Especially circuits with many resistors in some cases can not be simplified through the parallel or series connection of resistors. The problem solves only the star-delta-transform. With the help of this transform at least a circuit part of 3 resistors can be so converted, that then a parallel or series connection of 2 adjacent resistors is possible. To come to a solution for the entire circuit, you may must apply the transform at different circuit parts and in different directions.
 
The first script calculates the delta-circuit from the star-circuit.

The second script calculates the star-circuit from the delta-circuit.

If you want to know, how looks the equations used into the script, then see at the mathematical background: Star-delta-transform, Delta-star-transform.