cs50/substitution.c

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

int validate_key(string key); // NQXPOMAFTRHLZGECYJIUWSKDVB
void substitute(string plaintext, string key);

int main(int argc, string argv[])
{
    // Print usage and exit if argc is not 2 or key is not integer
    if (argc != 2 || validate_key(argv[1]) > 0)
    {
        printf("Usage: ./substitution key\n");
        return 1;
    }
    else
    {
        string key = argv[1];
        string plaintext = get_string("plaintext:  ");
        substitute(plaintext, key);
    }
    return 0;
}

// Convert a string of numbers to integer
int validate_key(string key)
{
    // Sum of consecutive numbers: (b + a) x (b - a + 1) / 2
    // If key length is 26 then iterate each string element
    if (strlen(key) == 26)
    {
        int sum = 0;

        for (int i = 0; i < strlen(key); i++)
        {
            if (isalpha(key[i]))
            {
                sum += toupper(key[i]);
            }
        }

        if (sum == 2015) // Sum of 26 unique uppercase characters
        {
            return 0;
        }
    }
    return 1;
}

void substitute(string plaintext, string key)
{
    printf("ciphertext: ");

    for (int i = 0; i < strlen(plaintext); i++)
    {
        if (isalpha(plaintext[i]))
        {
            if (isupper(plaintext[i]))
            {
                printf("%c", key[plaintext[i] - 65]);
            }
            else
            {
                printf("%c", tolower(key[plaintext[i] - 97]));
            }
        }
        else
        {
            printf("%c", plaintext[i]);
        }
    }
    printf("\n");
}