feature: support relativePosition.radius in subscriptions and REST requests

[sbender9]

This feature allows ws subscriptions to be based on the relative position of the vessel. This allows the client to to only get information about other vessels that are within a specific distance. For example:

{
  context: {
    radius: 10,
    position: {
      longitude: -76.4639314,
      latitude: 39.0700403
    }
  },
  subscribe: [
    {
      path: 'navigation.*'
    }
  ]
}

[tkurki]

Very minor here, but as discussed in canboatjs JSON.stringify is surprisingly heavy and I'd like to start the habit of doing this like debug.enabled && debug(...)

[tkurki]

change this to updatePosition(delta, position) as this doesn't need to know anything about spark.request?

[tkurki]

relativePosition is not very clear, how about msg.context.subscriber.position? Ditto for spark.request.subscriberPosition.

[tkurki]

Doesn't checkPosition here get the relativePosition from the subscribeCommand, which is static and will never change? So we are updating the subscriber's position in spark.request, but that is never used in checkPosition?

Thinking this through how useful is the updating current position going to be in real life? It only has the lifetime of the ws connection and can be easily updated by hanging off the ws conenction and recconnecting every x hours. For this to be useful we would need to be dealing with orbital speeds or very small radii. What do you think? Ditch or keep?

[sbender9]

Yep. You’re right. I’ll ditch it

[sbender9]

I think the only thing left here is the naming. One option for the subscription:

{
          context: {
              contexts: "vessels.*",
              radius: 10,
              position: {
                longitude: -76.4639314,
                latitude: 39.0700403
              }
            }
          },
          subscribe: [
            {
              path: '*'
            }
          ]
        }

[sbender9]

Or just

{
  context: {
    radius: 1,
    position: {
      longitude: -76.4639314,
      latitude: 39.0700403
    }
  },
  subscribe: [
    {
      path: 'navigation.position'
    }
  ]
}