text / Data / Text.hs

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{-# LANGUAGE BangPatterns #-}
{-# OPTIONS_GHC -fno-warn-orphans #-}

-- |
-- Module      : Data.Text
-- Copyright   : (c) Tom Harper 2008-2009,
--               (c) Bryan O'Sullivan 2009,
--               (c) Duncan Coutts 2009
--
-- License     : BSD-style
-- Maintainer  : bos@serpentine.com, rtharper@aftereternity.co.uk,
--               duncan@haskell.org
-- Stability   : experimental
-- Portability : GHC
--
-- A time and space-efficient implementation of Unicode text using
-- packed Word16 arrays.  Suitable for performance critical use, both
-- in terms of large data quantities and high speed.
--
-- This module is intended to be imported @qualified@, to avoid name
-- clashes with "Prelude" functions, e.g.
--
-- > import qualified Data.Text as T

module Data.Text
    (
    -- * Fusion
    -- $fusion

    -- * Types
      Text

    -- * Creation and elimination
    , pack
    , unpack
    , singleton
    , empty

    -- * Basic interface
    , cons
    , snoc
    , append
    , uncons
    , head
    , last
    , tail
    , init
    , null
    , length

    -- * Transformations
    , map
    , intercalate
    , intersperse
    , transpose
    , reverse

    -- * Folds
    , foldl
    , foldl'
    , foldl1
    , foldl1'
    , foldr
    , foldr1

    -- ** Special folds
    , concat
    , concatMap
    , any
    , all
    , maximum
    , minimum

    -- * Construction

    -- ** Scans
    , scanl
    , scanl1
    , scanr
    , scanr1

    -- ** Accumulating maps
    , mapAccumL
    , mapAccumR

    -- ** Generation and unfolding
    , replicate
    , unfoldr
    , unfoldrN

    -- * Substrings

    -- ** Breaking strings
    , take
    , drop
    , takeWhile
    , dropWhile
    , splitAt
    , span
    , break
    , group
    , groupBy
    , inits
    , tails

    -- ** Breaking into many substrings
    , split
    , splitWith
    , breakSubstring

    -- ** Breaking into lines and words
    , lines
    --, lines'
    , words
    , unlines
    , unwords

    -- * Predicates
    , isPrefixOf
    , isSuffixOf
    , isInfixOf

    -- * Searching
    , elem
    , filter
    , find
    , partition

    -- , findSubstring
    
    -- * Indexing
    , index
    , findIndex
    , findIndices
    , elemIndex
    , elemIndices
    , count

    -- * Zipping and unzipping
    , zip
    , zipWith

    -- -* Ordered text
    -- , sort
    ) where

import Prelude (Char, Bool(..), Functor(..), Int, Maybe(..), String,
                Eq(..), Ord(..), (++),
                Read(..), Show(..),
                (&&), (||), (+), (-), (.), ($),
                not, return, otherwise)
import Control.Exception (assert)
import Data.Char (isSpace)
import Control.Monad.ST (ST)
import qualified Data.Text.Array as A
import qualified Data.List as L
import Data.Monoid (Monoid(..))
import Data.Word (Word16)
import Data.String (IsString(..))

import qualified Data.Text.Fusion as S
import qualified Data.Text.Fusion.Common as S
import Data.Text.Fusion (stream, reverseStream, unstream)

import Data.Text.Internal (Text(..), empty, text, textP)
import qualified Prelude as P
import Data.Text.Unsafe (iter, iter_, unsafeHead, unsafeTail)
import Data.Text.UnsafeChar (unsafeChr)
import qualified Data.Text.Encoding.Utf16 as U16

-- $fusion
--
-- Most of the functions in this module are subject to /array fusion/,
-- meaning that a pipeline of functions will usually allocate at most
-- one 'Text' value.

instance Eq Text where
    t1 == t2 = stream t1 == stream t2
    {-# INLINE (==) #-}

instance Ord Text where
    compare t1 t2 = compare (stream t1) (stream t2)
    {-# INLINE compare #-}

instance Show Text where
    showsPrec p ps r = showsPrec p (unpack ps) r

instance Read Text where
    readsPrec p str = [(pack x,y) | (x,y) <- readsPrec p str]

instance Monoid Text where
    mempty  = empty
    mappend = append
    mconcat = concat

instance IsString Text where
    fromString = pack

-- -----------------------------------------------------------------------------
-- * Conversion to/from 'Text'

-- | /O(n)/ Convert a 'String' into a 'Text'.
--
-- This function is subject to array fusion.
pack :: String -> Text
pack = unstream . S.streamList
{-# INLINE [1] pack #-}

-- | /O(n)/ Convert a Text into a String.
-- Subject to array fusion.
unpack :: Text -> String
unpack = S.unstreamList . stream
{-# INLINE [1] unpack #-}

-- | /O(1)/ Convert a character into a Text.
-- Subject to array fusion.
singleton :: Char -> Text
singleton = unstream . S.singleton
{-# INLINE [1] singleton #-}

-- -----------------------------------------------------------------------------
-- * Basic functions

-- | /O(n)/ Adds a character to the front of a 'Text'.  This function
-- is more costly than its 'List' counterpart because it requires
-- copying a new array.  Subject to array fusion.
cons :: Char -> Text -> Text
cons c t = unstream (S.cons c (stream t))
{-# INLINE cons #-}

-- | /O(n)/ Adds a character to the end of a 'Text'.  This copies the
-- entire array in the process.  Subject to array fusion.
snoc :: Text -> Char -> Text
snoc t c = unstream (S.snoc (stream t) c)
{-# INLINE snoc #-}

-- | /O(n)/ Appends one 'Text' to the other by copying both of them
-- into a new 'Text'.  Subject to array fusion.
append :: Text -> Text -> Text
append (Text arr1 off1 len1) (Text arr2 off2 len2) = Text (A.run x) 0 len
    where
      len = len1+len2
      x = do
        arr <- A.unsafeNew len :: ST s (A.MArray s Word16)
        copy arr1 off1 (len1+off1) arr 0
        copy arr2 off2 (len2+off2) arr len1
        return arr
            where
              copy arr i top arr' j
                  | i >= top  = return ()
                  | otherwise = do A.unsafeWrite arr' j (arr `A.unsafeIndex` i)
                                   copy arr (i+1) top arr' (j+1)
{-# INLINE append #-}

{-# RULES
"TEXT append -> fused" [~1] forall t1 t2.
    append t1 t2 = unstream (S.append (stream t1) (stream t2))
"TEXT append -> unfused" [1] forall t1 t2.
    unstream (S.append (stream t1) (stream t2)) = append t1 t2
 #-}

-- | /O(1)/ Returns the first character of a 'Text', which must be
-- non-empty.  Subject to array fusion.
head :: Text -> Char
head t = S.head (stream t)
{-# INLINE head #-}

-- | /O(1)/ Returns the first character and rest of a 'Text', or
-- 'Nothing' if empty. Subject to array fusion.
uncons :: Text -> Maybe (Char, Text)
uncons t@(Text arr off len)
    | len <= 0  = Nothing
    | otherwise = Just (c, textP arr (off+d) (len-d))
    where (c,d) = iter t 0
{-# INLINE uncons #-}

-- | Lifted from Control.Arrow and specialized.
second :: (b -> c) -> (a,b) -> (a,c)
second f (a, b) = (a, f b)

{-# RULES
"TEXT uncons -> fused" [~1] forall t.
    uncons t = fmap (second unstream) (S.uncons (stream t))
"TEXT uncons -> unfused" [1] forall t.
    fmap (second unstream) (S.uncons (stream t)) = uncons t
  #-}

-- | /O(1)/ Returns the last character of a 'Text', which must be
-- non-empty.  Subject to array fusion.
last :: Text -> Char
last (Text arr off len)
    | len <= 0                 = emptyError "last"
    | n < 0xDC00 || n > 0xDFFF = unsafeChr n
    | otherwise                = U16.chr2 n0 n
    where n  = A.unsafeIndex arr (off+len-1)
          n0 = A.unsafeIndex arr (off+len-2)
{-# INLINE [1] last #-}

{-# RULES
"TEXT last -> fused" [~1] forall t.
    last t = S.last (stream t)
"TEXT last -> unfused" [1] forall t.
    S.last (stream t) = last t
  #-}

-- | /O(1)/ Returns all characters after the head of a 'Text', which
-- must be non-empty.  Subject to array fusion.
tail :: Text -> Text
tail t@(Text arr off len)
    | len <= 0  = emptyError "tail"
    | otherwise = textP arr (off+d) (len-d)
    where d = iter_ t 0
{-# INLINE [1] tail #-}

{-# RULES
"TEXT tail -> fused" [~1] forall t.
    tail t = unstream (S.tail (stream t))
"TEXT tail -> unfused" [1] forall t.
    unstream (S.tail (stream t)) = tail t
 #-}

-- | /O(1)/ Returns all but the last character of a 'Text', which must
-- be non-empty.  Subject to array fusion.
init :: Text -> Text
init (Text arr off len) | len <= 0                   = emptyError "init"
                        | n >= 0xDC00 && n <= 0xDFFF = textP arr off (len-2)
                        | otherwise                  = textP arr off (len-1)
    where
      n = A.unsafeIndex arr (off+len-1)
{-# INLINE [1] init #-}

{-# RULES
"TEXT init -> fused" [~1] forall t.
    init t = unstream (S.init (stream t))
"TEXT init -> unfused" [1] forall t.
    unstream (S.init (stream t)) = init t
 #-}

-- | /O(1)/ Tests whether a 'Text' is empty or not.  Subject to array
-- fusion.
null :: Text -> Bool
null (Text _arr _off len) = assert (len >= 0) $ len <= 0
{-# INLINE [1] null #-}

{-# RULES
"TEXT null -> fused" [~1] forall t.
    null t = S.null (stream t)
"TEXT null -> unfused" [1] forall t.
    S.null (stream t) = null t
 #-}

-- | /O(n)/ Returns the number of characters in a 'Text'.
-- Subject to array fusion.
length :: Text -> Int
length t = S.length (stream t)
{-# INLINE length #-}

-- -----------------------------------------------------------------------------
-- * Transformations
-- | /O(n)/ 'map' @f @xs is the 'Text' obtained by applying @f@ to
-- each element of @xs@.  Subject to array fusion.
map :: (Char -> Char) -> Text -> Text
map f t = unstream (S.map f (stream t))
{-# INLINE [1] map #-}

-- | /O(n)/ The 'intercalate' function takes a 'Text' and a list of
-- 'Text's and concatenates the list after interspersing the first
-- argument between each element of the list.
intercalate :: Text -> [Text] -> Text
intercalate t ts = unstream (S.intercalate (stream t) (L.map stream ts))
{-# INLINE intercalate #-}

-- | /O(n)/ The 'intersperse' function takes a character and places it
-- between the characters of a 'Text'.  Subject to array fusion.
intersperse     :: Char -> Text -> Text
intersperse c t = unstream (S.intersperse c (stream t))
{-# INLINE intersperse #-}

-- | /O(n)/ Reverse the characters of a string. Subject to array fusion.
reverse :: Text -> Text
reverse t = S.reverse (stream t)
{-# INLINE reverse #-}

-- | /O(n)/ The 'transpose' function transposes the rows and columns
-- of its 'Text' argument.  Note that this function uses 'pack',
-- 'unpack', and the list version of transpose, and is thus not very
-- efficient.
transpose :: [Text] -> [Text]
transpose ts = P.map pack (L.transpose (P.map unpack ts))

-- -----------------------------------------------------------------------------
-- * Reducing 'Text's (folds)

-- | /O(n)/ 'foldl', applied to a binary operator, a starting value
-- (typically the left-identity of the operator), and a 'Text',
-- reduces the 'Text' using the binary operator, from left to right.
-- Subject to array fusion.
foldl :: (b -> Char -> b) -> b -> Text -> b
foldl f z t = S.foldl f z (stream t)
{-# INLINE foldl #-}

-- | /O(n)/ A strict version of 'foldl'.
-- Subject to array fusion.
foldl' :: (b -> Char -> b) -> b -> Text -> b
foldl' f z t = S.foldl' f z (stream t)
{-# INLINE foldl' #-}

-- | /O(n)/ A variant of 'foldl' that has no starting value argument,
-- and thus must be applied to a non-empty 'Text'.  Subject to array
-- fusion.
foldl1 :: (Char -> Char -> Char) -> Text -> Char
foldl1 f t = S.foldl1 f (stream t)
{-# INLINE foldl1 #-}

-- | /O(n)/ A strict version of 'foldl1'.
-- Subject to array fusion.
foldl1' :: (Char -> Char -> Char) -> Text -> Char
foldl1' f t = S.foldl1' f (stream t)
{-# INLINE foldl1' #-}

-- | /O(n)/ 'foldr', applied to a binary operator, a starting value
-- (typically the right-identity of the operator), and a 'Text',
-- reduces the 'Text' using the binary operator, from right to left.
-- Subject to array fusion.
foldr :: (Char -> b -> b) -> b -> Text -> b
foldr f z t = S.foldr f z (stream t)
{-# INLINE foldr #-}

-- | /O(n)/ A variant of 'foldr' that has no starting value argument, and
-- thust must be applied to a non-empty 'Text'.  Subject to array
-- fusion.
foldr1 :: (Char -> Char -> Char) -> Text -> Char
foldr1 f t = S.foldr1 f (stream t)
{-# INLINE foldr1 #-}

-- -----------------------------------------------------------------------------
-- ** Special folds

-- | /O(n)/ Concatenate a list of 'Text's. Subject to array fusion.
concat :: [Text] -> Text
concat ts = unstream (S.concat (L.map stream ts))
{-# INLINE concat #-}

-- | /O(n)/ Map a function over a 'Text' that results in a 'Text', and
-- concatenate the results.  This function is subject to array fusion.
--
-- Note: if in 'concatMap' @f@ @t@, @f@ is defined in terms of fusible
-- functions, it will also be fusible.
concatMap :: (Char -> Text) -> Text -> Text
concatMap f t = unstream (S.concatMap (stream . f) (stream t))
{-# INLINE concatMap #-}

-- | /O(n)/ 'any' @p@ @t@ determines whether any character in the
-- 'Text' @t@ satisifes the predicate @p@. Subject to array fusion.
any :: (Char -> Bool) -> Text -> Bool
any p t = S.any p (stream t)
{-# INLINE any #-}

-- | /O(n)/ 'all' @p@ @t@ determines whether all characters in the
-- 'Text' @t@ satisify the predicate @p@. Subject to array fusion.
all :: (Char -> Bool) -> Text -> Bool
all p t = S.all p (stream t)
{-# INLINE all #-}

-- | /O(n)/ 'maximum' returns the maximum value from a 'Text', which
-- must be non-empty. Subject to array fusion.
maximum :: Text -> Char
maximum t = S.maximum (stream t)
{-# INLINE maximum #-}

-- | /O(n)/ 'minimum' returns the minimum value from a 'Text', which
-- must be non-empty. Subject to array fusion.
minimum :: Text -> Char
minimum t = S.minimum (stream t)
{-# INLINE minimum #-}

-- -----------------------------------------------------------------------------
-- * Building 'Text's

-- | /O(n)/ 'scanl' is similar to 'foldl', but returns a list of
-- successive reduced values from the left. This function is subject
-- to array fusion.
--
-- > scanl f z [x1, x2, ...] == [z, z `f` x1, (z `f` x1) `f` x2, ...]
--
-- Note that
--
-- > last (scanl f z xs) == foldl f z xs.
scanl :: (Char -> Char -> Char) -> Char -> Text -> Text
scanl f z t = unstream (S.scanl f z (stream t))
{-# INLINE scanl #-}

-- | /O(n)/ 'scanl1' is a variant of 'scanl' that has no starting
-- value argument.  This function is subject to array fusion.
--
-- > scanl1 f [x1, x2, ...] == [x1, x1 `f` x2, ...]
scanl1 :: (Char -> Char -> Char) -> Text -> Text
scanl1 f t | null t    = empty
           | otherwise = scanl f (unsafeHead t) (unsafeTail t)
{-# INLINE scanl1 #-}

-- | /O(n)/ 'scanr' is the right-to-left dual of 'scanl'.
--
-- > scanr f v == reverse . scanl (flip f) v . reverse
scanr :: (Char -> Char -> Char) -> Char -> Text -> Text
scanr f z = S.reverse . S.reverseScanr f z . reverseStream
{-# INLINE scanr #-}

-- | /O(n)/ 'scanr1' is a variant of 'scanr' that has no starting
-- value argument.  This function is subject to array fusion.
scanr1 :: (Char -> Char -> Char) -> Text -> Text
scanr1 f t | null t    = empty
           | otherwise = scanr f (last t) (init t)
{-# INLINE scanr1 #-}

-- | /O(n)/ Like a combination of 'map' and 'foldl'. Applies a
-- function to each element of a 'Text', passing an accumulating
-- parameter from left to right, and returns a final 'Text'.
mapAccumL :: (a -> Char -> (a,Char)) -> a -> Text -> (a, Text)
mapAccumL f s t = case uncons t of
                    Nothing -> (s, empty)
                    Just (x, xs) -> (s'', cons y ys)
                        where (s', y ) = f s x
                              (s'',ys) = mapAccumL f s' xs

-- | The 'mapAccumR' function behaves like a combination of 'map' and
-- 'foldr'; it applies a function to each element of a 'Text', passing
-- an accumulating parameter from right to left, and returning a final
-- value of this accumulator together with the new 'Text'.
mapAccumR :: (a -> Char -> (a,Char)) -> a -> Text -> (a, Text)
mapAccumR f s t = case uncons t of
                    Nothing -> (s, empty)
                    Just (x, xs) ->  (s'', cons y ys)
                        where (s'',y ) = f s' x
                              (s', ys) = mapAccumR f s xs

-- -----------------------------------------------------------------------------
-- ** Generating and unfolding 'Text's

-- | /O(n)/ 'replicate' @n@ @c@ is a 'Text' of length @n@ with @c@ the
-- value of every element. Subject to fusion.
replicate :: Int -> Char -> Text
replicate n c = unstream (S.replicate n c)
{-# INLINE replicate #-}

-- | /O(n)/, where @n@ is the length of the result. The 'unfoldr'
-- function is analogous to the List 'L.unfoldr'. 'unfoldr' builds a
-- 'Text' from a seed value. The function takes the element and
-- returns 'Nothing' if it is done producing the 'Text', otherwise
-- 'Just' @(a,b)@.  In this case, @a@ is the next 'Char' in the
-- string, and @b@ is the seed value for further production. Subject
-- to fusion.
unfoldr     :: (a -> Maybe (Char,a)) -> a -> Text
unfoldr f s = unstream (S.unfoldr f s)
{-# INLINE unfoldr #-}

-- | /O(n)/ Like 'unfoldr', 'unfoldrN' builds a 'Text' from a seed
-- value. However, the length of the result should be limited by the
-- first argument to 'unfoldrN'. This function is more efficient than
-- 'unfoldr' when the maximum length of the result is known and
-- correct, otherwise its performance is similar to 'unfoldr'. Subject
-- to fusion.
unfoldrN     :: Int -> (a -> Maybe (Char,a)) -> a -> Text
unfoldrN n f s = unstream (S.unfoldrN n f s)
{-# INLINE unfoldrN #-}

-- -----------------------------------------------------------------------------
-- * Substrings

-- | /O(n)/ 'take' @n@, applied to a 'Text', returns the prefix of the
-- 'Text' of length @n@, or the 'Text' itself if @n@ is greater than
-- the length of the Text. Subject to fusion.
take :: Int -> Text -> Text
take n t@(Text arr off len)
    | n <= 0    = empty
    | n >= len  = t
    | otherwise = Text arr off (loop 0 0)
  where
      loop !i !cnt
           | i >= len || cnt >= n = i
           | otherwise            = loop (i+d) (cnt+1)
           where d = iter_ t i
{-# INLINE [1] take #-}

{-# RULES
"TEXT take -> fused" [~1] forall n t.
    take n t = unstream (S.take n (stream t))
"TEXT take -> unfused" [1] forall n t.
    unstream (S.take n (stream t)) = take n t
  #-}

-- | /O(n)/ 'drop' @n@, applied to a 'Text', returns the suffix of the
-- 'Text' of length @n@, or the empty 'Text' if @n@ is greater than the
-- length of the 'Text'. Subject to fusion.
drop :: Int -> Text -> Text
drop n t@(Text arr off len)
    | n <= 0    = t
    | n >= len  = empty
    | otherwise = loop 0 0
  where loop !i !cnt
            | i >= len || cnt >= n   = Text arr (off+i) (len-i)
            | otherwise              = loop (i+d) (cnt+1)
            where d = iter_ t i
{-# INLINE [1] drop #-}

{-# RULES
"TEXT drop -> fused" [~1] forall n t.
    drop n t = unstream (S.drop n (stream t))
"TEXT drop -> unfused" [1] forall n t.
    unstream (S.drop n (stream t)) = drop n t
  #-}

-- | /O(n)/ 'takeWhile', applied to a predicate @p@ and a 'Text', returns
-- the longest prefix (possibly empty) of elements that satisfy @p@.
-- This function is subject to array fusion.
takeWhile :: (Char -> Bool) -> Text -> Text
takeWhile p t@(Text arr off len) = loop 0
  where loop !i | i >= len    = t
                | p c         = loop (i+d)
                | otherwise   = textP arr off i
            where (c,d)       = iter t i
{-# INLINE [1] takeWhile #-}

{-# RULES
"TEXT takeWhile -> fused" [~1] forall p t.
    takeWhile p t = unstream (S.takeWhile p (stream t))
"TEXT takeWhile -> unfused" [1] forall p t.
    unstream (S.takeWhile p (stream t)) = takeWhile p t
  #-}

-- | /O(n)/ 'dropWhile' @p@ @xs@ returns the suffix remaining after
-- 'takeWhile' @p@ @xs@. This function is subject to array fusion.
dropWhile :: (Char -> Bool) -> Text -> Text
dropWhile p t@(Text arr off len) = loop 0 0
  where loop !i !l | l >= len  = empty
                   | p c       = loop (i+d) (l+d)
                   | otherwise = Text arr (off+i) (len-l)
            where (c,d)        = iter t i
{-# INLINE [1] dropWhile #-}

{-# RULES
"TEXT dropWhile -> fused" [~1] forall p t.
    dropWhile p t = unstream (S.dropWhile p (stream t))
"TEXT dropWhile -> unfused" [1] forall p t.
    unstream (S.dropWhile p (stream t)) = dropWhile p t
  #-}

-- | /O(n)/ 'splitAt' @n t@ returns a pair whose first element is a
-- prefix of @t@ of length @n@, and whose second is the remainder of
-- the string. It is equivalent to @('take' n t, 'drop' n t)@.
splitAt :: Int -> Text -> (Text, Text)
splitAt n t@(Text arr off len)
    | n <= 0    = (empty, t)
    | n >= len  = (t, empty)
    | otherwise = (Text arr off k, Text arr (off+k) (len-k))
  where k = loop 0 0
        loop !i !cnt
            | i >= len || cnt >= n = i
            | otherwise            = loop (i+d) (cnt+1)
            where d                = iter_ t i
{-# INLINE splitAt #-}

-- | /O(n)/ 'span', applied to a predicate @p@ and text @t@, returns a
-- pair whose first element is the longest prefix (possibly empty) of
-- @t@ of elements that satisfy @p@, and whose second is the remainder
-- of the list.
span :: (Char -> Bool) -> Text -> (Text, Text)
span p t@(Text arr off len) = (textP arr off k, textP arr (off+k) (len-k))
  where k = loop 0
        loop !i | i >= len || not (p c) = i
                | otherwise             = loop (i+d)
            where (c,d)                 = iter t i
{-# INLINE span #-}

-- | /O(n)/ 'break' is like 'span', but the prefix returned is over
-- elements that fail the predicate @p@.
break :: (Char -> Bool) -> Text -> (Text, Text)
break p = span (not . p)
{-# INLINE break #-}

-- | /O(n)/ Group characters in a string according to a predicate.
groupBy :: (Char -> Char -> Bool) -> Text -> [Text]
groupBy p = loop
  where
    loop t@(Text arr off len)
        | null t    = []
        | otherwise = text arr off n : loop (text arr (off+n) (len-n))
        where (c,d) = iter t 0
              n     = d + findAIndexOrEnd (not . p c) (Text arr (off+d) (len-d))

-- | Returns the /array/ index (in units of 'Word16') at which a
-- character may be found.  This is /not/ the same as the logical
-- index returned by e.g. 'findIndex'.
findAIndexOrEnd :: (Char -> Bool) -> Text -> Int
findAIndexOrEnd q t@(Text _arr _off len) = go 0
    where go !i | i >= len || q c       = i
                | otherwise             = go (i+d)
                where (c,d)             = iter t i
    
-- | /O(n)/ Group characters in a string by equality.
group :: Text -> [Text]
group = groupBy (==)

-- | /O(n)/ Return all initial segments of the given 'Text', shortest
-- first.
inits :: Text -> [Text]
inits t@(Text arr off len) = loop 0
    where loop i | i >= len = [t]
                 | otherwise = Text arr off i : loop (i + iter_ t i)

-- | /O(n)/ Return all final segments of the given 'Text', longest
-- first.
tails :: Text -> [Text]
tails t | null t    = [empty]
        | otherwise = t : tails (unsafeTail t)

-- | /O(n)/ Break a 'Text' into pieces separated by the 'Char'
-- argument, consuming the delimiter. I.e.
--
-- > split '\n' "a\nb\nd\ne" == ["a","b","d","e"]
-- > split 'a'  "aXaXaXa"    == ["","X","X","X",""]
-- > split 'x'  "x"          == ["",""]
-- 
-- and
--
-- > intercalate (singleton c) . split c == id
-- > split == splitWith . (==)
-- 
-- As for all splitting functions in this library, this function does
-- not copy the substrings, it just constructs new 'Text's that are
-- slices of the original.
split :: Char -> Text -> [Text]
split c = splitWith (==c)
{-# INLINE split #-}

-- | /O(n)/ Splits a 'Text' 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.  eg.
--
-- > splitWith (=='a') "aabbaca" == ["","","bb","c",""]
-- > splitWith (=='a') []        == []
splitWith :: (Char -> Bool) -> Text -> [Text]
splitWith _ (Text _off _arr 0) = []
splitWith p t = loop t
    where loop s | null s'   = [l]
                 | otherwise = l : loop (unsafeTail s')
              where (l, s') = break p s
{-# INLINE splitWith #-}

-- ----------------------------------------------------------------------------
-- * Searching

-------------------------------------------------------------------------------
-- ** Searching by equality

-- | /O(n)/ 'elem' is the 'Text' membership predicate.
elem :: Char -> Text -> Bool
elem c t = S.elem c (stream t)
{-# INLINE elem #-}

-------------------------------------------------------------------------------
-- ** Searching with a predicate

-- | /O(n)/ The 'find' function takes a predicate and a 'Text',
-- and returns the first element in matching the predicate, or 'Nothing'
-- if there is no such element.
find :: (Char -> Bool) -> Text -> Maybe Char
find p t = S.find p (stream t)
{-# INLINE find #-}

-- | /O(n)/ The 'partition' function takes a predicate and a 'Text',
-- and returns the pair of 'Text's with elements which do and do not
-- satisfy the predicate, respectively; i.e.
--
-- > partition p t == (filter p t, filter (not . p) t)
partition :: (Char -> Bool) -> Text -> (Text, Text)
partition p t = (filter p t, filter (not . p) t)
{-# INLINE partition #-}

-- | /O(n)/ Break a string on a substring, returning a pair of the
-- part of the string prior to the match, and the rest of the string.
--
-- The following relationship holds:
--
-- > break (==c) l == breakSubstring (singleton c) l
--
-- For example, to tokenise a string, dropping delimiters:
--
-- > tokenise x y = h : if null t then [] else tokenise x (drop (length x) t)
-- >     where (h,t) = breakSubstring x y
--
-- To skip to the first occurence of a string:
--
-- > snd (breakSubstring x y)
--
-- To take the parts of a string before a delimiter:
--
-- > fst (breakSubstring x y)
--
breakSubstring :: Text -- ^ String to search for
               -> Text -- ^ String to search in
               -> (Text,Text) -- ^ Head and tail of string broken at substring

breakSubstring pat src = search 0 src
  where
    search !n !s
        | null s             = (src,empty)      -- not found
        | pat `isPrefixOf` s = (take n src,s)
        | otherwise          = search (n+1) (unsafeTail s)
{-# INLINE breakSubstring #-}

-- | /O(n)/ 'filter', applied to a predicate and a 'Text',
-- returns a 'Text' containing those characters that satisfy the
-- predicate.
filter :: (Char -> Bool) -> Text -> Text
filter p t = unstream (S.filter p (stream t))
{-# INLINE filter #-}


-------------------------------------------------------------------------------
-- ** Indexing 'Text's

-- | /O(n)/ 'Text' index (subscript) operator, starting from 0.
index :: Text -> Int -> Char
index t n = S.index (stream t) n
{-# INLINE index #-}

-- | /O(n)/ The 'findIndex' function takes a predicate and a 'Text'
-- and returns the index of the first element in the 'Text' satisfying
-- the predicate. This function is subject to fusion.
findIndex :: (Char -> Bool) -> Text -> Maybe Int
findIndex p t = S.findIndex p (stream t)
{-# INLINE findIndex #-}

-- | The 'findIndices' function extends 'findIndex', by returning the
-- indices of all elements satisfying the predicate, in ascending
-- order. This function is subject to fusion.
findIndices :: (Char -> Bool) -> Text -> [Int]
findIndices p t = S.findIndices p (stream t)
{-# INLINE findIndices #-}

-- | /O(n)/ The 'elemIndex' function returns the index of the first
-- element in the given 'Text' which is equal to the query element, or
-- 'Nothing' if there is no such element. This function is subject to
-- fusion.
elemIndex :: Char -> Text -> Maybe Int
elemIndex c t = S.elemIndex c (stream t)
{-# INLINE elemIndex #-}

-- | /O(n)/ The 'elemIndices' function returns the index of every
-- element in the given 'Text' which is equal to the query
-- element. This function is subject to fusion.
elemIndices :: Char -> Text -> [Int]
elemIndices c t = S.elemIndices c (stream t)
{-# INLINE elemIndices #-}

-- | /O(n)/ The 'count' function returns the number of times the query
-- element appears in the given 'Text'. This function is subject to
-- fusion.
count :: Char -> Text -> Int
count c t = S.count c (stream t)
{-# INLINE count #-}

-------------------------------------------------------------------------------
-- * Zipping

-- | /O(n)/ 'zip' takes two 'Text's and returns a list of
-- corresponding pairs of bytes. If one input 'Text' is short,
-- excess elements of the longer 'Text' are discarded. This is
-- equivalent to a pair of 'unpack' operations.
zip :: Text -> Text -> [(Char,Char)]
zip a b = S.unstreamList $ S.zipWith (,) (stream a) (stream b)
{-# INLINE [0] zip #-}

-- | /O(n)/ 'zipWith' generalises 'zip' by zipping with the function
-- given as the first argument, instead of a tupling function.
zipWith :: (Char -> Char -> Char) -> Text -> Text -> Text
zipWith f t1 t2 = unstream (S.zipWith f (stream t1) (stream t2))
{-# INLINE [0] zipWith #-}

-- | /O(n)/ Breaks a 'Text' up into a list of words, delimited by 'Char's
-- representing white space.
words :: Text -> [Text]
words t@(Text arr off len) = loop 0 0
  where
    loop !start !n
        | n >= len = if start == n
                     then []
                     else [Text arr (start+off) (n-start)]
        | isSpace c =
            if start == n
            then loop (start+1) (start+1)
            else Text arr (start+off) (n-start) : loop (n+d) (n+d)
        | otherwise = loop start (n+d)
        where (c,d) = iter t n
{-# INLINE words #-}

-- | /O(n)/ Breaks a 'Text' up into a list of 'Text's at
-- newline 'Char's. The resulting strings do not contain newlines.
lines :: Text -> [Text]
lines ps | null ps   = []
         | otherwise = h : if null t
                           then []
                           else lines (unsafeTail t)
    where (h,t) = span (/= '\n') ps
{-# INLINE lines #-}

-- | /O(n)/ Portably breaks a 'Text' up into a list of 'Text's at line
-- boundaries.
--
-- A line boundary is considered to be either a line feed, a carriage
-- return immediately followed by a line feed, or a carriage return.
-- This accounts for both Unix and Windows line ending conventions,
-- and for the old convention used on Mac OS 9 and earlier.
{-
lines' :: Text -> [Text]
lines' ps | null ps   = []
          | otherwise = h : case uncons t of
                              Nothing -> []
                              Just (c,t')
                                  | c == '\n' -> lines t'
                                  | c == '\r' -> case uncons t' of
                                                   Just ('\n',t'') -> lines t''
                                                   _               -> lines t'
    where (h,t)    = span notEOL ps
          notEOL c = c /= '\n' && c /= '\r'
{-# INLINE lines' #-}
-}

-- | /O(n)/ Joins lines, after appending a terminating newline to
-- each.
unlines :: [Text] -> Text
unlines = concat . L.map (`snoc` '\n')
{-# INLINE unlines #-}

-- | /O(n)/ Joins words using single space characters.
unwords :: [Text] -> Text
unwords = intercalate (singleton ' ')
{-# INLINE unwords #-}

-- | /O(n)/ The 'isPrefixOf' function takes two 'Text's and returns
-- 'True' iff the first is a prefix of the second.  This function is
-- subject to fusion.
isPrefixOf :: Text -> Text -> Bool
isPrefixOf a@(Text _ _ alen) b@(Text _ _ blen) =
    alen <= blen && S.isPrefixOf (stream a) (stream b)
{-# INLINE [1] isPrefixOf #-}

{-# RULES
"TEXT isPrefixOf -> fused" [~1] forall s t.
    isPrefixOf s t = S.isPrefixOf (stream s) (stream t)
"TEXT isPrefixOf -> unfused" [1] forall s t.
    S.isPrefixOf (stream s) (stream t) = isPrefixOf s t
  #-}

-- | /O(n)/ The 'isSuffixOf' function takes two 'Text's and returns
-- 'True' iff the first is a suffix of the second.
isSuffixOf :: Text -> Text -> Bool
isSuffixOf a@(Text _aarr _aoff alen) b@(Text barr boff blen) =
    d >= 0 && a == b'
  where d              = blen - alen
        b' | d == 0    = b
           | otherwise = Text barr (boff+d) alen
{-# INLINE isSuffixOf #-}

-- | /O(n)/ The 'isInfixOf' function takes two 'Text's and returns
-- 'True' iff the first is contained, wholly and intact, anywhere
-- within the second.
isInfixOf :: Text -> Text -> Bool
isInfixOf needle haystack = L.any (isPrefixOf needle) (tails haystack)
{-# INLINE isInfixOf #-}
-- TODO: a better implementation

emptyError :: String -> a
emptyError fun = P.error ("Data.Text." ++ fun ++ ": empty input")
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