Source code for src.ch05.p2_decode_null

"""Decode plaintext message from null cipher."""
from src.ch04.challenge.c1_encode_route import format_plaintext


def decode_null(interval: int, ciphertext: str) -> str:
    """Decode message from null cipher.

    For every word specified by **interval** in **ciphertext**, generate a
    string using each **interval** letter.

    Args:
        interval (int): nth letter of every nth word to form a string.
        ciphertext (str): String with null cipher encoded message. Spaces
            and punctuation are okay, but will be removed. Uppercase
            converted to lowercase.

    Returns:
        String containing nth letter of every nth word in
        **ciphertext**.

    Example:
        >>> from src.ch05.p2_decode_null import decode_null
        >>> ciphertext = 'national aeronautics space administration'
        >>> decode_null(1, ciphertext)
        'nasa'

    """
    plaintext = []
    clean_text = format_plaintext(ciphertext)
    for i, word in enumerate(clean_text):
        if i % interval == 0:
            plaintext.append(word[interval - 1])
    return ''.join(plaintext)


def main():
    """Demonstrate null cipher decoder.

    Tip:
        The website `bestwordlist.com`_ helped a metric ton.

    .. _bestwordlist.com: https://www.bestwordlist.com

    """
    print('I can decode a sequential null cipher. Don\'t be gettin\' fancy '
          'by\nskipping words or I\'ll give you such a pinch!\n')
    ciphertext = ('Amino acids are not actors, but are mere blips in the dark '
                  'afterlife.')
    interval = 4
    print(f'Analyzing: {ciphertext}')
    print(f'Using every {interval}th word and letter.\n')
    print(f'Result: {decode_null(interval, ciphertext)}')


if __name__ == '__main__':
    main()