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"""

One of the several implementations of Lempel–Ziv–Welch compression algorithm

https://en.wikipedia.org/wiki/Lempel%E2%80%93Ziv%E2%80%93Welch

"""

import math

import os

import sys

def read_file_binary(file_path: str) -> str:

"""

Reads given file as bytes and returns them as a long string

"""

result = ""

try:

with open(file_path, "rb") as binary_file:

data = binary_file.read()

for dat in data:

curr_byte = f"{dat:08b}"

result += curr_byte

return result

except OSError:

print("File not accessible")

sys.exit()

def add_key_to_lexicon(

lexicon: dict, curr_string: str, index: int, last_match_id: str

) -> None:

"""

Adds new strings (curr_string + "0", curr_string + "1") to the lexicon

"""

lexicon.pop(curr_string)

lexicon[curr_string + "0"] = last_match_id

if math.log2(index).is_integer():

for curr_key in lexicon:

lexicon[curr_key] = "0" + lexicon[curr_key]

lexicon[curr_string + "1"] = bin(index)[2:]

def compress_data(data_bits: str) -> str:

"""

Compresses given data_bits using Lempel–Ziv–Welch compression algorithm

and returns the result as a string

"""

lexicon = {"0": "0", "1": "1"}

result, curr_string = "", ""

index = len(lexicon)

for i in range(len(data_bits)):

curr_string += data_bits[i]

if curr_string not in lexicon:

continue

last_match_id = lexicon[curr_string]

result += last_match_id

add_key_to_lexicon(lexicon, curr_string, index, last_match_id)

index += 1

curr_string = ""

while curr_string != "" and curr_string not in lexicon:

curr_string += "0"

if curr_string != "":

last_match_id = lexicon[curr_string]

result += last_match_id

return result

def add_file_length(source_path: str, compressed: str) -> str:

"""

Adds given file's length in front (using Elias gamma coding) of the compressed

string

"""

file_length = os.path.getsize(source_path)

file_length_binary = bin(file_length)[2:]

length_length = len(file_length_binary)

return "0" * (length_length - 1) + file_length_binary + compressed

def write_file_binary(file_path: str, to_write: str) -> None:

"""

Writes given to_write string (should only consist of 0's and 1's) as bytes in the

file

"""

byte_length = 8

try:

with open(file_path, "wb") as opened_file:

result_byte_array = [

to_write[i : i + byte_length]

for i in range(0, len(to_write), byte_length)

]

if len(result_byte_array[-1]) % byte_length == 0:

result_byte_array.append("10000000")

else:

result_byte_array[-1] += "1" + "0" * (

byte_length - len(result_byte_array[-1]) - 1

)

for elem in result_byte_array:

opened_file.write(int(elem, 2).to_bytes(1, byteorder="big"))

except OSError:

print("File not accessible")

sys.exit()

def compress(source_path, destination_path: str) -> None:

"""

Reads source file, compresses it and writes the compressed result in destination

file

"""

data_bits = read_file_binary(source_path)

compressed = compress_data(data_bits)

compressed = add_file_length(source_path, compressed)

write_file_binary(destination_path, compressed)

if __name__ == "__main__":

compress(sys.argv[1], sys.argv[2])

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