#include <stdio.h>
#include <cs50.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
#include <math.h>

//Get the key right from the command line argument
int main(int argc, string argv[])
{
    //if the key has more or less than a single character and if the keyword is not alphabetical, end the program
    if (argc != 2)
    {
        printf("You must enter a single string as the key\n");
        return 1;
    }
    for (int k = 0, n = strlen(argv[1]); k < n; k++)
    {
        while (!isalpha(argv[1][k]))
        {
            printf("The key must be alphabetical\n");
            return 1;
        }
    }

    //Prompt the user for the plaintext
    string plaintext = get_string("Write a text to encrypt: ");

    string keyword = argv[1];

    int s = strlen(plaintext);

    //Print the response previous to the ciphering
    printf("ciphertext: ");

    //iterate over every plaintext and keyword character
    for (int i = 0, j = 0, n = strlen(keyword); i < s; i++)
    {

        //check if the plaintext character is alphabetic
        if (isalpha(plaintext[i]))
        {
            //Preserve upper cases and lower cases of each character
            if (isupper(plaintext[i]))
            {
                printf("%c", ((plaintext[i] - 65 + keyword[j] - 65) % 26 + 65));
                j = (j + 1) % n;
            }

            if (islower(plaintext[i]))
            {
                printf("%c", ((plaintext[i] - 97 + keyword[j] - 97) % 26 + 97));
                j = (j + 1) % n;
            }
        }

        //Print the non-alphabetic characters without applying the keyword
        else
        {
            printf("%c", plaintext[i]);
        }

    }

    printf("\n");
    return 0;

}