include/ftl/details/match.h

/*
 * Copyright 2022 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#pragma once

#include <type_traits>
#include <variant>

namespace android::ftl::details {

template <typename... Ms>
struct Matcher : Ms... {
  using Ms::operator()...;
};

// Deduction guide.
template <typename... Ms>
Matcher(Ms...) -> Matcher<Ms...>;

template <typename Matcher, typename... Ts>
constexpr bool is_exhaustive_match_v = (std::is_invocable_v<Matcher, Ts> && ...);

template <typename...>
struct Match;

template <typename T, typename U, typename... Ts>
struct Match<T, U, Ts...> {
  template <typename Variant, typename Matcher>
  static decltype(auto) match(Variant& variant, const Matcher& matcher) {
    if (auto* const ptr = std::get_if<T>(&variant)) {
      return matcher(*ptr);
    } else {
      return Match<U, Ts...>::match(variant, matcher);
    }
  }
};

template <typename T>
struct Match<T> {
  template <typename Variant, typename Matcher>
  static decltype(auto) match(Variant& variant, const Matcher& matcher) {
    return matcher(std::get<T>(variant));
  }
};

}  // namespace android::ftl::details