Application programming interface

Exception

exception PpmdError: This exception is raised when an error occurs.

Simple compression/decompression

This section contains:

function compress()

function decompress()

compress(bytes_or_str: Union[bytes, bytearray, memoryview, str], max_order: int, mem_size: int, variant: str)

Compress bytes_or_str, return the compressed data.

Parameters:

bytes_or_str (bytes-like object or str) – Data to be compressed. When it is type of str, encoded with “UTF-8” encoding before compress.
max_order (int) – maximum order of PPMd algorithm
mem_size (int) – memory size used for building PPMd model
variant (str) – PPMd variant name, only accept “H” or “I”

Returns:

Compressed data

Return type:

bytes

compressed_data = compress(data)

decompress_str(data: Union[bytes, memoryview], max_order: int, mem_size: int, encoding: str, variant: str)

Decompress data, return the decompressed text.

When encoding specified, return the decoded data as str type by specified encoding. Otherwise it returns data decoding by default “UTF-8”.

Parameters:

data (bytes-like object) – Data to be decompressed.
max_order (int) – maximum order of PPMd algorithm
mem_size (int) – memory size used for building PPMd model
encoding (str) – Encoding name to use when decoding raw decompressed data
variant (str) – PPMd variant name, only accept “H” or “I”

Returns:

Decompressed text

Return type:

str

Raises:

PpmdError – If decompression fails.

decompressed_text = decompress_str(data)

decompress(data: Union[bytes, memoryview], max_order: int, mem_size: int, variant: str)

Decompress data, return the decompressed data.

Parameters:

data (bytes-like object) – Data to be decompressed
max_order (int) – maximum order of PPMd algorithm
mem_size (int) – memory size used for building PPMd model
variant (str) – PPMd variant name, only accept “H” or “I”

Returns:

Decompressed data

Return type:

bytes

Raises:

PpmdError – If decompression fails.

decompressed_data = decompress(data)

Streaming compression

class PpmdCompressor

A streaming compressor. It’s thread-safe at method level.

__init__(self, max_order: int, mem_size: int, variant: str, restore_method: int)

Initialize a PpmdCompressor object. restore_method param is affected only when variant is “I”.

Parameters:

max_order (int) – maximum order of PPMd algorithm
mem_size (int) – memory size used for building PPMd model
variant (str) – PPMd variant name, only accept “H” or “I”
restore_method (int) – PPMD8_RESTORE_METHOD_RESTART(0) or PPMD8_RESTORE_METHOD_CUTOFF(1)

compress(self, data)

Provide data to the compressor object.

Parameters:: data (bytes-like object) – Data to be compressed.
Returns:: A chunk of compressed data if possible, or b'' otherwise.
Return type:: bytes

flush(self)

Flush any remaining data in internal buffer.

The compressor object can not be used after this method is called.

Returns:: Flushed data.
Return type:: bytes

c = PpmdCompressor()

dat1 = c.compress(b'123456')
dat2 = c.compress(b'abcdef')
dat3 = c.flush()

Streaming decompression

class PpmdDecompressor

A streaming decompressor. Thread-safe at method level. A restore_method param is affected only when variant is “I”.

__init__(self, max_order: int, mem_size: int, variant: str, restore_method: int)

Initialize a PpmdDecompressor object.

Parameters:

max_order (int) – maximum order of PPMd algorithm
mem_size (int) – memory size used for building PPMd model
variant (str) – PPMd variant name, only accept “H” or “I”
restore_method (int) – PPMD8_RESTORE_METHOD_RESTART(0) or PPMD8_RESTORE_METHOD_CUTOFF(1)

decompress(self, data, max_length=-1)

Decompress data, returning decompressed data as a bytes object.

Parameters:

data (bytes-like object) – Data to be decompressed.
max_length (int) – Maximum size of returned data. When it’s negative, the output size is unlimited. When it’s non-negative, returns at most max_length bytes of decompressed data. If this limit is reached and further output can (or may) be produced, the needs_input attribute will be set to False. In this case, the next call to this method may provide data as b'' to obtain more of the output.

needs_input

If the max_length output limit in decompress() method has been reached, and the decompressor has (or may has) unconsumed input data, it will be set to False. In this case, pass b'' to decompress() method may output further data.

If ignore this attribute when there is unconsumed input data, there will be a little performance loss because of extra memory copy. This flag can be True even all input data are consumed, when decompressor can be able to accept more data in some case.

eof: True means the end of the first frame has been reached. If decompress data after that, an EOFError exception will be raised. This flag can be False even all input data are consumed, when decompressor can be able to accept more data in some case.

unused_data: A bytes object. When PpmdDecompressor object stops after end mark, unused input data after the end mark. Otherwise this will be b''.

d1 = PpmdDecompressor()

decompressed_dat = d1.decompress(dat1)
decompressed_dat += d1.decompress(dat2)
decompressed_dat += d1.decompress(dat3)