zlib-0.6.1.1: Compression and decompression in the gzip and zlib formats

Copyright(c) 2006-2015 Duncan Coutts
LicenseBSD-style
Maintainerduncan@community.haskell.org
Safe HaskellTrustworthy
LanguageHaskell2010

Codec.Compression.Zlib.Internal

Contents

Description

Pure and IO stream based interfaces to lower level zlib wrapper

Synopsis

Pure interface

compress :: Format -> CompressParams -> ByteString -> ByteString Source

Compress a data stream provided as a lazy ByteString.

There are no expected error conditions. All input data streams are valid. It is possible for unexpected errors to occur, such as running out of memory, or finding the wrong version of the zlib C library, these are thrown as exceptions.

decompress :: Format -> DecompressParams -> ByteString -> ByteString Source

Decompress a data stream provided as a lazy ByteString.

It will throw an exception if any error is encountered in the input data. If you need more control over error handling then use one the incremental versions, decompressST or decompressIO.

Monadic incremental interface

The pure compress and decompress functions are streaming in the sense that they can produce output without demanding all input, however they need the input data stream as a lazy ByteString. Having the input data stream as a lazy ByteString often requires using lazy I/O which is not appropriate in all cicumstances.

For these cases an incremental interface is more appropriate. This interface allows both incremental input and output. Chunks of input data are supplied one by one (e.g. as they are obtained from an input source like a file or network source). Output is also produced chunk by chunk.

The incremental input and output is managed via the CompressStream and DecompressStream types. They represents the unfolding of the process of compressing and decompressing. They operates in either the ST or IO monads. They can be lifted into other incremental abstractions like pipes or conduits, or they can be used directly in the following style.

Using incremental compression

In a loop:

  • Inspect the status of the stream
  • When it is CompressInputRequired then you should call the action, passing a chunk of input (or empty when no more input is available) to get the next state of the stream and continue the loop.
  • When it is CompressOutputAvailable then do something with the given chunk of output, and call the action to get the next state of the stream and continue the loop.
  • When it is CompressStreamEnd then terminate the loop.

Note that you cannot stop as soon as you have no more input, you need to carry on until all the output has been collected, i.e. until you get to CompressStreamEnd.

Here is an example where we get input from one file handle and send the compressed output to another file handle.

go :: Handle -> Handle -> CompressStream IO -> IO ()
go inh outh (CompressInputRequired next) = do
   inchunk <- BS.hGet inh 4096
   go inh outh =<< next inchunk
go inh outh (CompressOutputAvailable outchunk next) =
   BS.hPut outh outchunk
   go inh outh =<< next
go _ _ CompressStreamEnd = return ()

The same can be achieved with foldCompressStream:

foldCompressStream
  (\next -> do inchunk <- BS.hGet inh 4096; next inchunk)
  (\outchunk next -> do BS.hPut outh outchunk; next)
  (return ())

data CompressStream m Source

The unfolding of the compression process, where you provide a sequence of uncompressed data chunks as input and receive a sequence of compressed data chunks as output. The process is incremental, in that the demand for input and provision of output are interleaved.

compressST :: Format -> CompressParams -> CompressStream (ST s) Source

Incremental compression in the ST monad. Using ST makes it possible to write pure lazy functions while making use of incremental compression.

compressIO :: Format -> CompressParams -> CompressStream IO Source

Incremental compression in the IO monad.

foldCompressStream :: Monad m => ((ByteString -> m a) -> m a) -> (ByteString -> m a -> m a) -> m a -> CompressStream m -> m a Source

A fold over the CompressStream in the given monad.

One way to look at this is that it runs the stream, using callback functions for the three stream events.

foldCompressStreamWithInput :: (ByteString -> a -> a) -> a -> (forall s. CompressStream (ST s)) -> ByteString -> a Source

A variant on foldCompressStream that is pure rather than operating in a monad and where the input is provided by a lazy ByteString. So we only have to deal with the output and end parts, making it just like a foldr on a list of output chunks.

For example:

toChunks = foldCompressStreamWithInput (:) []

Using incremental decompression

The use of DecompressStream is very similar to CompressStream but with a few differences:

Otherwise the same loop style applies, and there are fold functions.

data DecompressStream m Source

The unfolding of the decompression process, where you provide a sequence of compressed data chunks as input and receive a sequence of uncompressed data chunks as output. The process is incremental, in that the demand for input and provision of output are interleaved.

To indicate the end of the input supply an empty input chunk. Note that for gzipFormat with the default decompressAllMembers True you will have to do this, as the decompressor will look for any following members. With decompressAllMembers False the decompressor knows when the data ends and will produce DecompressStreamEnd without you having to supply an empty chunk to indicate the end of the input.

data DecompressError Source

The possible error cases when decompressing a stream.

This can be shown to give a human readable error message.

Constructors

TruncatedInput

The compressed data stream ended prematurely. This may happen if the input data stream was truncated.

DictionaryRequired

It is possible to do zlib compression with a custom dictionary. This allows slightly higher compression ratios for short files. However such compressed streams require the same dictionary when decompressing. This error is for when we encounter a compressed stream that needs a dictionary, and it's not provided.

DictionaryMismatch

If the stream requires a dictionary and you provide one with the wrong DictionaryHash then you will get this error.

DataFormatError String

If the compressed data stream is corrupted in any way then you will get this error, for example if the input data just isn't a compressed zlib data stream. In particular if the data checksum turns out to be wrong then you will get all the decompressed data but this error at the end, instead of the normal sucessful StreamEnd.

decompressST :: Format -> DecompressParams -> DecompressStream (ST s) Source

Incremental decompression in the ST monad. Using ST makes it possible to write pure lazy functions while making use of incremental decompression.

decompressIO :: Format -> DecompressParams -> DecompressStream IO Source

Incremental decompression in the IO monad.

foldDecompressStream :: Monad m => ((ByteString -> m a) -> m a) -> (ByteString -> m a -> m a) -> (ByteString -> m a) -> (DecompressError -> m a) -> DecompressStream m -> m a Source

A fold over the DecompressStream in the given monad.

One way to look at this is that it runs the stream, using callback functions for the four stream events.

foldDecompressStreamWithInput :: (ByteString -> a -> a) -> (ByteString -> a) -> (DecompressError -> a) -> (forall s. DecompressStream (ST s)) -> ByteString -> a Source

A variant on foldCompressStream that is pure rather than operating in a monad and where the input is provided by a lazy ByteString. So we only have to deal with the output, end and error parts, making it like a foldr on a list of output chunks.

For example:

toChunks = foldDecompressStreamWithInput (:) [] throw

The compression parameter types

data CompressParams Source

The full set of parameters for compression. The defaults are defaultCompressParams.

The compressBufferSize is the size of the first output buffer containing the compressed data. If you know an approximate upper bound on the size of the compressed data then setting this parameter can save memory. The default compression output buffer size is 16k. If your extimate is wrong it does not matter too much, the default buffer size will be used for the remaining chunks.

Instances

defaultCompressParams :: CompressParams Source

The default set of parameters for compression. This is typically used with the compressWith function with specific parameters overridden.

data DecompressParams Source

The full set of parameters for decompression. The defaults are defaultDecompressParams.

The decompressBufferSize is the size of the first output buffer, containing the uncompressed data. If you know an exact or approximate upper bound on the size of the decompressed data then setting this parameter can save memory. The default decompression output buffer size is 32k. If your extimate is wrong it does not matter too much, the default buffer size will be used for the remaining chunks.

One particular use case for setting the decompressBufferSize is if you know the exact size of the decompressed data and want to produce a strict ByteString. The compression and deccompression functions use lazy ByteStrings but if you set the decompressBufferSize correctly then you can generate a lazy ByteString with exactly one chunk, which can be converted to a strict ByteString in O(1) time using concat . toChunks.

defaultDecompressParams :: DecompressParams Source

The default set of parameters for decompression. This is typically used with the compressWith function with specific parameters overridden.

data Format Source

The format used for compression or decompression. There are three variations.

Constructors

GZip

Deprecated: Use gzipFormat. Format constructors will be hidden in version 0.7

Zlib

Deprecated: Use zlibFormat. Format constructors will be hidden in version 0.7

Raw

Deprecated: Use rawFormat. Format constructors will be hidden in version 0.7

GZipOrZlib

Deprecated: Use gzipOrZlibFormat. Format constructors will be hidden in version 0.7

gzipFormat :: Format Source

The gzip format uses a header with a checksum and some optional meta-data about the compressed file. It is intended primarily for compressing individual files but is also sometimes used for network protocols such as HTTP. The format is described in detail in RFC #1952 http://www.ietf.org/rfc/rfc1952.txt

zlibFormat :: Format Source

The zlib format uses a minimal header with a checksum but no other meta-data. It is especially designed for use in network protocols. The format is described in detail in RFC #1950 http://www.ietf.org/rfc/rfc1950.txt

rawFormat :: Format Source

The 'raw' format is just the compressed data stream without any additional header, meta-data or data-integrity checksum. The format is described in detail in RFC #1951 http://www.ietf.org/rfc/rfc1951.txt

gzipOrZlibFormat :: Format Source

This is not a format as such. It enabled zlib or gzip decoding with automatic header detection. This only makes sense for decompression.

data CompressionLevel Source

The compression level parameter controls the amount of compression. This is a trade-off between the amount of compression and the time required to do the compression.

Constructors

DefaultCompression

Deprecated: Use defaultCompression. CompressionLevel constructors will be hidden in version 0.7

NoCompression

Deprecated: Use noCompression. CompressionLevel constructors will be hidden in version 0.7

BestSpeed

Deprecated: Use bestSpeed. CompressionLevel constructors will be hidden in version 0.7

BestCompression

Deprecated: Use bestCompression. CompressionLevel constructors will be hidden in version 0.7

CompressionLevel Int 

defaultCompression :: CompressionLevel Source

The default compression level is 6 (that is, biased towards higher compression at expense of speed).

noCompression :: CompressionLevel Source

No compression, just a block copy.

bestSpeed :: CompressionLevel Source

The fastest compression method (less compression)

bestCompression :: CompressionLevel Source

The slowest compression method (best compression).

compressionLevel :: Int -> CompressionLevel Source

A specific compression level between 0 and 9.

data Method Source

The compression method

Constructors

Deflated

Deprecated: Use deflateMethod. Method constructors will be hidden in version 0.7

deflateMethod :: Method Source

'Deflate' is the only method supported in this version of zlib. Indeed it is likely to be the only method that ever will be supported.

data WindowBits Source

This specifies the size of the compression window. Larger values of this parameter result in better compression at the expense of higher memory usage.

The compression window size is the value of the the window bits raised to the power 2. The window bits must be in the range 8..15 which corresponds to compression window sizes of 256b to 32Kb. The default is 15 which is also the maximum size.

The total amount of memory used depends on the window bits and the MemoryLevel. See the MemoryLevel for the details.

Constructors

WindowBits Int 
DefaultWindowBits

Deprecated: Use defaultWindowBits. WindowBits constructors will be hidden in version 0.7

defaultWindowBits :: WindowBits Source

The default WindowBits is 15 which is also the maximum size.

windowBits :: Int -> WindowBits Source

A specific compression window size, specified in bits in the range 8..15

data MemoryLevel Source

The MemoryLevel parameter specifies how much memory should be allocated for the internal compression state. It is a tradoff between memory usage, compression ratio and compression speed. Using more memory allows faster compression and a better compression ratio.

The total amount of memory used for compression depends on the WindowBits and the MemoryLevel. For decompression it depends only on the WindowBits. The totals are given by the functions:

compressTotal windowBits memLevel = 4 * 2^windowBits + 512 * 2^memLevel
decompressTotal windowBits = 2^windowBits

For example, for compression with the default windowBits = 15 and memLevel = 8 uses 256Kb. So for example a network server with 100 concurrent compressed streams would use 25Mb. The memory per stream can be halved (at the cost of somewhat degraded and slower compressionby) by reducing the windowBits and memLevel by one.

Decompression takes less memory, the default windowBits = 15 corresponds to just 32Kb.

Constructors

DefaultMemoryLevel

Deprecated: Use defaultMemoryLevel. MemoryLevel constructors will be hidden in version 0.7

MinMemoryLevel

Deprecated: Use minMemoryLevel. MemoryLevel constructors will be hidden in version 0.7

MaxMemoryLevel

Deprecated: Use maxMemoryLevel. MemoryLevel constructors will be hidden in version 0.7

MemoryLevel Int 

defaultMemoryLevel :: MemoryLevel Source

The default memory level. (Equivalent to memoryLevel 8)

minMemoryLevel :: MemoryLevel Source

Use minimum memory. This is slow and reduces the compression ratio. (Equivalent to memoryLevel 1)

maxMemoryLevel :: MemoryLevel Source

Use maximum memory for optimal compression speed. (Equivalent to memoryLevel 9)

memoryLevel :: Int -> MemoryLevel Source

A specific level in the range 1..9

data CompressionStrategy Source

The strategy parameter is used to tune the compression algorithm.

The strategy parameter only affects the compression ratio but not the correctness of the compressed output even if it is not set appropriately.

Constructors

DefaultStrategy

Deprecated: Use defaultStrategy. CompressionStrategy constructors will be hidden in version 0.7

Filtered

Deprecated: Use filteredStrategy. CompressionStrategy constructors will be hidden in version 0.7

HuffmanOnly

Deprecated: Use huffmanOnlyStrategy. CompressionStrategy constructors will be hidden in version 0.7

defaultStrategy :: CompressionStrategy Source

Use this default compression strategy for normal data.

filteredStrategy :: CompressionStrategy Source

Use the filtered compression strategy for data produced by a filter (or predictor). Filtered data consists mostly of small values with a somewhat random distribution. In this case, the compression algorithm is tuned to compress them better. The effect of this strategy is to force more Huffman coding and less string matching; it is somewhat intermediate between defaultCompressionStrategy and huffmanOnlyCompressionStrategy.

huffmanOnlyStrategy :: CompressionStrategy Source

Use the Huffman-only compression strategy to force Huffman encoding only (no string match).