compact!

static VALUE
rb_hash_compact_bang(VALUE hash)
{
    rb_hash_modify_check(hash);
    if (RHASH(hash)->ntbl) {
        st_index_t n = RHASH(hash)->ntbl->num_entries;
        rb_hash_foreach(hash, delete_if_nil, hash);
        if (n != RHASH(hash)->ntbl->num_entries)
            return hash;
    }
    return Qnil;
}