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 three resistors can be so converted, that then a parallel or series connection of two 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 left script calculates the delta-circuit from the star-circuit.
The right 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.