Add support for JSON merge diff generation

Summary: Add method to recursively compute RFC7386-compliant merge-diff between two folly::dynamic objects.

Reviewed By: yfeldblum

Differential Revision: D6794339

fbshipit-source-id: f2b807404faa922d5a705f858825329a95fb0d38

folly/dynamic.cpp

@@ -321,6 +321,34 @@ void dynamic::destroy() noexcept {
   u_.nul = nullptr;
 }
 
+dynamic dynamic::merge_diff(const dynamic& source, const dynamic& target) {
+  if (!source.isObject() || source.type() != target.type()) {
+    return target;
+  }
+
+  dynamic diff = object;
+
+  // added/modified keys
+  for (const auto& pair : target.items()) {
+    auto it = source.find(pair.first);
+    if (it == source.items().end()) {
+      diff[pair.first] = pair.second;
+    } else {
+      diff[pair.first] = merge_diff(source[pair.first], target[pair.first]);
+    }
+  }
+
+  // removed keys
+  for (const auto& pair : source.items()) {
+    auto it = target.find(pair.first);
+    if (it == target.items().end()) {
+      diff[pair.first] = nullptr;
+    }
+  }
+
+  return diff;
+}
+
 //////////////////////////////////////////////////////////////////////
 
 } // namespace folly

folly/dynamic.h

@@ -471,6 +471,11 @@ struct dynamic : private boost::operators<dynamic> {
    */
   void merge_patch(const dynamic& patch);
 
+  /*
+   * Computes JSON merge patch (RFC7386) needed to mutate from source to target
+   */
+  static dynamic merge_diff(const dynamic& source, const dynamic& target);
+
   /*
    * Erase an element from a dynamic object, by key.
    *

folly/test/DynamicTest.cpp

@@ -718,3 +718,41 @@ TEST(Dynamic, MergePatchRemoveNonExistent) {
   EXPECT_EQ("d", target["c"].getString());
   EXPECT_EQ(2, target.size());
 }
+
+TEST(Dynamic, MergeDiffFlatObjects) {
+  dynamic source = dynamic::object("a", 0)("b", 1)("c", 2);
+  dynamic target = dynamic::object("a", 1)("b", 2);
+  auto patch = dynamic::merge_diff(source, target);
+
+  EXPECT_EQ(3, patch.size());
+  EXPECT_EQ(1, patch["a"].getInt());
+  EXPECT_EQ(2, patch["b"].getInt());
+  EXPECT_TRUE(patch["c"].isNull());
+
+  source.merge_patch(patch);
+  EXPECT_EQ(source, target);
+}
+
+TEST(Dynamic, MergeDiffNestedObjects) {
+  dynamic source = dynamic::object("a", dynamic::object("b", 1)("c", 2))(
+      "d", dynamic::array(1, 2, 3));
+  dynamic target = dynamic::object("a", dynamic::object("b", 2))(
+      "d", dynamic::array(2, 3, 4));
+
+  auto patch = dynamic::merge_diff(source, target);
+
+  EXPECT_EQ(2, patch.size());
+  EXPECT_EQ(2, patch["a"].size());
+
+  EXPECT_EQ(2, patch["a"]["b"].getInt());
+  EXPECT_TRUE(patch["a"]["c"].isNull());
+
+  EXPECT_TRUE(patch["d"].isArray());
+  EXPECT_EQ(3, patch["d"].size());
+  EXPECT_EQ(2, patch["d"][0].getInt());
+  EXPECT_EQ(3, patch["d"][1].getInt());
+  EXPECT_EQ(4, patch["d"][2].getInt());
+
+  source.merge_patch(patch);
+  EXPECT_EQ(source, target);
+}