Data/ByteString/Short.hs

    {-# LANGUAGE CPP #-}
    #if __GLASGOW_HASKELL__ >= 701
    {-# LANGUAGE Trustworthy #-}
    #endif

    -- |
    -- Module      : Data.ByteString.Short
    -- Copyright   : (c) Duncan Coutts 2012-2013
    -- License     : BSD-style
    --
    -- Maintainer  : duncan@community.haskell.org
    -- Stability   : stable
    -- Portability : ghc only
    --
    -- A compact representation suitable for storing short byte strings in memory.
    --
    -- In typical use cases it can be imported alongside "Data.ByteString", e.g.
    --
    -- > import qualified Data.ByteString       as B
    -- > import qualified Data.ByteString.Short as B
    -- >          (ShortByteString, toShort, fromShort)
    --
    -- Other 'ShortByteString' operations clash with "Data.ByteString" or "Prelude"
    -- functions however, so they should be imported @qualified@ with a different
    -- alias e.g.
    --
    -- > import qualified Data.ByteString.Short as B.Short
    --
    module Data.ByteString.Short (

        -- * The @ShortByteString@ type

        ShortByteString,

        -- ** Memory overhead
        -- | With GHC, the memory overheads are as follows, expressed in words and
        -- in bytes (words are 4 and 8 bytes on 32 or 64bit machines respectively).
        --
        -- * 'ByteString' unshared: 9 words; 36 or 72 bytes.
        --
        -- * 'ByteString' shared substring: 5 words; 20 or 40 bytes.
        --
        -- * 'ShortByteString': 4 words; 16 or 32 bytes.
        --
        -- For the string data itself, both 'ShortByteString' and 'ByteString' use
        -- one byte per element, rounded up to the nearest word. For example,
        -- including the overheads, a length 10 'ShortByteString' would take
        -- @16 + 12 = 28@ bytes on a 32bit platform and @32 + 16 = 48@ bytes on a
        -- 64bit platform.
        --
        -- These overheads can all be reduced by 1 word (4 or 8 bytes) when the
        -- 'ShortByteString' or 'ByteString' is unpacked into another constructor.
        --
        -- For example:
        --
        -- > data ThingId = ThingId {-# UNPACK #-} !Int
        -- >                        {-# UNPACK #-} !ShortByteString
        --
        -- This will take @1 + 1 + 3@ words (the @ThingId@ constructor +
        -- unpacked @Int@ + unpacked @ShortByteString@), plus the words for the
        -- string data.

        -- ** Heap fragmentation
        -- | With GHC, the 'ByteString' representation uses /pinned/ memory,
        -- meaning it cannot be moved by the GC. This is usually the right thing to
        -- do for larger strings, but for small strings using pinned memory can
        -- lead to heap fragmentation which wastes space. The 'ShortByteString'
        -- type (and the @Text@ type from the @text@ package) use /unpinned/ memory
        -- so they do not contribute to heap fragmentation. In addition, with GHC,
        -- small unpinned strings are allocated in the same way as normal heap
        -- allocations, rather than in a separate pinned area.

        -- * Conversions
        toShort,
        fromShort,
        pack,
        unpack,

        -- * Other operations
        empty, null, length, index,
      ) where

    import Data.ByteString.Short.Internal
    import Prelude ()