Documentation for TransformD64 in Bitcoin Core

[rollmeister]

Hi,

I am wanting to adapt the fixed 64 byte SHA256D hasher currently in Bitcoin Core for my own application.

After examining it, from transform 2 onwards I believe it uses custom constants calculated against the initial state, however I need to use TransformD64 with any number of different custom states.

Trying to find documentation how this function works and is built (i.e. how to calculate the custom constants) has proved fruitless.

Can anybody point me in the right direction?

Regards,
Rollmeister.