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

// Compile this program with:
//    cc -std=c11 -Wall -Werror -o rotate rotate.c

#define ROT 13

//  The rotate function returns the character ROT positions further along the
//  alphabetic character sequence from c, or c if c is not lower-case

char rotate(char c)
{
    // Check if c is lower-case or not
    if(islower(c)) {
        // The ciphered character is ROT positions beyond c,
        // allowing for wrap-around
        return ('a' + (c - 'a' + ROT) % 26);
    }
    else {
        return c;
    }
}

//  Execution of the whole program begins at the main function

int main(int argcount, char *argvalue[])
{
    // Exit with an error if the number of arguments (including
    // the name of the executable) is not precisely 2
    if(argcount != 2) {
        fprintf(stderr, "%s: program expected 1 argument, but instead received %i\n",
                    argvalue[0], argcount-1);
        exit(EXIT_FAILURE);
    }
    else {
        // Calculate the length of the first argument
        int length = strlen(argvalue[1]);

        // Loop for every character in the text
        for(int i = 0; i < length; i++) {
            // Determine and print the ciphered character
            printf("%c", rotate(argvalue[1][i]));
        }

        // Print one final new-line character
        printf("\n");

        // Exit indicating success
        exit(EXIT_SUCCESS);
    }
    return 0;
}