How does EnergyPlus find the direct and diffuse radiation codes to calculate the incident solar radiation on the building walls??

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I am using EnergyPlus to develope a new module.This module requires the calculation of solar radiation of titled surfaces. But it seems that the source code of E+ does not mention much about the calculation of solar radiation on the wall.I have browsed the github.com website for a long time.Not much useful information I have got.

I want to know where can I find the formula code for calculating solar radiation on building walls . where can I find the direct and diffuse radiation codes to calculate the incident solar radiation on the building walls?

Somebody who can help me? I would be greatly appreciated.It is very important to me.Thanks a lot!!!