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112 lines
3.1 KiB
Python
112 lines
3.1 KiB
Python
"""
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One of the several implementations of Lempel–Ziv–Welch decompression 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 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 decompress_data(data_bits: str) -> str:
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"""
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Decompresses 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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lexicon[curr_string] = last_match_id + "0"
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if math.log2(index).is_integer():
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newLex = {}
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for curr_key in list(lexicon):
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newLex["0" + curr_key] = lexicon.pop(curr_key)
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lexicon = newLex
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lexicon[bin(index)[2:]] = last_match_id + "1"
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index += 1
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curr_string = ""
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return result
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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[:-1]:
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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 remove_prefix(data_bits: str) -> str:
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"""
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Removes size prefix, that compressed file should have
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Returns the result
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"""
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counter = 0
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for letter in data_bits:
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if letter == "1":
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break
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counter += 1
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data_bits = data_bits[counter:]
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data_bits = data_bits[counter + 1 :]
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return data_bits
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def compress(source_path: str, destination_path: str) -> None:
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"""
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Reads source file, decompresses it and writes the result in destination file
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"""
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data_bits = read_file_binary(source_path)
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data_bits = remove_prefix(data_bits)
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decompressed = decompress_data(data_bits)
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write_file_binary(destination_path, decompressed)
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if __name__ == "__main__":
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compress(sys.argv[1], sys.argv[2])
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