zlib- Compression and decompression in the gzip and zlib formats

Portabilityportable (H98 + FFI)
Safe HaskellNone




Pure stream based interface to lower level zlib wrapper



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

Compress a data stream.

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.

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.

defaultCompressParams :: CompressParamsSource

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


decompress :: Format -> DecompressParams -> ByteString -> ByteStringSource

Decompress a data stream.

It will throw an exception if any error is encountered in the input data. If you need more control over error handling then use decompressWithErrors.

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 :: DecompressParamsSource

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

The compression parameter types

data Format Source

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




gzipFormat :: FormatSource

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 :: FormatSource

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 :: FormatSource

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 :: FormatSource

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.

defaultCompression :: CompressionLevelSource

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

noCompression :: CompressionLevelSource

No compression, just a block copy.

bestSpeed :: CompressionLevelSource

The fastest compression method (less compression)

bestCompression :: CompressionLevelSource

The slowest compression method (best compression).

compressionLevel :: Int -> CompressionLevelSource

A specific compression level between 0 and 9.

data Method Source

The compression method



deflateMethod :: MethodSource

'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.

defaultWindowBits :: WindowBitsSource

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

windowBits :: Int -> WindowBitsSource

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.

defaultMemoryLevel :: MemoryLevelSource

The default memory level. (Equivalent to memoryLevel 8)

minMemoryLevel :: MemoryLevelSource

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

maxMemoryLevel :: MemoryLevelSource

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

memoryLevel :: Int -> MemoryLevelSource

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.

defaultStrategy :: CompressionStrategySource

Use this default compression strategy for normal data.

filteredStrategy :: CompressionStrategySource

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 :: CompressionStrategySource

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

Low-level API to get explicit error reports

decompressWithErrors :: Format -> DecompressParams -> ByteString -> DecompressStreamSource

Like decompress but returns a DecompressStream data structure that contains an explicit representation of the error conditions that one may encounter when decompressing.

Note that in addition to errors in the input data, it is possible for other unexpected errors to occur, such as out of memory, or finding the wrong version of the zlib C library, these are still thrown as exceptions (because representing them as data would make this function impure).

data DecompressStream Source

A sequence of chunks of data produced from decompression.

The difference from a simple list is that it contains a representation of errors as data rather than as exceptions. This allows you to handle error conditions explicitly.


StreamChunk ByteString DecompressStream 
StreamError DecompressError String

An error code and a human readable error message.

data DecompressError Source

The possible error cases when decompressing a stream.



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


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.


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.

foldDecompressStream :: (ByteString -> a -> a) -> a -> (DecompressError -> String -> a) -> DecompressStream -> aSource

Fold an DecompressionStream. Just like foldr but with an extra error case. For example to convert to a list and translate the errors into exceptions:

 foldDecompressStream (:) [] (\code msg -> error msg)

fromDecompressStream :: DecompressStream -> ByteStringSource

Convert a DecompressStream to a lazy ByteString. If any decompression errors are encountered then they are thrown as exceptions.

This is a special case of foldDecompressStream.