import cs50
import sys

def main():
    # make sure the user gives the right parameters
    if len(sys.argv) != 2:
        print("python caesar.py key")
        exit(1)

    # make sure the input are valid alabetical characters
    if sys.argv[1].isalpha() == False:
        print("plase provide valid input")
        exit(2)
    else:
        key = sys.argv[1]

    # prompt the user for text to be ciphered
    print("plaintext: ", end="")
    plaintext = cs50.get_string()
    print("ciphertext: ", end="")

    # prepare to do the ciphering
    key_index = 0
    key_len = len(sys.argv[1])
    ciphertext = []

    # loop and cipher
    for i in plaintext:
        if i.isalpha():
            cipher_key = ord(key[key_index % key_len].lower()) - 97
            ciphertext.append(cipher_this(i, cipher_key))
            cipher_key += 1
        else:
            ciphertext.append(i)

    # print the final solution
    print("".join(ciphertext))

    exit(0)

def cipher_this(char, key):
    if char.isupper():
        return chr(((ord(char) - 65 + key) % 26) + 65)
    elif char.islower():
        return chr(((ord(char) - 97 + key) % 26) + 97)

if __name__=="__main__":
    main()