#!/usr/bin/python3
# -*- coding: utf-8 -*-

# Vigenere Cipher (Polyalphabetic Substitution Cipher)
# https://www.nostarch.com/crackingcodes (BSD Licensed)

LETTERS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"


def main() -> None:
    # This text can be copy/pasted from https://invpy.com/vigenereCipher.py:
    myMessage = """If there is no struggle there is no progress. Those \
who profess to favor freedom and yet deprecate agitation are men who wa\
nt crops without plowing up the ground; they want rain without thunder \
and lightning. They want the ocean without the awful roar of its many w\
aters. This struggle may be a moral one, or it may be a physical one, a\
nd it may be both moral and physical, but it must be a struggle. Power \
concedes nothing without a demand. It never did and it never will. -- F\
rederick Douglass"""
    myKey = "ASIMOV"
    myMode = "encrypt"  # Set to either 'encrypt' or 'decrypt'.

    if myMode == "encrypt":
        translated = encryptMessage(myKey, myMessage)
    elif myMode == "decrypt":
        translated = decryptMessage(myKey, myMessage)

    print("%sed message:" % (myMode.title()))
    print(translated)
    print()
    print("The message has been copied to the clipboard.")


def encryptMessage(key: str, message: str) -> str:
    return translateMessage(key, message, "encrypt")


def decryptMessage(key: str, message: str) -> str:
    return translateMessage(key, message, "decrypt")


def translateMessage(key: str, message: str, mode: str) -> str:
    translated = []  # Stores the encrypted/decrypted message string.

    keyIndex = 0
    key = key.upper()

    for symbol in message:  # Loop through each symbol in message.
        num = LETTERS.find(symbol.upper())
        if num != -1:  # -1 means symbol.upper() was not found in LETTERS.
            if mode == "encrypt":
                num += LETTERS.find(key[keyIndex])  # Add if encrypting.
            elif mode == "decrypt":
                num -= LETTERS.find(key[keyIndex])  # Subtract if decrypting.

            num %= len(LETTERS)  # Handle any wraparound.

            # Add the encrypted/decrypted symbol to the end of translated:
            if symbol.isupper():
                translated.append(LETTERS[num])
            elif symbol.islower():
                translated.append(LETTERS[num].lower())

            keyIndex += 1  # Move to the next letter in the key.
            if keyIndex == len(key):
                keyIndex = 0
        else:
            # Append the symbol without encrypting/decrypting.
            translated.append(symbol)

    return "".join(translated)


# If vigenereCipher.py is run (instead of imported as a module) call
# the main() function.
if __name__ == "__main__":
    main()