Custom aggragation function that creates an array

Hi,

I want to write a custom agg function that, given an "int4 index",
increments the element at "index" of an array and, at the end, returns
the array. The array will always be int4[].

I need it in C, since plpgsql is way slower (and I need to use it in
5M+ rows). I did it, but I also need to create the array "on demand",
not as initcond (the array size will be a parameter).

I suppose that, since I'll be using "construct_array" (if it makes
sense), I'll need a new MemoryContext?
I've looked at array_agg_transfn, but I think that's a lot more
complicated because it'll repalloc, so it has to keep
the memorycontext...

Since I'm not familiar with these things, would something like this
work?

if (PG_ARGISNULL(0))
{ // first time, alloc array
arr_context = AllocSetContextCreate(rcontext,

"myCustomArrayResult",
ALLOCSET_DEFAULT_MINSIZE,
ALLOCSET_DEFAULT_INITSIZE,
ALLOCSET_DEFAULT_MAXSIZE);
oldcontext = MemoryContextSwitchTo(arr_context);
transarray = construct_array([...]);
MemoryContextSwitchTo(oldcontext );
}
else
{
transarray = PG_GETARG_ARRAYTYPE_P(0);
}

This is how the C function is written right now:

ArrayType *transarray;
int32 index = PG_GETARG_INT32(1);
int32 *array;

if (AggCheckCallContext(fcinfo, NULL))
{
// code above would go in place of this line:
transarray = PG_GETARG_ARRAYTYPE_P(0);
}
else
transarray = PG_GETARG_ARRAYTYPE_P_COPY(0);

array = (int32*)ARR_DATA_PTR(transarray);
array[index]++;

PG_RETURN_ARRAYTYPE_P(transarray);

(checks on boundaries etc omitted)

The problem is that it expects the array to be already available, so I had to
do:

CREATE AGGREGATE cust_agg_func (int4)
(
sfunc = cust_agg_funct_add,
stype = int4[],
initcond = '{0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0}'
);

I would like to have:

CREATE AGGREGATE cust_agg_func (int4 index, int4
array_size_to_be_created)
(
sfunc = cust_agg_funct_with_array_creation_add,
stype = int4[]
);