Interference and Fermat's principle

Fermat’s principle states that light always takes the path for which the optical length is stationary.

begin{align}
deltaint n(vec{r}) :mathrm ds = 0.
end{align}

Furthermore, it is equivalent to Huygens’ principle. Huygens’ principle states that each point on a wavefront is the source of a new spherical wave. Huygens’s principle is often used to describe interference patterns. How would you describe interference patterns with Fermat’s principle?