FTL: Add Optional<T>::transform

Bug: 185536303
Test: ftl_test
Change-Id: If4cb9894c615499af04bb9793d9f900485e18fe2

3 files changed, 149 insertions, 0 deletions

diff --git a/include/ftl/optional.h b/include/ftl/optional.h
new file mode 100644
index 0000000000..daf4502bd2
--- /dev/null
+++ b/include/ftl/optional.h
@@ -0,0 +1,71 @@
+/*
+ * 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 <functional>
+#include <optional>
+#include <type_traits>
+#include <utility>
+
+namespace android::ftl {
+
+// Superset of std::optional<T> with monadic operations, as proposed in https://wg21.link/P0798R8.
+//
+// TODO: Remove in C++23.
+//
+template <typename T>
+struct Optional final : std::optional<T> {
+  using std::optional<T>::optional;
+
+  using std::optional<T>::has_value;
+  using std::optional<T>::value;
+
+  // Returns Optional<U> where F is a function that maps T to U.
+  template <typename F>
+  constexpr auto transform(F&& f) const& {
+    using U = std::remove_cv_t<std::invoke_result_t<F, decltype(value())>>;
+    if (has_value()) return Optional<U>(std::invoke(std::forward<F>(f), value()));
+    return Optional<U>();
+  }
+
+  template <typename F>
+  constexpr auto transform(F&& f) & {
+    using U = std::remove_cv_t<std::invoke_result_t<F, decltype(value())>>;
+    if (has_value()) return Optional<U>(std::invoke(std::forward<F>(f), value()));
+    return Optional<U>();
+  }
+
+  template <typename F>
+  constexpr auto transform(F&& f) const&& {
+    using U = std::invoke_result_t<F, decltype(std::move(value()))>;
+    if (has_value()) return Optional<U>(std::invoke(std::forward<F>(f), std::move(value())));
+    return Optional<U>();
+  }
+
+  template <typename F>
+  constexpr auto transform(F&& f) && {
+    using U = std::invoke_result_t<F, decltype(std::move(value()))>;
+    if (has_value()) return Optional<U>(std::invoke(std::forward<F>(f), std::move(value())));
+    return Optional<U>();
+  }
+};
+
+// Deduction guide.
+template <typename T>
+Optional(T) -> Optional<T>;
+
+}  // namespace android::ftl
diff --git a/libs/ftl/Android.bp b/libs/ftl/Android.bp
index c010a2e58a..8f89e7d151 100644
--- a/libs/ftl/Android.bp
+++ b/libs/ftl/Android.bp
@@ -20,6 +20,7 @@ cc_test {
         "fake_guard_test.cpp",
         "flags_test.cpp",
         "future_test.cpp",
+        "optional_test.cpp",
         "small_map_test.cpp",
         "small_vector_test.cpp",
         "static_vector_test.cpp",
diff --git a/libs/ftl/optional_test.cpp b/libs/ftl/optional_test.cpp
new file mode 100644
index 0000000000..6a8c8f9d3d
--- /dev/null
+++ b/libs/ftl/optional_test.cpp
@@ -0,0 +1,77 @@
+/*
+ * 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/optional.h>
+#include <ftl/static_vector.h>
+#include <ftl/string.h>
+#include <gtest/gtest.h>
+
+#include <functional>
+#include <numeric>
+#include <utility>
+
+using namespace std::placeholders;
+using namespace std::string_literals;
+
+namespace android::test {
+
+using ftl::Optional;
+using ftl::StaticVector;
+
+TEST(Optional, Transform) {
+  // Empty.
+  EXPECT_EQ(std::nullopt, Optional<int>().transform([](int) { return 0; }));
+
+  // By value.
+  EXPECT_EQ(0, Optional(0).transform([](int x) { return x; }));
+  EXPECT_EQ(100, Optional(99).transform([](int x) { return x + 1; }));
+  EXPECT_EQ("0b100"s, Optional(4).transform(std::bind(ftl::to_string<int>, _1, ftl::Radix::kBin)));
+
+  // By reference.
+  {
+    Optional opt = 'x';
+    EXPECT_EQ('z', opt.transform([](char& c) {
+      c = 'y';
+      return 'z';
+    }));
+
+    EXPECT_EQ('y', opt);
+  }
+
+  // By rvalue reference.
+  {
+    std::string out;
+    EXPECT_EQ("xyz"s, Optional("abc"s).transform([&out](std::string&& str) {
+      out = std::move(str);
+      return "xyz"s;
+    }));
+
+    EXPECT_EQ(out, "abc"s);
+  }
+
+  // Chaining.
+  EXPECT_EQ(14u, Optional(StaticVector{"upside"s, "down"s})
+                     .transform([](StaticVector<std::string, 3>&& v) {
+                       v.push_back("cake"s);
+                       return v;
+                     })
+                     .transform([](const StaticVector<std::string, 3>& v) {
+                       return std::accumulate(v.begin(), v.end(), std::string());
+                     })
+                     .transform([](const std::string& s) { return s.length(); }));
+}
+
+}  // namespace android::test