OBJ Parser with Boost Spirit - Ignoring comments

问题:

I'm trying to write a basic OBJ file loader using the Boost Spirit library. Although I got it working using the standard std::ifstreams, I'm wondering if it's possible to do a phrase_parse on the entire file using a memory mapped file, since it seems to provide the best performance as posted here.

I have the following code, which seems to work well, but it breaks when there is a comment in the file. So, my question is how do you ignore a comment that starts with a '#' in the OBJ file using Spririt?

struct vertex {
    double x, y, z;
};

BOOST_FUSION_ADAPT_STRUCT(
                          vertex,
                          (double, x)
                          (double, y)
                          (double, z)
                          )
std::vector<vertex> b_vertices         
boost::iostreams::mapped_file mmap(
                                           path,
                                           boost::iostreams::mapped_file::readonly);
        const char* f = mmap.const_data();
        const char* l = f + mmap.size();


        using namespace boost::spirit::qi;

      bool ok = phrase_parse(f,l,(("v" >> double_ >> double_ >> double_) |
                               ("vn" >> double_ >> double_>> double_)) % eol ,
                               blank, b_vertices);

The above code works well when there are no comments or any other data except vertices/normals. But when there is a different type of data the parser fails (as it should) and I'm wondering if there is a way to make it work without going back to parsing every line as it is slower (almost 2.5x in my tests). Thank you!

回答1:

The simplest way that comes to mind is to simply make comments skippable:

bool ok = qi::phrase_parse(
        f,l,
         (
               ("v"  >> qi::double_ >> qi::double_ >> qi::double_) |
               ("vn" >> qi::double_ >> qi::double_ >> qi::double_)
          ) 
          % qi::eol,
        ('#' >> *(qi::char_ - qi::eol) >> qi::eol | qi::blank), b_vertices);

Note that this also 'recognizes' comments if # appears somewhere inside the line. This is probably just fine (as it would make the parsing fail, unless it was a comment trailing on an otherwise valid input line).

See it Live on Coliru

Alternatively, use some phoenix magic to handle "comment lines" just as you handle a "vn" or "v" line.

回答2:

I realize that my comment/post is not directly related code but I'm for not reinventing the wheel if possible and I would have wanted to know about this library. I was working with a handwritten OBJ/Wavefront loader but in my research I found this library Tiny Obj Loader. This library is written C++ with no dependencies excetp C++ STL. It handles the edge cases for the Wavefront spec fairly well and it is very fast. The thing that the user has to do is convert the Tiny OBJ objects into their code. TinyObjLoader has been adopted by quite a number of projects as well. I do apologize for not directly answering the question and my desire is to get knowledge about this great library out.