Improving fold function - c++

Question

I have implemented a simple fold function in C++ that accepts a lambda, and can fold multiple vectors at the same time at compile time. I am wondering if it could be simplified in some manner (I have provided both a recursive version and an iteratively recursive version - I am unsure which should have better performance): https://godbolt.org/z/39pW81
Performance optimizations are also welcome - in that regard is any of the two approaches faster?
template<int I, typename type_identity, typename type_head, int N, typename ...type_tail, int ...N_tail, typename Function>
auto foldHelperR(Function&& func, const type_identity& id, const tvecn<type_head, N>& head, const tvecn<type_tail, N_tail>&... tail)
{
if constexpr (I>0)
{
return func(foldHelperR<I-1>(std::forward<Function>(func), id, head, tail...), head[I], tail[I]...);
}
else
{
return func(id, head[0], tail[0]...);
}
}
template<int I, typename type_identity, typename type_head, int N, typename ...type_tail, int ...N_tail, typename Function>
auto foldHelperI(Function&& func, const type_identity id, const tvecn<type_head, N>& head, const tvecn<type_tail, N_tail>&... tail)
{
if constexpr (I<N-1)
{
return foldHelperI<I+1>(std::forward<Function>(func), func(id, head[I], tail[I]...), head, tail...);
}
else
{
return func(id, head[N-1], tail[N-1]...);
}
}
template<typename type_identity, typename type_head, int N_head, typename ...type_tail, int ...N_tail, typename Function = void (const type_identity&, const type_head&, const type_tail&...)>
constexpr auto fold(Function&& func, const type_identity& id, const tvecn<type_head, N_head>& head, const tvecn<type_tail, N_tail>&... tail)
{
static_assert(std::is_invocable_v<Function, const type_identity&, const type_head&, const type_tail &...>,
"The function cannot be invoked with these zip arguments (possibly wrong argument count).");
static_assert(all_equal_v<N_head, N_tail...>, "Vector sizes must match.");
//return foldHelperR<N_head-1>(std::forward<Function>(func), id, head, tail...);
return foldHelperI<0>(std::forward<Function>(func), id, head, tail...);
}
int main()
{
tvecn<int,3> a(1,2,3);
return fold([](auto x, auto y, auto z) {return x+y+z;}, 0, a, a);
}

Answer 1

and can fold multiple vectors at the same time at compile time
Not exactly: if you want to operate compile-time
(1) you have to define constexpr the tvecn constructor and
(2) you have to define constexpr the foldhelper function and
(3) you have to declare constexpr a
// VVVVVVVVV
constexpr tvecn<int,3> a(1,2,3);
(4) you have to place the result of fold in a constexpr variable (or, more generally speaking, in a place where the value is required compile time, as the size field of a C-style array, or a template value parameter, or a static_assert() test)
constexpr auto f = fold([](auto x, auto y, auto z) {return x+y+z;},
0, a, a);
I am wondering if it could be simplified in some manner
Sure.
First of all: if you can, avoid to reinventing the weel: your tvecn is a simplified version of std::array.
Suggestion: use std::array (if you can obviously)
Second: you tagged C++17 so you can use folding
Suggestion: use it also for all_equal
template <auto V0, auto ... Vs>
struct all_equal : public std::bool_constant<((V0 == Vs) && ...)>
{ };
template<auto ...N_pack>
constexpr bool all_equal_v = all_equal<N_pack...>::value;
More in general: when you have to define a custom type traits that has to provide a number, inherit (if possible) from std::integral_constant (or std::bool_constant, or std::true_type, or std::false_type: all std::integral_constant specializations). So you automatically inherit all std::integral_constant facilities.
Third: almost all C++ standard uses std::size_t, not int, for sizes.
Suggestion: when you have to do with sizes, use std::size_t, not int. This way you can avoid a lot of annoying troubles.
Fourth: from main() you should return only EXIT_SUCCESS (usually zero) or EXIT_FAILURE (usually 1)
Suggestion: avoid things as
return fold([](auto x, auto y, auto z) {return x+y+z;}, 0, a, a);
Fifth: never underestimate the power of the comma operator.
Suggestion: avoid recursion at all and use template folding also for the helper function; by example
template <std::size_t ... Is, typename F, typename T, typename ... As>
constexpr auto foldHelperF (std::index_sequence<Is...>,
F const & f, T id, As const & ... arrs)
{ return ( ..., (id = [&](auto i){ return f(id, arrs[i]...); }(Is))); }
that you can call as follows from fold()
return foldHelperF(std::make_index_sequence<N_head>{},
std::forward<Function>(func),
id, head, tail...);
The following is a full compiling, and simplified, example
#include <array>
#include <utility>
#include <iostream>
#include <type_traits>
template <auto V0, auto ... Vs>
struct all_equal : public std::bool_constant<((V0 == Vs) && ...)>
{ };
template<auto ...N_pack>
constexpr bool all_equal_v = all_equal<N_pack...>::value;
template <std::size_t ... Is, typename F, typename T, typename ... As>
constexpr auto foldHelperF (std::index_sequence<Is...>,
F const & f, T id, As const & ... arrs)
{ return ( ..., (id = [&](auto i){ return f(id, arrs[i]...); }(Is))); }
template <typename type_identity, typename type_head, std::size_t N_head,
typename ...type_tail, std::size_t ...N_tail,
typename Function = void (type_identity const &,
type_head const &,
type_tail const & ...)>
constexpr auto fold (Function && func, type_identity const & id,
std::array<type_head, N_head> const & head,
std::array<type_tail, N_tail> const & ... tail)
{
static_assert( std::is_invocable_v<Function, const type_identity&,
const type_head&, const type_tail &...>,
"The function cannot be invoked with these zip arguments"
" (possibly wrong argument count).");
static_assert( all_equal_v<N_head, N_tail...>,
"Vector sizes must match.");
return foldHelperF(std::make_index_sequence<N_head>{},
std::forward<Function>(func),
id, head, tail...);
}
int main()
{
constexpr std::array<int, 3u> b{2, 5, 7};
constexpr auto f = fold([](auto x, auto y, auto z) {return x+y+z;},
0, b, b);
std::cout << f << std::endl;
}

Answer 2

With Fold expression, it might be:
template <typename F, typename Init, std::size_t... Is, typename... Arrays>
constexpr auto fold_impl(F&& f, Init init, std::index_sequence<Is...>, Arrays&&... arrays)
{
auto l = [&](Init init, std::size_t i){ return f(init, arrays[i]...); };
return ((init = l(init, Is)), ...);
}
template <typename F, typename Init, typename Array, typename ... Arrays>
constexpr auto fold(F&& f, Init init, Array&& array, Arrays&&... arrays)
{
static_assert(((arrays.size() == array.size()) && ...));
return fold_impl(f, init, std::make_index_sequence<array.size()>{}, array, arrays...);
}
Demo