Class FieldGroup

depends/protobuf/google/protobuf/compiler/cpp/cpp_message.cc:178–220

FieldGroup is just a helper for OptimizePadding below. It holds a vector of fields that are grouped together because they have compatible alignment, and a preferred location in the final field ordering.

// fields that are grouped together because they have compatible alignment, and
// a preferred location in the final field ordering.
class FieldGroup {
 public:
  FieldGroup()
      : preferred_location_(0) {}

  // A group with a single field.
  FieldGroup(float preferred_location, const FieldDescriptor* field)
      : preferred_location_(preferred_location),
        fields_(1, field) {}

  // Append the fields in 'other' to this group.
  void Append(const FieldGroup& other) {
    if (other.fields_.empty()) {
      return;
    }
    // Preferred location is the average among all the fields, so we weight by
    // the number of fields on each FieldGroup object.
    preferred_location_ =
        (preferred_location_ * fields_.size() +
         (other.preferred_location_ * other.fields_.size())) /
        (fields_.size() + other.fields_.size());
    fields_.insert(fields_.end(), other.fields_.begin(), other.fields_.end());
  }

  void SetPreferredLocation(float location) { preferred_location_ = location; }
  const vector<const FieldDescriptor*>& fields() const { return fields_; }

  // FieldGroup objects sort by their preferred location.
  bool operator<(const FieldGroup& other) const {
    return preferred_location_ < other.preferred_location_;
  }

 private:
  // "preferred_location_" is an estimate of where this group should go in the
  // final list of fields.  We compute this by taking the average index of each
  // field in this group in the original ordering of fields.  This is very
  // approximate, but should put this group close to where its member fields
  // originally went.
  float preferred_location_;
  vector<const FieldDescriptor*> fields_;
  // We rely on the default copy constructor and operator= so this type can be
  // used in a vector.
};

// Reorder 'fields' so that if the fields are output into a c++ class in the new
// order, the alignment padding is minimized.  We try to do this while keeping