Computing inverse of merge_patch() from

[MHebes]

This is to merge_patch as diff is to patch, that is:

json source = // ...
json target = // ...
std::optional<json> patch = inverse_merge_patch(source, target);

json patched = source;
patched.merge_patch(*patch);

EXPECT_EQ(patched, target)

Returns an optional because the RFC was unclear to me what happens in the case of two exact json literal values. Also made the recursion simpler.

Borrowed a lot of code from the json::diff implementation.

using json = nlohmann::json;

/// Returns a merge-patch object which, when applied to `source` with
/// `merge_patch`, would yield `target`.
std::optional<json> inverse_merge_patch(const json& source, const json& target,
                                        const std::string& path = "") {
  // target is not an object -> return if different
  if (target.type() != json::value_t::object) {
    if (target == source) return std::nullopt;
    return target;
  };

  if (source.type() != target.type()) {
    // type changed! full replace
    return target;
  }

  // same types! myst both be objects
  std::optional<json> result;

  // first pass: traverse this object's elements
  for (auto it = source.cbegin(); it != source.cend(); ++it) {
    // escape the key name to be used in a JSON patch
    const auto path_key = path + "/" + nlohmann::detail::escape(it.key());

    if (target.find(it.key()) != target.end()) {
      // recursive call to compare object values at key it
      auto temp_diff =
          inverse_merge_patch(it.value(), target[it.key()], path_key);
      if (temp_diff.has_value()) {
        if (!result.has_value()) result = json(json::value_t::object);
        (*result)[it.key()] = std::move(*temp_diff);
      }
    } else {
      if (!result.has_value()) result = json(json::value_t::object);
      // found a key that is not in o -> remove it
      (*result)[it.key()] = json(json::value_t::null);
    }
  }

  // second pass: traverse other object's elements
  for (auto it = target.cbegin(); it != target.cend(); ++it) {
    if (source.find(it.key()) == source.end()) {
      // found a key that is not in this -> add it
      if (!result.has_value()) result = json(json::value_t::object);
      (*result)[it.key()] = it.value();
    }
  }

  return result;
}

Example:

[Matheus-Garbelini]

Any chance to integrate this to the main library?