Topology Internals

The topology extension has SQL-facing documentation in the user manual. This page is for implementation notes that matter when changing the C topology engine or its PostgreSQL wrappers.

An edge end star is the collection of directed edge ends incident to one topology node. The current implementation lives in liblwgeom/topo/lwt_edgeend_star.h and liblwgeom/topo/lwt_edgeend_star.c, with individual edge ends represented by liblwgeom/topo/lwt_edgeend.h.

lwt_edgeEndStar_addEdge() adds the outgoing end when the edge starts at the star node, the incoming end when the edge ends there, or both ends for a closed edge incident to the same node. Each edge end records its edge pointer, direction, and azimuth.

The star is sorted lazily by azimuth. lwt_edgeEndStar_getNextCW() and lwt_edgeEndStar_getNextCCW() ensure sorting, find the requested directed edge end, and return the neighboring edge end around the node, wrapping at either end of the sorted array.

This structure is an internal navigation helper for topology algorithms that need adjacent edge-end order around a node. User-facing topology behavior belongs in the manual; the old Trac EdgeStar page was only a pointer to this header and should not remain as a parallel source page.