Cracking the code interview: Problem 1 .6 basic string compression

I created a GitHub repository containing my code solutions for some of the exercises proposed in the book. I updated my solution for the problem 1.6 here.

Description

A function that compress the original string and store the compressed string in compress ex. aabcccccaaa will become a2blc5a3. If the compressed string is longer than the original it returns the original string.

Here's the code:

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

const char *compression(const char *original, char *compress) {
    size_t len, i, j, nc;

    /*memorize the first char of original in compress*/
    char previous = original[0];
    len = strlen(original);
    nc = 1;

    for (i = 1, j = 0; i < len; ++i) {
        if (original[i] == previous) {
            ++nc;
        } else {
            compress[j++] = previous;
            compress[j++] = nc + '0';
            nc = 1;
            previous = original[i];

        }

    }
    /* there are characters still in the stream */
    compress[j++] = previous;
    compress[j++] = nc + '0';
    compress[j] = '\0';

    if (strlen(compress) >= len) return original;
    return compress;
}

int main() {
    char buffer[20] = {0};
    printf("%s\n", compression("aabcccccaaa", buffer));
    return 0;
}

I'm looking for possible improvement of the code or better algorithm to solve the problem.