compression.lz77

LZ77 compression algorithm - lossless data compression published in papers by Abraham Lempel and Jacob Ziv in 1977 - also known as LZ1 or sliding-window compression - form the basis for many variations including LZW, LZSS, LZMA and others

It uses a “sliding window” method. Within the sliding window we have:

• search buffer

• look ahead buffer

len(sliding_window) = len(search_buffer) + len(look_ahead_buffer)

LZ77 manages a dictionary that uses triples composed of:

• Offset into search buffer, it’s the distance between the start of a phrase and the beginning of a file.

• Length of the match, it’s the number of characters that make up a phrase.

• The indicator is represented by a character that is going to be encoded next.

As a file is parsed, the dictionary is dynamically updated to reflect the compressed data contents and size.

Examples: “cabracadabrarrarrad” <-> [(0, 0, ‘c’), (0, 0, ‘a’), (0, 0, ‘b’), (0, 0, ‘r’),

(3, 1, ‘c’), (2, 1, ‘d’), (7, 4, ‘r’), (3, 5, ‘d’)]

“ababcbababaa” <-> [(0, 0, ‘a’), (0, 0, ‘b’), (2, 2, ‘c’), (4, 3, ‘a’), (2, 2, ‘a’)] “aacaacabcabaaac” <-> [(0, 0, ‘a’), (1, 1, ‘c’), (3, 4, ‘b’), (3, 3, ‘a’), (1, 2, ‘c’)]

Sources: en.wikipedia.org/wiki/LZ77_and_LZ78

Attributes

__author__
__version__
lz77_compressor

Classes

LZ77Compressor	Class containing compress and decompress methods using LZ77 compression algorithm.
Token	Dataclass representing triplet called token consisting of length, offset

Module Contents

class compression.lz77.LZ77Compressor(window_size: int = 13, lookahead_buffer_size: int = 6)

Class containing compress and decompress methods using LZ77 compression algorithm.

_find_encoding_token(text: str, search_buffer: str) → Token

Finds the encoding token for the first character in the text.

Tests:

>>> lz77_compressor = LZ77Compressor()
>>> lz77_compressor._find_encoding_token("abrarrarrad", "abracad").offset
7
>>> lz77_compressor._find_encoding_token("adabrarrarrad", "cabrac").length
1
>>> lz77_compressor._find_encoding_token("abc", "xyz").offset
0
>>> lz77_compressor._find_encoding_token("", "xyz").offset
Traceback (most recent call last):
    ...
ValueError: We need some text to work with.
>>> lz77_compressor._find_encoding_token("abc", "").offset
0

_match_length_from_index(text: str, window: str, text_index: int, window_index: int) → int

Calculate the longest possible match of text and window characters from text_index in text and window_index in window.

Args:

text: _description_ window: sliding window text_index: index of character in text window_index: index of character in sliding window

Returns:

The maximum match between text and window, from given indexes.

Tests:

>>> lz77_compressor = LZ77Compressor(13, 6)
>>> lz77_compressor._match_length_from_index("rarrad", "adabrar", 0, 4)
5
>>> lz77_compressor._match_length_from_index("adabrarrarrad",
...     "cabrac", 0, 1)
1

compress(text: str) → list[Token]

Compress the given string text using LZ77 compression algorithm.

Args:

text: string to be compressed

Returns:

output: the compressed text as a list of Tokens

>>> lz77_compressor = LZ77Compressor()
>>> str(lz77_compressor.compress("ababcbababaa"))
'[(0, 0, a), (0, 0, b), (2, 2, c), (4, 3, a), (2, 2, a)]'
>>> str(lz77_compressor.compress("aacaacabcabaaac"))
'[(0, 0, a), (1, 1, c), (3, 4, b), (3, 3, a), (1, 2, c)]'

decompress(tokens: list[Token]) → str

Convert the list of tokens into an output string.

Args:

tokens: list containing triplets (offset, length, char)

Returns:

output: decompressed text

Tests:

>>> lz77_compressor = LZ77Compressor()
>>> lz77_compressor.decompress([Token(0, 0, 'c'), Token(0, 0, 'a'),
... Token(0, 0, 'b'), Token(0, 0, 'r'), Token(3, 1, 'c'),
... Token(2, 1, 'd'), Token(7, 4, 'r'), Token(3, 5, 'd')])
'cabracadabrarrarrad'
>>> lz77_compressor.decompress([Token(0, 0, 'a'), Token(0, 0, 'b'),
... Token(2, 2, 'c'), Token(4, 3, 'a'), Token(2, 2, 'a')])
'ababcbababaa'
>>> lz77_compressor.decompress([Token(0, 0, 'a'), Token(1, 1, 'c'),
... Token(3, 4, 'b'), Token(3, 3, 'a'), Token(1, 2, 'c')])
'aacaacabcabaaac'

lookahead_buffer_size = 6

search_buffer_size = 7

window_size = 13

class compression.lz77.Token

Dataclass representing triplet called token consisting of length, offset and indicator. This triplet is used during LZ77 compression.

__repr__() → str

>>> token = Token(1, 2, "c")
>>> repr(token)
'(1, 2, c)'
>>> str(token)
'(1, 2, c)'

indicator: str

length: int

offset: int

compression.lz77.__author__ = 'Lucia Harcekova'

compression.lz77.__version__ = '0.1'

compression.lz77.lz77_compressor