Codec.Compression.Zlib.Raw

Compression and decompression of data streams in the raw deflate format.

The format is described in detail in RFC #1951: http://www.ietf.org/rfc/rfc1951.txt

See also the zlib home page: http://zlib.net/

Simple compression and decompression

compress :: ByteString -> ByteString

decompress :: ByteString -> ByteString

Extended api with control over compression parameters

compressWith :: CompressParams -> ByteString -> ByteString

decompressWith :: DecompressParams -> ByteString -> ByteString

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

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 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.

Constructors

DecompressParams

Fields

decompressWindowBits :: !WindowBits

decompressBufferSize :: !Int

defaultDecompressParams :: DecompressParams

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

The compression parameter types

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

NoCompression

BestSpeed

BestCompression

CompressionLevel Int

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

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

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

MinMemoryLevel

MaxMemoryLevel

MemoryLevel Int

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

Filtered

HuffmanOnly

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).