std::string to_json(
    const google::protobuf::Message& m,
    std::function<std::string (const std::string&)> name_filter) {
  std::vector<std::pair<std::string, std::string>> key_value_pairs;

  auto refl = m.GetReflection();
  std::vector<const google::protobuf::FieldDescriptor*> field_descriptors;
  refl->ListFields(m, &field_descriptors);

  for (auto fd : field_descriptors) {
    if (!refl->HasField(m, fd)) {
      continue;
    }

    string value;

    if (fd->is_repeated()) {
      const int n = refl->FieldSize(m, fd);
      std::vector<std::string> vals(n);

      for (int i = 0; i < n; i++) {
        switch (fd->type()) {
          case google::protobuf::FieldDescriptor::TYPE_DOUBLE:
            vals.push_back(std::to_string(refl->GetRepeatedDouble(m, fd, i)));
            break;
          case google::protobuf::FieldDescriptor::TYPE_FLOAT:
            vals.push_back(std::to_string(refl->GetRepeatedFloat(m, fd, i)));
            break;
          case google::protobuf::FieldDescriptor::TYPE_INT64:
            vals.push_back(std::to_string(refl->GetRepeatedInt64(m, fd, i)));
            break;
          case google::protobuf::FieldDescriptor::TYPE_UINT64:
            vals.push_back(std::to_string(refl->GetRepeatedUInt64(m, fd, i)));
            break;
          case google::protobuf::FieldDescriptor::TYPE_INT32:
            vals.push_back(std::to_string(refl->GetRepeatedInt32(m, fd, i)));
            break;
          case google::protobuf::FieldDescriptor::TYPE_FIXED64:
            vals.push_back(std::to_string(refl->GetRepeatedInt64(m, fd, i)));
            break;
          case google::protobuf::FieldDescriptor::TYPE_FIXED32:
            vals.push_back(std::to_string(refl->GetRepeatedInt32(m, fd, i)));
            break;
          case google::protobuf::FieldDescriptor::TYPE_BOOL:
            vals.push_back(std::to_string(refl->GetRepeatedBool(m, fd, i)));
            break;
          case google::protobuf::FieldDescriptor::TYPE_STRING:
            vals.push_back(refl->GetRepeatedString(m, fd, i));
            break;
          case google::protobuf::FieldDescriptor::TYPE_GROUP:
            throw "Groups support not implemented.";
            break;
          case google::protobuf::FieldDescriptor::TYPE_MESSAGE:
            vals.push_back(
                to_json(refl->GetRepeatedMessage(m, fd, i), name_filter));
            break;
          case google::protobuf::FieldDescriptor::TYPE_BYTES:
            throw "Can't serialize binary to json.";
            break;
          case google::protobuf::FieldDescriptor::TYPE_UINT32:
            vals.push_back(std::to_string(refl->GetRepeatedUInt32(m, fd, i)));
            break;
          case google::protobuf::FieldDescriptor::TYPE_ENUM:
            throw "Can't serialize enums to json.";
            break;
          case google::protobuf::FieldDescriptor::TYPE_SFIXED32:
            vals.push_back(std::to_string(refl->GetRepeatedInt32(m, fd, i)));
            break;
          case google::protobuf::FieldDescriptor::TYPE_SFIXED64:
            vals.push_back(std::to_string(refl->GetRepeatedInt64(m, fd, i)));
            break;
          case google::protobuf::FieldDescriptor::TYPE_SINT32:
            vals.push_back(std::to_string(refl->GetRepeatedInt32(m, fd, i)));
            break;
          case google::protobuf::FieldDescriptor::TYPE_SINT64:
            vals.push_back(std::to_string(refl->GetRepeatedInt64(m, fd, i)));
            break;
          default:
            break;
        }

        value += '[';
        for (const auto& v : vals) {
          value += v;
          value += ',';
        }
        if (value[value.length() - 1] == ',') {
          value[value.length() - 1] = ']';
        } else {
          value += ']';
        }
      }
    } else {
      switch (fd->type()) {
        case google::protobuf::FieldDescriptor::TYPE_DOUBLE:
          value = std::to_string(refl->GetDouble(m, fd));
          break;
        case google::protobuf::FieldDescriptor::TYPE_FLOAT:
          value = std::to_string(refl->GetFloat(m, fd));
          break;
        case google::protobuf::FieldDescriptor::TYPE_INT64:
          value = std::to_string(refl->GetInt64(m, fd));
          break;
        case google::protobuf::FieldDescriptor::TYPE_UINT64:
          value = std::to_string(refl->GetUInt64(m, fd));
          break;
        case google::protobuf::FieldDescriptor::TYPE_INT32:
          value = std::to_string(refl->GetInt32(m, fd));
          break;
        case google::protobuf::FieldDescriptor::TYPE_FIXED64:
          value = std::to_string(refl->GetInt64(m, fd));
          break;
        case google::protobuf::FieldDescriptor::TYPE_FIXED32:
          value = std::to_string(refl->GetInt32(m, fd));
          break;
        case google::protobuf::FieldDescriptor::TYPE_BOOL:
          value = std::to_string(refl->GetBool(m, fd));
          break;
        case google::protobuf::FieldDescriptor::TYPE_STRING:
          value += '"';
          value += refl->GetString(m, fd);
          value += '"';
          break;
        case google::protobuf::FieldDescriptor::TYPE_GROUP:
          throw "Groups support not implemented.";
          break;
        case google::protobuf::FieldDescriptor::TYPE_MESSAGE:
          value += '"';
          value += to_json(refl->GetMessage(m, fd), name_filter);
          value += '"';
          break;
        case google::protobuf::FieldDescriptor::TYPE_BYTES:
          throw "Can't serialize binary to json.";
          break;
        case google::protobuf::FieldDescriptor::TYPE_UINT32:
          value = std::to_string(refl->GetUInt32(m, fd));
          break;
        case google::protobuf::FieldDescriptor::TYPE_ENUM:
          throw "Can't serialize enums to json.";
          break;
        case google::protobuf::FieldDescriptor::TYPE_SFIXED32:
          value = std::to_string(refl->GetInt32(m, fd));
          break;
        case google::protobuf::FieldDescriptor::TYPE_SFIXED64:
          value = std::to_string(refl->GetInt64(m, fd));
          break;
        case google::protobuf::FieldDescriptor::TYPE_SINT32:
          value = std::to_string(refl->GetInt32(m, fd));
          break;
        case google::protobuf::FieldDescriptor::TYPE_SINT64:
          value = std::to_string(refl->GetInt64(m, fd));
          break;
        default:
          break;
      }
    }

    key_value_pairs.push_back(std::make_pair(name_filter(fd->name()), value));
  }

  string ret = "{";
  for (const auto& p : key_value_pairs) {
    ret += '"';
    ret += p.first;
    ret += "\":";
    ret += p.second;
    ret += ',';
  }

  if (ret[ret.length() - 1] == ',') {
    ret[ret.length() - 1] = '}';
  } else {
    ret += '}';
  }

  return ret;
}