minimal Stan user func with external files

Makefile

model: model.stan support.o support.hpp
	STANCFLAGS=--allow-undefined USER_HEADER=$$PWD/support.hpp LDLIBS_OS=$$PWD/support.o make -C ~/.cmdstan/cmdstan-2.28.2/ $$PWD/model

model.stan

functions {
  real usersum(matrix m);
}

parameters {
  real theta;
}

model {
  matrix[5,5] m = rep_matrix(-theta*theta, 5, 5);
  target += usersum(m);
}

support.c

double support_sum(int n, const double*const els) {
        double acc = 0.0;
        for (int i=0; i<n; i++) acc += els[i];
        return acc;
}

void support_sum_adj(int n, const double g, double*adj){
	for (int i=0; i<n; i++)
		adj[i] += g;
}

support.h

extern "C" double support_sum(int n, const double*els);
extern "C" void support_sum_adj(int n, const double g, double*adj);

support.hpp

#include <stan/math.hpp>
#include <stan/math/rev/core/reverse_pass_callback.hpp>
#include "support.h"

namespace model_model_namespace {

using namespace stan;
using namespace std;

double usersum(const Eigen::MatrixXd& m, ostream *pstream__) {
	return support_sum(m.size(), m.data());
}

template <typename M>
promote_args_t<value_type_t<M>>
usersum(const M& m, ostream *pstream__) {
	using value_t = value_type_t<M>; // var_value<double>
	using ret_type = promote_var_matrix_t<M,M>; // Eigen::Matrix<stan::math::var_value<double>, -1, -1, 0>
	Eigen::MatrixXd mv = value_of(m);
	const double *md = mv.data();
	value_t ret(support_sum(m.size(), md));
	arena_t<M> arena_m(m);
	arena_t<value_t> aret(ret);
	math::reverse_pass_callback([aret, arena_m]() mutable {
		Eigen::MatrixXd adj(arena_m.rows(), arena_m.cols());
		support_sum_adj(adj.size(), aret.adj(), adj.data());
		arena_m.adj() += adj;
	});
	return ret;
}
}