Loop over types in the variant?

[Yurlungur]

I wonder if it's possible to achieve vectorization (for the EOS's that vectorize) for the scalar calls by looping over EOS type in the variant first. I bet this kind of loop construct can be done at compile time. Not sure this is a priority but I wanted to write the idea down somewhere. Would also like to pick your brain @dholladay00 .

[dholladay00]

@Yurlungur this might be a good place to stash that code you had put in our Mattermost convo as well.

[Yurlungur]

👍 good idea @dholladay00 . Something like this allows us to loop over EOS type. I had it in mind for the rewrite of the EOS builder #28 .

#include <iostream>
#include <string>
#include <variant>
#include <type_traits>
#include <tuple>

template <typename... Ts>
struct type_list {};

template <typename... Ts>
struct tl_to_Variant_struct {
  using vt = std::variant<Ts...>;
};

template <typename... Ts>
constexpr auto tl_to_variant(type_list<Ts...>) {
  return tl_to_Variant_struct<Ts...>{};
}

template<typename T, typename... Ts>
constexpr bool contains(){ 
    return std::disjunction_v<std::is_same<T, Ts>...>; 
}

struct Foo {};
struct Bar {};

template<typename T>
struct Mod {
    Mod(T t) {};
};

static constexpr const auto base_list = type_list<Foo, Bar>{};
static constexpr const auto mod_list = type_list<Foo, Bar, Mod<Foo>, Mod<Bar>>{};

using Var_t = typename decltype(tl_to_variant(mod_list))::vt;

template<typename Variant, template <class...> typename T>
Variant Modify(Variant var) {
    return var;
}

template<typename Variant, template <class...> typename T, typename U, typename... Rest>
Variant Modify(Variant var) {
    if (std::holds_alternative<U>(var)) {
        return Variant(T<U>(std::get<U>(var)));
    } else {
        return Modify<Variant,T,Rest...>(var);
    }
}

int main() {
    Var_t f = Foo();
    auto m = Modify<Var_t, Mod, Foo, Bar>(f);
    return 0;
}

[jhp-lanl]

Interesting... this does look a lot more extensible than the EOS builder machinery

[Yurlungur]

I haven't worked out all the kinks yet, but I think it should let us write code like this (I hope):

EOS my_eos = IdealGas();
if (shifted) {
  my_eos = Modify<Shifted, ... >(my_eos, shift);
}
if (scaled) {
  my_eos = Modify<Scaled,...>(my_eos, scale);
}
// etc