Run-length encoding of a string in C Solution, output & explanation

#include <stdio.h>

void rle(const char *s, char *out, size_t size) {
    size_t used = 0;
    out[0] = '\0';
    while (*s && used < size) {
        char c = *s;
        int run = 0;
        while (*s == c) {
            s++;
            run++;
        }
        int w = snprintf(out + used, size - used, "%c%d", c, run);
        if (w < 0)
            break;
        used += (size_t) w;
    }
}

int main(void) {
    char out[32];
    rle("aaabbcddddd", out, sizeof out);
    printf("%s\n", out);
    return 0;
}

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