vigenere

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

char shift_character(char c, char key);

int main(int argc, string argv[])
{
    if (argc != 2)
    {
        printf("Usage: ./vigenere k\n");
        return 1;
    }
    string key = argv[1];
    int key_index = 0;
    int max_index = strlen(key) - 1;
    for (int i = 0; i <= max_index; i++)
    {
        if (!isalpha(key[i]))
        {
            return 1;
        }
    }

    string plain_text = get_string("plaintext: ");
    printf("ciphertext: ");
    for (int i = 0, input_length = strlen(plain_text); i < input_length; i++)
    {
        if (!isalpha(plain_text[i]))
        {
            printf("%c", plain_text[i]);
            continue;
        }
        printf("%c", shift_character(plain_text[i], key[key_index]));
        key_index++;
        if (key_index > max_index)
        {
            key_index = 0;
        }
    }
    printf("\n");
}

char shift_character(char c, char key)
{
    key = toupper(key) - 'A';
    int offset = ((islower(c)) ? 'a' : 'A');
    int alphabit_index = c - offset;
    int shifted_alpha_index = (alphabit_index + key) % 26;
    return shifted_alpha_index + offset;
}