[mkgmap-dev] highwayCount

Thu Feb 7 15:54:04 GMT 2013

Hi Gerd

Some background:

The term "highwayCount" is inaccurate and the count is unimportant
the only important concept is if a point on a line is a "node" or not.

The term "node" is used in lots of different contexts of course, so
what I mean in this context is:

1. a point that is used in more than one line is a "node"
2. a boundary node is a "node" - it is joined to a line in another
    tile.
3. house numbers are tied to "node"s so you can have a "node" in the
    middle of a line, just to support a house number range.

I used to say "routing node", but case (3) which I just discovered
recently kind of makes that name incorrect too.

> My understanding is this:
> 1) highwayCount should be increased for each coord instance that is part of
> a road.

Not necessarily, it is just an implementation, a one bit counter would
be enough.

> I think the right place to do that is in an extra loop at the beginning of
> StyledConverter.removeShortArcsByMergingNodes().
> A coord instance that is part of more than one road is considered to become
> a road junction.

Its not just actual road junctions, two lines that join end to end
as part of a single road in real life are still connected by a "node".

> 2) in many places I see something like this:
> 						if(co.getOnBoundary()) {
> 							// this point lies on a boundary
> 							// make sure it becomes a node
> 							co.incHighwayCount();
> 						}
> I think this is an abuse of the highwayCount. The highwayCount is
> used to determine junctions of roads (e.g. in the frigRoundabout() method)
> and I don't see why the attribute "onBoundary" should change the
> "highwayCount"

Agreed that the name is not appropriate and is confusing in this case.
But the functionality is correct, a boundary node should be a "node".

> If "onBoundary = true" and "highwayCount > 1" both have a meaning like
> "isRoadJunction"

isRoadJunction() is too narrow a name, all road junctions are nodes
but not all nodes are road junction. But perhaps isNode()

> I suggest that we create a new flag with this name.
>
> Does that make sense?

I'm fine with changing to better names + better comments so it
doesn't cause confusion. Not so sure about algorithm changes that
produce different results until you have identified a specific bug.

> If yes, how can I verify the results of my changes ?
> With my logic, I see ~20% fewer coords with highwayCount > 1^, so
> the change is big.

> The unit tests do not help much, SimpleRouteTest failes because
> the NOD size is different, but I doubt that the expected size is a must?

Correct, the size checks are just there to prevent accidental
breakage, if you are fixing a bug that is expected to change the
size then the test has to be changed too after very careful checking
that the results are correct.

We don't have many good ways of checking results. The ultimate check
is of course is testing how well routing works on an actual device (or
range of devices, since they are all a bit different).

All there is at the moment is NetDisplay and NodDisplay in the display
repo. We need better tools to display things are a higher level and
automatically cross check between the different sections.

..Steve