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Add createOfLast in MutArray module and update Array.createOfLast #2900

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Dec 15, 2024
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Add createOfLast in MutArray module and update Array.createOfLast #2900

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4 changes: 4 additions & 0 deletions core/src/Streamly/Internal/Data/Array.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -210,6 +210,10 @@ last = getIndexRev 0
-- Folds with Array as the container
-------------------------------------------------------------------------------

-- NOTE: We could possible write this in terms of "MutArray.createOfLast" but
-- this causes regression. This is probably because mapping inside "Fold.ifThen"
-- is more efficient than mapping over "Fold.ifTen".
--
-- | @createOfLast n@ folds a maximum of @n@ elements from the end of the input
-- stream to an 'Array'.
--
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15 changes: 15 additions & 0 deletions core/src/Streamly/Internal/Data/MutArray.hs
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Expand Up @@ -25,6 +25,7 @@ module Streamly.Internal.Data.MutArray
, compactSepByByte_
, compactEndByByte_
, compactEndByLn_
, createOfLast
-- * Unboxed IORef
, module Streamly.Internal.Data.IORef.Unboxed

Expand Down Expand Up @@ -57,10 +58,13 @@ import Streamly.Internal.Data.Serialize.Type (Serialize)
import Streamly.Internal.Data.Stream.Type (Stream)
import Streamly.Internal.Data.Unbox (Unbox)
import Streamly.Internal.Data.Unfold.Type (Unfold(..))
import Streamly.Internal.Data.Fold.Type (Fold)

import qualified Streamly.Internal.Data.RingArray as RingArray
import qualified Streamly.Internal.Data.Serialize.Type as Serialize
import qualified Streamly.Internal.Data.Stream.Nesting as Stream
import qualified Streamly.Internal.Data.Stream.Type as Stream
import qualified Streamly.Internal.Data.Fold.Type as Fold
-- import qualified Streamly.Internal.Data.Stream.Transform as Stream
import qualified Streamly.Internal.Data.Unfold as Unfold

Expand Down Expand Up @@ -337,3 +341,14 @@ compactEndByLn_ :: MonadIO m
=> Stream m (MutArray Word8)
-> Stream m (MutArray Word8)
compactEndByLn_ = compactEndByByte_ 10

-- | @createOfLast n@ folds a maximum of @n@ elements from the end of the input
-- stream to an 'MutArray'.
--
{-# INLINE createOfLast #-}
createOfLast :: (Unbox a, MonadIO m) => Int -> Fold m a (MutArray a)
createOfLast n =
Fold.ifThen
(pure (n <= 0))
(Fold.fromPure empty)
(Fold.rmapM RingArray.toMutArray $ RingArray.createOfLast n)
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