|Portability||portable (H98 + FFI)|
Pure stream based interface to lower level bzlib wrapper
- compressDefault :: BlockSize -> ByteString -> ByteString
- decompressDefault :: ByteString -> ByteString
- data BlockSize
- compressFull :: BlockSize -> Verbosity -> WorkFactor -> ByteString -> ByteString
- decompressFull :: Verbosity -> MemoryLevel -> ByteString -> ByteString
- data WorkFactor
- data MemoryLevel
- data Verbosity
Compression and decompression
The block size affects both the compression ratio achieved, and the amount of memory needed for compression and decompression.
Larger block sizes give rapidly diminishing marginal returns. Most of the compression comes from the first two or three hundred k of block size, a fact worth bearing in mind when using bzip2 on small machines. It is also important to appreciate that the decompression memory requirement is set at compression time by the choice of block size.
- In general, try and use the largest block size memory constraints allow, since that maximises the compression achieved.
- Compression and decompression speed are virtually unaffected by block size.
Another significant point applies to files which fit in a single block -
that means most files you'd encounter using a large block size. The amount
of real memory touched is proportional to the size of the file, since the
file is smaller than a block. For example, compressing a file 20,000 bytes
long with the flag
will cause the compressor to allocate
around 7600k of memory, but only touch 400k + 20000 * 8 = 560 kbytes of it.
Similarly, the decompressor will allocate 3700k but only touch 100k + 20000
* 4 = 180 kbytes.
The default block size is also the maximum.
A specific block size between 1 and 9.
The same but with the full set of parameters
WorkFactor parameter controls how the compression phase behaves when
presented with worst case, highly repetitive, input data. If compression
runs into difficulties caused by repetitive data, the library switches from
the standard sorting algorithm to a fallback algorithm. The fallback is
slower than the standard algorithm by perhaps a factor of three, but always
behaves reasonably, no matter how bad the input.
Lower values of
WorkFactor reduce the amount of effort the standard
algorithm will expend before resorting to the fallback. You should set this
parameter carefully; too low, and many inputs will be handled by the
fallback algorithm and so compress rather slowly, too high, and your
average-to-worst case compression times can become very large. The default
value of 30 gives reasonable behaviour over a wide range of circumstances.
- Note that the compressed output generated is the same regardless of whether or not the fallback algorithm is used.
The default work factor is 30.
Allowable values range from 1 to 250 inclusive.
For files compressed with the default 900k block size, decompression will require about 3700k to decompress. To support decompression of any file in less than 4Mb there is the option to decompress using approximately half this amount of memory, about 2300k. Decompression speed is also halved, so you should use this option only where necessary.
Use minimum memory dusing decompression. This halves the memory needed but also halves the decompression speed.