invert

static VALUE
fsdbm_invert(VALUE obj)
{
    datum key, val;
    struct dbmdata *dbmp;
    DBM *dbm;
    VALUE keystr, valstr;
    VALUE hash = rb_hash_new();

    GetDBM2(obj, dbmp, dbm);
    for (key = sdbm_firstkey(dbm); key.dptr; key = sdbm_nextkey(dbm)) {
        val = sdbm_fetch(dbm, key);
        keystr = rb_external_str_new(key.dptr, key.dsize);
        valstr = rb_external_str_new(val.dptr, val.dsize);
        rb_hash_aset(hash, valstr, keystr);
    }
    return hash;
}