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

int main(int argc, string argv[])
{
    //Check correct # of characters
    if (argc != 2)
    {
        printf("Error!\n");
        return 1;
    }
    //reject if there are non-alphabet characters in keyword
    for (int i = 0; i < strlen(argv[1]); i++)
    {
        if (!isalpha(argv[1][i]))
        {
            printf("Error!\n");
            return 1;
        }
    }

    int keylength = strlen(argv[1]);
    if (argc == 2)
    {
        //get the plaintext
        string plain = get_string("plaintext: ");
        int plainlength = strlen(plain);
        printf("ciphertext: ");
        //for each character in the plaintext
        for (int i = 0, s = 0; i < plainlength; i++)
        {
            //if it's an alphabet
            if (isalpha(plain[i]))
            {
                int key = (tolower(argv[1][s % keylength])) - 97;// key
                //if it's lowercase
                if (islower(plain[i]))
                {
                    int lowercipher = (((plain[i] + key) - 97) % 26) + 97;
                    printf("%c", lowercipher);
                    s++;
                }
                // if it's uppercase, do same thing as lowercase but change to uppercase in the end
                if (isupper(plain[i]))
                {
                    int uppercipher = (((tolower(plain[i]) + key) - 97) % 26) + 97;
                    printf("%c", toupper(uppercipher));
                    s++;
                }
            }
            //if it's any other character, print that character
            else
            {
                printf("%c", plain[i]);
            }
        }
        //line break after plaintext is converted to ciphertext
        printf("\n");
    }
}