substitution.c

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

int main(int argc, string argv[])
{
    if (argc != 2)
    {
        printf("Usage: ./substitution key\n");
        return 1;
    }

    string key = argv[1];
    if (strlen(key) != 26)
    {
        printf("Key must contain 26 characters.\n");
        return 1;
    }

    // iterate through all key char
    for (int i = 0; i < strlen(key); i++)
    {
        // normalize key as uppercase letters
        key[i] = toupper(key[i]);

        // special characters are not allowed
        bool is_within_a_to_z = key[i] >= 'A' && key[i] <= 'Z';

        if (!is_within_a_to_z)
        {
            printf("Special characters are not allowed.\n");
            return 1;
        }

        // duplicated keys are not allowed
        for (int j = 0; j < i; j++)
        {
            // check if any previous key char is the same as current key char
            if (key[i] == key[j])
            {
                printf("Duplicated keys are not allowed.\n");
                return 1;   
            }
        }
    }

    string text = get_string("plaintext:  ");

    for (int i = 0; i < strlen(text); i++)
    {
        // substitute only alphabets
        if ((text[i] >= 'a' && text[i] <= 'z') || (text[i] >= 'A' && text[i] <= 'Z'))
        {
            // normalize it to upper
            char normalized = toupper(text[i]);

            // upper case starts at 'A' so 'A'-'A' = 0, 'B'-'A' = 1 and so on
            char cipher_char = key[normalized - 'A'];

            // check if plaintext char is lowercase, if so, map cipher char to lowercase
            if (islower(text[i]))
            {
                cipher_char = tolower(cipher_char);
            }
            
            // update text[i] as cipher char
            text[i] = cipher_char;
        }
    }

    printf("ciphertext: %s\n", text);

    return 0;
}