Merge pull request #1791 from stan-dev/feature/apply-jac-ops

adds an apply function and cleans adj_jac_apply to use it

Author: bbbales2

stan/math/prim/functor.hpp

@@ -1,6 +1,7 @@
 #ifndef STAN_MATH_PRIM_FUNCTOR_HPP
 #define STAN_MATH_PRIM_FUNCTOR_HPP
 
+#include <stan/math/prim/functor/apply.hpp>
 #include <stan/math/prim/functor/coupled_ode_observer.hpp>
 #include <stan/math/prim/functor/coupled_ode_system.hpp>
 #include <stan/math/prim/functor/finite_diff_gradient.hpp>

stan/math/prim/functor/apply.hpp

@@ -0,0 +1,52 @@
+#ifndef STAN_MATH_PRIM_FUNCTOR_APPLY_HPP
+#define STAN_MATH_PRIM_FUNCTOR_APPLY_HPP
+
+#include <functional>
+#include <tuple>
+#include <utility>
+
+namespace stan {
+namespace math {
+namespace internal {
+/*
+ * Invoke the functor f with arguments given in t and indexed in the index
+ * sequence I
+ *
+ * @tparam F Type of functor
+ * @tparam Tuple Type of tuple containing arguments
+ * @tparam I Parameter pack of an index sequence going from 0 to
+ * std::tuple_size<T>::value - 1 inclusive
+ * @param f functor callable
+ * @param t tuple of arguments
+ * @param i placeholder variable for index sequence
+ */
+template <class F, class Tuple, std::size_t... I>
+constexpr decltype(auto) apply_impl(F&& f, Tuple&& t,
+                                    std::index_sequence<I...> i) {
+  return f(std::forward<decltype(std::get<I>(t))>(std::get<I>(t))...);
+}
+}  // namespace internal
+/*
+ * Call the functor f with the tuple of arguments t, like:
+ *
+ * f(std::get<0>(t), std::get<1>(t), ...)
+ *
+ * TODO: replace this with implementation in C++ std when C++17 is available
+ *
+ * @tparam F Type of functor
+ * @tparam Tuple Type of tuple containing arguments
+ * @param f functor callable
+ * @param t tuple of arguments
+ */
+template <class F, class Tuple>
+constexpr decltype(auto) apply(F&& f, Tuple&& t) {
+  return internal::apply_impl(
+      std::forward<F>(f), std::forward<Tuple>(t),
+      std::make_index_sequence<
+          std::tuple_size<std::remove_reference_t<Tuple>>{}>{});
+}
+
+}  // namespace math
+}  // namespace stan
+
+#endif