KfVisit.h

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/* Copyright (C) 2025 GSI Helmholtzzentrum fuer Schwerionenforschung, Darmstadt
   SPDX-License-Identifier: GPL-3.0-only
   Authors: Sergei Zharko [committer] */

#pragma once
 
#if defined(__GNUC__) && __GNUC__ < 12 || defined(__clang_major__) && __clang_major__ < 15
 
#include <functional>
#include <type_traits>
#include <utility>
#include <variant>
 
namespace cbm::algo::kf
{
  namespace detail
  {
    template<typename ExpectedRet>
    struct AlternativeSelector {
      ~AlternativeSelector() = delete;

      template<size_t VisitedIndex, size_t VariantSize, typename Visitor, typename Variant>
      static constexpr ExpectedRet SelectAndInvoke(Visitor&& aVisitor, Variant&& aVariant)
      {
        if constexpr (VisitedIndex < VariantSize) {
          // Check, if the visited alternative is held by the variant
          using ActualRet = decltype(
            std::invoke(std::forward<Visitor>(aVisitor), std::get<VisitedIndex>(std::forward<Variant>(aVariant))));
          static_assert(std::is_same_v<ActualRet, ExpectedRet>, "kf::visit: visitor must have a single return type");
          if (auto* held = std::get_if<VisitedIndex>(&(std::forward<Variant>(aVariant)))) {
            // This alternative is held by the variant => invoke the visitor on it
            return std::invoke(std::forward<Visitor>(aVisitor), *held);
          }
          else {
            // Try the next alternative
            return SelectAndInvoke<VisitedIndex + 1, VariantSize>(std::forward<Visitor>(aVisitor),
                                                                  std::forward<Variant>(aVariant));
          }
        }
        __builtin_unreachable();  // No return is reachable
      }
    };
  }  // namespace detail

  template<typename Visitor, typename Variant>
  constexpr decltype(auto) visit(Visitor&& aVisitor, Variant&& aVariant)
  {
    using ExpectedRet =
      decltype(std::invoke(std::forward<Visitor>(aVisitor), std::get<0>(std::forward<Variant>(aVariant))));
    constexpr size_t Size = std::variant_size_v<std::remove_cv_t<std::remove_reference_t<Variant>>>;
    return detail::AlternativeSelector<ExpectedRet>::template SelectAndInvoke<0, Size>(std::forward<Visitor>(aVisitor),
                                                                                       std::forward<Variant>(aVariant));
  }
}  // namespace cbm::algo::kf
 
#else  // Modern compiler
 
#include <variant>
 
namespace cbm::algo::kf
{
  using std::visit;  //< use a standard visit
}
 
#endif