Data/Text.hs

    {-# LANGUAGE BangPatterns, CPP, MagicHash, Rank2Types, UnboxedTuples #-}
    {-# OPTIONS_GHC -fno-warn-orphans #-}
    #if __GLASGOW_HASKELL__ >= 702
    {-# LANGUAGE Trustworthy #-}
    #endif
    #if __GLASGOW_HASKELL__ >= 708
    {-# LANGUAGE TypeFamilies #-}
    #endif

    -- |
    -- Module      : Data.Text
    -- Copyright   : (c) 2009, 2010, 2011, 2012 Bryan O'Sullivan,
    --               (c) 2009 Duncan Coutts,
    --               (c) 2008, 2009 Tom Harper
    --
    -- License     : BSD-style
    -- Maintainer  : bos@serpentine.com
    -- Portability : GHC
    --
    -- A time and space-efficient implementation of Unicode text.
    -- Suitable for performance critical use, both in terms of large data
    -- quantities and high speed.
    --
    -- /Note/: Read below the synopsis for important notes on the use of
    -- this module.
    --
    -- This module is intended to be imported @qualified@, to avoid name
    -- clashes with "Prelude" functions, e.g.
    --
    -- > import qualified Data.Text as T
    --
    -- To use an extended and very rich family of functions for working
    -- with Unicode text (including normalization, regular expressions,
    -- non-standard encodings, text breaking, and locales), see the
    -- <http://hackage.haskell.org/package/text-icu text-icu package >.
    --

    module Data.Text
        (
        -- * Strict vs lazy types
        -- $strict

        -- * Acceptable data
        -- $replacement

        -- * Definition of character
        -- $character_definition

        -- * Fusion
        -- $fusion

        -- * Types
          Text

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

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

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

        -- ** Case conversion
        -- $case
        , toCaseFold
        , toLower
        , toUpper
        , toTitle

        -- ** Justification
        , justifyLeft
        , justifyRight
        , center

        -- * 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
        , takeEnd
        , drop
        , dropEnd
        , takeWhile
        , takeWhileEnd
        , dropWhile
        , dropWhileEnd
        , dropAround
        , strip
        , stripStart
        , stripEnd
        , splitAt
        , breakOn
        , breakOnEnd
        , break
        , span
        , group
        , groupBy
        , inits
        , tails

        -- ** Breaking into many substrings
        -- $split
        , splitOn
        , split
        , chunksOf

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

        -- * Predicates
        , isPrefixOf
        , isSuffixOf
        , isInfixOf

        -- ** View patterns
        , stripPrefix
        , stripSuffix
        , commonPrefixes

        -- * Searching
        , filter
        , breakOnAll
        , find
        , partition

        -- , findSubstring

        -- * Indexing
        -- $index
        , index
        , findIndex
        , count

        -- * Zipping
        , zip
        , zipWith

        -- -* Ordered text
        -- , sort

        -- * Low level operations
        , copy
        , unpackCString#
        ) where

    import Prelude (Char, Bool(..), Int, Maybe(..), String,
                    Eq(..), Ord(..), Ordering(..), (++),
                    Read(..),
                    (&&), (||), (+), (-), (.), ($), ($!), (>>),
                    not, return, otherwise, quot)
    #if defined(HAVE_DEEPSEQ)
    import Control.DeepSeq (NFData(rnf))
    #endif
    #if defined(ASSERTS)
    import Control.Exception (assert)
    #endif
    import Data.Char (isSpace)
    import Data.Data (Data(gfoldl, toConstr, gunfold, dataTypeOf), constrIndex,
                      Constr, mkConstr, DataType, mkDataType, Fixity(Prefix))
    import Control.Monad (foldM)
    import Control.Monad.ST (ST)
    import qualified Data.Text.Array as A
    import qualified Data.List as L
    import Data.Binary (Binary(get, put))
    import Data.Monoid (Monoid(..))
    #if MIN_VERSION_base(4,9,0)
    import Data.Semigroup (Semigroup(..))
    #endif
    import Data.String (IsString(..))
    import qualified Data.Text.Internal.Fusion as S
    import qualified Data.Text.Internal.Fusion.Common as S
    import Data.Text.Encoding (decodeUtf8', encodeUtf8)
    import Data.Text.Internal.Fusion (stream, reverseStream, unstream)
    import Data.Text.Internal.Private (span_)
    import Data.Text.Internal (Text(..), empty, firstf, mul, safe, text)
    import Data.Text.Show (singleton, unpack, unpackCString#)
    import qualified Prelude as P
    import Data.Text.Unsafe (Iter(..), iter, iter_, lengthWord16, reverseIter,
                             reverseIter_, unsafeHead, unsafeTail)
    import Data.Text.Internal.Unsafe.Char (unsafeChr)
    import qualified Data.Text.Internal.Functions as F
    import qualified Data.Text.Internal.Encoding.Utf16 as U16
    import Data.Text.Internal.Search (indices)
    #if defined(__HADDOCK__)
    import Data.ByteString (ByteString)
    import qualified Data.Text.Lazy as L
    import Data.Int (Int64)
    #endif
    import GHC.Base (eqInt, neInt, gtInt, geInt, ltInt, leInt)
    #if __GLASGOW_HASKELL__ >= 708
    import qualified GHC.Exts as Exts
    #endif
    #if MIN_VERSION_base(4,7,0)
    import Text.Printf (PrintfArg, formatArg, formatString)
    #endif

    -- $character_definition
    --
    -- This package uses the term /character/ to denote Unicode /code points/.
    --
    -- Note that this is not the same thing as a grapheme (e.g. a
    -- composition of code points that form one visual symbol). For
    -- instance, consider the grapheme \"&#x00e4;\". This symbol has two
    -- Unicode representations: a single code-point representation
    -- @U+00E4@ (the @LATIN SMALL LETTER A WITH DIAERESIS@ code point),
    -- and a two code point representation @U+0061@ (the \"@A@\" code
    -- point) and @U+0308@ (the @COMBINING DIAERESIS@ code point).

    -- $strict
    --
    -- This package provides both strict and lazy 'Text' types.  The
    -- strict type is provided by the "Data.Text" module, while the lazy
    -- type is provided by the "Data.Text.Lazy" module. Internally, the
    -- lazy @Text@ type consists of a list of strict chunks.
    --
    -- The strict 'Text' type requires that an entire string fit into
    -- memory at once.  The lazy 'Data.Text.Lazy.Text' type is capable of
    -- streaming strings that are larger than memory using a small memory
    -- footprint.  In many cases, the overhead of chunked streaming makes
    -- the lazy 'Data.Text.Lazy.Text' type slower than its strict
    -- counterpart, but this is not always the case.  Sometimes, the time
    -- complexity of a function in one module may be different from the
    -- other, due to their differing internal structures.
    --
    -- Each module provides an almost identical API, with the main
    -- difference being that the strict module uses 'Int' values for
    -- lengths and counts, while the lazy module uses 'Data.Int.Int64'
    -- lengths.

    -- $replacement
    --
    -- A 'Text' value is a sequence of Unicode scalar values, as defined
    -- in
    -- <http://www.unicode.org/versions/Unicode5.2.0/ch03.pdf#page=35 §3.9, definition D76 of the Unicode 5.2 standard >.
    -- As such, a 'Text' cannot contain values in the range U+D800 to
    -- U+DFFF inclusive. Haskell implementations admit all Unicode code
    -- points
    -- (<http://www.unicode.org/versions/Unicode5.2.0/ch03.pdf#page=13 §3.4, definition D10 >)
    -- as 'Char' values, including code points from this invalid range.
    -- This means that there are some 'Char' values that are not valid
    -- Unicode scalar values, and the functions in this module must handle
    -- those cases.
    --
    -- Within this module, many functions construct a 'Text' from one or
    -- more 'Char' values. Those functions will substitute 'Char' values
    -- that are not valid Unicode scalar values with the replacement
    -- character \"&#xfffd;\" (U+FFFD).  Functions that perform this
    -- inspection and replacement are documented with the phrase
    -- \"Performs replacement on invalid scalar values\".
    --
    -- (One reason for this policy of replacement is that internally, a
    -- 'Text' value is represented as packed UTF-16 data. Values in the
    -- range U+D800 through U+DFFF are used by UTF-16 to denote surrogate
    -- code points, and so cannot be represented. The functions replace
    -- invalid scalar values, instead of dropping them, as a security
    -- measure. For details, see
    -- <http://unicode.org/reports/tr36/#Deletion_of_Noncharacters Unicode Technical Report 36, §3.5 >.)

    -- $fusion
    --
    -- Most of the functions in this module are subject to /fusion/,
    -- meaning that a pipeline of such functions will usually allocate at
    -- most one 'Text' value.
    --
    -- As an example, consider the following pipeline:
    --
    -- > import Data.Text as T
    -- > import Data.Text.Encoding as E
    -- > import Data.ByteString (ByteString)
    -- >
    -- > countChars :: ByteString -> Int
    -- > countChars = T.length . T.toUpper . E.decodeUtf8
    --
    -- From the type signatures involved, this looks like it should
    -- allocate one 'Data.ByteString.ByteString' value, and two 'Text'
    -- values. However, when a module is compiled with optimisation
    -- enabled under GHC, the two intermediate 'Text' values will be
    -- optimised away, and the function will be compiled down to a single
    -- loop over the source 'Data.ByteString.ByteString'.
    --
    -- Functions that can be fused by the compiler are documented with the
    -- phrase \"Subject to fusion\".

    instance Eq Text where
        Text arrA offA lenA == Text arrB offB lenB
            | lenA == lenB = A.equal arrA offA arrB offB lenA
            | otherwise    = False
        {-# INLINE (==) #-}

    instance Ord Text where
        compare = compareText

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

    #if MIN_VERSION_base(4,9,0)
    -- | Non-orphan 'Semigroup' instance only defined for
    -- @base-4.9.0.0@ and later; orphan instances for older GHCs are
    -- provided by
    -- the [semigroups](http://hackage.haskell.org/package/semigroups)
    -- package
    --
    -- @since 1.2.2.0
    instance Semigroup Text where
        (<>) = append
    #endif

    instance Monoid Text where
        mempty  = empty
    #if MIN_VERSION_base(4,9,0)
        mappend = (<>) -- future-proof definition
    #else
        mappend = append
    #endif
        mconcat = concat

    instance IsString Text where
        fromString = pack

    #if __GLASGOW_HASKELL__ >= 708
    -- | @since 1.2.0.0
    instance Exts.IsList Text where
        type Item Text = Char
        fromList       = pack
        toList         = unpack
    #endif

    #if defined(HAVE_DEEPSEQ)
    instance NFData Text where rnf !_ = ()
    #endif

    -- | @since 1.2.1.0
    instance Binary Text where
        put t = put (encodeUtf8 t)
        get   = do
          bs <- get
          case decodeUtf8' bs of
            P.Left exn -> P.fail (P.show exn)
            P.Right a -> P.return a

    -- | This instance preserves data abstraction at the cost of inefficiency.
    -- We omit reflection services for the sake of data abstraction.
    --
    -- This instance was created by copying the updated behavior of
    -- @"Data.Set".@'Data.Set.Set' and @"Data.Map".@'Data.Map.Map'. If you
    -- feel a mistake has been made, please feel free to submit
    -- improvements.
    --
    -- The original discussion is archived here:
    -- <http://groups.google.com/group/haskell-cafe/browse_thread/thread/b5bbb1b28a7e525d/0639d46852575b93 could we get a Data instance for Data.Text.Text? >
    --
    -- The followup discussion that changed the behavior of 'Data.Set.Set'
    -- and 'Data.Map.Map' is archived here:
    -- <http://markmail.org/message/trovdc6zkphyi3cr#query:+page:1+mid:a46der3iacwjcf6n+state:results Proposal: Allow gunfold for Data.Map, ... >

    instance Data Text where
      gfoldl f z txt = z pack `f` (unpack txt)
      toConstr _ = packConstr
      gunfold k z c = case constrIndex c of
        1 -> k (z pack)
        _ -> P.error "gunfold"
      dataTypeOf _ = textDataType

    #if MIN_VERSION_base(4,7,0)
    -- | Only defined for @base-4.7.0.0@ and later
    --
    -- @since 1.2.2.0
    instance PrintfArg Text where
      formatArg txt = formatString $ unpack txt
    #endif

    packConstr :: Constr
    packConstr = mkConstr textDataType "pack" [] Prefix

    textDataType :: DataType
    textDataType = mkDataType "Data.Text.Text" [packConstr]

    -- | /O(n)/ Compare two 'Text' values lexicographically.
    compareText :: Text -> Text -> Ordering
    compareText ta@(Text _arrA _offA lenA) tb@(Text _arrB _offB lenB)
        | lenA == 0 && lenB == 0 = EQ
        | otherwise              = go 0 0
      where
        go !i !j
            | i >= lenA || j >= lenB = compare lenA lenB
            | a < b                  = LT
            | a > b                  = GT
            | otherwise              = go (i+di) (j+dj)
          where Iter a di = iter ta i
                Iter b dj = iter tb j

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

    -- | /O(n)/ Convert a 'String' into a 'Text'.  Subject to
    -- fusion.  Performs replacement on invalid scalar values.
    pack :: String -> Text
    pack = unstream . S.map safe . S.streamList
    {-# INLINE [1] pack #-}

    -- -----------------------------------------------------------------------------
    -- * 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 fusion.  Performs replacement on
    -- invalid scalar values.
    cons :: Char -> Text -> Text
    cons c t = unstream (S.cons (safe c) (stream t))
    {-# INLINE cons #-}

    infixr 5 `cons`

    -- | /O(n)/ Adds a character to the end of a 'Text'.  This copies the
    -- entire array in the process, unless fused.  Subject to fusion.
    -- Performs replacement on invalid scalar values.
    snoc :: Text -> Char -> Text
    snoc t c = unstream (S.snoc (stream t) (safe c))
    {-# INLINE snoc #-}

    -- | /O(n)/ Appends one 'Text' to the other by copying both of them
    -- into a new 'Text'.  Subject to fusion.
    append :: Text -> Text -> Text
    append a@(Text arr1 off1 len1) b@(Text arr2 off2 len2)
        | len1 == 0 = b
        | len2 == 0 = a
        | len > 0   = Text (A.run x) 0 len
        | otherwise = overflowError "append"
        where
          len = len1+len2
          x :: ST s (A.MArray s)
          x = do
            arr <- A.new len
            A.copyI arr 0 arr1 off1 len1
            A.copyI arr len1 arr2 off2 len
            return arr
    {-# NOINLINE 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 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 fusion.
    uncons :: Text -> Maybe (Char, Text)
    uncons t@(Text arr off len)
        | len <= 0  = Nothing
        | otherwise = Just $ let !(Iter c d) = iter t 0
                             in (c, text arr (off+d) (len-d))
    {-# INLINE [1] uncons #-}

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

    -- | /O(1)/ Returns the last character of a 'Text', which must be
    -- non-empty.  Subject to 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 fusion.
    tail :: Text -> Text
    tail t@(Text arr off len)
        | len <= 0  = emptyError "tail"
        | otherwise = text 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 fusion.
    init :: Text -> Text
    init (Text arr off len) | len <= 0                   = emptyError "init"
                            | n >= 0xDC00 && n <= 0xDFFF = text arr off (len-2)
                            | otherwise                  = text 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)/ Returns all but the last character and the last character of a
    -- 'Text', or 'Nothing' if empty.
    --
    -- @since 1.2.3.0
    unsnoc :: Text -> Maybe (Text, Char)
    unsnoc (Text arr off len)
        | len <= 0                 = Nothing
        | n < 0xDC00 || n > 0xDFFF = Just (text arr off (len-1), unsafeChr n)
        | otherwise                = Just (text arr off (len-2), U16.chr2 n0 n)
        where n  = A.unsafeIndex arr (off+len-1)
              n0 = A.unsafeIndex arr (off+len-2)
    {-# INLINE [1] unsnoc #-}

    -- | /O(1)/ Tests whether a 'Text' is empty or not.  Subject to
    -- fusion.
    null :: Text -> Bool
    null (Text _arr _off len) =
    #if defined(ASSERTS)
        assert (len >= 0) $
    #endif
        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(1)/ Tests whether a 'Text' contains exactly one character.
    -- Subject to fusion.
    isSingleton :: Text -> Bool
    isSingleton = S.isSingleton . stream
    {-# INLINE isSingleton #-}

    -- | /O(n)/ Returns the number of characters in a 'Text'.
    -- Subject to fusion.
    length :: Text -> Int
    length t = S.length (stream t)
    {-# INLINE [0] length #-}
    -- length needs to be phased after the compareN/length rules otherwise
    -- it may inline before the rules have an opportunity to fire.

    -- | /O(n)/ Compare the count of characters in a 'Text' to a number.
    -- Subject to fusion.
    --
    -- This function gives the same answer as comparing against the result
    -- of 'length', but can short circuit if the count of characters is
    -- greater than the number, and hence be more efficient.
    compareLength :: Text -> Int -> Ordering
    compareLength t n = S.compareLengthI (stream t) n
    {-# INLINE [1] compareLength #-}

    {-# RULES
    "TEXT compareN/length -> compareLength" [~1] forall t n.
        compare (length t) n = compareLength t n
      #-}

    {-# RULES
    "TEXT ==N/length -> compareLength/==EQ" [~1] forall t n.
        eqInt (length t) n = compareLength t n == EQ
      #-}

    {-# RULES
    "TEXT /=N/length -> compareLength//=EQ" [~1] forall t n.
        neInt (length t) n = compareLength t n /= EQ
      #-}

    {-# RULES
    "TEXT <N/length -> compareLength/==LT" [~1] forall t n.
        ltInt (length t) n = compareLength t n == LT
      #-}

    {-# RULES
    "TEXT <=N/length -> compareLength//=GT" [~1] forall t n.
        leInt (length t) n = compareLength t n /= GT
      #-}

    {-# RULES
    "TEXT >N/length -> compareLength/==GT" [~1] forall t n.
        gtInt (length t) n = compareLength t n == GT
      #-}

    {-# RULES
    "TEXT >=N/length -> compareLength//=LT" [~1] forall t n.
        geInt (length t) n = compareLength t n /= LT
      #-}

    -- -----------------------------------------------------------------------------
    -- * Transformations
    -- | /O(n)/ 'map' @f@ @t@ is the 'Text' obtained by applying @f@ to
    -- each element of @t@.
    --
    -- Example:
    --
    -- >>> let message = pack "I am not angry. Not at all."
    -- >>> T.map (\c -> if c == '.' then '!' else c) message
    -- "I am not angry! Not at all!"
    --
    -- Subject to fusion.  Performs replacement on invalid scalar values.
    map :: (Char -> Char) -> Text -> Text
    map f t = unstream (S.map (safe . 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.
    --
    -- Example:
    --
    -- >>> T.intercalate "NI!" ["We", "seek", "the", "Holy", "Grail"]
    -- "WeNI!seekNI!theNI!HolyNI!Grail"
    intercalate :: Text -> [Text] -> Text
    intercalate t = concat . (F.intersperse t)
    {-# INLINE intercalate #-}

    -- | /O(n)/ The 'intersperse' function takes a character and places it
    -- between the characters of a 'Text'.
    --
    -- Example:
    --
    -- >>> T.intersperse '.' "SHIELD"
    -- "S.H.I.E.L.D"
    --
    -- Subject to fusion.  Performs replacement on invalid scalar values.
    intersperse     :: Char -> Text -> Text
    intersperse c t = unstream (S.intersperse (safe c) (stream t))
    {-# INLINE intersperse #-}

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

    -- | /O(m+n)/ Replace every non-overlapping occurrence of @needle@ in
    -- @haystack@ with @replacement@.
    --
    -- This function behaves as though it was defined as follows:
    --
    -- @
    -- replace needle replacement haystack =
    --   'intercalate' replacement ('splitOn' needle haystack)
    -- @
    --
    -- As this suggests, each occurrence is replaced exactly once.  So if
    -- @needle@ occurs in @replacement@, that occurrence will /not/ itself
    -- be replaced recursively:
    --
    -- >>> replace "oo" "foo" "oo"
    -- "foo"
    --
    -- In cases where several instances of @needle@ overlap, only the
    -- first one will be replaced:
    --
    -- >>> replace "ofo" "bar" "ofofo"
    -- "barfo"
    --
    -- In (unlikely) bad cases, this function's time complexity degrades
    -- towards /O(n*m)/.
    replace :: Text
            -- ^ @needle@ to search for.  If this string is empty, an
            -- error will occur.
            -> Text
            -- ^ @replacement@ to replace @needle@ with.
            -> Text
            -- ^ @haystack@ in which to search.
            -> Text
    replace needle@(Text _      _      neeLen)
                   (Text repArr repOff repLen)
          haystack@(Text hayArr hayOff hayLen)
      | neeLen == 0 = emptyError "replace"
      | L.null ixs  = haystack
      | len > 0     = Text (A.run x) 0 len
      | otherwise   = empty
      where
        ixs = indices needle haystack
        len = hayLen - (neeLen - repLen) `mul` L.length ixs
        x :: ST s (A.MArray s)
        x = do
          marr <- A.new len
          let loop (i:is) o d = do
                let d0 = d + i - o
                    d1 = d0 + repLen
                A.copyI marr d  hayArr (hayOff+o) d0
                A.copyI marr d0 repArr repOff d1
                loop is (i + neeLen) d1
              loop []     o d = A.copyI marr d hayArr (hayOff+o) len
          loop ixs 0 0
          return marr

    -- ----------------------------------------------------------------------------
    -- ** Case conversions (folds)

    -- $case
    --
    -- When case converting 'Text' values, do not use combinators like
    -- @map toUpper@ to case convert each character of a string
    -- individually, as this gives incorrect results according to the
    -- rules of some writing systems.  The whole-string case conversion
    -- functions from this module, such as @toUpper@, obey the correct
    -- case conversion rules.  As a result, these functions may map one
    -- input character to two or three output characters. For examples,
    -- see the documentation of each function.
    --
    -- /Note/: In some languages, case conversion is a locale- and
    -- context-dependent operation. The case conversion functions in this
    -- module are /not/ locale sensitive. Programs that require locale
    -- sensitivity should use appropriate versions of the
    -- <http://hackage.haskell.org/package/text-icu-0.6.3.7/docs/Data-Text-ICU.html#g:4 case mapping functions from the text-icu package >.

    -- | /O(n)/ Convert a string to folded case.  Subject to fusion.
    --
    -- This function is mainly useful for performing caseless (also known
    -- as case insensitive) string comparisons.
    --
    -- A string @x@ is a caseless match for a string @y@ if and only if:
    --
    -- @toCaseFold x == toCaseFold y@
    --
    -- The result string may be longer than the input string, and may
    -- differ from applying 'toLower' to the input string.  For instance,
    -- the Armenian small ligature \"&#xfb13;\" (men now, U+FB13) is case
    -- folded to the sequence \"&#x574;\" (men, U+0574) followed by
    -- \"&#x576;\" (now, U+0576), while the Greek \"&#xb5;\" (micro sign,
    -- U+00B5) is case folded to \"&#x3bc;\" (small letter mu, U+03BC)
    -- instead of itself.
    toCaseFold :: Text -> Text
    toCaseFold t = unstream (S.toCaseFold (stream t))
    {-# INLINE toCaseFold #-}

    -- | /O(n)/ Convert a string to lower case, using simple case
    -- conversion.  Subject to fusion.
    --
    -- The result string may be longer than the input string.  For
    -- instance, \"&#x130;\" (Latin capital letter I with dot above,
    -- U+0130) maps to the sequence \"i\" (Latin small letter i, U+0069)
    -- followed by \" &#x307;\" (combining dot above, U+0307).
    toLower :: Text -> Text
    toLower t = unstream (S.toLower (stream t))
    {-# INLINE toLower #-}

    -- | /O(n)/ Convert a string to upper case, using simple case
    -- conversion.  Subject to fusion.
    --
    -- The result string may be longer than the input string.  For
    -- instance, the German \"&#xdf;\" (eszett, U+00DF) maps to the
    -- two-letter sequence \"SS\".
    toUpper :: Text -> Text
    toUpper t = unstream (S.toUpper (stream t))
    {-# INLINE toUpper #-}

    -- | /O(n)/ Convert a string to title case, using simple case
    -- conversion. Subject to fusion.
    --
    -- The first letter of the input is converted to title case, as is
    -- every subsequent letter that immediately follows a non-letter.
    -- Every letter that immediately follows another letter is converted
    -- to lower case.
    --
    -- The result string may be longer than the input string. For example,
    -- the Latin small ligature &#xfb02; (U+FB02) is converted to the
    -- sequence Latin capital letter F (U+0046) followed by Latin small
    -- letter l (U+006C).
    --
    -- /Note/: this function does not take language or culture specific
    -- rules into account. For instance, in English, different style
    -- guides disagree on whether the book name \"The Hill of the Red
    -- Fox\" is correctly title cased&#x2014;but this function will
    -- capitalize /every/ word.
    --
    -- @since 1.0.0.0
    toTitle :: Text -> Text
    toTitle t = unstream (S.toTitle (stream t))
    {-# INLINE toTitle #-}

    -- | /O(n)/ Left-justify a string to the given length, using the
    -- specified fill character on the right. Subject to fusion.
    -- Performs replacement on invalid scalar values.
    --
    -- Examples:
    --
    -- >>> justifyLeft 7 'x' "foo"
    -- "fooxxxx"
    --
    -- >>> justifyLeft 3 'x' "foobar"
    -- "foobar"
    justifyLeft :: Int -> Char -> Text -> Text
    justifyLeft k c t
        | len >= k  = t
        | otherwise = t `append` replicateChar (k-len) c
      where len = length t
    {-# INLINE [1] justifyLeft #-}

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

    -- | /O(n)/ Right-justify a string to the given length, using the
    -- specified fill character on the left.  Performs replacement on
    -- invalid scalar values.
    --
    -- Examples:
    --
    -- >>> justifyRight 7 'x' "bar"
    -- "xxxxbar"
    --
    -- >>> justifyRight 3 'x' "foobar"
    -- "foobar"
    justifyRight :: Int -> Char -> Text -> Text
    justifyRight k c t
        | len >= k  = t
        | otherwise = replicateChar (k-len) c `append` t
      where len = length t
    {-# INLINE justifyRight #-}

    -- | /O(n)/ Center a string to the given length, using the specified
    -- fill character on either side.  Performs replacement on invalid
    -- scalar values.
    --
    -- Examples:
    --
    -- >>> center 8 'x' "HS"
    -- "xxxHSxxx"
    center :: Int -> Char -> Text -> Text
    center k c t
        | len >= k  = t
        | otherwise = replicateChar l c `append` t `append` replicateChar r c
      where len = length t
            d   = k - len
            r   = d `quot` 2
            l   = d - r
    {-# INLINE center #-}

    -- | /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.
    --
    -- Examples:
    --
    -- >>> transpose ["green","orange"]
    -- ["go","rr","ea","en","ng","e"]
    --
    -- >>> transpose ["blue","red"]
    -- ["br","le","ud","e"]
    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 fusion.
    foldl :: (a -> Char -> a) -> a -> Text -> a
    foldl f z t = S.foldl f z (stream t)
    {-# INLINE foldl #-}

    -- | /O(n)/ A strict version of 'foldl'.  Subject to fusion.
    foldl' :: (a -> Char -> a) -> a -> Text -> a
    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 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 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 fusion.
    foldr :: (Char -> a -> a) -> a -> Text -> a
    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 thus must be applied to a non-empty 'Text'.  Subject to
    -- 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.
    concat :: [Text] -> Text
    concat ts = case ts' of
                  [] -> empty
                  [t] -> t
                  _ -> Text (A.run go) 0 len
      where
        ts' = L.filter (not . null) ts
        len = sumP "concat" $ L.map lengthWord16 ts'
        go :: ST s (A.MArray s)
        go = do
          arr <- A.new len
          let step i (Text a o l) =
                let !j = i + l in A.copyI arr i a o j >> return j
          foldM step 0 ts' >> return arr

    -- | /O(n)/ Map a function over a 'Text' that results in a 'Text', and
    -- concatenate the results.
    concatMap :: (Char -> Text) -> Text -> Text
    concatMap f = concat . foldr ((:) . f) []
    {-# INLINE concatMap #-}

    -- | /O(n)/ 'any' @p@ @t@ determines whether any character in the
    -- 'Text' @t@ satisfies the predicate @p@. Subject to 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@ satisfy the predicate @p@. Subject to 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 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 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. Subject to fusion.
    -- Performs replacement on invalid scalar values.
    --
    -- > 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 g z (stream t))
        where g a b = safe (f a b)
    {-# INLINE scanl #-}

    -- | /O(n)/ 'scanl1' is a variant of 'scanl' that has no starting
    -- value argument.  Subject to fusion.  Performs replacement on
    -- invalid scalar values.
    --
    -- > 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'.  Performs
    -- replacement on invalid scalar values.
    --
    -- > scanr f v == reverse . scanl (flip f) v . reverse
    scanr :: (Char -> Char -> Char) -> Char -> Text -> Text
    scanr f z = S.reverse . S.reverseScanr g z . reverseStream
        where g a b = safe (f a b)
    {-# INLINE scanr #-}

    -- | /O(n)/ 'scanr1' is a variant of 'scanr' that has no starting
    -- value argument.  Subject to fusion.  Performs replacement on
    -- invalid scalar values.
    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'.  Performs
    -- replacement on invalid scalar values.
    mapAccumL :: (a -> Char -> (a,Char)) -> a -> Text -> (a, Text)
    mapAccumL f z0 = S.mapAccumL g z0 . stream
        where g a b = second safe (f a b)
    {-# INLINE mapAccumL #-}

    -- | The 'mapAccumR' function behaves like a combination of 'map' and
    -- a strict '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'.
    -- Performs replacement on invalid scalar values.
    mapAccumR :: (a -> Char -> (a,Char)) -> a -> Text -> (a, Text)
    mapAccumR f z0 = second reverse . S.mapAccumL g z0 . reverseStream
        where g a b = second safe (f a b)
    {-# INLINE mapAccumR #-}

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

    -- | /O(n*m)/ 'replicate' @n@ @t@ is a 'Text' consisting of the input
    -- @t@ repeated @n@ times.
    replicate :: Int -> Text -> Text
    replicate n t@(Text a o l)
        | n <= 0 || l <= 0       = empty
        | n == 1                 = t
        | isSingleton t          = replicateChar n (unsafeHead t)
        | otherwise              = Text (A.run x) 0 len
      where
        len = l `mul` n
        x :: ST s (A.MArray s)
        x = do
          arr <- A.new len
          let loop !d !i | i >= n    = return arr
                         | otherwise = let m = d + l
                                       in A.copyI arr d a o m >> loop m (i+1)
          loop 0 0
    {-# INLINE [1] replicate #-}

    {-# RULES
    "TEXT replicate/singleton -> replicateChar" [~1] forall n c.
        replicate n (singleton c) = replicateChar n c
      #-}

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

    -- | /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.  Performs replacement on invalid scalar values.
    unfoldr     :: (a -> Maybe (Char,a)) -> a -> Text
    unfoldr f s = unstream (S.unfoldr (firstf safe . 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.  Performs replacement on invalid scalar values.
    unfoldrN     :: Int -> (a -> Maybe (Char,a)) -> a -> Text
    unfoldrN n f s = unstream (S.unfoldrN n (firstf safe . 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 (iterN n t)
    {-# INLINE [1] take #-}

    iterN :: Int -> Text -> Int
    iterN n t@(Text _arr _off len) = loop 0 0
      where loop !i !cnt
                | i >= len || cnt >= n = i
                | otherwise            = loop (i+d) (cnt+1)
              where d = iter_ t i

    {-# 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)/ 'takeEnd' @n@ @t@ returns the suffix remaining after
    -- taking @n@ characters from the end of @t@.
    --
    -- Examples:
    --
    -- >>> takeEnd 3 "foobar"
    -- "bar"
    --
    -- @since 1.1.1.0
    takeEnd :: Int -> Text -> Text
    takeEnd n t@(Text arr off len)
        | n <= 0    = empty
        | n >= len  = t
        | otherwise = text arr (off+i) (len-i)
      where i = iterNEnd n t

    iterNEnd :: Int -> Text -> Int
    iterNEnd n t@(Text _arr _off len) = loop (len-1) n
      where loop i !m
              | m <= 0    = i+1
              | i <= 0    = 0
              | otherwise = loop (i+d) (m-1)
              where d = reverseIter_ t i

    -- | /O(n)/ 'drop' @n@, applied to a 'Text', returns the suffix of the
    -- 'Text' after the first @n@ characters, 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 = text arr (off+i) (len-i)
      where i = iterN n t
    {-# 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)/ 'dropEnd' @n@ @t@ returns the prefix remaining after
    -- dropping @n@ characters from the end of @t@.
    --
    -- Examples:
    --
    -- >>> dropEnd 3 "foobar"
    -- "foo"
    --
    -- @since 1.1.1.0
    dropEnd :: Int -> Text -> Text
    dropEnd n t@(Text arr off len)
        | n <= 0    = t
        | n >= len  = empty
        | otherwise = text arr off (iterNEnd n t)

    -- | /O(n)/ 'takeWhile', applied to a predicate @p@ and a 'Text',
    -- returns the longest prefix (possibly empty) of elements that
    -- satisfy @p@.  Subject to 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   = text arr off i
                where Iter 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)/ 'takeWhileEnd', applied to a predicate @p@ and a 'Text',
    -- returns the longest suffix (possibly empty) of elements that
    -- satisfy @p@.  Subject to fusion.
    -- Examples:
    --
    -- >>> takeWhileEnd (=='o') "foo"
    -- "oo"
    --
    -- @since 1.2.2.0
    takeWhileEnd :: (Char -> Bool) -> Text -> Text
    takeWhileEnd p t@(Text arr off len) = loop (len-1) len
      where loop !i !l | l <= 0    = t
                       | p c       = loop (i+d) (l+d)
                       | otherwise = text arr (off+l) (len-l)
                where (c,d)        = reverseIter t i
    {-# INLINE [1] takeWhileEnd #-}

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

    -- | /O(n)/ 'dropWhile' @p@ @t@ returns the suffix remaining after
    -- 'takeWhile' @p@ @t@. Subject to 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 Iter 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)/ 'dropWhileEnd' @p@ @t@ returns the prefix remaining after
    -- dropping characters that satisfy the predicate @p@ from the end of
    -- @t@.  Subject to fusion.
    --
    -- Examples:
    --
    -- >>> dropWhileEnd (=='.') "foo..."
    -- "foo"
    dropWhileEnd :: (Char -> Bool) -> Text -> Text
    dropWhileEnd p t@(Text arr off len) = loop (len-1) len
      where loop !i !l | l <= 0    = empty
                       | p c       = loop (i+d) (l+d)
                       | otherwise = Text arr off l
                where (c,d)        = reverseIter t i
    {-# INLINE [1] dropWhileEnd #-}

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

    -- | /O(n)/ 'dropAround' @p@ @t@ returns the substring remaining after
    -- dropping characters that satisfy the predicate @p@ from both the
    -- beginning and end of @t@.  Subject to fusion.
    dropAround :: (Char -> Bool) -> Text -> Text
    dropAround p = dropWhile p . dropWhileEnd p
    {-# INLINE [1] dropAround #-}

    -- | /O(n)/ Remove leading white space from a string.  Equivalent to:
    --
    -- > dropWhile isSpace
    stripStart :: Text -> Text
    stripStart = dropWhile isSpace
    {-# INLINE [1] stripStart #-}

    -- | /O(n)/ Remove trailing white space from a string.  Equivalent to:
    --
    -- > dropWhileEnd isSpace
    stripEnd :: Text -> Text
    stripEnd = dropWhileEnd isSpace
    {-# INLINE [1] stripEnd #-}

    -- | /O(n)/ Remove leading and trailing white space from a string.
    -- Equivalent to:
    --
    -- > dropAround isSpace
    strip :: Text -> Text
    strip = dropAround isSpace
    {-# INLINE [1] strip #-}

    -- | /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 = let k = iterN n t
                      in (text arr off k, text arr (off+k) (len-k))

    -- | /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 = case span_ p t of
                 (# hd,tl #) -> (hd,tl)
    {-# 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 Iter 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 Iter 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)

    -- $split
    --
    -- Splitting functions in this library do not perform character-wise
    -- copies to create substrings; they just construct new 'Text's that
    -- are slices of the original.

    -- | /O(m+n)/ Break a 'Text' into pieces separated by the first 'Text'
    -- argument (which cannot be empty), consuming the delimiter. An empty
    -- delimiter is invalid, and will cause an error to be raised.
    --
    -- Examples:
    --
    -- >>> splitOn "\r\n" "a\r\nb\r\nd\r\ne"
    -- ["a","b","d","e"]
    --
    -- >>> splitOn "aaa"  "aaaXaaaXaaaXaaa"
    -- ["","X","X","X",""]
    --
    -- >>> splitOn "x"    "x"
    -- ["",""]
    --
    -- and
    --
    -- > intercalate s . splitOn s         == id
    -- > splitOn (singleton c)             == split (==c)
    --
    -- (Note: the string @s@ to split on above cannot be empty.)
    --
    -- In (unlikely) bad cases, this function's time complexity degrades
    -- towards /O(n*m)/.
    splitOn :: Text
            -- ^ String to split on. If this string is empty, an error
            -- will occur.
            -> Text
            -- ^ Input text.
            -> [Text]
    splitOn pat@(Text _ _ l) src@(Text arr off len)
        | l <= 0          = emptyError "splitOn"
        | isSingleton pat = split (== unsafeHead pat) src
        | otherwise       = go 0 (indices pat src)
      where
        go !s (x:xs) =  text arr (s+off) (x-s) : go (x+l) xs
        go  s _      = [text arr (s+off) (len-s)]
    {-# INLINE [1] splitOn #-}

    {-# RULES
    "TEXT splitOn/singleton -> split/==" [~1] forall c t.
        splitOn (singleton c) t = split (==c) t
      #-}

    -- | /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.
    --
    -- >>> split (=='a') "aabbaca"
    -- ["","","bb","c",""]
    --
    -- >>> split (=='a') ""
    -- [""]
    split :: (Char -> Bool) -> Text -> [Text]
    split _ t@(Text _off _arr 0) = [t]
    split p t = loop t
        where loop s | null s'   = [l]
                     | otherwise = l : loop (unsafeTail s')
                  where (# l, s' #) = span_ (not . p) s
    {-# INLINE split #-}

    -- | /O(n)/ Splits a 'Text' into components of length @k@.  The last
    -- element may be shorter than the other chunks, depending on the
    -- length of the input. Examples:
    --
    -- >>> chunksOf 3 "foobarbaz"
    -- ["foo","bar","baz"]
    --
    -- >>> chunksOf 4 "haskell.org"
    -- ["hask","ell.","org"]
    chunksOf :: Int -> Text -> [Text]
    chunksOf k = go
      where
        go t = case splitAt k t of
                 (a,b) | null a    -> []
                       | otherwise -> a : go b
    {-# INLINE chunksOf #-}

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

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

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

    -- | /O(n+m)/ Find the first instance of @needle@ (which must be
    -- non-'null') 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.
    --
    -- Examples:
    --
    -- >>> breakOn "::" "a::b::c"
    -- ("a","::b::c")
    --
    -- >>> breakOn "/" "foobar"
    -- ("foobar","")
    --
    -- Laws:
    --
    -- > append prefix match == haystack
    -- >   where (prefix, match) = breakOn needle haystack
    --
    -- If you need to break a string by a substring repeatedly (e.g. you
    -- want to break on every instance of a substring), use 'breakOnAll'
    -- instead, as it has lower startup overhead.
    --
    -- In (unlikely) bad cases, this function's time complexity degrades
    -- towards /O(n*m)/.
    breakOn :: Text -> Text -> (Text, Text)
    breakOn pat src@(Text arr off len)
        | null pat  = emptyError "breakOn"
        | otherwise = case indices pat src of
                        []    -> (src, empty)
                        (x:_) -> (text arr off x, text arr (off+x) (len-x))
    {-# INLINE breakOn #-}

    -- | /O(n+m)/ 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 :: Text -> Text -> (Text, Text)
    breakOnEnd pat src = (reverse b, reverse a)
        where (a,b) = breakOn (reverse pat) (reverse src)
    {-# INLINE breakOnEnd #-}

    -- | /O(n+m)/ Find all non-overlapping instances of @needle@ in
    -- @haystack@.  Each element of the returned list consists of a pair:
    --
    -- * The entire string prior to the /k/th match (i.e. the prefix)
    --
    -- * The /k/th match, followed by the remainder of the string
    --
    -- Examples:
    --
    -- >>> breakOnAll "::" ""
    -- []
    --
    -- >>> breakOnAll "/" "a/b/c/"
    -- [("a","/b/c/"),("a/b","/c/"),("a/b/c","/")]
    --
    -- In (unlikely) bad cases, this function's time complexity degrades
    -- towards /O(n*m)/.
    --
    -- The @needle@ parameter may not be empty.
    breakOnAll :: Text              -- ^ @needle@ to search for
               -> Text              -- ^ @haystack@ in which to search
               -> [(Text, Text)]
    breakOnAll pat src@(Text arr off slen)
        | null pat  = emptyError "breakOnAll"
        | otherwise = L.map step (indices pat src)
      where
        step       x = (chunk 0 x, chunk x (slen-x))
        chunk !n !l  = text arr (n+off) l
    {-# INLINE breakOnAll #-}

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

    -- $index
    --
    -- If you think of a 'Text' value as an array of 'Char' values (which
    -- it is not), you run the risk of writing inefficient code.
    --
    -- An idiom that is common in some languages is to find the numeric
    -- offset of a character or substring, then use that number to split
    -- or trim the searched string.  With a 'Text' value, this approach
    -- would require two /O(n)/ operations: one to perform the search, and
    -- one to operate from wherever the search ended.
    --
    -- For example, suppose you have a string that you want to split on
    -- the substring @\"::\"@, such as @\"foo::bar::quux\"@. Instead of
    -- searching for the index of @\"::\"@ and taking the substrings
    -- before and after that index, you would instead use @breakOnAll \"::\"@.

    -- | /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. Subject to fusion.
    findIndex :: (Char -> Bool) -> Text -> Maybe Int
    findIndex p t = S.findIndex p (stream t)
    {-# INLINE findIndex #-}

    -- | /O(n+m)/ The 'count' function returns the number of times the
    -- query string appears in the given 'Text'. An empty query string is
    -- invalid, and will cause an error to be raised.
    --
    -- In (unlikely) bad cases, this function's time complexity degrades
    -- towards /O(n*m)/.
    count :: Text -> Text -> Int
    count pat src
        | null pat        = emptyError "count"
        | isSingleton pat = countChar (unsafeHead pat) src
        | otherwise       = L.length (indices pat src)
    {-# INLINE [1] count #-}

    {-# RULES
    "TEXT count/singleton -> countChar" [~1] forall c t.
        count (singleton c) t = countChar c t
      #-}

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

    -------------------------------------------------------------------------------
    -- * 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 zip #-}

    -- | /O(n)/ 'zipWith' generalises 'zip' by zipping with the function
    -- given as the first argument, instead of a tupling function.
    -- Performs replacement on invalid scalar values.
    zipWith :: (Char -> Char -> Char) -> Text -> Text -> Text
    zipWith f t1 t2 = unstream (S.zipWith g (stream t1) (stream t2))
        where g a b = safe (f a b)
    {-# INLINE 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 Iter 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.  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)
      #-}

    -- | /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+m)/ The 'isInfixOf' function takes two 'Text's and returns
    -- 'True' iff the first is contained, wholly and intact, anywhere
    -- within the second.
    --
    -- In (unlikely) bad cases, this function's time complexity degrades
    -- towards /O(n*m)/.
    isInfixOf :: Text -> Text -> Bool
    isInfixOf needle haystack
        | null needle        = True
        | isSingleton needle = S.elem (unsafeHead needle) . S.stream $ haystack
        | otherwise          = not . L.null . indices needle $ haystack
    {-# INLINE [1] isInfixOf #-}

    {-# RULES
    "TEXT isInfixOf/singleton -> S.elem/S.stream" [~1] forall n h.
        isInfixOf (singleton n) h = S.elem n (S.stream h)
      #-}

    -------------------------------------------------------------------------------
    -- * View patterns

    -- | /O(n)/ Return the suffix of the second string if its prefix
    -- matches the entire first string.
    --
    -- Examples:
    --
    -- >>> stripPrefix "foo" "foobar"
    -- Just "bar"
    --
    -- >>> stripPrefix ""    "baz"
    -- Just "baz"
    --
    -- >>> stripPrefix "foo" "quux"
    -- Nothing
    --
    -- This is particularly useful with the @ViewPatterns@ extension to
    -- GHC, as follows:
    --
    -- > {-# LANGUAGE ViewPatterns #-}
    -- > import Data.Text as T
    -- >
    -- > fnordLength :: Text -> Int
    -- > fnordLength (stripPrefix "fnord" -> Just suf) = T.length suf
    -- > fnordLength _                                 = -1
    stripPrefix :: Text -> Text -> Maybe Text
    stripPrefix p@(Text _arr _off plen) t@(Text arr off len)
        | p `isPrefixOf` t = Just $! text arr (off+plen) (len-plen)
        | otherwise        = Nothing

    -- | /O(n)/ Find the longest non-empty common prefix of two strings
    -- and return it, along with the suffixes of each string at which they
    -- no longer match.
    --
    -- If the strings do not have a common prefix or either one is empty,
    -- this function returns 'Nothing'.
    --
    -- Examples:
    --
    -- >>> commonPrefixes "foobar" "fooquux"
    -- Just ("foo","bar","quux")
    --
    -- >>> commonPrefixes "veeble" "fetzer"
    -- Nothing
    --
    -- >>> commonPrefixes "" "baz"
    -- Nothing
    commonPrefixes :: Text -> Text -> Maybe (Text,Text,Text)
    commonPrefixes t0@(Text arr0 off0 len0) t1@(Text arr1 off1 len1) = go 0 0
      where
        go !i !j | i < len0 && j < len1 && a == b = go (i+d0) (j+d1)
                 | i > 0     = Just (Text arr0 off0 i,
                                     text arr0 (off0+i) (len0-i),
                                     text arr1 (off1+j) (len1-j))
                 | otherwise = Nothing
          where Iter a d0 = iter t0 i
                Iter b d1 = iter t1 j

    -- | /O(n)/ Return the prefix of the second string if its suffix
    -- matches the entire first string.
    --
    -- Examples:
    --
    -- >>> stripSuffix "bar" "foobar"
    -- Just "foo"
    --
    -- >>> stripSuffix ""    "baz"
    -- Just "baz"
    --
    -- >>> stripSuffix "foo" "quux"
    -- Nothing
    --
    -- This is particularly useful with the @ViewPatterns@ extension to
    -- GHC, as follows:
    --
    -- > {-# LANGUAGE ViewPatterns #-}
    -- > import Data.Text as T
    -- >
    -- > quuxLength :: Text -> Int
    -- > quuxLength (stripSuffix "quux" -> Just pre) = T.length pre
    -- > quuxLength _                                = -1
    stripSuffix :: Text -> Text -> Maybe Text
    stripSuffix p@(Text _arr _off plen) t@(Text arr off len)
        | p `isSuffixOf` t = Just $! text arr off (len-plen)
        | otherwise        = Nothing

    -- | Add a list of non-negative numbers.  Errors out on overflow.
    sumP :: String -> [Int] -> Int
    sumP fun = go 0
      where go !a (x:xs)
                | ax >= 0   = go ax xs
                | otherwise = overflowError fun
              where ax = a + x
            go a  _         = a

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

    overflowError :: String -> a
    overflowError fun = P.error $ "Data.Text." ++ fun ++ ": size overflow"

    -- | /O(n)/ Make a distinct copy of the given string, sharing no
    -- storage with the original string.
    --
    -- As an example, suppose you read a large string, of which you need
    -- only a small portion.  If you do not use 'copy', the entire original
    -- array will be kept alive in memory by the smaller string. Making a
    -- copy \"breaks the link\" to the original array, allowing it to be
    -- garbage collected if there are no other live references to it.
    copy :: Text -> Text
    copy (Text arr off len) = Text (A.run go) 0 len
      where
        go :: ST s (A.MArray s)
        go = do
          marr <- A.new len
          A.copyI marr 0 arr off len
          return marr

    -------------------------------------------------
    -- NOTE: the named chunk below used by doctest;
    --       verify the doctests via `doctest -fobject-code Data/Text.hs`

    -- $setup
    -- >>> :set -XOverloadedStrings
    -- >>> import qualified Data.Text as T