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126 lines
3.5 KiB
Python
126 lines
3.5 KiB
Python
"""
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One of the several implementations of Lempel–Ziv–Welch compression algorithm
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https://en.wikipedia.org/wiki/Lempel%E2%80%93Ziv%E2%80%93Welch
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"""
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import math
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import os
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import sys
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def read_file_binary(file_path: str) -> str:
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"""
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Reads given file as bytes and returns them as a long string
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"""
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result = ""
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try:
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with open(file_path, "rb") as binary_file:
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data = binary_file.read()
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for dat in data:
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curr_byte = f"{dat:08b}"
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result += curr_byte
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return result
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except OSError:
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print("File not accessible")
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sys.exit()
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def add_key_to_lexicon(
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lexicon: dict, curr_string: str, index: int, last_match_id: int
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) -> None:
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"""
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Adds new strings (curr_string + "0", curr_string + "1") to the lexicon
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"""
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lexicon.pop(curr_string)
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lexicon[curr_string + "0"] = last_match_id
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if math.log2(index).is_integer():
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for curr_key in lexicon:
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lexicon[curr_key] = "0" + lexicon[curr_key]
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lexicon[curr_string + "1"] = bin(index)[2:]
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def compress_data(data_bits: str) -> str:
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"""
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Compresses given data_bits using Lempel–Ziv–Welch compression algorithm
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and returns the result as a string
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"""
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lexicon = {"0": "0", "1": "1"}
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result, curr_string = "", ""
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index = len(lexicon)
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for i in range(len(data_bits)):
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curr_string += data_bits[i]
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if curr_string not in lexicon:
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continue
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last_match_id = lexicon[curr_string]
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result += last_match_id
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add_key_to_lexicon(lexicon, curr_string, index, last_match_id)
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index += 1
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curr_string = ""
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while curr_string != "" and curr_string not in lexicon:
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curr_string += "0"
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if curr_string != "":
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last_match_id = lexicon[curr_string]
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result += last_match_id
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return result
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def add_file_length(source_path: str, compressed: str) -> str:
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"""
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Adds given file's length in front (using Elias gamma coding) of the compressed
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string
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"""
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file_length = os.path.getsize(source_path)
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file_length_binary = bin(file_length)[2:]
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length_length = len(file_length_binary)
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return "0" * (length_length - 1) + file_length_binary + compressed
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def write_file_binary(file_path: str, to_write: str) -> None:
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"""
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Writes given to_write string (should only consist of 0's and 1's) as bytes in the
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file
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"""
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byte_length = 8
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try:
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with open(file_path, "wb") as opened_file:
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result_byte_array = [
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to_write[i : i + byte_length]
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for i in range(0, len(to_write), byte_length)
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]
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if len(result_byte_array[-1]) % byte_length == 0:
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result_byte_array.append("10000000")
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else:
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result_byte_array[-1] += "1" + "0" * (
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byte_length - len(result_byte_array[-1]) - 1
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)
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for elem in result_byte_array:
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opened_file.write(int(elem, 2).to_bytes(1, byteorder="big"))
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except OSError:
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print("File not accessible")
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sys.exit()
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def compress(source_path, destination_path: str) -> None:
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"""
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Reads source file, compresses it and writes the compressed result in destination
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file
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"""
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data_bits = read_file_binary(source_path)
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compressed = compress_data(data_bits)
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compressed = add_file_length(source_path, compressed)
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write_file_binary(destination_path, compressed)
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if __name__ == "__main__":
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compress(sys.argv[1], sys.argv[2])
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