# Functions
def int_check(question):
    """Checks users enter an integer."""

    error = "Please enter an integer between 0 and 26."

    while True:

        try:
            # Return the response if it's an integer
            response = int(input(question))

            if 0 < response < 26:
                return response
        
            else:
                print(error)

        except ValueError:

            print(error)


def string_check(question, valid_ans_list):
    """Checks that users enter the full word
    or the first letter of a word from a list of valid responses"""

    result = ''

    while True:

        response = input(question).lower()

        for item in valid_ans_list:

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

            # check if it's the first letter
            elif response == item[:1]:
                return item

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


def encrypt_func():
    '''Encrypts user input by custom amount'''

    result = ''

    plain_text = input("Plain text: ")

    shift = int_check("Shift amount: ")

    for char in plain_text:
        if char.isupper():
            FIRST_CHAR_CODE = 65
            LAST_CHAR_CODE = 90

        if char.islower():
            FIRST_CHAR_CODE = 97
            LAST_CHAR_CODE = 122

        if char.isalpha():
            char_code = ord(char)
            new_char_code = char_code + shift

            if new_char_code > LAST_CHAR_CODE:
                new_char_code -= CHAR_RANGE
            
            if new_char_code < FIRST_CHAR_CODE:
                new_char_code += CHAR_RANGE

            new_char = chr(new_char_code)
            result += new_char
        else:
            result += char

    print("Encrypted Text: " + result)
    print()


def decrypt_func():
    '''Gives all possible deciphered texts'''

    cipher_text = input("Cipher Text: ")
    
    shift = 1

    result = ""

    while shift < 25:
        for char in cipher_text:
            if char.isupper():
                FIRST_CHAR_CODE = 65
                LAST_CHAR_CODE = 90

            if char.islower():
                FIRST_CHAR_CODE = 97
                LAST_CHAR_CODE = 122
            
            if char.isalpha():
                char_code = ord(char)
                new_char_code = char_code + shift

                if new_char_code > LAST_CHAR_CODE:
                    new_char_code -= CHAR_RANGE
                
                if new_char_code < FIRST_CHAR_CODE:
                    new_char_code += CHAR_RANGE

                new_char = chr(new_char_code)
                result += new_char

            else:
                result += char
        
        shift + 1



# Main Routine
print()
print("=== Caesar Cipher Encrypter & Decrypter ===")
print()

CHAR_RANGE = 26

mode = string_check("Encrypt or Decrypt? ", ['encrypt', 'decrypt'])

if mode == "encrypt":
    encrypt_func()
else:
    decrypt_func()