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.. _user_guide:
**********
User Guide
**********
PPM, Prediction by partial matching, is a wellknown compression technique
based on context modeling and prediction. PPM models use a set of previous
symbols in the uncompressed symbol stream to predict the next symbol in the
stream.
PPMd is an implementation of PPMII by Dmitry Shkarin.
The ppmd-cffi package uses core C files from p7zip.
The library has a bare function and no metadata/header handling functions.
This means you should know compression parameters and input/output data
sizes.
Getting started
===============
Install
-------
The ppmd-cffi is written by Python and C language bound with CFFI, and can be downloaded
from PyPI(aka. Python Package Index) using standard 'pip' command as like follows;
.. code-block:: bash
$ pip install ppmd-cffi
Command line
============
`ppmd-cffi` provide command line script to hande `.ppmd` file.
To compress file
.. code-block:: bash
$ ppmd target.dat
To decompress ppmd file
.. code-block:: bash
$ ppmd -x target.ppmd
To decompress to STDOUT
.. code-block:: bash
$ ppmd -x -c target.ppmd
Programming Interfaces
======================
`.ppmd` file comression/decompression
=====================================
ppmd-cffi project provide two functions which compress and decompress `.ppmd` archive file.
`PpmdCompressor` class provide compress function `compress()` and `PpmdDecompressor` class
provide extraction function `decompress()`.
Both classes takes `version=` argument which default is 8, means PPMd Ver. I.
Also classes takes `target`, `fname` and `ftime` arguments which is a target file and its properties.
`target` should be a file-like object which has `write()` method.
`fname` and `ftime` is a file property which is stored in archive as meta data.
`fname` should be string, and `ftime` should be a datetime object.
`order` and `mem_in_mb` parameters will be vary.
Compression with PPMd ver. H
----------------------------
.. code-block:: python
targetfile = pathlib.Path('target.dat')
fname = 'target.dat'
ftime = datetime.utcfromtimestamp(targetfile.stat().st_mtime)
archivefile = 'archive.ppmd'
order = 6
mem_in_mb = 16
with archivefile.open('wb') as target:
with targetfile.open('rb') as src:
with PpmdCompressor(target, fname, ftime, order, mem_in_mb, version=7) as compressor:
compressor.compress(src)
Compression with PPMd ver. I
----------------------------
.. code-block:: python
targetfile = pathlib.Path('target.dat')
fname = 'target.dat'
ftime = datetime.utcfromtimestamp(targetfile.stat().st_mtime)
archivefile = 'archive.ppmd'
order = 6
mem_in_mb = 8
with archivefile.open('wb') as target:
with targetfile.open('rb') as src:
with PpmdCompressor(target, fname, ftime, order, mem_in_mb, version=8) as compressor:
compressor.compress(src)
Decompression
-------------
When construct `PpmdDecompressor` object, it read header from specified archive file.
The header hold a compression parameters such as PPMd version, order and memory size.
It also has a `filename` and `timestamp`.
`PpmdDecompressor` select a proper decoder based on header data.
You need to handle `filename` and `timestamp` by your self.
A decompressor method will write data to specified file-like object, which should have
`write()` method.
.. code-block:: python
targetfile = pathlib.Path('target.ppmd')
with targetfile.open('rb') as target:
with PpmdDecompressor(target, target_size) as decompressor:
extractedfile = pathlib.Path(parent.joinpath(decompressor.filename))
with extractedfile.open('wb') as ofile:
decompressor.decompress(ofile)
timestamp = datetime_to_timestamp(decompressor.ftime)
os.utime(str(extractedfile), times=(timestamp, timestamp))
Bare encoding/decoding PPMd data
================================
There are several classes to handle bare PPMd data. Note: mem parameter should be
as bytes not MB.
* Ppmd7Encoder(dst, order, mem)
* Ppmd7Decoder(src, order, mem)
* Ppmd8Encoder(det, order, mem, restore)
* Ppmd8Decoder(src, order, mem, restore)
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