str_replace good?

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


char* str_replace(const char *src, const char *from, const char *to) {

    if(src == NULL || from == NULL || to == NULL) return NULL;

    // helper pointers
    const char * sp; // src pointer
    const char * mp; // match pointer
    char * dp; // dest pointer (for writing)


    // lengths
    size_t from_len = strlen(from);
    size_t to_len = strlen(to);
    size_t src_len = strlen(src);

    // count matches
    size_t count = 0;
    sp = src;
    while(1) {
        mp = strstr(sp, from); // find next match
        if(mp == NULL) break; // end of matches
        count++;
        sp = mp + from_len; // advance past match
    }

    // compute dest size
    size_t len = src_len;
    long int sub = (from_len - to_len)*count;

    // sanity check
    if(from_len >= to_len) {
        if(sub > len || sub < 0) return NULL;
    } else {
        if((len-sub) < 0 || sub > 0) return NULL;
    }

    // allocate output buffer
    size_t actual_size = (len - sub) + 1;
    char *dest = malloc( actual_size );

    // fill output buffer
    if(dest != NULL) {

        sp = src; // src pointer
        dp = dest; // dest pointer

        while(1) {
            mp = strstr(sp, from); // find next match
            if(mp == NULL) break; // end of matches

            // copy chunk before match
            memcpy(dp, sp, mp - sp);
            dp += mp - sp;

            // copy replacement
            memcpy(dp, to, to_len);
            dp += to_len;

            // advance past match
            sp = mp + from_len;
        }

        // copy tail
        size_t k = src_len - (sp - src) + 1;
        memcpy(dp, sp, k);

    }

    return dest;
}