CODEWITHSUNDEEP
c++c++17variadic-templatestemplate-meta-programmingfold-expression
lightxbulb
lightxbulb

Reputation: 1321

Improving fold function

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);
}

Upvotes: 1

Views: 279

Answers (2)

max66
max66

Reputation: 66230

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;
 }

Upvotes: 3

Jarod42
Jarod42

Reputation: 218323

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

Upvotes: 2

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