import string


# Functions go here
def make_statement(statement, decoration):
    """Emphasises headings by adding decoration at the start and end"""

    print(f"{decoration * 3} {statement} {decoration * 3}")


def string_check(question, valid_answers=('yes', 'no'), num_letters=1):
    """Checks that users enter the full word or the 'n' letters of a word from a list of valid responses"""

    while True:

        response = input(question).lower()

        for item in valid_answers:

            # check if the response is the entire word
            if response == item:
                return item

            # check if it's the first letter
            elif response == item[:num_letters]:
                return item

        print(f"Please choose an option from {valid_answers}")


def not_blank(question):
    """Checks that a user response is not blank"""

    while True:
        response = input(question)

        if response != "":
            return response

        print("Sorry, this can't be blank. Please try again.\n")


def instructions():
    make_statement("Instructions", "🎲")

    print('''
Select whether you want to encrypt or decrypt, then enter your message and the number of letters you want it to be 
shifted by.

The program will record the messages you have encrypted/decrypted, what they were shifted by and how many times you 
have used the program. Once you have entered the exit code ('xxx'), the program will display your user history and write the data as a text file.
    ''')


def encode():
    encoded_message = ""
    for letter in message:
        if letter in alphabet:
            new_position = alphabet.index(letter) + shift_number
            new_position = new_position % len(alphabet)
            encoded_message = encoded_message + alphabet[new_position]
        elif letter in upper_alphabet:
            new_position = upper_alphabet.index(letter) + shift_number
            new_position = new_position % len(upper_alphabet)
            encoded_message = encoded_message + upper_alphabet[new_position]
        else:
            encoded_message = encoded_message + letter

    print("This is your encoded message:", encoded_message)
    return encoded_message


def decode():
    decoded_message = ""
    for letter in message:
        if letter in alphabet:
            new_position = alphabet.index(letter) - shift_number
            new_position = new_position % len(alphabet)
            decoded_message = decoded_message + alphabet[new_position]
        elif letter in upper_alphabet:
            new_position = upper_alphabet.index(letter) - shift_number
            new_position = new_position % len(upper_alphabet)
            decoded_message = decoded_message + upper_alphabet[new_position]
        else:
            decoded_message = decoded_message + letter

    print("This is your decoded message:", decoded_message)
    return decoded_message


# initialise alphabet + enc_dec
alphabet = list(string.ascii_lowercase)
upper_alphabet = list(string.ascii_uppercase)
messages_encoded = 0
messages_decoded = 0
cipher_types = ('encode', 'decode', 'xxxxx')

# Main routine goes here
make_statement("Caesar Cipher Program", "⚖")

print()
want_instructions = string_check("Do you want to see the instructions? ")

if want_instructions == "yes":
    instructions()

print()

while True:
    # ask user for their message (and check it's not blank)
    print()
    message = not_blank("What is your message? ")

    # if name is exit code, break out of loop
    if message == "xxx":
        break

    # Ask for their shift_number
    shift_number = int(input("How many letters would you like to shift your message by? "))

    # ask user for cipher type (encode / decode / enc / dec)
    cipher_type = string_check("Type of cipher (encode/decode): ", cipher_types, 3)
    if cipher_type == 'encode':
        encode()
        messages_encoded += 1
        prior_message = string_check("Do you want to decode your prior message? ")
        if prior_message == 'yes':
            message = encode()
            decode()
            messages_decoded += 1
        elif prior_message == 'no':
            continue
    if cipher_type == 'decode':
        decode()
        messages_decoded += 1
        prior_message = string_check("Do you want to encode your prior message? ")
        if prior_message == 'yes':
            message = decode()
            encode()
            messages_encoded += 1
        elif prior_message == 'no':
            continue
    elif cipher_type == "xxx":
        break


print(f"You have encoded {messages_encoded}, and decoded {messages_decoded}. You have used this program "
      f"{messages_encoded + messages_decoded} times.")