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

Copyright | (c) 2006-2015 Duncan Coutts
License | BSD-style
Maintainer | duncan@community.haskell.org
Safe Haskell | Trustworthy
Language | Haskell2010

Codec.Compression.Zlib.Internal

Contents

- Pure interface
- Monadic incremental interface
  - Using incremental compression
  - Using incremental decompression
- The compression parameter types

Description

Pure and IO stream based interfaces to lower level zlib wrapper

Synopsis

- compress :: Format -> CompressParams -> ByteString -> ByteString
- decompress :: Format -> DecompressParams -> ByteString -> ByteString
- data CompressStream m
  - = CompressInputRequired {
    - compressSupplyInput :: ByteString -> m (CompressStream m)
    }
  - | CompressOutputAvailable {
    - compressOutput :: !ByteString
    - compressNext :: m (CompressStream m)
    }
  - | CompressStreamEnd
- compressST :: Format -> CompressParams -> CompressStream (ST s)
- compressIO :: Format -> CompressParams -> CompressStream IO
- foldCompressStream :: Monad m => ((ByteString -> m a) -> m a) -> (ByteString -> m a -> m a) -> m a -> CompressStream m -> m a
- foldCompressStreamWithInput :: (ByteString -> a -> a) -> a -> (forall s. CompressStream (ST s)) -> ByteString -> a
- data DecompressStream m
  - = DecompressInputRequired {
    - decompressSupplyInput :: ByteString -> m (DecompressStream m)
    }
  - | DecompressOutputAvailable {
    - decompressOutput :: !ByteString
    - decompressNext :: m (DecompressStream m)
    }
  - | DecompressStreamEnd {
    - decompressUnconsumedInput :: ByteString
    }
  - | DecompressStreamError {
    - decompressStreamError :: DecompressError
    }
- data DecompressError
  - = TruncatedInput
  - | DictionaryRequired
  - | DictionaryMismatch
  - | DataFormatError String
- decompressST :: Format -> DecompressParams -> DecompressStream (ST s)
- decompressIO :: Format -> DecompressParams -> DecompressStream IO
- foldDecompressStream :: Monad m => ((ByteString -> m a) -> m a) -> (ByteString -> m a -> m a) -> (ByteString -> m a) -> (DecompressError -> m a) -> DecompressStream m -> m a
- foldDecompressStreamWithInput :: (ByteString -> a -> a) -> (ByteString -> a) -> (DecompressError -> a) -> (forall s. DecompressStream (ST s)) -> ByteString -> a
- data CompressParams = CompressParams {
  - compressLevel :: !CompressionLevel
  - compressMethod :: !Method
  - compressWindowBits :: !WindowBits
  - compressMemoryLevel :: !MemoryLevel
  - compressStrategy :: !CompressionStrategy
  - compressBufferSize :: !Int
  - compressDictionary :: Maybe ByteString
  }
- defaultCompressParams :: CompressParams
- data DecompressParams = DecompressParams {
  - decompressWindowBits :: !WindowBits
  - decompressBufferSize :: !Int
  - decompressDictionary :: Maybe ByteString
  - decompressAllMembers :: Bool
  }
- defaultDecompressParams :: DecompressParams
- data Format
  - = GZip
  - | Zlib
  - | Raw
  - | GZipOrZlib
- gzipFormat :: Format
- zlibFormat :: Format
- rawFormat :: Format
- gzipOrZlibFormat :: Format
- data CompressionLevel
  - = DefaultCompression
  - | NoCompression
  - | BestSpeed
  - | BestCompression
  - | CompressionLevel Int
- defaultCompression :: CompressionLevel
- noCompression :: CompressionLevel
- bestSpeed :: CompressionLevel
- bestCompression :: CompressionLevel
- compressionLevel :: Int -> CompressionLevel
- data Method = Deflated
- deflateMethod :: Method
- data WindowBits
  - = WindowBits Int
  - | DefaultWindowBits
- defaultWindowBits :: WindowBits
- windowBits :: Int -> WindowBits
- data MemoryLevel
  - = DefaultMemoryLevel
  - | MinMemoryLevel
  - | MaxMemoryLevel
  - | MemoryLevel Int
- defaultMemoryLevel :: MemoryLevel
- minMemoryLevel :: MemoryLevel
- maxMemoryLevel :: MemoryLevel
- memoryLevel :: Int -> MemoryLevel
- data CompressionStrategy
  - = DefaultStrategy
  - | Filtered
  - | HuffmanOnly
- defaultStrategy :: CompressionStrategy
- filteredStrategy :: CompressionStrategy
- huffmanOnlyStrategy :: CompressionStrategy

# Pure interface

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

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

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

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.

Constructors

CompressInputRequired |
Fields compressSupplyInput :: ByteString -> m (CompressStream m)
CompressOutputAvailable |
Fields compressOutput :: !ByteString compressNext :: m (CompressStream m)
CompressStreamEnd |

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

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

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

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

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:

- There is the extra possibility of a `DecompressStreamError`
- There can be extra trailing data after a compressed stream, and the `DecompressStreamEnd` includes that.

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

data DecompressStream m

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.

Constructors

DecompressInputRequired |
Fields decompressSupplyInput :: ByteString -> m (DecompressStream m)
DecompressOutputAvailable |
Fields decompressOutput :: !ByteString decompressNext :: m (DecompressStream m)
DecompressStreamEnd | Includes any trailing unconsumed input data.
Fields decompressUnconsumedInput :: ByteString
DecompressStreamError | An error code
Fields decompressStreamError :: DecompressError

data DecompressError

The possible error cases when decompressing a stream.

This can be `show`n 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`.

Instances

Eq DecompressError |
Show DecompressError |
Exception DecompressError |

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

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

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

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

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

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.

Constructors

CompressParams |
Fields compressLevel :: !CompressionLevel compressMethod :: !Method compressWindowBits :: !WindowBits compressMemoryLevel :: !MemoryLevel compressStrategy :: !CompressionStrategy compressBufferSize :: !Int compressDictionary :: Maybe ByteString

Instances

Show CompressParams |

defaultCompressParams :: CompressParams

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

data DecompressParams

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 `ByteString`s 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``.

Constructors

DecompressParams |
Fields decompressWindowBits :: !WindowBits decompressBufferSize :: !Int decompressDictionary :: Maybe ByteString decompressAllMembers :: Bool

Instances

Show DecompressParams |

defaultDecompressParams :: DecompressParams

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

data Format

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

Instances

Bounded Format |
Enum Format |
Eq Format |
Ord Format |
Show Format |
Generic Format |
type Rep Format |

gzipFormat :: Format

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

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

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

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

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 |

Instances

Eq CompressionLevel |
Show CompressionLevel |
Generic CompressionLevel |
type Rep CompressionLevel |

defaultCompression :: CompressionLevel

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

noCompression :: CompressionLevel

No compression, just a block copy.

bestSpeed :: CompressionLevel

The fastest compression method (less compression)

bestCompression :: CompressionLevel

The slowest compression method (best compression).

compressionLevel :: Int -> CompressionLevel

A specific compression level between 0 and 9.

data Method

The compression method

Constructors

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

Instances

Bounded Method |
Enum Method |
Eq Method |
Ord Method |
Show Method |
Generic Method |
type Rep Method |

deflateMethod :: Method

'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

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

Instances

Eq WindowBits |
Ord WindowBits |
Show WindowBits |
Generic WindowBits |
type Rep WindowBits |

defaultWindowBits :: WindowBits

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

windowBits :: Int -> WindowBits

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

data MemoryLevel

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 |

Instances

Eq MemoryLevel |
Show MemoryLevel |
Generic MemoryLevel |
type Rep MemoryLevel |

defaultMemoryLevel :: MemoryLevel

The default memory level. (Equivalent to ``memoryLevel` 8`)

minMemoryLevel :: MemoryLevel

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

maxMemoryLevel :: MemoryLevel

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

memoryLevel :: Int -> MemoryLevel

A specific level in the range `1..9`

data CompressionStrategy

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

Instances

Bounded CompressionStrategy |
Enum CompressionStrategy |
Eq CompressionStrategy |
Ord CompressionStrategy |
Show CompressionStrategy |
Generic CompressionStrategy |
type Rep CompressionStrategy |

defaultStrategy :: CompressionStrategy

Use this default compression strategy for normal data.

filteredStrategy :: CompressionStrategy

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

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