    {-# LANGUAGE BangPatterns, DeriveDataTypeable #-}
    -- |
    -- Module      : Data.Text.Lazy.Internal
    -- Copyright   : (c) 2009, 2010 Bryan O'Sullivan
    --
    -- License     : BSD-style
    -- Maintainer  : bos@serpentine.com, rtomharper@googlemail.com,
    --               duncan@haskell.org
    -- Stability   : experimental
    -- Portability : GHC
    --
    -- A module containing private 'Text' internals. This exposes the
    -- 'Text' representation and low level construction functions.
    -- Modules which extend the 'Text' system may need to use this module.
    --
    -- You should not use this module unless you are determined to monkey
    -- with the internals, as the functions here do just about nothing to
    -- preserve data invariants.  You have been warned!

    module Data.Text.Lazy.Internal
        (
          Text(..)
        , chunk
        , empty
        , foldrChunks
        , foldlChunks
        -- * Data type invariant and abstraction functions

        -- $invariant
        , strictInvariant
        , lazyInvariant
        , showStructure

        -- * Chunk allocation sizes
        , defaultChunkSize
        , smallChunkSize
        , chunkOverhead
        ) where

    import Data.Text ()
    import Data.Text.UnsafeShift (shiftL)
    import Data.Typeable (Typeable)
    import Foreign.Storable (sizeOf)
    import qualified Data.Text.Internal as T

    data Text = Empty
              | Chunk {-# UNPACK #-} !T.Text Text
                deriving (Typeable)

    -- $invariant
    --
    -- The data type invariant for lazy 'Text': Every 'Text' is either 'Empty' or
    -- consists of non-null 'T.Text's.  All functions must preserve this,
    -- and the QC properties must check this.

    -- | Check the invariant strictly.
    strictInvariant :: Text -> Bool
    strictInvariant Empty = True
    strictInvariant x@(Chunk (T.Text _ _ len) cs)
        | len > 0   = strictInvariant cs
        | otherwise = error $ "Data.Text.Lazy: invariant violation: "
                      ++ showStructure x

    -- | Check the invariant lazily.
    lazyInvariant :: Text -> Text
    lazyInvariant Empty = Empty
    lazyInvariant x@(Chunk c@(T.Text _ _ len) cs)
        | len > 0   = Chunk c (lazyInvariant cs)
        | otherwise = error $ "Data.Text.Lazy: invariant violation: "
                      ++ showStructure x

    -- | Display the internal structure of a lazy 'Text'.
    showStructure :: Text -> String
    showStructure Empty           = "Empty"
    showStructure (Chunk t Empty) = "Chunk " ++ show t ++ " Empty"
    showStructure (Chunk t ts)    =
        "Chunk " ++ show t ++ " (" ++ showStructure ts ++ ")"

    -- | Smart constructor for 'Chunk'. Guarantees the data type invariant.
    chunk :: T.Text -> Text -> Text
    {-# INLINE chunk #-}
    chunk t@(T.Text _ _ len) ts | len == 0 = ts
                                | otherwise = Chunk t ts

    -- | Smart constructor for 'Empty'.
    empty :: Text
    {-# INLINE [0] empty #-}
    empty = Empty

    -- | Consume the chunks of a lazy 'Text' with a natural right fold.
    foldrChunks :: (T.Text -> a -> a) -> a -> Text -> a
    foldrChunks f z = go
      where go Empty        = z
            go (Chunk c cs) = f c (go cs)
    {-# INLINE foldrChunks #-}

    -- | Consume the chunks of a lazy 'Text' with a strict, tail-recursive,
    -- accumulating left fold.
    foldlChunks :: (a -> T.Text -> a) -> a -> Text -> a
    foldlChunks f z = go z
      where go !a Empty        = a
            go !a (Chunk c cs) = go (f a c) cs
    {-# INLINE foldlChunks #-}

    -- | Currently set to 16 KiB, less the memory management overhead.
    defaultChunkSize :: Int
    defaultChunkSize = 16384 - chunkOverhead
    {-# INLINE defaultChunkSize #-}

    -- | Currently set to 128 bytes, less the memory management overhead.
    smallChunkSize :: Int
    smallChunkSize = 128 - chunkOverhead
    {-# INLINE smallChunkSize #-}

    -- | The memory management overhead. Currently this is tuned for GHC only.
    chunkOverhead :: Int
    chunkOverhead = sizeOf (undefined :: Int) `shiftL` 1
    {-# INLINE chunkOverhead #-}