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Optimize inner loop of packUptoLenBytes and packUptoLenChars #292

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Oct 7, 2020
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Optimize inner loop of packUptoLenBytes and packUptoLenChars #292

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24 changes: 13 additions & 11 deletions Data/ByteString/Internal.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -96,7 +96,7 @@ import Prelude hiding (concat, null)
import qualified Data.List as List

import Foreign.ForeignPtr (ForeignPtr, withForeignPtr)
import Foreign.Ptr (Ptr, FunPtr, plusPtr)
import Foreign.Ptr (Ptr, FunPtr, plusPtr, minusPtr)
import Foreign.Storable (Storable(..))

#if MIN_VERSION_base(4,5,0) || __GLASGOW_HASKELL__ >= 703
Expand Down Expand Up @@ -357,19 +357,21 @@ unsafePackLiteral addr# =

packUptoLenBytes :: Int -> [Word8] -> (ByteString, [Word8])
packUptoLenBytes len xs0 =
unsafeCreateUptoN' len $ \p -> go p len xs0
where
go !_ !n [] = return (len-n, [])
go !_ !0 xs = return (len, xs)
go !p !n (x:xs) = poke p x >> go (p `plusPtr` 1) (n-1) xs
unsafeCreateUptoN' len $ \p0 ->
let p_end = plusPtr p0 len
go !p [] = return (p `minusPtr` p0, [])
go !p xs | p == p_end = return (len, xs)
go !p (x:xs) = poke p x >> go (p `plusPtr` 1) xs
in go p0 xs0

packUptoLenChars :: Int -> [Char] -> (ByteString, [Char])
packUptoLenChars len cs0 =
unsafeCreateUptoN' len $ \p -> go p len cs0
where
go !_ !n [] = return (len-n, [])
go !_ !0 cs = return (len, cs)
go !p !n (c:cs) = poke p (c2w c) >> go (p `plusPtr` 1) (n-1) cs
unsafeCreateUptoN' len $ \p0 ->
let p_end = plusPtr p0 len
go !p [] = return (p `minusPtr` p0, [])
go !p cs | p == p_end = return (len, cs)
go !p (c:cs) = poke p (c2w c) >> go (p `plusPtr` 1) cs
in go p0 cs0

-- Unpacking bytestrings into lists efficiently is a tradeoff: on the one hand
-- we would like to write a tight loop that just blasts the list into memory, on
Expand Down