File: Metered.hs

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{- Metered IO
 -
 - Copyright 2012, 2013 Joey Hess <joey@kitenet.net>
 -
 - License: BSD-2-clause
 -}

{-# LANGUAGE TypeSynonymInstances #-}

module Utility.Metered where

import Common

import qualified Data.ByteString.Lazy as L
import qualified Data.ByteString as S
import System.IO.Unsafe
import Foreign.Storable (Storable(sizeOf))
import System.Posix.Types

{- An action that can be run repeatedly, updating it on the bytes processed.
 -
 - Note that each call receives the total number of bytes processed, so
 - far, *not* an incremental amount since the last call. -}
type MeterUpdate = (BytesProcessed -> IO ())

{- Total number of bytes processed so far. -}
newtype BytesProcessed = BytesProcessed Integer
	deriving (Eq, Ord, Show)

class AsBytesProcessed a where
	toBytesProcessed :: a -> BytesProcessed
	fromBytesProcessed :: BytesProcessed -> a

instance AsBytesProcessed Integer where
	toBytesProcessed i = BytesProcessed i
	fromBytesProcessed (BytesProcessed i) = i

instance AsBytesProcessed Int where
	toBytesProcessed i = BytesProcessed $ toInteger i
	fromBytesProcessed (BytesProcessed i) = fromInteger i

instance AsBytesProcessed FileOffset where
	toBytesProcessed sz = BytesProcessed $ toInteger sz
	fromBytesProcessed (BytesProcessed sz) = fromInteger sz

addBytesProcessed :: AsBytesProcessed v => BytesProcessed -> v -> BytesProcessed
addBytesProcessed (BytesProcessed i) v = 
	let (BytesProcessed n) = toBytesProcessed v
	in BytesProcessed $! i + n

zeroBytesProcessed :: BytesProcessed
zeroBytesProcessed = BytesProcessed 0

{- Sends the content of a file to an action, updating the meter as it's
 - consumed. -}
withMeteredFile :: FilePath -> MeterUpdate -> (L.ByteString -> IO a) -> IO a
withMeteredFile f meterupdate a = withBinaryFile f ReadMode $ \h ->
	hGetContentsMetered h meterupdate >>= a

{- Sends the content of a file to a Handle, updating the meter as it's
 - written. -}
streamMeteredFile :: FilePath -> MeterUpdate -> Handle -> IO ()
streamMeteredFile f meterupdate h = withMeteredFile f meterupdate $ L.hPut h

{- Writes a ByteString to a Handle, updating a meter as it's written. -}
meteredWrite :: MeterUpdate -> Handle -> L.ByteString -> IO ()
meteredWrite meterupdate h = go zeroBytesProcessed . L.toChunks 
  where
	go _ [] = return ()
	go sofar (c:cs) = do
		S.hPut h c
		let sofar' = addBytesProcessed sofar $ S.length c
		meterupdate sofar'
		go sofar' cs

meteredWriteFile :: MeterUpdate -> FilePath -> L.ByteString -> IO ()
meteredWriteFile meterupdate f b = withBinaryFile f WriteMode $ \h ->
	meteredWrite meterupdate h b

{- This is like L.hGetContents, but after each chunk is read, a meter
 - is updated based on the size of the chunk.
 -
 - Note that the meter update is run in unsafeInterleaveIO, which means that
 - it can be run at any time. It's even possible for updates to run out
 - of order, as different parts of the ByteString are consumed.
 -
 - All the usual caveats about using unsafeInterleaveIO apply to the
 - meter updates, so use caution.
 -}
hGetContentsMetered :: Handle -> MeterUpdate -> IO L.ByteString
hGetContentsMetered h meterupdate = lazyRead zeroBytesProcessed
  where
	lazyRead sofar = unsafeInterleaveIO $ loop sofar

	loop sofar = do
		c <- S.hGetSome h defaultChunkSize
		if S.null c
			then do
				hClose h
				return $ L.empty
			else do
				let sofar' = addBytesProcessed sofar $
					S.length c
				meterupdate sofar'
				{- unsafeInterleaveIO causes this to be
				 - deferred until the data is read from the
				 - ByteString. -}
				cs <- lazyRead sofar'
				return $ L.append (L.fromChunks [c]) cs

{- Same default chunk size Lazy ByteStrings use. -}
defaultChunkSize :: Int
defaultChunkSize = 32 * k - chunkOverhead
  where
	k = 1024
	chunkOverhead = 2 * sizeOf (undefined :: Int) -- GHC specific