what is dda_unit doing?

```
void dda_unit(vec3 cen, vec3 invdir, out float tmax) {
    float t1, t2;
    tmax = 1e9f;
    for (int i = 0; i < 3; ++i) {
        t1 = - cen[i] * invdir[i];
        t2 = t1 +  invdir[i];
        tmax = min(tmax, max(t1, t2));
    }
}
```
I may have trouble understanding it...could you help me..many thanks!..
It looks like the t1 is trying to go back to the origin...