FTL: Add Optional<T>::or_else

Bug: 185536303
Test: ftl_test
Change-Id: Idf0bac4711111c8cb28e8624bb146a76ec1ff38b

3 files changed, 63 insertions, 1 deletions

diff --git a/include/ftl/details/optional.h b/include/ftl/details/optional.h
index bff7c1e000..e45c1f5e23 100644
--- a/include/ftl/details/optional.h
+++ b/include/ftl/details/optional.h
@@ -54,5 +54,15 @@ struct and_then_result {
 template <typename F, typename T>
 using and_then_result_t = typename and_then_result<F, T>::type;
 
+template <typename F, typename T>
+struct or_else_result {
+  using type = remove_cvref_t<std::invoke_result_t<F>>;
+  static_assert(std::is_same_v<type, std::optional<T>> || std::is_same_v<type, Optional<T>>,
+                "or_else function must return an optional T");
+};
+
+template <typename F, typename T>
+using or_else_result_t = typename or_else_result<F, T>::type;
+
 }  // namespace details
 }  // namespace android::ftl
diff --git a/include/ftl/optional.h b/include/ftl/optional.h
index a818128c92..94d8e3d7cb 100644
--- a/include/ftl/optional.h
+++ b/include/ftl/optional.h
@@ -96,13 +96,25 @@ struct Optional final : std::optional<T> {
     return R();
   }
 
+  // Returns this Optional<T> if not nullopt, or else the Optional<T> returned by the function F.
+  template <typename F>
+  constexpr auto or_else(F&& f) const& -> details::or_else_result_t<F, T> {
+    if (has_value()) return *this;
+    return std::forward<F>(f)();
+  }
+
+  template <typename F>
+  constexpr auto or_else(F&& f) && -> details::or_else_result_t<F, T> {
+    if (has_value()) return std::move(*this);
+    return std::forward<F>(f)();
+  }
+
   // Delete new for this class. Its base doesn't have a virtual destructor, and
   // if it got deleted via base class pointer, it would cause undefined
   // behavior. There's not a good reason to allocate this object on the heap
   // anyway.
   static void* operator new(size_t) = delete;
   static void* operator new[](size_t) = delete;
-
 };
 
 template <typename T, typename U>
diff --git a/libs/ftl/optional_test.cpp b/libs/ftl/optional_test.cpp
index 6b3b6c49e5..91bf7bc5b6 100644
--- a/libs/ftl/optional_test.cpp
+++ b/libs/ftl/optional_test.cpp
@@ -164,6 +164,46 @@ TEST(Optional, AndThen) {
                      }));
 }
 
+TEST(Optional, OrElse) {
+  // Non-empty.
+  {
+    const Optional opt = false;
+    EXPECT_EQ(false, opt.or_else([] { return Optional(true); }));
+    EXPECT_EQ('x', Optional('x').or_else([] { return std::make_optional('y'); }));
+  }
+
+  // Empty.
+  {
+    const Optional<int> opt;
+    EXPECT_EQ(123, opt.or_else([]() -> Optional<int> { return 123; }));
+    EXPECT_EQ("abc"s, Optional<std::string>().or_else([] { return Optional("abc"s); }));
+  }
+  {
+    bool empty = false;
+    EXPECT_EQ(Optional<float>(), Optional<float>().or_else([&empty]() -> Optional<float> {
+      empty = true;
+      return std::nullopt;
+    }));
+    EXPECT_TRUE(empty);
+  }
+
+  // Chaining.
+  using StringVector = StaticVector<std::string, 3>;
+  EXPECT_EQ(999, Optional(StaticVector{"1"s, "0"s, "0"s})
+                     .and_then([](StringVector&& v) -> Optional<StringVector> {
+                       if (v.push_back("0"s)) return v;
+                       return {};
+                     })
+                     .or_else([] {
+                       return Optional(StaticVector{"9"s, "9"s, "9"s});
+                     })
+                     .transform([](const StringVector& v) {
+                       return std::accumulate(v.begin(), v.end(), std::string());
+                     })
+                     .and_then(parse_int)
+                     .or_else([] { return Optional(-1); }));
+}
+
 // Comparison.
 namespace {