#include <stdio.h>

int encrypt(int num);
int decrypt(int encryptedNum);

int encryptMenu(void);
int decryptMenu(void);

int main(void)
{
    char choice = '0';
START:
    printf("%s\n\n%s\n%s\n\n%s", "Do you want to encrypt or decrypt a code?",
           "[E\\e] - To Encrypt",
           "[D\\d] - To Decrypt",
           "Your answer: ");
    scanf("%c", &choice);
    switch (choice)
    {
    case 'e':
    case 'E':
        encryptMenu(); // prints encryption menu
        break;
    case 'd':
    case 'D':
        decryptMenu(); // prints decryption menu
        break;
    default:
        puts("Type in either 'e' or 'd'.");
        goto START;
        break;
    }
}

int encrypt(int num)
{
    int d1, d2, d3, d4; // 4 digits of the number to be separated
    int result;         // encrypted integer to be returned
    // Separate digits
    d4 = num % 10;
    num /= 10;
    d3 = num % 10;
    num /= 10;
    d2 = num % 10;
    num /= 10;
    d1 = num;
    // Add 7 to each
    d1 += 7;
    d2 += 7;
    d3 += 7;
    d4 += 7;
    // Get the remainder when divided by 10
    d1 %= 10;
    d2 %= 10;
    d3 %= 10;
    d4 %= 10;
    // Swap digit 1 with 3, and 2 with 4
    result = d3 * 1000 + d4 * 100 + d1 * 10 + d2;
    return result;
}
int decrypt(int encryptedNum)
{
    int d1, d2, d3, d4; // 4 digits of the number to be separated
    int result;         // encrypted integer to be returned
    // Separate digits
    d4 = encryptedNum % 10;
    encryptedNum /= 10;
    d3 = encryptedNum % 10;
    encryptedNum /= 10;
    d2 = encryptedNum % 10;
    encryptedNum /= 10;
    d1 = encryptedNum;
    // Add 10, if the number was previously divided
    if (d1 < 8 && d1 > 0)
        d1 += 10;
    if (d2 < 8 && d2 > 0)
        d2 += 10;
    if (d3 < 8 && d3 > 0)
        d3 += 10;
    if (d4 < 8 && d4 > 0)
        d4 += 10;
    // Substract 7 from each
    d1 -= 7;
    d2 -= 7;
    d3 -= 7;
    d4 -= 7;
    // Swap digit 1 back with 3, and 2 with 4
    result = d3 * 1000 + d4 * 100 + d1 * 10 + d2;
    return result;
}

int encryptMenu(void)
{
    int input;
    printf("\n%s", "Enter a 4-digit integer: ");
    scanf("%d", &input);
    if (input < 1000 || input > 9999) // make sure input is a 4-digit integer
    {
        puts("4-digit integer is required!");
        encryptMenu(); // go back and ask for an input again
    }
    printf("\nYour encrypted integer is: %d\n\n", encrypt(input));
    main();
}
int decryptMenu(void)
{
    int input;
    printf("\n%s", "Enter an encrypted 4-digit integer: ");
    scanf("%d", &input);
    if (input < 1000 || input > 9999) // make sure input is a 4-digit integer
    {
        puts("4-digit integer is required!");
        decryptMenu(); // go back and ask for an input again
    }
    printf("\nYour encrypted integer is: %d\n\n", decrypt(input));
    main();
}