According GitHub commit: https://github.com/MikhanGusev/gdal/commit/12dc0678c78329f65f885958103e056491fadcd1
Automatic graph building in GNM
Similar to my old concept there can be several automatic graph building algorithms in GNM, because this mechanism is implemented as the pattern "Strategy". Firstly we must initialize the strategy with its own settings, than set the strategy for the GNMConnectivity via SetAutoConnectStrategy() and finally call the GNMConnectivity::AutoConnect(). The system graph layer will be filled with new connections.
Now there is only one algorithm in GDAL source tree - the one, which I'd implemented already in an old API. Now it is integrated to the GNM and builds graph basing on the set of line and point layer ids of the current GNMConnectivity.
So now GNM has a capability not just to create a set of obligatory layers over a dataset, but to create a real connectivity among features in this dataset, if it is needed.
* We can modify any built connection via GNMConnectivity::ReconnectFeatures() and delete it via GNMConnectivity::DisconnectFeatures().
* If the graph is already built and we add some not connected features to the network we can anytime call AutoConnect() again and it will add additional connections not rebuilding old.
* The graph can be cleared anytime via GNMConnectivity::DisconnectAll().
* Virtual AutoConnect() method intends to be a generic for future derived classes for the concrete network formats. For example in GNM pgRouting Connectivity class it can call pgr_createTopology.
GNMConnectivity::ConnectAllLinePoint() uses AutoConnect() but firstly counts all line and point layers, passing their ids as the parameters when the connection strategy is being created.