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

int main (int argc, string argv[])
{
    //error if no key given
    if (argc < 2)
    {
        printf("\nI'll need a key at the command prompt!\n\n");
        return 1;
    }

    //error if too much keys given
    else if (argc > 2)
    {
        printf("\nThat's too much!\n\n");
        return 1;
    }

    //and for the right amount of arguments we can continue
    else
    {
        string key = argv[1];

        //check for invalid chars in keyword
        for (int i = 0, n = strlen(key); i < n; i++)
        {
            if (!(isalpha(key[i])))
            {
                printf("\nI'll need a keyword, only letters!\n\n");
                return 1;
            }
        }

        //close the checkalpha iteration and ask for phrase to crypt
        string plaintext = GetString();
        int k = 0; //to correct for skipped plaintextchars

        //iterate through plaintext array we just got
        for (int i = 0, n = strlen(plaintext); i < n; i++)
        {
            int j = i % strlen(key);

            //branch for all non-alphabetic chars
            if (!(isalpha(plaintext[i])))
            {
                printf("%c", plaintext[i]);
                k++;
            }

            //branch for all alphabetic chars in plaintext
            else
            {
                //declare conversion constant for upper & lower case letters
                int a = 65;
                int b = 97;

                //branch for all uppercase chars in plaintext
                if (isupper(plaintext[i]))
                {
                    int convertedPlaintext = plaintext[i] - a;

                    if (isupper(key[j]))
                    {
                        int convertedKey = key[j - k] - a;
                        int result = (convertedPlaintext + convertedKey) % 26;
                        printf("%c", result + a);
                    }

                    else //islower
                    {
                        int convertedKey = key[j - k] - b;
                        int result = (convertedPlaintext + convertedKey) % 26;
                        printf("%c", result + a);
                    }
                }

                //branch for all lower case chars in OGPhrase
                else
                {
                    int convertedPlaintext = plaintext[i] - b;

                    if (isupper(key[j]))
                    {
                        int convertedKey = key[j - k] - a; //is a still 65 here scopewise?
                        int result = (convertedPlaintext + convertedKey) % 26;
                        printf("%c", result + b);
                    }

                    else
                    {
                        int convertedKey = key[j - k] - b;
                        int result = (convertedPlaintext + convertedKey) % 26;
                        printf("%c", result + b);
                    }
                }
            }
        }
        printf("\n");
        return 0;
    }
}