{-# LANGUAGE BangPatterns, Rank2Types #-}
-- |
-- Module      : Data.Text.Fusion.Common
-- Copyright   : (c) Bryan O'Sullivan 2009
--
-- License     : BSD-style
-- Maintainer  : bos@serpentine.com, rtomharper@googlemail.com,
--               duncan@haskell.org
-- Stability   : experimental
-- Portability : GHC
--
-- Common stream fusion functionality for text.

module Data.Text.Fusion.Common
    (
    -- * Creation and elimination
      singleton
    , streamList
    , unstreamList

    -- * Basic interface
    , cons
    , snoc
    , append
    , head
    , uncons
    , last
    , tail
    , init
    , null
    , lengthI
    , compareLengthI
    , isSingleton

    -- * Transformations
    , map
    , intercalate
    , intersperse

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

    -- ** Justification
    , justifyLeftI

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

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

    -- * Construction
    -- ** Scans
    , scanl

    -- ** Accumulating maps
    -- , mapAccumL

    -- ** Generation and unfolding
    , replicateCharI
    , replicateI
    , unfoldr
    , unfoldrNI

    -- * Substrings
    -- ** Breaking strings
    , take
    , drop
    , takeWhile
    , dropWhile

    -- * Predicates
    , isPrefixOf

    -- * Searching
    , elem
    , filter

    -- * Indexing
    , findBy
    , indexI
    , findIndexI
    , countCharI

    -- * Zipping and unzipping
    , zipWith
    ) where

import Prelude (Bool(..), Char, Eq(..), Int, Integral, Maybe(..),
                Ord(..), Ordering(..), String, (.), ($), (+), (-), (*), (++),
                (&&), fromIntegral, otherwise)
import qualified Data.List as L
import qualified Prelude as P
import Data.Int (Int64)
import Data.Text.Fusion.Internal
import Data.Text.Fusion.CaseMapping (foldMapping, lowerMapping, upperMapping)
import Data.Text.Fusion.Size

singleton :: Char -> Stream Char
singleton c = Stream next False 1
    where next False = Yield c True
          next True  = Done
{-# INLINE singleton #-}

streamList :: [a] -> Stream a
{-# INLINE [0] streamList #-}
streamList s  = Stream next s unknownSize
    where next []       = Done
          next (x:xs)   = Yield x xs

unstreamList :: Stream a -> [a]
unstreamList (Stream next s0 _len) = unfold s0
    where unfold !s = case next s of
                        Done       -> []
                        Skip s'    -> unfold s'
                        Yield x s' -> x : unfold s'
{-# INLINE [0] unstreamList #-}

{-# RULES "STREAM streamList/unstreamList fusion" forall s. streamList (unstreamList s) = s #-}

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

data C s = C0 !s
         | C1 !s

-- | /O(n)/ Adds a character to the front of a Stream Char.
cons :: Char -> Stream Char -> Stream Char
cons w (Stream next0 s0 len) = Stream next (C1 s0) (len+1)
    where
      next (C1 s) = Yield w (C0 s)
      next (C0 s) = case next0 s of
                          Done -> Done
                          Skip s' -> Skip (C0 s')
                          Yield x s' -> Yield x (C0 s')
{-# INLINE [0] cons #-}

-- | /O(n)/ Adds a character to the end of a stream.
snoc :: Stream Char -> Char -> Stream Char
snoc (Stream next0 xs0 len) w = Stream next (J xs0) (len+1)
  where
    next (J xs) = case next0 xs of
      Done        -> Yield w N
      Skip xs'    -> Skip    (J xs')
      Yield x xs' -> Yield x (J xs')
    next N = Done
{-# INLINE [0] snoc #-}

data E l r = L !l
           | R !r

-- | /O(n)/ Appends one Stream to the other.
append :: Stream Char -> Stream Char -> Stream Char
append (Stream next0 s01 len1) (Stream next1 s02 len2) =
    Stream next (L s01) (len1 + len2)
    where
      next (L s1) = case next0 s1 of
                         Done        -> Skip    (R s02)
                         Skip s1'    -> Skip    (L s1')
                         Yield x s1' -> Yield x (L s1')
      next (R s2) = case next1 s2 of
                          Done        -> Done
                          Skip s2'    -> Skip    (R s2')
                          Yield x s2' -> Yield x (R s2')
{-# INLINE [0] append #-}

-- | /O(1)/ Returns the first character of a Text, which must be non-empty.
-- Subject to array fusion.
head :: Stream Char -> Char
head (Stream next s0 _len) = loop_head s0
    where
      loop_head !s = case next s of
                      Yield x _ -> x
                      Skip s'   -> loop_head s'
                      Done      -> streamError "head" "Empty stream"
{-# INLINE [0] head #-}

-- | /O(1)/ Returns the first character and remainder of a 'Stream
-- Char', or 'Nothing' if empty.  Subject to array fusion.
uncons :: Stream Char -> Maybe (Char, Stream Char)
uncons (Stream next s0 len) = loop_uncons s0
    where
      loop_uncons !s = case next s of
                         Yield x s1 -> Just (x, Stream next s1 (len-1))
                         Skip s'    -> loop_uncons s'
                         Done       -> Nothing
{-# INLINE [0] uncons #-}

-- | /O(n)/ Returns the last character of a 'Stream Char', which must
-- be non-empty.
last :: Stream Char -> Char
last (Stream next s0 _len) = loop0_last s0
    where
      loop0_last !s = case next s of
                        Done       -> emptyError "last"
                        Skip s'    -> loop0_last  s'
                        Yield x s' -> loop_last x s'
      loop_last !x !s = case next s of
                         Done        -> x
                         Skip s'     -> loop_last x  s'
                         Yield x' s' -> loop_last x' s'
{-# INLINE[0] last #-}

-- | /O(1)/ Returns all characters after the head of a Stream Char, which must
-- be non-empty.
tail :: Stream Char -> Stream Char
tail (Stream next0 s0 len) = Stream next (C0 s0) (len-1)
    where
      next (C0 s) = case next0 s of
                      Done       -> emptyError "tail"
                      Skip s'    -> Skip (C0 s')
                      Yield _ s' -> Skip (C1 s')
      next (C1 s) = case next0 s of
                      Done       -> Done
                      Skip s'    -> Skip    (C1 s')
                      Yield x s' -> Yield x (C1 s')
{-# INLINE [0] tail #-}

data Init s = Init0 !s
            | Init1 {-# UNPACK #-} !Char !s

-- | /O(1)/ Returns all but the last character of a Stream Char, which
-- must be non-empty.
init :: Stream Char -> Stream Char
init (Stream next0 s0 len) = Stream next (Init0 s0) (len-1)
    where
      next (Init0 s) = case next0 s of
                         Done       -> emptyError "init"
                         Skip s'    -> Skip (Init0 s')
                         Yield x s' -> Skip (Init1 x s')
      next (Init1 x s)  = case next0 s of
                            Done        -> Done
                            Skip s'     -> Skip    (Init1 x s')
                            Yield x' s' -> Yield x (Init1 x' s')
{-# INLINE [0] init #-}

-- | /O(1)/ Tests whether a Stream Char is empty or not.
null :: Stream Char -> Bool
null (Stream next s0 _len) = loop_null s0
    where
      loop_null !s = case next s of
                       Done      -> True
                       Yield _ _ -> False
                       Skip s'   -> loop_null s'
{-# INLINE[0] null #-}

-- | /O(n)/ Returns the number of characters in a string.
lengthI :: Integral a => Stream Char -> a
lengthI (Stream next s0 _len) = loop_length 0 s0
    where
      loop_length !z s  = case next s of
                           Done       -> z
                           Skip    s' -> loop_length z s'
                           Yield _ s' -> loop_length (z + 1) s'
{-# INLINE[0] lengthI #-}

-- | /O(n)/ Compares the count of characters in a string to a number.
-- Subject to fusion.
--
-- This function gives the same answer as comparing against the result
-- of 'lengthI', but can short circuit if the count of characters is
-- greater than the number, and hence be more efficient.
compareLengthI :: Integral a => Stream Char -> a -> Ordering
compareLengthI (Stream next s0 len) n = 
    case exactly len of
      Nothing -> loop_cmp 0 s0
      Just i  -> compare (fromIntegral i) n
    where
      loop_cmp !z s  = case next s of
                         Done       -> compare z n
                         Skip    s' -> loop_cmp z s'
                         Yield _ s' | z > n     -> GT
                                    | otherwise -> loop_cmp (z + 1) s'
{-# INLINE[0] compareLengthI #-}

-- | /O(n)/ Indicate whether a string contains exactly one element.
isSingleton :: Stream Char -> Bool
isSingleton (Stream next s0 _len) = loop 0 s0
    where
      loop !z s  = case next s of
                     Done            -> z == (1::Int)
                     Skip    s'      -> loop z s'
                     Yield _ s'
                         | z >= 1    -> False
                         | otherwise -> loop (z+1) s'
{-# INLINE[0] isSingleton #-}

-- ----------------------------------------------------------------------------
-- * Stream transformations

-- | /O(n)/ 'map' @f @xs is the Stream Char obtained by applying @f@
-- to each element of @xs@.
map :: (Char -> Char) -> Stream Char -> Stream Char
map f (Stream next0 s0 len) = Stream next s0 len
    where
      next !s = case next0 s of
                  Done       -> Done
                  Skip s'    -> Skip s'
                  Yield x s' -> Yield (f x) s'
{-# INLINE [0] map #-}

{-#
  RULES "STREAM map/map fusion" forall f g s.
     map f (map g s) = map (\x -> f (g x)) s
 #-}

data I s = I1 !s
         | I2 !s {-# UNPACK #-} !Char
         | I3 !s

-- | /O(n)/ Take a character and place it between each of the
-- characters of a 'Stream Char'.
intersperse :: Char -> Stream Char -> Stream Char
intersperse c (Stream next0 s0 len) = Stream next (I1 s0) len
    where
      next (I1 s) = case next0 s of
        Done       -> Done
        Skip s'    -> Skip (I1 s')
        Yield x s' -> Skip (I2 s' x)
      next (I2 s x)  = Yield x (I3 s)
      next (I3 s) = case next0 s of
        Done       -> Done
        Skip s'    -> Skip    (I3 s')
        Yield x s' -> Yield c (I2 s' x)
{-# INLINE [0] intersperse #-}

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

-- $case
--
-- With Unicode text, it is incorrect to use combinators like @map
-- toUpper@ to case convert each character of a string individually.
-- Instead, use the whole-string case conversion functions from this
-- module.  For correctness in different writing systems, these
-- functions may map one input character to two or three output
-- characters.

caseConvert :: (forall s. Char -> s -> Step (CC s) Char)
            -> Stream Char -> Stream Char
caseConvert remap (Stream next0 s0 len) = Stream next (CC s0 '\0' '\0') len
  where
    next (CC s '\0' _) =
        case next0 s of
          Done       -> Done
          Skip s'    -> Skip (CC s' '\0' '\0')
          Yield c s' -> remap c s'
    next (CC s a b)  =  Yield a (CC s b '\0')

-- | /O(n)/ Convert a string to folded case.  This function is mainly
-- useful for performing caseless (or 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 men now (U+FB13) is case folded to the
-- bigram men now (U+0574 U+0576), while the micro sign (U+00B5) is
-- case folded to the Greek small letter letter mu (U+03BC) instead of
-- itself.
toCaseFold :: Stream Char -> Stream Char
toCaseFold = caseConvert foldMapping
{-# INLINE [0] toCaseFold #-}

-- | /O(n)/ Convert a string to upper case, using simple case
-- conversion.  The result string may be longer than the input string.
-- For instance, the German eszett (U+00DF) maps to the two-letter
-- sequence SS.
toUpper :: Stream Char -> Stream Char
toUpper = caseConvert upperMapping
{-# INLINE [0] toUpper #-}

-- | /O(n)/ Convert a string to lower case, using simple case
-- conversion.  The result string may be longer than the input string.
-- For instance, the Latin capital letter I with dot above (U+0130)
-- maps to the sequence Latin small letter i (U+0069) followed by
-- combining dot above (U+0307).
toLower :: Stream Char -> Stream Char
toLower = caseConvert lowerMapping
{-# INLINE [0] toLower #-}

justifyLeftI :: Integral a => a -> Char -> Stream Char -> Stream Char
justifyLeftI k c (Stream next0 s0 len) =
    Stream next (s0 :*: S1 :*: 0) (larger (fromIntegral k) len)
  where
    next (s :*: S1 :*: n) =
        case next0 s of
          Done       -> next (s :*: S2 :*: n)
          Skip s'    -> Skip (s' :*: S1 :*: n)
          Yield x s' -> Yield x (s' :*: S1 :*: n+1)
    next (s :*: S2 :*: n)
        | n < k       = Yield c (s :*: S2 :*: n+1)
        | otherwise   = Done
    {-# INLINE next #-}
{-# INLINE [0] justifyLeftI #-}

-- ----------------------------------------------------------------------------
-- * Reducing Streams (folds)

-- | foldl, applied to a binary operator, a starting value (typically the
-- left-identity of the operator), and a Stream, reduces the Stream using the
-- binary operator, from left to right.
foldl :: (b -> Char -> b) -> b -> Stream Char -> b
foldl f z0 (Stream next s0 _len) = loop_foldl z0 s0
    where
      loop_foldl z !s = case next s of
                          Done -> z
                          Skip s' -> loop_foldl z s'
                          Yield x s' -> loop_foldl (f z x) s'
{-# INLINE [0] foldl #-}

-- | A strict version of foldl.
foldl' :: (b -> Char -> b) -> b -> Stream Char -> b
foldl' f z0 (Stream next s0 _len) = loop_foldl' z0 s0
    where
      loop_foldl' !z !s = case next s of
                            Done -> z
                            Skip s' -> loop_foldl' z s'
                            Yield x s' -> loop_foldl' (f z x) s'
{-# INLINE [0] foldl' #-}

-- | foldl1 is a variant of foldl that has no starting value argument,
-- and thus must be applied to non-empty Streams.
foldl1 :: (Char -> Char -> Char) -> Stream Char -> Char
foldl1 f (Stream next s0 _len) = loop0_foldl1 s0
    where
      loop0_foldl1 !s = case next s of
                          Skip s' -> loop0_foldl1 s'
                          Yield x s' -> loop_foldl1 x s'
                          Done -> emptyError "foldl1"
      loop_foldl1 z !s = case next s of
                           Done -> z
                           Skip s' -> loop_foldl1 z s'
                           Yield x s' -> loop_foldl1 (f z x) s'
{-# INLINE [0] foldl1 #-}

-- | A strict version of foldl1.
foldl1' :: (Char -> Char -> Char) -> Stream Char -> Char
foldl1' f (Stream next s0 _len) = loop0_foldl1' s0
    where
      loop0_foldl1' !s = case next s of
                           Skip s' -> loop0_foldl1' s'
                           Yield x s' -> loop_foldl1' x s'
                           Done -> emptyError "foldl1"
      loop_foldl1' !z !s = case next s of
                             Done -> z
                             Skip s' -> loop_foldl1' z s'
                             Yield x s' -> loop_foldl1' (f z x) s'
{-# INLINE [0] foldl1' #-}

-- | 'foldr', applied to a binary operator, a starting value (typically the
-- right-identity of the operator), and a stream, reduces the stream using the
-- binary operator, from right to left.
foldr :: (Char -> b -> b) -> b -> Stream Char -> b
foldr f z (Stream next s0 _len) = loop_foldr s0
    where
      loop_foldr !s = case next s of
                        Done -> z
                        Skip s' -> loop_foldr s'
                        Yield x s' -> f x (loop_foldr s')
{-# INLINE [0] foldr #-}

-- | foldr1 is a variant of 'foldr' that has no starting value argument,
-- and thus must be applied to non-empty streams.
-- Subject to array fusion.
foldr1 :: (Char -> Char -> Char) -> Stream Char -> Char
foldr1 f (Stream next s0 _len) = loop0_foldr1 s0
  where
    loop0_foldr1 !s = case next s of
      Done       -> emptyError "foldr1"
      Skip    s' -> loop0_foldr1  s'
      Yield x s' -> loop_foldr1 x s'

    loop_foldr1 x !s = case next s of
      Done        -> x
      Skip     s' -> loop_foldr1 x s'
      Yield x' s' -> f x (loop_foldr1 x' s')
{-# INLINE [0] foldr1 #-}

intercalate :: Stream Char -> [Stream Char] -> Stream Char
intercalate s = concat . (L.intersperse s)
{-# INLINE [0] intercalate #-}

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

-- | /O(n)/ Concatenate a list of streams. Subject to array fusion.
concat :: [Stream Char] -> Stream Char
concat = L.foldr append empty
{-# INLINE [0] concat #-}

-- | Map a function over a stream that results in a stream and concatenate the
-- results.
concatMap :: (Char -> Stream Char) -> Stream Char -> Stream Char
concatMap f = foldr (append . f) empty
{-# INLINE [0] concatMap #-}

-- | /O(n)/ any @p @xs determines if any character in the stream
-- @xs@ satisifes the predicate @p@.
any :: (Char -> Bool) -> Stream Char -> Bool
any p (Stream next0 s0 _len) = loop_any s0
    where
      loop_any !s = case next0 s of
                      Done                   -> False
                      Skip s'                -> loop_any s'
                      Yield x s' | p x       -> True
                                 | otherwise -> loop_any s'
{-# INLINE [0] any #-}

-- | /O(n)/ all @p @xs determines if all characters in the 'Text'
-- @xs@ satisify the predicate @p@.
all :: (Char -> Bool) -> Stream Char -> Bool
all p (Stream next0 s0 _len) = loop_all s0
    where
      loop_all !s = case next0 s of
                      Done                   -> True
                      Skip s'                -> loop_all s'
                      Yield x s' | p x       -> loop_all s'
                                 | otherwise -> False
{-# INLINE [0] all #-}

-- | /O(n)/ maximum returns the maximum value from a stream, which must be
-- non-empty.
maximum :: Stream Char -> Char
maximum (Stream next0 s0 _len) = loop0_maximum s0
    where
      loop0_maximum !s   = case next0 s of
                             Done       -> emptyError "maximum"
                             Skip s'    -> loop0_maximum s'
                             Yield x s' -> loop_maximum x s'
      loop_maximum !z !s = case next0 s of
                             Done            -> z
                             Skip s'         -> loop_maximum z s'
                             Yield x s'
                                 | x > z     -> loop_maximum x s'
                                 | otherwise -> loop_maximum z s'
{-# INLINE [0] maximum #-}

-- | /O(n)/ minimum returns the minimum value from a 'Text', which must be
-- non-empty.
minimum :: Stream Char -> Char
minimum (Stream next0 s0 _len) = loop0_minimum s0
    where
      loop0_minimum !s   = case next0 s of
                             Done       -> emptyError "minimum"
                             Skip s'    -> loop0_minimum s'
                             Yield x s' -> loop_minimum x s'
      loop_minimum !z !s = case next0 s of
                             Done            -> z
                             Skip s'         -> loop_minimum z s'
                             Yield x s'
                                 | x < z     -> loop_minimum x s'
                                 | otherwise -> loop_minimum z s'
{-# INLINE [0] minimum #-}

-- -----------------------------------------------------------------------------
-- * Building streams

scanl :: (Char -> Char -> Char) -> Char -> Stream Char -> Stream Char
scanl f z0 (Stream next0 s0 len) = Stream next (S1 :*: z0 :*: s0) (len+1) -- HINT maybe too low
  where
    {-# INLINE next #-}
    next (S1 :*: z :*: s) = Yield z (S2 :*: z :*: s)
    next (S2 :*: z :*: s) = case next0 s of
                              Yield x s' -> let !x' = f z x
                                            in Yield x' (S2 :*: x' :*: s')
                              Skip s'    -> Skip (S2 :*: z :*: s')
                              Done       -> Done
{-# INLINE [0] scanl #-}

-- -----------------------------------------------------------------------------
-- ** Accumulating maps

{-
-- | /O(n)/ Like a combination of 'map' and 'foldl'. Applies a
-- function to each element of a stream, passing an accumulating
-- parameter from left to right, and returns a final stream.
--
-- /Note/: Unlike the version over lists, this function does not
-- return a final value for the accumulator, because the nature of
-- streams precludes it.
mapAccumL :: (a -> b -> (a,b)) -> a -> Stream b -> Stream b
mapAccumL f z0 (Stream next0 s0 len) = Stream next (s0 :*: z0) len -- HINT depends on f
  where
    {-# INLINE next #-}
    next (s :*: z) = case next0 s of
                       Yield x s' -> let (z',y) = f z x
                                     in Yield y (s' :*: z')
                       Skip s'    -> Skip (s' :*: z)
                       Done       -> Done
{-# INLINE [0] mapAccumL #-}
-}

-- -----------------------------------------------------------------------------
-- ** Generating and unfolding streams

replicateCharI :: Integral a => a -> Char -> Stream Char
replicateCharI n c
    | n < 0     = empty
    | otherwise = Stream next 0 (fromIntegral n) -- HINT maybe too low
  where
    next i | i >= n    = Done
           | otherwise = Yield c (i + 1)
{-# INLINE [0] replicateCharI #-}

data RI s = RI !s {-# UNPACK #-} !Int64

replicateI :: Int64 -> Stream Char -> Stream Char
replicateI n (Stream next0 s0 len) =
    Stream next (RI s0 0) (fromIntegral (max 0 n) * len)
  where
    next (RI s k)
        | k >= n = Done
        | otherwise = case next0 s of
                        Done       -> Skip    (RI s0 (k+1))
                        Skip s'    -> Skip    (RI s' k)
                        Yield x s' -> Yield x (RI s' k)
{-# INLINE [0] replicateI #-}

-- | /O(n)/, where @n@ is the length of the result. The unfoldr function
-- is analogous to the List 'unfoldr'. unfoldr builds a stream
-- from a seed value. The function takes the element and returns
-- Nothing if it is done producing the stream or returns Just
-- (a,b), in which case, a is the next Char in the string, and b is
-- the seed value for further production.
unfoldr :: (a -> Maybe (Char,a)) -> a -> Stream Char
unfoldr f s0 = Stream next s0 1 -- HINT maybe too low
    where
      {-# INLINE next #-}
      next !s = case f s of
                 Nothing      -> Done
                 Just (w, s') -> Yield w s'
{-# INLINE [0] unfoldr #-}

-- | /O(n)/ Like 'unfoldr', 'unfoldrNI' builds a stream from a seed
-- value. However, the length of the result is limited by the
-- first argument to 'unfoldrNI'. This function is more efficient than
-- 'unfoldr' when the length of the result is known.
unfoldrNI :: Integral a => a -> (b -> Maybe (Char,b)) -> b -> Stream Char
unfoldrNI n f s0 | n <  0    = empty
                 | otherwise = Stream next (0 :*: s0) (fromIntegral (n*2)) -- HINT maybe too high
    where
      {-# INLINE next #-}
      next (z :*: s) = case f s of
          Nothing                  -> Done
          Just (w, s') | z >= n    -> Done
                       | otherwise -> Yield w ((z + 1) :*: s')
{-# INLINE unfoldrNI #-}

-------------------------------------------------------------------------------
--  * Substreams

-- | /O(n)/ take n, applied to a stream, returns the prefix of the
-- stream of length @n@, or the stream itself if @n@ is greater than the
-- length of the stream.
take :: Integral a => a -> Stream Char -> Stream Char
take n0 (Stream next0 s0 len) =
    Stream next (n0 :*: s0) (smaller len (fromIntegral (max 0 n0)))
    where
      {-# INLINE next #-}
      next (n :*: s) | n <= 0    = Done
                     | otherwise = case next0 s of
                                     Done -> Done
                                     Skip s' -> Skip (n :*: s')
                                     Yield x s' -> Yield x ((n-1) :*: s')
{-# INLINE [0] take #-}

-- | /O(n)/ drop n, applied to a stream, returns the suffix of the
-- stream after the first @n@ characters, or the empty stream if @n@
-- is greater than the length of the stream.
drop :: Integral a => a -> Stream Char -> Stream Char
drop n0 (Stream next0 s0 len) =
    Stream next (J n0 :*: s0) (len - fromIntegral (max 0 n0))
  where
    {-# INLINE next #-}
    next (J n :*: s)
      | n <= 0    = Skip (N :*: s)
      | otherwise = case next0 s of
          Done       -> Done
          Skip    s' -> Skip (J n    :*: s')
          Yield _ s' -> Skip (J (n-1) :*: s')
    next (N :*: s) = case next0 s of
      Done       -> Done
      Skip    s' -> Skip    (N :*: s')
      Yield x s' -> Yield x (N :*: s')
{-# INLINE [0] drop #-}

-- | takeWhile, applied to a predicate @p@ and a stream, returns the
-- longest prefix (possibly empty) of elements that satisfy p.
takeWhile :: (Char -> Bool) -> Stream Char -> Stream Char
takeWhile p (Stream next0 s0 len) = Stream next s0 len -- HINT maybe too high
    where
      {-# INLINE next #-}
      next !s = case next0 s of
                  Done    -> Done
                  Skip s' -> Skip s'
                  Yield x s' | p x       -> Yield x s'
                             | otherwise -> Done
{-# INLINE [0] takeWhile #-}

-- | dropWhile @p @xs returns the suffix remaining after takeWhile @p @xs.
dropWhile :: (Char -> Bool) -> Stream Char -> Stream Char
dropWhile p (Stream next0 s0 len) = Stream next (S1 :*: s0) len -- HINT maybe too high
    where
    {-# INLINE next #-}
    next (S1 :*: s)  = case next0 s of
      Done                   -> Done
      Skip    s'             -> Skip    (S1 :*: s')
      Yield x s' | p x       -> Skip    (S1 :*: s')
                 | otherwise -> Yield x (S2 :*: s')
    next (S2 :*: s) = case next0 s of
      Done       -> Done
      Skip    s' -> Skip    (S2 :*: s')
      Yield x s' -> Yield x (S2 :*: s')
{-# INLINE [0] dropWhile #-}

-- | /O(n)/ The 'isPrefixOf' function takes two 'Stream's and returns
-- 'True' iff the first is a prefix of the second.
isPrefixOf :: (Eq a) => Stream a -> Stream a -> Bool
isPrefixOf (Stream next1 s1 _) (Stream next2 s2 _) = loop (next1 s1) (next2 s2)
    where
      loop Done      _ = True
      loop _    Done = False
      loop (Skip s1')     (Skip s2')     = loop (next1 s1') (next2 s2')
      loop (Skip s1')     x2             = loop (next1 s1') x2
      loop x1             (Skip s2')     = loop x1          (next2 s2')
      loop (Yield x1 s1') (Yield x2 s2') = x1 == x2 &&
                                           loop (next1 s1') (next2 s2')
{-# INLINE [0] isPrefixOf #-}
{-# SPECIALISE isPrefixOf :: Stream Char -> Stream Char -> Bool #-}

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

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

-- | /O(n)/ elem is the stream membership predicate.
elem :: Char -> Stream Char -> Bool
elem w (Stream next s0 _len) = loop_elem s0
    where
      loop_elem !s = case next s of
                       Done -> False
                       Skip s' -> loop_elem s'
                       Yield x s' | x == w -> True
                                  | otherwise -> loop_elem s'
{-# INLINE [0] elem #-}

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

-- | /O(n)/ The 'findBy' function takes a predicate and a stream,
-- and returns the first element in matching the predicate, or 'Nothing'
-- if there is no such element.

findBy :: (Char -> Bool) -> Stream Char -> Maybe Char
findBy p (Stream next s0 _len) = loop_find s0
    where
      loop_find !s = case next s of
                       Done -> Nothing
                       Skip s' -> loop_find s'
                       Yield x s' | p x -> Just x
                                  | otherwise -> loop_find s'
{-# INLINE [0] findBy #-}

-- | /O(n)/ Stream index (subscript) operator, starting from 0.
indexI :: Integral a => Stream Char -> a -> Char
indexI (Stream next s0 _len) n0
  | n0 < 0    = streamError "index" "Negative index"
  | otherwise = loop_index n0 s0
  where
    loop_index !n !s = case next s of
      Done                   -> streamError "index" "Index too large"
      Skip    s'             -> loop_index  n    s'
      Yield x s' | n == 0    -> x
                 | otherwise -> loop_index (n-1) s'
{-# INLINE [0] indexI #-}

-- | /O(n)/ 'filter', applied to a predicate and a stream,
-- returns a stream containing those characters that satisfy the
-- predicate.
filter :: (Char -> Bool) -> Stream Char -> Stream Char
filter p (Stream next0 s0 len) = Stream next s0 len -- HINT maybe too high
  where
    next !s = case next0 s of
                Done                   -> Done
                Skip    s'             -> Skip    s'
                Yield x s' | p x       -> Yield x s'
                           | otherwise -> Skip    s'
{-# INLINE [0] filter #-}

{-# RULES
  "STREAM filter/filter fusion" forall p q s.
  filter p (filter q s) = filter (\x -> q x && p x) s
  #-}

-- | The 'findIndexI' function takes a predicate and a stream and
-- returns the index of the first element in the stream satisfying the
-- predicate.
findIndexI :: Integral a => (Char -> Bool) -> Stream Char -> Maybe a
findIndexI p s = case findIndicesI p s of
                  (i:_) -> Just i
                  _     -> Nothing
{-# INLINE [0] findIndexI #-}

-- | The 'findIndicesI' function takes a predicate and a stream and
-- returns all indices of the elements in the stream satisfying the
-- predicate.
findIndicesI :: Integral a => (Char -> Bool) -> Stream Char -> [a]
findIndicesI p (Stream next s0 _len) = loop_findIndex 0 s0
  where
    loop_findIndex !i !s = case next s of
      Done                   -> []
      Skip    s'             -> loop_findIndex i     s' -- hmm. not caught by QC
      Yield x s' | p x       -> i : loop_findIndex (i+1) s'
                 | otherwise -> loop_findIndex (i+1) s'
{-# INLINE [0] findIndicesI #-}

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

-- | zipWith generalises 'zip' by zipping with the function given as
-- the first argument, instead of a tupling function.
zipWith :: (a -> a -> b) -> Stream a -> Stream a -> Stream b
zipWith f (Stream next0 sa0 len1) (Stream next1 sb0 len2) =
    Stream next (sa0 :*: sb0 :*: N) (smaller len1 len2)
    where
      next (sa :*: sb :*: N) = case next0 sa of
                                 Done -> Done
                                 Skip sa' -> Skip (sa' :*: sb :*: N)
                                 Yield a sa' -> Skip (sa' :*: sb :*: J a)

      next (sa' :*: sb :*: J a) = case next1 sb of
                                    Done -> Done
                                    Skip sb' -> Skip (sa' :*: sb' :*: J a)
                                    Yield b sb' -> Yield (f a b) (sa' :*: sb' :*: N)
{-# INLINE [0] zipWith #-}

-- | /O(n)/ The 'countCharI' function returns the number of times the
-- query element appears in the given stream.
countCharI :: Integral a => Char -> Stream Char -> a
countCharI a (Stream next s0 _len) = loop 0 s0
  where
    loop !i !s = case next s of
      Done                   -> i
      Skip    s'             -> loop i s'
      Yield x s' | a == x    -> loop (i+1) s'
                 | otherwise -> loop i s'
{-# INLINE [0] countCharI #-}

streamError :: String -> String -> a
streamError func msg = P.error $ "Data.Text.Fusion.Common." ++ func ++ ": " ++ msg

emptyError :: String -> a
emptyError func = internalError func "Empty input"

internalError :: String -> a
internalError func = streamError func "Internal error"