FTL: Add std::variant matcher

Bug: 185536303
Test: ftl_test
Change-Id: Id6357fa04ffe0c17b17c99b49b5a5262d68925bf

4 files changed, 170 insertions, 0 deletions

diff --git a/include/ftl/details/match.h b/include/ftl/details/match.h
new file mode 100644
index 0000000000..51b99d2f13
--- /dev/null
+++ b/include/ftl/details/match.h
@@ -0,0 +1,59 @@
+/*
+ * 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
diff --git a/include/ftl/match.h b/include/ftl/match.h
new file mode 100644
index 0000000000..7318c45b7b
--- /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
diff --git a/libs/ftl/Android.bp b/libs/ftl/Android.bp
index a25a49397e..c1945fddb3 100644
--- a/libs/ftl/Android.bp
+++ b/libs/ftl/Android.bp
@@ -21,6 +21,7 @@ cc_test {
         "fake_guard_test.cpp",
         "flags_test.cpp",
         "future_test.cpp",
+        "match_test.cpp",
         "optional_test.cpp",
         "small_map_test.cpp",
         "small_vector_test.cpp",
diff --git a/libs/ftl/match_test.cpp b/libs/ftl/match_test.cpp
new file mode 100644
index 0000000000..a6cff2eed6
--- /dev/null
+++ b/libs/ftl/match_test.cpp
@@ -0,0 +1,48 @@
+/*
+ * 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.
+ */
+
+#include <ftl/match.h>
+#include <gtest/gtest.h>
+
+#include <chrono>
+#include <string>
+#include <variant>
+
+namespace android::test {
+
+// Keep in sync with example usage in header file.
+TEST(Match, Example) {
+  using namespace std::chrono;
+  using namespace std::chrono_literals;
+  using namespace std::string_literals;
+
+  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.
+  EXPECT_EQ("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; }));
+}
+
+}  // namespace android::test