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226 lines
7.9 KiB
Python
226 lines
7.9 KiB
Python
"""
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LZ77 compression algorithm
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- lossless data compression published in papers by Abraham Lempel and Jacob Ziv in 1977
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- also known as LZ1 or sliding-window compression
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- form the basis for many variations including LZW, LZSS, LZMA and others
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It uses a “sliding window” method. Within the sliding window we have:
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- search buffer
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- look ahead buffer
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len(sliding_window) = len(search_buffer) + len(look_ahead_buffer)
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LZ77 manages a dictionary that uses triples composed of:
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- Offset into search buffer, it's the distance between the start of a phrase and
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the beginning of a file.
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- Length of the match, it's the number of characters that make up a phrase.
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- The indicator is represented by a character that is going to be encoded next.
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As a file is parsed, the dictionary is dynamically updated to reflect the compressed
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data contents and size.
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Examples:
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"cabracadabrarrarrad" <-> [(0, 0, 'c'), (0, 0, 'a'), (0, 0, 'b'), (0, 0, 'r'),
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(3, 1, 'c'), (2, 1, 'd'), (7, 4, 'r'), (3, 5, 'd')]
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"ababcbababaa" <-> [(0, 0, 'a'), (0, 0, 'b'), (2, 2, 'c'), (4, 3, 'a'), (2, 2, 'a')]
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"aacaacabcabaaac" <-> [(0, 0, 'a'), (1, 1, 'c'), (3, 4, 'b'), (3, 3, 'a'), (1, 2, 'c')]
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Sources:
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en.wikipedia.org/wiki/LZ77_and_LZ78
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"""
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from dataclasses import dataclass
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__version__ = "0.1"
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__author__ = "Lucia Harcekova"
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@dataclass
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class Token:
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"""
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Dataclass representing triplet called token consisting of length, offset
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and indicator. This triplet is used during LZ77 compression.
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"""
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offset: int
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length: int
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indicator: str
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def __repr__(self) -> str:
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"""
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>>> token = Token(1, 2, "c")
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>>> repr(token)
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'(1, 2, c)'
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>>> str(token)
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'(1, 2, c)'
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"""
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return f"({self.offset}, {self.length}, {self.indicator})"
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class LZ77Compressor:
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"""
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Class containing compress and decompress methods using LZ77 compression algorithm.
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"""
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def __init__(self, window_size: int = 13, lookahead_buffer_size: int = 6) -> None:
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self.window_size = window_size
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self.lookahead_buffer_size = lookahead_buffer_size
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self.search_buffer_size = self.window_size - self.lookahead_buffer_size
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def compress(self, text: str) -> list[Token]:
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"""
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Compress the given string text using LZ77 compression algorithm.
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Args:
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text: string to be compressed
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Returns:
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output: the compressed text as a list of Tokens
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>>> lz77_compressor = LZ77Compressor()
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>>> str(lz77_compressor.compress("ababcbababaa"))
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'[(0, 0, a), (0, 0, b), (2, 2, c), (4, 3, a), (2, 2, a)]'
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>>> str(lz77_compressor.compress("aacaacabcabaaac"))
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'[(0, 0, a), (1, 1, c), (3, 4, b), (3, 3, a), (1, 2, c)]'
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"""
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output = []
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search_buffer = ""
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# while there are still characters in text to compress
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while text:
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# find the next encoding phrase
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# - triplet with offset, length, indicator (the next encoding character)
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token = self._find_encoding_token(text, search_buffer)
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# update the search buffer:
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# - add new characters from text into it
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# - check if size exceed the max search buffer size, if so, drop the
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# oldest elements
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search_buffer += text[: token.length + 1]
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if len(search_buffer) > self.search_buffer_size:
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search_buffer = search_buffer[-self.search_buffer_size :]
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# update the text
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text = text[token.length + 1 :]
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# append the token to output
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output.append(token)
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return output
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def decompress(self, tokens: list[Token]) -> str:
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"""
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Convert the list of tokens into an output string.
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Args:
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tokens: list containing triplets (offset, length, char)
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Returns:
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output: decompressed text
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Tests:
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>>> lz77_compressor = LZ77Compressor()
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>>> lz77_compressor.decompress([Token(0, 0, 'c'), Token(0, 0, 'a'),
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... Token(0, 0, 'b'), Token(0, 0, 'r'), Token(3, 1, 'c'),
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... Token(2, 1, 'd'), Token(7, 4, 'r'), Token(3, 5, 'd')])
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'cabracadabrarrarrad'
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>>> lz77_compressor.decompress([Token(0, 0, 'a'), Token(0, 0, 'b'),
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... Token(2, 2, 'c'), Token(4, 3, 'a'), Token(2, 2, 'a')])
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'ababcbababaa'
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>>> lz77_compressor.decompress([Token(0, 0, 'a'), Token(1, 1, 'c'),
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... Token(3, 4, 'b'), Token(3, 3, 'a'), Token(1, 2, 'c')])
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'aacaacabcabaaac'
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"""
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output = ""
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for token in tokens:
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for _ in range(token.length):
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output += output[-token.offset]
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output += token.indicator
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return output
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def _find_encoding_token(self, text: str, search_buffer: str) -> Token:
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"""Finds the encoding token for the first character in the text.
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Tests:
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>>> lz77_compressor = LZ77Compressor()
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>>> lz77_compressor._find_encoding_token("abrarrarrad", "abracad").offset
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7
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>>> lz77_compressor._find_encoding_token("adabrarrarrad", "cabrac").length
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1
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>>> lz77_compressor._find_encoding_token("abc", "xyz").offset
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0
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>>> lz77_compressor._find_encoding_token("", "xyz").offset
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Traceback (most recent call last):
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...
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ValueError: We need some text to work with.
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>>> lz77_compressor._find_encoding_token("abc", "").offset
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0
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"""
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if not text:
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raise ValueError("We need some text to work with.")
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# Initialise result parameters to default values
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length, offset = 0, 0
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if not search_buffer:
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return Token(offset, length, text[length])
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for i, character in enumerate(search_buffer):
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found_offset = len(search_buffer) - i
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if character == text[0]:
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found_length = self._match_length_from_index(text, search_buffer, 0, i)
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# if the found length is bigger than the current or if it's equal,
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# which means it's offset is smaller: update offset and length
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if found_length >= length:
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offset, length = found_offset, found_length
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return Token(offset, length, text[length])
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def _match_length_from_index(
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self, text: str, window: str, text_index: int, window_index: int
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) -> int:
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"""Calculate the longest possible match of text and window characters from
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text_index in text and window_index in window.
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Args:
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text: _description_
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window: sliding window
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text_index: index of character in text
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window_index: index of character in sliding window
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Returns:
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The maximum match between text and window, from given indexes.
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Tests:
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>>> lz77_compressor = LZ77Compressor(13, 6)
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>>> lz77_compressor._match_length_from_index("rarrad", "adabrar", 0, 4)
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5
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>>> lz77_compressor._match_length_from_index("adabrarrarrad",
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... "cabrac", 0, 1)
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1
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"""
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if not text or text[text_index] != window[window_index]:
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return 0
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return 1 + self._match_length_from_index(
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text, window + text[text_index], text_index + 1, window_index + 1
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)
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if __name__ == "__main__":
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from doctest import testmod
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testmod()
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# Initialize compressor class
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lz77_compressor = LZ77Compressor(window_size=13, lookahead_buffer_size=6)
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# Example
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TEXT = "cabracadabrarrarrad"
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compressed_text = lz77_compressor.compress(TEXT)
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print(lz77_compressor.compress("ababcbababaa"))
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decompressed_text = lz77_compressor.decompress(compressed_text)
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assert decompressed_text == TEXT, "The LZ77 algorithm returned the invalid result."
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