Can anyone help with protocol buffers?

[Cody Fendant]

I'm trying to read information from a service which only provides it in Protocol Buffer format.

I'd never heard of it until now, and this is what I understand about it:

1. It's binary
2. you don't just read it like JSON, you have to know the format of it first
3. you know the format because you get a proto file and parse that
4. this can be done by https://metacpan.org/pod/Google::ProtocolBuffers
5. Google::ProtocolBuffers returns a list of accessor objects, essentially modules created on the fly
6. Then you grab data from your binary file by using one of the accessors

I've got as far as step 5. Now what?

I have the binary file, and the proto file which describes the format, but I can't figure out how to access the data.

The data is basically hundreds of these entities (bus information for Sydney Australia):

entity {
  id: "42558_202397_3000_41_1"
  vehicle {
    trip {
      trip_id: "892878"
      start_time: "16:05:00"
      start_date: "20190609"
      schedule_relationship: SCHEDULED
      route_id: "3000_41"
    }
    position {
      latitude: -33.03611
      longitude: 151.66042
      bearing: 332.0
      speed: 2.2
    }
    timestamp: 1560060858
    congestion_level: UNKNOWN_CONGESTION_LEVEL
    vehicle {
      id: "42558_202397_3000_41_1"
      [transit_realtime.tfnsw_vehicle_descriptor] {
        air_conditioned: true
        wheelchair_accessible: 1
        vehicle_model: "Volvo~B7RLE~Bustech~VST"
        special_vehicle_attributes: 0
      }
    }
    occupancy_status: MANY_SEATS_AVAILABLE
  }
}

And this is what the module tells me are the accessors:

[
          'TransitRealtime::Alert::Cause',
          'TransitRealtime::Alert::Effect',
          'TransitRealtime::FeedHeader::Incrementality',
          'TransitRealtime::TripDescriptor::ScheduleRelationship',
          'TransitRealtime::TripUpdate::StopTimeUpdate::ScheduleRelati
+onship',
          'TransitRealtime::VehiclePosition::CongestionLevel',
          'TransitRealtime::VehiclePosition::OccupancyStatus',
          'TransitRealtime::VehiclePosition::VehicleStopStatus',
          'TransitRealtime::Alert',
          'TransitRealtime::EntitySelector',
          'TransitRealtime::FeedEntity',
          'TransitRealtime::FeedHeader',
          'TransitRealtime::FeedMessage',
          'TransitRealtime::Position',
          'TransitRealtime::TfnswVehicleDescriptor',
          'TransitRealtime::TimeRange',
          'TransitRealtime::TranslatedString',
          'TransitRealtime::TranslatedString::Translation',
          'TransitRealtime::TripDescriptor',
          'TransitRealtime::TripUpdate',
          'TransitRealtime::TripUpdate::StopTimeEvent',
          'TransitRealtime::TripUpdate::StopTimeUpdate',
          'TransitRealtime::VehicleDescriptor',
          'TransitRealtime::VehiclePosition'
        ];

What I would like to do is parse it like I would parse JSON. Say I want to see where that bus (route_id: "3000_41") is from that data, what should I do? What's the equivalent of parsing through like

foreach $entity ( @entities ){
    if($entity->route_id eq '3000_41'){
        print "bus is at " . $position->{latitude} . ', ' .  $position
+->{longitude};
    }
}