FTL: Add std::variant matcher
Bug: 185536303
Test: ftl_test
Change-Id: Id6357fa04ffe0c17b17c99b49b5a5262d68925bf
diff --git a/include/ftl/match.h b/include/ftl/match.h
new file mode 100644
index 0000000..7318c45
--- /dev/null
+++ b/include/ftl/match.h
@@ -0,0 +1,62 @@
+/*
+ * 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 <utility>
+#include <variant>
+
+#include <ftl/details/match.h>
+
+namespace android::ftl {
+
+// Concise alternative to std::visit that compiles to branches rather than a dispatch table. For
+// std::variant<T0, ..., TN> where N is small, this is slightly faster since the branches can be
+// inlined unlike the function pointers.
+//
+// using namespace std::chrono;
+// std::variant<seconds, minutes, hours> duration = 119min;
+//
+// // Mutable match.
+// ftl::match(duration, [](auto& d) { ++d; });
+//
+// // Immutable match. Exhaustive due to minutes being convertible to seconds.
+// assert("2 hours"s ==
+// ftl::match(duration,
+// [](const seconds& s) {
+// const auto h = duration_cast<hours>(s);
+// return std::to_string(h.count()) + " hours"s;
+// },
+// [](const hours& h) { return std::to_string(h.count() / 24) + " days"s; }));
+//
+template <typename... Ts, typename... Ms>
+decltype(auto) match(std::variant<Ts...>& variant, Ms&&... matchers) {
+ const auto matcher = details::Matcher{std::forward<Ms>(matchers)...};
+ static_assert(details::is_exhaustive_match_v<decltype(matcher), Ts&...>, "Non-exhaustive match");
+
+ return details::Match<Ts...>::match(variant, matcher);
+}
+
+template <typename... Ts, typename... Ms>
+decltype(auto) match(const std::variant<Ts...>& variant, Ms&&... matchers) {
+ const auto matcher = details::Matcher{std::forward<Ms>(matchers)...};
+ static_assert(details::is_exhaustive_match_v<decltype(matcher), const Ts&...>,
+ "Non-exhaustive match");
+
+ return details::Match<Ts...>::match(variant, matcher);
+}
+
+} // namespace android::ftl