Bicycle wheel spoke length calculator

Description of wheel:
Non-Drive side hub flange holes P.C.D.: a =
Drive side hub flange holes P.C.D.: b =
Distance between flange centres: c =
Axle width: d =
Axle end to non-drive side flange centre: e =
Rim outer diameter: f =
Radial distance between base of slot on spoke nipple to outer diameter of wheel rim: g =
Number of spokes:
Number of times spokes cross:

Non-driveside spoke length:
Driveside spoke length:

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What is the mathematics behind this utility?