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

int k;
int shift(char c);
int main(int argc, string argv[])
{
    if (argc == 2) // ensure one argument from user
    {
        for (int i = 0; i < strlen(argv[1]); i++) // iterate over length of argument input by user
        {
            if (isalpha(argv[1][i]) == 0) // ensure each character in argument is alphabetic
            {
                printf("Usage: ./vigenere keyword\n"); // error/failure message to user
                return 1;
            }
        }
    }

    string plain = get_string("plaintext: "); // get plaintext input from user
    printf("ciphertext: ");
    for (int i = 0; i < strlen(plain); i++) // iterate over length of plaintext input
    {
        k = shift(argv[1][i]);
        char c = plain[i]; // convert string to individual characters
        if (c < 65 || (c > 90 && c < 97) || c > 122) // leave non-alphabetical characters unchanged
        {
            printf("%c", c);
        }
        else if (((c + k % 26) > 90 && (c + k % 26) < 97) || (c + k % 26) > 122)
        // prevent printing of undesired non-alphabetical characters resulting from rotation
        {
            printf("%c", c + (k % 26 - 26));
        }
        else
        {
            printf("%c", c + (k % 26)); // apply key upon print
        }
        printf("\n");
    }
}

int shift(char c)
{
    if (isupper(c))
    {
        int x = (c - 65);
        return x;
    }
    else
    {
        int x = (c - 97);
        return x;
    }
}