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|
{-# LANGUAGE MultiWayIf #-}
{-# LANGUAGE NoImplicitPrelude #-}
-- |
-- Module : System.OsString.Data.ByteString.Short
-- Copyright : (c) Duncan Coutts 2012-2013, Julian Ospald 2022
-- License : BSD-style
--
-- Maintainer : hasufell@posteo.de
-- 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 System.OsString.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).
--
-- * 'B.ByteString' unshared: 8 words; 32 or 64 bytes.
--
-- * 'B.ByteString' shared substring: 4 words; 16 or 32 bytes.
--
-- * 'ShortByteString': 4 words; 16 or 32 bytes.
--
-- For the string data itself, both 'ShortByteString' and 'B.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 'B.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 'B.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.
-- * Introducing and eliminating 'ShortByteString's
empty,
singleton,
pack,
unpack,
fromShort,
toShort,
-- * Basic interface
snoc,
cons,
append,
last,
tail,
uncons,
uncons2,
head,
init,
unsnoc,
null,
length,
-- * Transforming ShortByteStrings
map,
reverse,
intercalate,
-- * Reducing 'ShortByteString's (folds)
foldl,
foldl',
foldl1,
foldl1',
foldr,
foldr',
foldr1,
foldr1',
-- ** Special folds
all,
any,
concat,
-- ** Generating and unfolding ByteStrings
replicate,
unfoldr,
unfoldrN,
-- * Substrings
-- ** Breaking strings
take,
takeEnd,
takeWhileEnd,
takeWhile,
drop,
dropEnd,
dropWhile,
dropWhileEnd,
breakEnd,
break,
span,
spanEnd,
splitAt,
split,
splitWith,
stripSuffix,
stripPrefix,
-- * Predicates
isInfixOf,
isPrefixOf,
isSuffixOf,
-- ** Search for arbitrary substrings
breakSubstring,
-- * Searching ShortByteStrings
-- ** Searching by equality
elem,
-- ** Searching with a predicate
find,
filter,
partition,
-- * Indexing ShortByteStrings
index,
indexMaybe,
(!?),
elemIndex,
elemIndices,
count,
findIndex,
findIndices,
-- * Low level conversions
-- ** Packing 'Foreign.C.String.CString's and pointers
packCString,
packCStringLen,
-- ** Using ShortByteStrings as 'Foreign.C.String.CString's
useAsCString,
useAsCStringLen,
) where
import Data.ByteString.Short.Internal
import System.OsString.Data.ByteString.Short.Internal
import Prelude (Maybe(..), Ord(..), Num(..), ($), otherwise)
import Data.Word (Word8)
uncons2 :: ShortByteString -> Maybe (Word8, Word8, ShortByteString)
uncons2 = \sbs ->
let l = length sbs
nl = l - 2
in if | l <= 1 -> Nothing
| otherwise -> let h = indexWord8Array (asBA sbs) 0
h' = indexWord8Array (asBA sbs) 1
t = create nl $ \mba -> copyByteArray (asBA sbs) 1 mba 0 nl
in Just (h, h', t)
|
