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

    char substitution_encrypt (char c, char *key)
    {
            return isupper (c) ? toupper (key[c - 65]) : (islower (c) ? tolower (key[c - 97]) : c);
    }

    void check_key (char *key)
    {
            if (strlen (key) != 26) {
                    fprintf (stderr, "Invalid key");
                    exit(EXIT_FAILURE);
            }

            int letter_bitmap = 0;
            for (char *c = key; *c != '\0'; c++) {
                    char curr = *c;
                    if (!isalpha (curr)) {
                            fprintf (stderr, "Invalid key");
                            exit (EXIT_FAILURE);
                    }

                    if (isupper (curr))
                            curr -= 65;
                    else if (islower (curr))
                            curr -= 97;

                    if (letter_bitmap & (1 << curr)) {
                            fprintf (stderr, "Invalid key");
                            exit (EXIT_FAILURE);
                    }

                    letter_bitmap |= (1 << curr);
            }
    }

    int main (int argc, char **argv)
    {
            if (argc != 2) {
                    fprintf (stderr, "Usage: ./substitution key\n");
                    exit (EXIT_FAILURE);
            }

            char *key = argv[1];

            check_key(key);

            char *text = get_string ("plaintext:  ");

            printf ("ciphertext: ");
            while (*text) {
                    putchar (substitution_encrypt (*text++, key));
            }
    }