{-# LANGUAGE TupleSections, BangPatterns, ConstraintKinds #-}

-- | This module extends "Data.List" with extra functions of a similar nature.
--   The package also exports the existing "Data.List" functions.
--   Some of the names and semantics were inspired by the
--   <https://hackage.haskell.org/package/text text> package.
module Data.List.Extra(
    module Data.List,
    -- * String operations
    lower, upper, trim, trimStart, trimEnd, word1, line1,
    escapeHTML, escapeJSON,
    unescapeHTML, unescapeJSON,
    -- * Splitting
    dropEnd, takeEnd, splitAtEnd, breakEnd, spanEnd,
    dropWhileEnd', takeWhileEnd,
    stripSuffix, stripInfix, stripInfixEnd,
    dropPrefix, dropSuffix,
    wordsBy, linesBy,
    breakOn, breakOnEnd, splitOn, split, chunksOf,
    -- * Basics
    notNull, list, unsnoc, cons, snoc, drop1, mconcatMap,
    -- * Enum operations
    enumerate,
    -- * List operations
    groupSort, groupSortOn, groupSortBy,
    nubOrd, nubOrdBy, nubOrdOn,
    nubOn, groupOn,
    nubSort, nubSortBy, nubSortOn,
    maximumOn, minimumOn,
    disjoint, allSame, anySame,
    repeatedly, for, firstJust,
    concatUnzip, concatUnzip3,
    zipFrom, zipWithFrom,
    replace, merge, mergeBy,
    ) where

import Partial
import Data.List
import Data.Maybe
import Data.Function
import Data.Char
import Data.Tuple.Extra
import Data.Monoid
import Numeric
import Data.Functor
import Prelude


-- | Apply some operation repeatedly, producing an element of output
--   and the remainder of the list.
--
-- > \xs -> repeatedly (splitAt 3) xs  == chunksOf 3 xs
-- > \xs -> repeatedly word1 (trim xs) == words xs
-- > \xs -> repeatedly line1 xs == lines xs
repeatedly :: ([a] -> (b, [a])) -> [a] -> [b]
repeatedly f [] = []
repeatedly f as = b : repeatedly f as'
    where (b, as') = f as


-- | /DEPRECATED/ Use @flip map@ directly, since this function clashes with @Data.Traversable.for@.
--
--   Flipped version of 'map'.
{-# DEPRECATED for "Use flip map directly, since this function clashes with Data.Traversable.for" #-}
for :: [a] -> (a -> b) -> [b]
for = flip map

-- | Are two lists disjoint, with no elements in common.
--
-- > disjoint [1,2,3] [4,5] == True
-- > disjoint [1,2,3] [4,1] == False
disjoint :: Eq a => [a] -> [a] -> Bool
disjoint xs = null . intersect xs

-- | Is there any element which occurs more than once.
--
-- > anySame [1,1,2] == True
-- > anySame [1,2,3] == False
-- > anySame (1:2:1:undefined) == True
-- > anySame [] == False
-- > \xs -> anySame xs == (length (nub xs) < length xs)
anySame :: Eq a => [a] -> Bool
anySame = f []
    where
        f seen (x:xs) = x `elem` seen || f (x:seen) xs
        f seen [] = False

-- | Are all elements the same.
--
-- > allSame [1,1,2] == False
-- > allSame [1,1,1] == True
-- > allSame [1]     == True
-- > allSame []      == True
-- > allSame (1:1:2:undefined) == False
-- > \xs -> allSame xs == (length (nub xs) <= 1)
allSame :: Eq a => [a] -> Bool
allSame [] = True
allSame (x:xs) = all (x ==) xs


-- | A composition of 'not' and 'null'.
--
-- > notNull []  == False
-- > notNull [1] == True
-- > \xs -> notNull xs == not (null xs)
notNull :: [a] -> Bool
notNull = not . null

-- | Non-recursive transform over a list, like 'maybe'.
--
-- > list 1 (\v _ -> v - 2) [5,6,7] == 3
-- > list 1 (\v _ -> v - 2) []      == 1
-- > \nil cons xs -> maybe nil (uncurry cons) (uncons xs) == list nil cons xs
list :: b -> (a -> [a] -> b) -> [a] -> b
list nil cons [] = nil
list nil cons (x:xs) = cons x xs

-- | If the list is empty returns 'Nothing', otherwise returns the 'init' and the 'last'.
--
-- > unsnoc "test" == Just ("tes",'t')
-- > unsnoc ""     == Nothing
-- > \xs -> unsnoc xs == if null xs then Nothing else Just (init xs, last xs)
unsnoc :: [a] -> Maybe ([a], a)
unsnoc [] = Nothing
unsnoc [x] = Just ([], x)
unsnoc (x:xs) = Just (x:a, b)
    where Just (a,b) = unsnoc xs

-- | Append an element to the start of a list, an alias for '(:)'.
--
-- > cons 't' "est" == "test"
-- > \x xs -> uncons (cons x xs) == Just (x,xs)
cons :: a -> [a] -> [a]
cons = (:)

-- | Append an element to the end of a list, takes /O(n)/ time.
--
-- > snoc "tes" 't' == "test"
-- > \xs x -> unsnoc (snoc xs x) == Just (xs,x)
snoc :: [a] -> a -> [a]
snoc xs x = xs ++ [x]


-- | Enumerate all the values of an 'Enum', from 'minBound' to 'maxBound'.
--
-- > enumerate == [False, True]
enumerate :: (Enum a, Bounded a) => [a]
enumerate = [minBound..maxBound]

-- | Take a number of elements from the end of the list.
--
-- > takeEnd 3 "hello"  == "llo"
-- > takeEnd 5 "bye"    == "bye"
-- > takeEnd (-1) "bye" == ""
-- > \i xs -> takeEnd i xs `isSuffixOf` xs
-- > \i xs -> length (takeEnd i xs) == min (max 0 i) (length xs)
takeEnd :: Int -> [a] -> [a]
takeEnd i xs = f xs (drop i xs)
    where f (x:xs) (y:ys) = f xs ys
          f xs _ = xs

-- | Drop a number of elements from the end of the list.
--
-- > dropEnd 3 "hello"  == "he"
-- > dropEnd 5 "bye"    == ""
-- > dropEnd (-1) "bye" == "bye"
-- > \i xs -> dropEnd i xs `isPrefixOf` xs
-- > \i xs -> length (dropEnd i xs) == max 0 (length xs - max 0 i)
-- > \i -> take 3 (dropEnd 5 [i..]) == take 3 [i..]
dropEnd :: Int -> [a] -> [a]
dropEnd i xs = f xs (drop i xs)
    where f (x:xs) (y:ys) = x : f xs ys
          f _ _ = []


-- | @'splitAtEnd' n xs@ returns a split where the second element tries to
--   contain @n@ elements.
--
-- > splitAtEnd 3 "hello" == ("he","llo")
-- > splitAtEnd 3 "he"    == ("", "he")
-- > \i xs -> uncurry (++) (splitAt i xs) == xs
-- > \i xs -> splitAtEnd i xs == (dropEnd i xs, takeEnd i xs)
splitAtEnd :: Int -> [a] -> ([a], [a])
splitAtEnd i xs = f xs (drop i xs)
    where f (x:xs) (y:ys) = first (x:) $ f xs ys
          f xs _ = ([], xs)


-- | 'zip' against an enumeration.
--   Never truncates the output - raises an error if the enumeration runs out.
--
-- > \i xs -> zip [i..] xs == zipFrom i xs
-- > zipFrom False [1..3] == undefined
zipFrom :: (Partial, Enum a) => a -> [b] -> [(a, b)]
zipFrom = zipWithFrom (,)

-- | 'zipFrom' generalised to any combining operation.
--   Never truncates the output - raises an error if the enumeration runs out.
--
-- > \i xs -> zipWithFrom (,) i xs == zipFrom i xs
zipWithFrom :: (Partial, Enum a) => (a -> b -> c) -> a -> [b] -> [c]
zipWithFrom f a xs = go a xs
    where
        -- if we aren't strict in the accumulator, it's highly like to be a space leak
        go !a [] = []
        go !a (x:xs) = f a x : go (succ a) xs


-- | A merging of 'unzip' and 'concat'.
--
-- > concatUnzip [("a","AB"),("bc","C")] == ("abc","ABC")
concatUnzip :: [([a], [b])] -> ([a], [b])
concatUnzip = (concat *** concat) . unzip

-- | A merging of 'unzip3' and 'concat'.
--
-- > concatUnzip3 [("a","AB",""),("bc","C","123")] == ("abc","ABC","123")
concatUnzip3 :: [([a],[b],[c])] -> ([a],[b],[c])
concatUnzip3 xs = (concat a, concat b, concat c)
    where (a,b,c) = unzip3 xs


-- | A version of 'takeWhile' operating from the end.
--
-- > takeWhileEnd even [2,3,4,6] == [4,6]
takeWhileEnd :: (a -> Bool) -> [a] -> [a]
takeWhileEnd f = reverse . takeWhile f . reverse


-- | Remove spaces from the start of a string, see 'trim'.
trimStart :: String -> String
trimStart = dropWhile isSpace

-- | Remove spaces from the end of a string, see 'trim'.
trimEnd :: String -> String
trimEnd = dropWhileEnd isSpace

-- | Remove spaces from either side of a string. A combination of 'trimEnd' and 'trimStart'.
--
-- > trim      "  hello   " == "hello"
-- > trimStart "  hello   " == "hello   "
-- > trimEnd   "  hello   " == "  hello"
-- > \s -> trim s == trimEnd (trimStart s)
trim :: String -> String
trim = trimEnd . trimStart

-- | Convert a string to lower case.
--
-- > lower "This is A TEST" == "this is a test"
-- > lower "" == ""
lower :: String -> String
lower = map toLower

-- | Convert a string to upper case.
--
-- > upper "This is A TEST" == "THIS IS A TEST"
-- > upper "" == ""
upper :: String -> String
upper = map toUpper


-- | Split the first word off a string. Useful for when starting to parse the beginning
--   of a string, but you want to accurately preserve whitespace in the rest of the string.
--
-- > word1 "" == ("", "")
-- > word1 "keyword rest of string" == ("keyword","rest of string")
-- > word1 "  keyword\n  rest of string" == ("keyword","rest of string")
-- > \s -> fst (word1 s) == concat (take 1 $ words s)
-- > \s -> words (snd $ word1 s) == drop 1 (words s)
word1 :: String -> (String, String)
word1 = second (dropWhile isSpace) . break isSpace . dropWhile isSpace

-- | Split the first line off a string.
--
-- > line1 "" == ("", "")
-- > line1 "test" == ("test","")
-- > line1 "test\n" == ("test","")
-- > line1 "test\nrest" == ("test","rest")
-- > line1 "test\nrest\nmore" == ("test","rest\nmore")
line1 :: String -> (String, String)
line1 = second drop1 . break (== '\n')

-- | Escape a string such that it can be inserted into an HTML document or @\"@ attribute
--   without any special interpretation. This requires escaping the @<@, @>@, @&@ and @\"@ characters.
--   Note that it will escape @\"@ and @\'@ even though that is not required in an HTML body (but is not harmful).
--
-- > escapeHTML "this is a test" == "this is a test"
-- > escapeHTML "<b>\"g&t\"</n>" == "&lt;b&gt;&quot;g&amp;t&quot;&lt;/n&gt;"
-- > escapeHTML "t'was another test" == "t&#39;was another test"
escapeHTML :: String -> String
escapeHTML = concatMap f
    where
        f '>' = "&gt;"
        f '<' = "&lt;"
        f '&' = "&amp;"
        f '\"' = "&quot;"
        f '\'' = "&#39;"
        f x = [x]

-- | Invert of 'escapeHTML' (does not do general HTML unescaping)
--
-- > \xs -> unescapeHTML (escapeHTML xs) == xs
unescapeHTML :: String -> String
unescapeHTML ('&':xs)
    | Just xs <- stripPrefix "lt;" xs = '<' : unescapeHTML xs
    | Just xs <- stripPrefix "gt;" xs = '>' : unescapeHTML xs
    | Just xs <- stripPrefix "amp;" xs = '&' : unescapeHTML xs
    | Just xs <- stripPrefix "quot;" xs = '\"' : unescapeHTML xs
    | Just xs <- stripPrefix "#39;" xs = '\'' : unescapeHTML xs
unescapeHTML (x:xs) = x : unescapeHTML xs
unescapeHTML [] = []


-- | Escape a string so it can form part of a JSON literal.
--   This requires escaping the special whitespace and control characters. Additionally,
--   Note that it does /not/ add quote characters around the string.
--
-- > escapeJSON "this is a test" == "this is a test"
-- > escapeJSON "\ttab\nnewline\\" == "\\ttab\\nnewline\\\\"
-- > escapeJSON "\ESC[0mHello" == "\\u001b[0mHello"
escapeJSON :: String -> String
escapeJSON x = concatMap f x
    where f '\"' = "\\\""
          f '\\' = "\\\\"
          -- the spaces are technically optional, but we include them so the JSON is readable
          f '\b' = "\\b"
          f '\f' = "\\f"
          f '\n' = "\\n"
          f '\r' = "\\r"
          f '\t' = "\\t"
          f x | isControl x = "\\u" ++ takeEnd 4 ("0000" ++ showHex (ord x) "")
          f x = [x]

-- | General JSON unescaping, inversion of 'escapeJSON' and all other JSON escapes.
--
-- > \xs -> unescapeJSON (escapeJSON xs) == xs
unescapeJSON :: String -> String
unescapeJSON ('\\':x:xs)
    | x == '\"' = '\"' : unescapeJSON xs
    | x == '\\' = '\\' : unescapeJSON xs
    | x == '/' = '/' : unescapeJSON xs
    | x == 'b' = '\b' : unescapeJSON xs
    | x == 'f' = '\f' : unescapeJSON xs
    | x == 'n' = '\n' : unescapeJSON xs
    | x == 'r' = '\r' : unescapeJSON xs
    | x == 't' = '\t' : unescapeJSON xs
    | x == 'u', let (a,b) = splitAt 4 xs, length a == 4, [(i, "")] <- readHex a = chr i : unescapeJSON b
unescapeJSON (x:xs) = x : unescapeJSON xs
unescapeJSON [] = []


-- | A version of 'group' where the equality is done on some extracted value.
groupOn :: Eq b => (a -> b) -> [a] -> [[a]]
groupOn f = groupBy ((==) `on2` f)
    -- redefine on so we avoid duplicate computation for most values.
    where (.*.) `on2` f = \x -> let fx = f x in \y -> fx .*. f y


-- | A version of 'nub' where the equality is done on some extracted value.
--   @nubOn f@ is equivalent to @nubBy ((==) `on` f)@, but has the
--   performance advantage of only evaluating @f@ once for each element in the
--   input list.
nubOn :: Eq b => (a -> b) -> [a] -> [a]
nubOn f = map snd . nubBy ((==) `on` fst) . map (\x -> let y = f x in y `seq` (y, x))

-- | A version of 'maximum' where the comparison is done on some extracted value.
maximumOn :: Ord b => (a -> b) -> [a] -> a
maximumOn f = maximumBy (compare `on` f)

-- | A version of 'minimum' where the comparison is done on some extracted value.
minimumOn :: Ord b => (a -> b) -> [a] -> a
minimumOn f = minimumBy (compare `on` f)

-- | A combination of 'group' and 'sort'.
--
-- > groupSort [(1,'t'),(3,'t'),(2,'e'),(2,'s')] == [(1,"t"),(2,"es"),(3,"t")]
-- > \xs -> map fst (groupSort xs) == sort (nub (map fst xs))
-- > \xs -> concatMap snd (groupSort xs) == map snd (sortOn fst xs)
groupSort :: Ord k => [(k, v)] -> [(k, [v])]
groupSort = map (\x -> (fst $ head x, map snd x)) . groupOn fst . sortOn fst

-- | A combination of 'group' and 'sort', using a part of the value to compare on.
--
-- > groupSortOn length ["test","of","sized","item"] == [["of"],["test","item"],["sized"]]
groupSortOn :: Ord b => (a -> b) -> [a] -> [[a]]
groupSortOn f = map (map snd) . groupBy ((==) `on` fst) . sortBy (compare `on` fst) . map (f &&& id)

-- | A combination of 'group' and 'sort', using a predicate to compare on.
--
-- > groupSortBy (compare `on` length) ["test","of","sized","item"] == [["of"],["test","item"],["sized"]]
groupSortBy :: (a -> a -> Ordering) -> [a] -> [[a]]
groupSortBy f = groupBy (\a b -> f a b == EQ) . sortBy f


-- | Merge two lists which are assumed to be ordered.
--
-- > merge "ace" "bd" == "abcde"
-- > \xs ys -> merge (sort xs) (sort ys) == sort (xs ++ ys)
merge :: Ord a => [a] -> [a] -> [a]
merge = mergeBy compare


-- | Like 'merge', but with a custom ordering function.
mergeBy :: (a -> a -> Ordering) -> [a] -> [a] -> [a]
mergeBy f xs [] = xs
mergeBy f [] ys = ys
mergeBy f (x:xs) (y:ys)
    | f x y /= GT = x : mergeBy f xs (y:ys)
    | otherwise = y : mergeBy f (x:xs) ys


-- | Replace a subsequence everywhere it occurs. The first argument must
--   not be the empty list.
--
-- > replace "el" "_" "Hello Bella" == "H_lo B_la"
-- > replace "el" "e" "Hello"       == "Helo"
-- > replace "" "e" "Hello"         == undefined
-- > \xs ys -> not (null xs) ==> replace xs xs ys == ys
replace :: (Partial, Eq a) => [a] -> [a] -> [a] -> [a]
replace [] _ _ = error "Extra.replace, first argument cannot be empty"
replace from to xs | Just xs <- stripPrefix from xs = to ++ replace from to xs
replace from to (x:xs) = x : replace from to xs
replace from to [] = []


-- | Break, but from the end.
--
-- > breakEnd isLower "youRE" == ("you","RE")
-- > breakEnd isLower "youre" == ("youre","")
-- > breakEnd isLower "YOURE" == ("","YOURE")
-- > \f xs -> breakEnd (not . f) xs == spanEnd f  xs
breakEnd :: (a -> Bool) -> [a] -> ([a], [a])
breakEnd f = swap . both reverse . break f . reverse

-- | Span, but from the end.
--
-- > spanEnd isUpper "youRE" == ("you","RE")
-- > spanEnd (not . isSpace) "x y z" == ("x y ","z")
-- > \f xs -> uncurry (++) (spanEnd f xs) == xs
-- > \f xs -> spanEnd f xs == swap (both reverse (span f (reverse xs)))
spanEnd :: (a -> Bool) -> [a] -> ([a], [a])
spanEnd f = breakEnd (not . f)


-- | A variant of 'words' with a custom test. In particular,
--   adjacent separators are discarded, as are leading or trailing separators.
--
-- > wordsBy (== ':') "::xyz:abc::123::" == ["xyz","abc","123"]
-- > \s -> wordsBy isSpace s == words s
wordsBy :: (a -> Bool) -> [a] -> [[a]]
wordsBy f s = case dropWhile f s of
    [] -> []
    x:xs -> (x:w) : wordsBy f (drop1 z)
        where (w,z) = break f xs

-- | A variant of 'lines' with a custom test. In particular,
--   if there is a trailing separator it will be discarded.
--
-- > linesBy (== ':') "::xyz:abc::123::" == ["","","xyz","abc","","123",""]
-- > \s -> linesBy (== '\n') s == lines s
-- > linesBy (== ';') "my;list;here;" == ["my","list","here"]
linesBy :: (a -> Bool) -> [a] -> [[a]]
linesBy f [] = []
linesBy f s = cons $ case break f s of
    (l, s) -> (l,) $ case s of
        [] -> []
        _:s -> linesBy f s
  where
    cons ~(h, t) = h : t -- to fix a space leak, see the GHC defn of lines

-- | Find the first element of a list for which the operation returns 'Just', along
--   with the result of the operation. Like 'find' but useful where the function also
--   computes some expensive information that can be reused. Particular useful
--   when the function is monadic, see 'firstJustM'.
--
-- > firstJust id [Nothing,Just 3]  == Just 3
-- > firstJust id [Nothing,Nothing] == Nothing
firstJust :: (a -> Maybe b) -> [a] -> Maybe b
firstJust f = listToMaybe . mapMaybe f


-- | Equivalent to @drop 1@, but likely to be faster and a single lexeme.
--
-- > drop1 ""         == ""
-- > drop1 "test"     == "est"
-- > \xs -> drop 1 xs == drop1 xs
drop1 :: [a] -> [a]
drop1 [] = []
drop1 (x:xs) = xs


-- | Version on `concatMap` generalised to a `Monoid` rather than just a list.
--
-- > mconcatMap Sum [1,2,3] == Sum 6
-- > \f xs -> mconcatMap f xs == concatMap f xs
mconcatMap :: Monoid b => (a -> b) -> [a] -> b
mconcatMap f = mconcat . map f


-- | Find the first instance of @needle@ in @haystack@.
-- The first element of the returned tuple
-- is the prefix of @haystack@ before @needle@ is matched.  The second
-- is the remainder of @haystack@, starting with the match.
-- If you want the remainder /without/ the match, use 'stripInfix'.
--
-- > breakOn "::" "a::b::c" == ("a", "::b::c")
-- > breakOn "/" "foobar"   == ("foobar", "")
-- > \needle haystack -> let (prefix,match) = breakOn needle haystack in prefix ++ match == haystack
breakOn :: Eq a => [a] -> [a] -> ([a], [a])
breakOn needle haystack | needle `isPrefixOf` haystack = ([], haystack)
breakOn needle [] = ([], [])
breakOn needle (x:xs) = first (x:) $ breakOn needle xs

-- | Similar to 'breakOn', but searches from the end of the
-- string.
--
-- The first element of the returned tuple is the prefix of @haystack@
-- up to and including the last match of @needle@.  The second is the
-- remainder of @haystack@, following the match.
--
-- > breakOnEnd "::" "a::b::c" == ("a::b::", "c")
breakOnEnd :: Eq a => [a] -> [a] -> ([a], [a])
breakOnEnd needle haystack = both reverse $ swap $ breakOn (reverse needle) (reverse haystack)


-- | Break a list into pieces separated by the first
-- list argument, consuming the delimiter. An empty delimiter is
-- invalid, and will cause an error to be raised.
--
-- > splitOn "\r\n" "a\r\nb\r\nd\r\ne" == ["a","b","d","e"]
-- > splitOn "aaa"  "aaaXaaaXaaaXaaa"  == ["","X","X","X",""]
-- > splitOn "x"    "x"                == ["",""]
-- > splitOn "x"    ""                 == [""]
-- > \s x -> s /= "" ==> intercalate s (splitOn s x) == x
-- > \c x -> splitOn [c] x                           == split (==c) x
splitOn :: (Partial, Eq a) => [a] -> [a] -> [[a]]
splitOn [] _ = error "splitOn, needle may not be empty"
splitOn _ [] = [[]]
splitOn needle haystack = a : if null b then [] else splitOn needle $ drop (length needle) b
    where (a,b) = breakOn needle haystack


-- | Splits a list into components delimited by separators,
-- where the predicate returns True for a separator element.  The
-- resulting components do not contain the separators.  Two adjacent
-- separators result in an empty component in the output.
--
-- > split (== 'a') "aabbaca" == ["","","bb","c",""]
-- > split (== 'a') ""        == [""]
-- > split (== ':') "::xyz:abc::123::" == ["","","xyz","abc","","123","",""]
-- > split (== ',') "my,list,here" == ["my","list","here"]
split :: (a -> Bool) -> [a] -> [[a]]
split f [] = [[]]
split f (x:xs) | f x = [] : split f xs
split f (x:xs) | y:ys <- split f xs = (x:y) : ys


-- | A version of 'dropWhileEnd' but with different strictness properties.
--   The function 'dropWhileEnd' can be used on an infinite list and tests the property
--   on each character. In contrast, 'dropWhileEnd'' is strict in the spine of the list
--   but only tests the trailing suffix.
--   This version usually outperforms 'dropWhileEnd' if the list is short or the test is expensive.
--   Note the tests below cover both the prime and non-prime variants.
--
-- > dropWhileEnd  isSpace "ab cde  " == "ab cde"
-- > dropWhileEnd' isSpace "ab cde  " == "ab cde"
-- > last (dropWhileEnd  even [undefined,3]) == undefined
-- > last (dropWhileEnd' even [undefined,3]) == 3
-- > head (dropWhileEnd  even (3:undefined)) == 3
-- > head (dropWhileEnd' even (3:undefined)) == undefined
dropWhileEnd' :: (a -> Bool) -> [a] -> [a]
dropWhileEnd' p = foldr (\x xs -> if null xs && p x then [] else x : xs) []


-- | Drops the given prefix from a list.
--   It returns the original sequence if the sequence doesn't start with the given prefix.
--
-- > dropPrefix "Mr. " "Mr. Men" == "Men"
-- > dropPrefix "Mr. " "Dr. Men" == "Dr. Men"
dropPrefix :: Eq a => [a] -> [a] -> [a]
dropPrefix a b = fromMaybe b $ stripPrefix a b


-- | Drops the given suffix from a list.
--   It returns the original sequence if the sequence doesn't end with the given suffix.
--
-- > dropSuffix "!" "Hello World!"  == "Hello World"
-- > dropSuffix "!" "Hello World!!" == "Hello World!"
-- > dropSuffix "!" "Hello World."  == "Hello World."
dropSuffix :: Eq a => [a] -> [a] -> [a]
dropSuffix a b = fromMaybe b $ stripSuffix a b

-- | Return the prefix of the second list if its suffix
--   matches the entire first list.
--
-- Examples:
--
-- > stripSuffix "bar" "foobar" == Just "foo"
-- > stripSuffix ""    "baz"    == Just "baz"
-- > stripSuffix "foo" "quux"   == Nothing
stripSuffix :: Eq a => [a] -> [a] -> Maybe [a]
stripSuffix a b = reverse <$> stripPrefix (reverse a) (reverse b)


-- | Return the the string before and after the search string,
--   or 'Nothing' if the search string is not present.
--
-- Examples:
--
-- > stripInfix "::" "a::b::c" == Just ("a", "b::c")
-- > stripInfix "/" "foobar"   == Nothing
stripInfix :: Eq a => [a] -> [a] -> Maybe ([a], [a])
stripInfix needle haystack | Just rest <- stripPrefix needle haystack = Just ([], rest)
stripInfix needle [] = Nothing
stripInfix needle (x:xs) = first (x:) <$> stripInfix needle xs


-- | Similar to 'stripInfix', but searches from the end of the
-- string.
--
-- > stripInfixEnd "::" "a::b::c" == Just ("a::b", "c")
stripInfixEnd :: Eq a => [a] -> [a] -> Maybe ([a], [a])
stripInfixEnd needle haystack = both reverse . swap <$> stripInfix (reverse needle) (reverse haystack)


-- | Split a list into chunks of a given size. The last chunk may contain
--   fewer than n elements. The chunk size must be positive.
--
-- > chunksOf 3 "my test" == ["my ","tes","t"]
-- > chunksOf 3 "mytest"  == ["myt","est"]
-- > chunksOf 8 ""        == []
-- > chunksOf 0 "test"    == undefined
chunksOf :: Partial => Int -> [a] -> [[a]]
chunksOf i xs | i <= 0 = error $ "chunksOf, number must be positive, got " ++ show i
chunksOf i xs = repeatedly (splitAt i) xs


-- | /O(n log n)/. The 'nubSort' function sorts and removes duplicate elements from a list.
-- In particular, it keeps only the first occurrence of each element.
--
-- > nubSort "this is a test" == " aehist"
-- > \xs -> nubSort xs == nub (sort xs)
nubSort :: Ord a => [a] -> [a]
nubSort = nubSortBy compare

-- | A version of 'nubSort' which operates on a portion of the value.
--
-- > nubSortOn length ["a","test","of","this"] == ["a","of","test"]
nubSortOn :: Ord b => (a -> b) -> [a] -> [a]
nubSortOn f = nubSortBy (compare `on` f)

-- | A version of 'nubSort' with a custom predicate.
--
-- > nubSortBy (compare `on` length) ["a","test","of","this"] == ["a","of","test"]
nubSortBy :: (a -> a -> Ordering) -> [a] -> [a]
nubSortBy cmp = f . sortBy cmp
    where f (x1:x2:xs) | cmp x1 x2 == EQ = f (x1:xs)
          f (x:xs) = x : f xs
          f [] = []

-- | /O(n log n)/. The 'nubOrd' function removes duplicate elements from a list.
-- In particular, it keeps only the first occurrence of each element.
-- Unlike the standard 'nub' operator, this version requires an 'Ord' instance
-- and consequently runs asymptotically faster.
--
-- > nubOrd "this is a test" == "this ae"
-- > nubOrd (take 4 ("this" ++ undefined)) == "this"
-- > \xs -> nubOrd xs == nub xs
nubOrd :: Ord a => [a] -> [a]
nubOrd = nubOrdBy compare

-- | A version of 'nubOrd' which operates on a portion of the value.
--
-- > nubOrdOn length ["a","test","of","this"] == ["a","test","of"]
nubOrdOn :: Ord b => (a -> b) -> [a] -> [a]
nubOrdOn f = map snd . nubOrdBy (compare `on` fst) . map (f &&& id)

-- | A version of 'nubOrd' with a custom predicate.
--
-- > nubOrdBy (compare `on` length) ["a","test","of","this"] == ["a","test","of"]
nubOrdBy :: (a -> a -> Ordering) -> [a] -> [a]
nubOrdBy cmp xs = f E xs
    where f seen [] = []
          f seen (x:xs) | memberRB cmp x seen = f seen xs
                        | otherwise = x : f (insertRB cmp x seen) xs

---------------------------------------------------------------------
-- OKASAKI RED BLACK TREE
-- Taken from https://www.cs.kent.ac.uk/people/staff/smk/redblack/Untyped.hs

data Color = R | B deriving Show
data RB a = E | T Color (RB a) a (RB a) deriving Show

{- Insertion and membership test as by Okasaki -}
insertRB :: (a -> a -> Ordering) -> a -> RB a -> RB a
insertRB cmp x s =
    T B a z b
    where
    T _ a z b = ins s
    ins E = T R E x E
    ins s@(T B a y b) = case cmp x y of
        LT -> balance (ins a) y b
        GT -> balance a y (ins b)
        EQ -> s
    ins s@(T R a y b) = case cmp x y of
        LT -> T R (ins a) y b
        GT -> T R a y (ins b)
        EQ -> s

memberRB :: (a -> a -> Ordering) -> a -> RB a -> Bool
memberRB cmp x E = False
memberRB cmp x (T _ a y b) = case cmp x y of
    LT -> memberRB cmp x a
    GT -> memberRB cmp x b
    EQ -> True

{- balance: first equation is new,
   to make it work with a weaker invariant -}
balance :: RB a -> a -> RB a -> RB a
balance (T R a x b) y (T R c z d) = T R (T B a x b) y (T B c z d)
balance (T R (T R a x b) y c) z d = T R (T B a x b) y (T B c z d)
balance (T R a x (T R b y c)) z d = T R (T B a x b) y (T B c z d)
balance a x (T R b y (T R c z d)) = T R (T B a x b) y (T B c z d)
balance a x (T R (T R b y c) z d) = T R (T B a x b) y (T B c z d)
balance a x b = T B a x b