Codec.Compression.Zlib.Raw

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.

The default CompressionLevel.

No compression, just a block copy.

The fastest compression method (less compression).

The slowest compression method (best compression).

A specific compression level in the range 0..9. Throws an error for arguments outside of this range.

• 0 stands for noCompression,

• 1 stands for bestSpeed,

• 6 stands for defaultCompression,

• 9 stands for bestCompression.

The compression method

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

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 9..15 which corresponds to compression window sizes of 512b 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.

The default WindowBits. Equivalent to windowBits 15. which is also the maximum size.

A specific compression window size, specified in bits in the range 9..15. Throws an error for arguments outside of this range.

The MemoryLevel parameter specifies how much memory should be allocated for the internal compression state. It is a trade-off 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 compression) by reducing the windowBits and memLevel by one.

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

The default MemoryLevel. Equivalent to memoryLevel 8.

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

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

A specific memory level in the range 1..9. Throws an error for arguments outside of this range.

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.

Use this default compression strategy for normal data.

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 defaultStrategy and huffmanOnlyStrategy.

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

Use rleStrategy to limit match distances to one (run-length encoding). rleStrategy is designed to be almost as fast as huffmanOnlyStrategy, but give better compression for PNG image data.

fixedStrategy prevents the use of dynamic Huffman codes, allowing for a simpler decoder for special applications.