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aka_constexpr_map.hh
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Fri, Nov 8, 06:28

aka_constexpr_map.hh
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#include <array>
#include <tuple>
#include <type_traits>
#ifndef AKANTU_CONSTEXPR_MAP_HH
#define AKANTU_CONSTEXPR_MAP_HH
namespace akantu {
namespace details {
template <class InputIt, class UnaryPredicate>
constexpr InputIt my_find_if(InputIt first, InputIt last,
UnaryPredicate && p) {
for (; first != last; ++first) {
if (std::forward<UnaryPredicate>(p)(*first)) {
return first;
}
}
return last;
}
// Author Jason Turner C++ Weekly ep 233
template <typename Key, typename Value, std::size_t Size>
struct ConstexprMap {
std::array<std::pair<Key, Value>, Size> data;
[[nodiscard]] constexpr Value at(const Key & key) const {
const auto it =
my_find_if(data.begin(), data.end(),
[&key](const auto & val) { return val.first == key; });
if (it != data.end()) {
return it->second;
} else {
throw std::range_error("Key out of range");
}
}
[[nodiscard]] constexpr auto find(const Key & key) const {
const auto it =
my_find_if(data.begin(), data.end(),
[&key](const auto & val) { return val.first == key; });
return it;
}
[[nodiscard]] constexpr auto begin() const { return data.begin(); }
[[nodiscard]] constexpr auto end() const { return data.end(); }
};
// magic_switch from
// https://stackoverflow.com/questions/39915986/solutions-for-dynamic-dispatch-on-unrelated-types
template <class Function, class DynamicType, class Tuple,
class DefaultFunction, std::size_t... Is>
[[gnu::visibility("hidden")]] constexpr decltype(auto) static_switch_dispatch(
const Tuple &, Function && function, const DynamicType & type,
DefaultFunction && default_function, std::index_sequence<Is...> /*is*/) {
auto * function_pointer = std::addressof(function);
using FunctionPointer = decltype(function_pointer);
using Ret = decltype(function(std::tuple_element_t<0, Tuple>{}));
using TableEntry = Ret (*)(FunctionPointer);
constexpr std::array<std::pair<DynamicType, std::size_t>, sizeof...(Is)>
data{{{std::tuple_element_t<Is, Tuple>::value, Is}...}};
constexpr auto map =
ConstexprMap<DynamicType, std::size_t, data.size()>{{data}};
constexpr TableEntry table[] = {
[](FunctionPointer function_pointer) -> Ret {
return (*function_pointer)(std::tuple_element_t<Is, Tuple>{});
}...};
auto it = map.find(type);
if (it != map.end()) {
return table[it->second](function_pointer);
} else {
return default_function(type);
}
}
} // namespace details
} // namespace akantu
#endif // AKANTU_CONSTEXPR_MAP_HH

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