# Author: M. Yathurshan
# Black Formatter: True

"""
Implementation of SHA256 Hash function in a Python class and provides utilities
to find hash of string or hash of text from a file.

Usage: python sha256.py --string "Hello World!!"
       python sha256.py --file "hello_world.txt"
       When run without any arguments,
       it prints the hash of the string "Hello World!! Welcome to Cryptography"

References:
https://qvault.io/cryptography/how-sha-2-works-step-by-step-sha-256/
https://en.wikipedia.org/wiki/SHA-2
"""

import argparse
import struct
import unittest


class SHA256:
    """
    Class to contain the entire pipeline for SHA1 Hashing Algorithm

    >>> SHA256(b'Python').hash
    '18885f27b5af9012df19e496460f9294d5ab76128824c6f993787004f6d9a7db'

    >>> SHA256(b'hello world').hash
    'b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9'
    """

    def __init__(self, data: bytes) -> None:
        self.data = data

        # Initialize hash values
        self.hashes = [
            0x6A09E667,
            0xBB67AE85,
            0x3C6EF372,
            0xA54FF53A,
            0x510E527F,
            0x9B05688C,
            0x1F83D9AB,
            0x5BE0CD19,
        ]

        # Initialize round constants
        self.round_constants = [
            0x428A2F98,
            0x71374491,
            0xB5C0FBCF,
            0xE9B5DBA5,
            0x3956C25B,
            0x59F111F1,
            0x923F82A4,
            0xAB1C5ED5,
            0xD807AA98,
            0x12835B01,
            0x243185BE,
            0x550C7DC3,
            0x72BE5D74,
            0x80DEB1FE,
            0x9BDC06A7,
            0xC19BF174,
            0xE49B69C1,
            0xEFBE4786,
            0x0FC19DC6,
            0x240CA1CC,
            0x2DE92C6F,
            0x4A7484AA,
            0x5CB0A9DC,
            0x76F988DA,
            0x983E5152,
            0xA831C66D,
            0xB00327C8,
            0xBF597FC7,
            0xC6E00BF3,
            0xD5A79147,
            0x06CA6351,
            0x14292967,
            0x27B70A85,
            0x2E1B2138,
            0x4D2C6DFC,
            0x53380D13,
            0x650A7354,
            0x766A0ABB,
            0x81C2C92E,
            0x92722C85,
            0xA2BFE8A1,
            0xA81A664B,
            0xC24B8B70,
            0xC76C51A3,
            0xD192E819,
            0xD6990624,
            0xF40E3585,
            0x106AA070,
            0x19A4C116,
            0x1E376C08,
            0x2748774C,
            0x34B0BCB5,
            0x391C0CB3,
            0x4ED8AA4A,
            0x5B9CCA4F,
            0x682E6FF3,
            0x748F82EE,
            0x78A5636F,
            0x84C87814,
            0x8CC70208,
            0x90BEFFFA,
            0xA4506CEB,
            0xBEF9A3F7,
            0xC67178F2,
        ]

        self.preprocessed_data = self.preprocessing(self.data)
        self.final_hash()

    @staticmethod
    def preprocessing(data: bytes) -> bytes:
        padding = b"\x80" + (b"\x00" * (63 - (len(data) + 8) % 64))
        big_endian_integer = struct.pack(">Q", (len(data) * 8))
        return data + padding + big_endian_integer

    def final_hash(self) -> None:
        # Convert into blocks of 64 bytes
        self.blocks = [
            self.preprocessed_data[x : x + 64]
            for x in range(0, len(self.preprocessed_data), 64)
        ]

        for block in self.blocks:
            # Convert the given block into a list of 4 byte integers
            words = list(struct.unpack(">16L", block))
            # add 48 0-ed integers
            words += [0] * 48

            a, b, c, d, e, f, g, h = self.hashes

            for index in range(0, 64):
                if index > 15:
                    # modify the zero-ed indexes at the end of the array
                    s0 = (
                        self.ror(words[index - 15], 7)
                        ^ self.ror(words[index - 15], 18)
                        ^ (words[index - 15] >> 3)
                    )
                    s1 = (
                        self.ror(words[index - 2], 17)
                        ^ self.ror(words[index - 2], 19)
                        ^ (words[index - 2] >> 10)
                    )

                    words[index] = (
                        words[index - 16] + s0 + words[index - 7] + s1
                    ) % 0x100000000

                # Compression
                S1 = self.ror(e, 6) ^ self.ror(e, 11) ^ self.ror(e, 25)
                ch = (e & f) ^ ((~e & (0xFFFFFFFF)) & g)
                temp1 = (
                    h + S1 + ch + self.round_constants[index] + words[index]
                ) % 0x100000000
                S0 = self.ror(a, 2) ^ self.ror(a, 13) ^ self.ror(a, 22)
                maj = (a & b) ^ (a & c) ^ (b & c)
                temp2 = (S0 + maj) % 0x100000000

                h, g, f, e, d, c, b, a = (
                    g,
                    f,
                    e,
                    ((d + temp1) % 0x100000000),
                    c,
                    b,
                    a,
                    ((temp1 + temp2) % 0x100000000),
                )

            mutated_hash_values = [a, b, c, d, e, f, g, h]

            # Modify final values
            self.hashes = [
                ((element + mutated_hash_values[index]) % 0x100000000)
                for index, element in enumerate(self.hashes)
            ]

        self.hash = "".join([hex(value)[2:].zfill(8) for value in self.hashes])

    def ror(self, value: int, rotations: int) -> int:
        """
        Right rotate a given unsigned number by a certain amount of rotations
        """
        return 0xFFFFFFFF & (value << (32 - rotations)) | (value >> rotations)


class SHA256HashTest(unittest.TestCase):
    """
    Test class for the SHA256 class. Inherits the TestCase class from unittest
    """

    def test_match_hashes(self) -> None:
        import hashlib

        msg = bytes("Test String", "utf-8")
        self.assertEqual(SHA256(msg).hash, hashlib.sha256(msg).hexdigest())


def main() -> None:
    """
    Provides option 'string' or 'file' to take input
    and prints the calculated SHA-256 hash
    """

    # unittest.main()

    import doctest

    doctest.testmod()

    parser = argparse.ArgumentParser()
    parser.add_argument(
        "-s",
        "--string",
        dest="input_string",
        default="Hello World!! Welcome to Cryptography",
        help="Hash the string",
    )
    parser.add_argument(
        "-f", "--file", dest="input_file", help="Hash contents of a file"
    )

    args = parser.parse_args()

    input_string = args.input_string

    # hash input should be a bytestring
    if args.input_file:
        with open(args.input_file, "rb") as f:
            hash_input = f.read()
    else:
        hash_input = bytes(input_string, "utf-8")

    print(SHA256(hash_input).hash)


if __name__ == "__main__":
    main()