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Make primMapListBounded into a good consumer #469

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Make primMapListBounded into a good consumer #469

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43 changes: 23 additions & 20 deletions Data/ByteString/Builder/Prim.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -512,12 +512,6 @@ primMapByteStringFixed = primMapByteStringBounded . toB
primMapLazyByteStringFixed :: FixedPrim Word8 -> (L.ByteString -> Builder)
primMapLazyByteStringFixed = primMapLazyByteStringBounded . toB

-- IMPLEMENTATION NOTE: Sadly, 'encodeListWith' cannot be used for foldr/build
-- fusion. Its performance relies on hoisting several variables out of the
-- inner loop. That's not possible when writing 'encodeListWith' as a 'foldr'.
-- If we had stream fusion for lists, then we could fuse 'encodeListWith', as
-- 'encodeWithStream' can keep control over the execution.


-- | Create a 'Builder' that encodes values with the given 'BoundedPrim'.
--
Expand Down Expand Up @@ -578,20 +572,29 @@ primBounded w x =
--
-- because it moves several variables out of the inner loop.
{-# INLINE primMapListBounded #-}
primMapListBounded :: BoundedPrim a -> [a] -> Builder
primMapListBounded w xs0 =
builder $ step xs0
where
step xs1 k (BufferRange op0 ope0) =
go xs1 op0
where
go [] !op = k (BufferRange op ope0)
go xs@(x':xs') !op
| op `plusPtr` bound <= ope0 = runB w x' op >>= go xs'
| otherwise =
return $ bufferFull bound op (step xs k)

bound = I.sizeBound w
primMapListBounded :: forall a. BoundedPrim a -> [a] -> Builder
primMapListBounded w = let
bound = I.sizeBound w

adjustRange :: (Ptr Word8 -> Ptr Word8 -> IO (BuildSignal r)) -> BuildStep r
adjustRange action (BufferRange bufStart bufEnd)
= action (bufEnd `plusPtr` negate bound) bufStart

augmentContinuation
:: a -> (Ptr Word8 -> Ptr Word8 -> IO (BuildSignal r))
-> Ptr Word8 -> Ptr Word8 -> IO (BuildSignal r)
augmentContinuation x kont = \stop start -> if start <= stop
then runB w x start >>= kont stop
else return $ bufferFull bound start $ adjustRange $
\newStop newStart -> runB w x newStart >>= kont newStop

finalContinuation
:: BuildStep r -> Ptr Word8 -> Ptr Word8 -> IO (BuildSignal r)
finalContinuation kont
= \stop start -> kont (BufferRange start $ stop `plusPtr` bound)

in \xs -> builder $ \kont ->
adjustRange $ foldr augmentContinuation (finalContinuation kont) xs

-- TODO: Add 'foldMap/encodeWith' its variants
-- TODO: Ensure rewriting 'primBounded w . f = primBounded (w #. f)'
Expand Down