    unzipWith f pairs = map ( \ (a, b) -> f a b ) pairs

    seqList :: [a] -> b -> b
    seqList [] b = b
    seqList (x:xs) b = x `seq` seqList xs b

    -- Global variables:

    global :: a -> IORef a
    global a = unsafePerformIO (newIORef a)

    consIORef :: IORef [a] -> a -> IO ()
    consIORef var x = do
      atomicModifyIORef var (\xs -> (x:xs,()))

    globalM :: IO a -> IORef a
    globalM ma = unsafePerformIO (ma >>= newIORef)

    -- Module names:

    looksLikeModuleName :: String -> Bool
    looksLikeModuleName [] = False
    looksLikeModuleName (c:cs) = isUpper c && go cs
      where go [] = True
            go ('.':cs) = looksLikeModuleName cs
            go (c:cs)   = (isAlphaNum c || c == '_' || c == '\'') && go cs

    {-
    Akin to @Prelude.words@, but acts like the Bourne shell, treating
    quoted strings as Haskell Strings, and also parses Haskell [String]
    syntax.
    -}

    getCmd :: String -> Either String             -- Error
                               (String, String) -- (Cmd, Rest)
    getCmd s = case break isSpace $ dropWhile isSpace s of
               ([], _) -> Left ("Couldn't find command in " ++ show s)
               res -> Right res

    toCmdArgs :: String -> Either String             -- Error
                                  (String, [String]) -- (Cmd, Args)
    toCmdArgs s = case getCmd s of
                  Left err -> Left err
                  Right (cmd, s') -> case toArgs s' of
                                     Left err -> Left err
                                     Right args -> Right (cmd, args)

    toArgs :: String -> Either String   -- Error
                               [String] -- Args
    toArgs str
        = case dropWhile isSpace str of
          s@('[':_) -> case reads s of
                       [(args, spaces)]
                        | all isSpace spaces ->
                           Right args
                       _ ->
                           Left ("Couldn't read " ++ show str ++ "as [String]")
          s -> toArgs' s
     where
      toArgs' s = case dropWhile isSpace s of
                  [] -> Right []
                  ('"' : _) -> case reads s of
                               [(arg, rest)]
                                  -- rest must either be [] or start with a space
                                | all isSpace (take 1 rest) ->
                                   case toArgs' rest of
                                   Left err -> Left err
                                   Right args -> Right (arg : args)
                               _ ->
                                   Left ("Couldn't read " ++ show s ++ "as String")
                  s' -> case break isSpace s' of
                        (arg, s'') -> case toArgs' s'' of
                                      Left err -> Left err
                                      Right args -> Right (arg : args)

    {-
    -- -----------------------------------------------------------------------------
    -- Floats
    -}

    readRational__ :: ReadS Rational -- NB: doesn't handle leading "-"
    readRational__ r = do
         (n,d,s) <- readFix r
         (k,t)   <- readExp s
         return ((n%1)*10^^(k-d), t)
     where
         readFix r = do
            (ds,s)  <- lexDecDigits r
            (ds',t) <- lexDotDigits s
            return (read (ds++ds'), length ds', t)

         readExp (e:s) | e `elem` "eE" = readExp' s
         readExp s                     = return (0,s)

         readExp' ('+':s) = readDec s
         readExp' ('-':s) = do (k,t) <- readDec s
                               return (-k,t)
         readExp' s       = readDec s

         readDec s = do
            (ds,r) <- nonnull isDigit s
            return (foldl1 (\n d -> n * 10 + d) [ ord d - ord '0' | d <- ds ],
                    r)

         lexDecDigits = nonnull isDigit

         lexDotDigits ('.':s) = return (span isDigit s)
         lexDotDigits s       = return ("",s)

         nonnull p s = do (cs@(_:_),t) <- return (span p s)
                          return (cs,t)

    readRational :: String -> Rational -- NB: *does* handle a leading "-"
    readRational top_s
      = case top_s of
          '-' : xs -> - (read_me xs)
          xs       -> read_me xs
      where
        read_me s
          = case (do { (x,"") <- readRational__ s ; return x }) of
              [x] -> x
              []  -> error ("readRational: no parse:"        ++ top_s)
              _   -> error ("readRational: ambiguous parse:" ++ top_s)

    -----------------------------------------------------------------------------
    -- read helpers

    maybeRead :: Read a => String -> Maybe a
    maybeRead str = case reads str of
                    [(x, "")] -> Just x
                    _         -> Nothing

    maybeReadFuzzy :: Read a => String -> Maybe a
    maybeReadFuzzy str = case reads str of
                         [(x, s)]
                          | all isSpace s ->
                             Just x
                         _ ->
                             Nothing

    -----------------------------------------------------------------------------
    -- Verify that the 'dirname' portion of a FilePath exists.
    --
    doesDirNameExist :: FilePath -> IO Bool
    doesDirNameExist fpath = doesDirectoryExist (takeDirectory fpath)

    -----------------------------------------------------------------------------
    -- Backwards compatibility definition of getModificationTime

    getModificationUTCTime :: FilePath -> IO UTCTime
    getModificationUTCTime = getModificationTime

    -- --------------------------------------------------------------
    -- check existence & modification time at the same time

    modificationTimeIfExists :: FilePath -> IO (Maybe UTCTime)
    modificationTimeIfExists f = do
      (do t <- getModificationUTCTime f; return (Just t))
            `catchIO` \e -> if isDoesNotExistError e
                            then return Nothing
                            else ioError e

    -- split a string at the last character where 'pred' is True,
    -- returning a pair of strings. The first component holds the string
    -- up (but not including) the last character for which 'pred' returned
    -- True, the second whatever comes after (but also not including the
    -- last character).
    --
    -- If 'pred' returns False for all characters in the string, the original
    -- string is returned in the first component (and the second one is just
    -- empty).
    splitLongestPrefix :: String -> (Char -> Bool) -> (String,String)
    splitLongestPrefix str pred
      | null r_pre = (str,           [])
      | otherwise  = (reverse (tail r_pre), reverse r_suf)
                               -- 'tail' drops the char satisfying 'pred'
      where (r_suf, r_pre) = break pred (reverse str)

    escapeSpaces :: String -> String
    escapeSpaces = foldr (\c s -> if isSpace c then '\\':c:s else c:s) ""

    type Suffix = String

    --------------------------------------------------------------
    -- * Search path
    --------------------------------------------------------------

    data Direction = Forwards | Backwards

    reslash :: Direction -> FilePath -> FilePath
    reslash d = f
        where f ('/'  : xs) = slash : f xs
              f ('\\' : xs) = slash : f xs
              f (x    : xs) = x     : f xs
              f ""          = ""
              slash = case d of
                      Forwards -> '/'
                      Backwards -> '\\'

    makeRelativeTo :: FilePath -> FilePath -> FilePath
    this `makeRelativeTo` that = directory </> thisFilename
        where (thisDirectory, thisFilename) = splitFileName this
              thatDirectory = dropFileName that
              directory = joinPath $ f (splitPath thisDirectory)
                                       (splitPath thatDirectory)

              f (x : xs) (y : ys)
               | x == y = f xs ys
              f xs ys = replicate (length ys) ".." ++ xs

    {-
    ************************************************************************
    *                                                                      *
    \subsection[Utils-Data]{Utils for defining Data instances}
    *                                                                      *
    ************************************************************************

    These functions helps us to define Data instances for abstract types.
    -}

    abstractConstr :: String -> Constr
    abstractConstr n = mkConstr (abstractDataType n) ("{abstract:"++n++"}") [] Prefix

    abstractDataType :: String -> DataType
    abstractDataType n = mkDataType n [abstractConstr n]

    {-
    ************************************************************************
    *                                                                      *
    \subsection[Utils-C]{Utils for printing C code}
    *                                                                      *
    ************************************************************************
    -}

    charToC :: Word8 -> String
    charToC w =
      case chr (fromIntegral w) of
            '\"' -> "\\\""
            '\'' -> "\\\'"
            '\\' -> "\\\\"
            c | c >= ' ' && c <= '~' -> [c]
              | otherwise -> ['\\',
                             chr (ord '0' + ord c `div` 64),
                             chr (ord '0' + ord c `div` 8 `mod` 8),
                             chr (ord '0' + ord c         `mod` 8)]

    {-
    ************************************************************************
    *                                                                      *
    \subsection[Utils-Hashing]{Utils for hashing}
    *                                                                      *
    ************************************************************************
    -}

    -- | A sample hash function for Strings.  We keep multiplying by the
    -- golden ratio and adding.  The implementation is:
    --
    -- > hashString = foldl' f golden
    -- >   where f m c = fromIntegral (ord c) * magic + hashInt32 m
    -- >         magic = 0xdeadbeef
    --
    -- Where hashInt32 works just as hashInt shown above.
    --
    -- Knuth argues that repeated multiplication by the golden ratio
    -- will minimize gaps in the hash space, and thus it's a good choice
    -- for combining together multiple keys to form one.
    --
    -- Here we know that individual characters c are often small, and this
    -- produces frequent collisions if we use ord c alone.  A
    -- particular problem are the shorter low ASCII and ISO-8859-1
    -- character strings.  We pre-multiply by a magic twiddle factor to
    -- obtain a good distribution.  In fact, given the following test:
    --
    -- > testp :: Int32 -> Int
    -- > testp k = (n - ) . length . group . sort . map hs . take n $ ls
    -- >   where ls = [] : [c : l | l <- ls, c <- ['\0'..'\xff']]
    -- >         hs = foldl' f golden
    -- >         f m c = fromIntegral (ord c) * k + hashInt32 m
    -- >         n = 100000
    --
    -- We discover that testp magic = 0.
    hashString :: String -> Int32
    hashString = foldl' f golden
       where f m c = fromIntegral (ord c) * magic + hashInt32 m
             magic = fromIntegral (0xdeadbeef :: Word32)

    golden :: Int32
    golden = 1013904242 -- = round ((sqrt 5 - 1) * 2^32) :: Int32
    -- was -1640531527 = round ((sqrt 5 - 1) * 2^31) :: Int32
    -- but that has bad mulHi properties (even adding 2^32 to get its inverse)
    -- Whereas the above works well and contains no hash duplications for
    -- [-32767..65536]

    -- | A sample (and useful) hash function for Int32,
    -- implemented by extracting the uppermost 32 bits of the 64-bit
    -- result of multiplying by a 33-bit constant.  The constant is from
    -- Knuth, derived from the golden ratio:
    --
    -- > golden = round ((sqrt 5 - 1) * 2^32)
    --
    -- We get good key uniqueness on small inputs
    -- (a problem with previous versions):
    --  (length $ group $ sort $ map hashInt32 [-32767..65536]) == 65536 + 32768
    --
    hashInt32 :: Int32 -> Int32
    hashInt32 x = mulHi x golden + x

    -- hi 32 bits of a x-bit * 32 bit -> 64-bit multiply
    mulHi :: Int32 -> Int32 -> Int32
    mulHi a b = fromIntegral (r `shiftR` 32)
       where r :: Int64
             r = fromIntegral a * fromIntegral b