Parsing large json file

[hka]

I'm trying to parse a large json file that is too big to keep completely in memory on my machine. I found this: #927
which helps me part of the way.

The data I am trying to parse looks like this (it is from open street maps):

{
  "type": "FeatureCollection",
  "features": [
    {
      "type": "Feature",
      "properties": {
        "@id": "way/1630601",
        "highway": "secondary"
      },
      "geometry": {
        "type": "LineString",
        "coordinates": [
          [
            15.0498395,
            58.529443
          ],
          [
            15.0502197,
            58.529431
          ]
        ]
      },
      "id": "way/1630601"
    },
    {
      "type": "Feature",
      "properties": {
        "@id": "way/1634031"
      },
      "geometry": {
        "type": "LineString",
        "coordinates": [
          [
            15.0901486,
            58.4024008
          ]
        ]
      },
      "id": "way/1634031"
    }
  ]
}

If I parse using callbacks and check for json::parse_event_t::object_end I get a callback after each }, I would like to get a callback after each complete item in the array (so I get "properties" and "geometry" together). Any advice on how to do that?

[hka]

Maybe not the neatest way to do it but counting starts and ends did the trick!

void test(const char* path)
{
  using json = nlohmann::json;
  std::size_t state = 0;

  auto x = [&state](int depth, json::parse_event_t event, json& parsed) {
    if(event == json::parse_event_t::array_start && state == 0)
    {
      state = 1;
      return true;
    }
    if(event == json::parse_event_t::object_start)
    {
      ++state;
      return true;
    }
    if (event == json::parse_event_t::object_end)
    {
      --state;
      if(state == 1)
      {
        std::cout << std::endl << parsed.dump(2) << std::endl;
        return false; // do not store the object value
      }
      else
        return true;
    }
    else
    {
      return true;
    }
  };
  std::ifstream f(path);
  json::parse(f, x);
}

[hka]

A related question, is it possible to keep track of reading progress? How many bytes of the input file has been parsed or something similar?