#include <stdio.h>

// Code for encrypting
void encryptDecrypt(int mode) {
    char input[100] = {0}; //Maximum length of the string is 100 characters
    short pass; //Password should be within range 0~25
    if (mode == 0) {
        printf("Enter the string to be encrypted (no longer than 100 characters):\n");
    }
    else {
        printf("Enter the string to be decrypted (no longer than 100 characters):\n");
    }
    scanf("%s",input);
    printf("Enter your password (0-25): ");
    scanf("%hd", &pass);
    // Go through every character in input
    for (int i = 0; i < 100; i++){
        if (input[i] == '\0') {
            // '\0' means we have reached the string's end.
            break;
        }
        else {
            input[i] -= 'A'; //Calculate the chacter's place in the alphabet, from 0~25
            if (mode == 0) {
                input[i] += pass; //Add the offset of pass
            }
            else {
                input[i] -= pass; //Subtract the offset of pass
                input[i] += 26; //Avoid negative number
            }
            input[i] %= 26; //Prevent the number from overflow
            input[i] += 'A'; //Turn the number into ASCII again.
            printf("%c", input[i]);
        }
    }
}

int main() {
    char mode;
    printf("Slect Mode: (1) Encypt (2) Decrypt: ");
    mode = getchar();
    if (mode == '1') {
        encryptDecrypt(0);
    }
    else if (mode == '2') {
        encryptDecrypt(1);
    }
    else {
        printf("Unrecognized command.");
    }
    return 0;
}