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

char shift(char c, int by)
{
    int start = 0;
    if(c >= 'a' && c <= 'z')
    {
        start = 'a';
    }
    else if(c >= 'A' && c <= 'Z')
    {
        start = 'A';
    }
    else return c;
    c -= start;
    c += by;
    c %= 26;
    return c + start;
}

char *reverse_shifted(char *input, int by)
{
    if(!input)
    {
        return NULL;
    }
    int len = strlen(input);
    char *res = calloc(len, sizeof(char));
    if(!res)
    {
        return NULL;
    }
    int start = 0;
    int end = len + 1;
    for (int i = 0; i < len; i++)
    {
        char val = input[end--];
        val = shift(val, by);
        res[start++] = val;
    }
    return res;
}

int main(void)
{
    char *test1 = "!qxsbdc k cs csrd";
    char *res1 = reverse_shifted(test1, 42);
    if(!res1)
    {
        return EXIT_FAILURE;
    }
    printf("%s",res1);
    puts(res1);
    free(res1);
    return EXIT_SUCCESS;
}