Add variadic template for get_optional to support nested map (#932)

Summary:
Pull Request resolved: https://github.com/facebook/folly/pull/932

As title.

Reviewed By: yfeldblum

Differential Revision: D9817328

fbshipit-source-id: 261c68f21316e91f3ed752c4aeab9e9c95dfe16d

folly/MapUtil.h

@@ -216,6 +216,23 @@ auto extract_default(const KeysDefault&... keysDefault) ->
 }
 } // namespace detail
 
+/**
+ * Given a map of maps and a path of keys, return a Optional<V> if the nested
+ * key exists and None if the nested keys does not exist in the map.
+ */
+template <class Map, class Key1, class Key2, class... Keys>
+auto get_optional(
+    const Map& map,
+    const Key1& key1,
+    const Key2& key2,
+    const Keys&... keys)
+    -> folly::Optional<
+        typename detail::NestedMapType<Map, 2 + sizeof...(Keys)>::type> {
+  auto pos = map.find(key1);
+  return pos != map.end() ? get_optional(pos->second, key2, keys...)
+                          : folly::none;
+}
+
 /**
  * Given a map of maps and a path of keys, return a pointer to the nested value,
  * or nullptr if the key doesn't exist in the map.

folly/test/MapUtilTest.cpp

@@ -70,6 +70,34 @@ TEST(MapUtil, get_optional) {
   EXPECT_FALSE(get_optional(m, 2).hasValue());
 }
 
+TEST(MapUtil, get_optional_path_simple) {
+  using std::map;
+  map<int, map<int, map<int, map<int, int>>>> m{{1, {{2, {{3, {{4, 5}}}}}}}};
+  EXPECT_EQ(folly::Optional<int>(5), get_optional(m, 1, 2, 3, 4));
+  EXPECT_TRUE(get_optional(m, 1, 2, 3, 4));
+  EXPECT_FALSE(get_optional(m, 1, 2, 3, 0));
+  EXPECT_TRUE(get_optional(m, 1, 2, 3));
+  EXPECT_FALSE(get_optional(m, 1, 2, 0));
+  EXPECT_TRUE(get_optional(m, 1, 2));
+  EXPECT_FALSE(get_optional(m, 1, 0));
+  EXPECT_TRUE(get_optional(m, 1));
+  EXPECT_FALSE(get_optional(m, 0));
+}
+
+TEST(MapUtil, get_optional_path_mixed) {
+  using std::map;
+  using std::string;
+  using std::unordered_map;
+  unordered_map<string, map<int, map<string, int>>> m{{"a", {{1, {{"b", 2}}}}}};
+  EXPECT_EQ(folly::Optional<int>(2), get_optional(m, "a", 1, "b"));
+  EXPECT_TRUE(get_optional(m, "a", 1, "b"));
+  EXPECT_FALSE(get_optional(m, "b", 1, "b"));
+  EXPECT_FALSE(get_optional(m, "a", 2, "b"));
+  EXPECT_FALSE(get_optional(m, "a", 1, "c"));
+  EXPECT_TRUE(get_optional(m, "a", 1));
+  EXPECT_TRUE(get_optional(m, "a"));
+}
+
 TEST(MapUtil, get_ref_default) {
   std::map<int, int> m;
   m[1] = 2;