Find if a string is a rotation of another.

rotation.c

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

int isrotation(char*, char*);

main() {
    int result = isrotation("byegood", "goodbye");
    if (result == 0) {
        printf("Nope.\n");
    } else {
        printf("Yup.\n");
    }
        
}

// Determines if str2 is a rotation of str1 by adding str1 to itself
// and checking if str2 is in the concatenation.
int isrotation(char* str1, char* str2) {
    
    char concat[strlen(str1)*2]; /* make a new string to hold str1 + str1 */
    strcat(concat, str1); 
    strcat(concat, str1); 
    char* is_in = strstr(concat, str2);
    
    if (is_in != NULL) {
        return 1;
    }
    return 0;
}