Functions in C with Ornate Data Structures

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I'm trying to write C functions to handle some of the number crunching
that I have been doing via backend processing. Specifically, I want
to be able to construct a function such that a query like:

select crunch_number(foo) from bar where [some condition];

...where `foo' is the name of some column and `bar' is some table
name. The function needs to create some ornate data structures (i.e.,
doubly linked lists), and outputs some summary statistic.

If my data types were simpler, I could simply use an AGGREGATE function.
Unfortunately, I don't know of any way to schlep something as complex
as a doubly-linked list of arrays of arbitrary precision numbers.

I suppose ideally I'd like some way of either:

-Being able to call a function on each row (like most user-defined
functions) which only returns a result on the last row; or
-Being able to pass the table name, column name, and selection
conditions to the function, and walk through the matching rows
inside the function, returning a single result upon completion

In terms of logical structure, this looks similar to functions to do
things like compute means or standard deviations. The complication (as
far as I can tell) is because I can't get by with a simple accumulator
variable/transform function.

Is there any clean way to accomplish this in Postgres? Any pointers
or suggestions would be appreciated.

- -Steve

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