[PD] using tabread4~~

[Peter P.]

I am trying to play back a table in varispeed and am getting artefacts.
I looked into vanilla's 'onset* message for tabread4~ but since I want
to play tables backwards as well a solution with an audio signal as
offset message seems simpler to me. Now iem_dp (double precision) has
tabread4~~ which features exactly that. 
The only thing that is missing is me understanding how to build a
wrapper around tabread4~~ so that in can be a drop-in replacement
abstraction for tabread4~. How can I derive the offset signal in the
first hand place? For messages I would devide the index by a certain
numer, for example 100000, truncate the result and use this as the
offset. Then I would use [modulo 100000] to get the left input for
tabread4~~.
Am I correct to go this way? 

How would I truncate an audio signal in Pd?

How would I calculate the modulo of an audio signal in Pd?

Is there a more elegant way to do this?

Thank you for all ideas!
best, Peter