Adding the same closed line to topology twice creates two nodes: ST_Scroll (or ST_Rebase) function

[strk]

As found in ticket #2172 adding a closed line to a topology using TopoGeo_AddLineString two times creates a second node being the leftmost (and lowermost) vertex in the closed line.

This is due to the way noding with existing edges is performed.

It is not necessarely a bug but can be annoying as it produces a more complex topology that needed.

In order to fix this there should be a primitive function for rotating a closed linestring so to start/end on a given point number. I don't think we have one, do we ?

[robe]

I like ST_Rebound. Definitions that make me think why this term is appropriate:

​http://www.thefreedictionary.com/rebound

1. in the act of springing back
2. To reecho; resound.

[robe]

most relevant definition:

return to a former condition

[robe]

strk - push this back to 2.0 if you don't think it requires exposing a front facing function.

[strk]

I like ST_Rebase more, in that the function needs not deal to any concept of "former condition". Given a closed linestring, "Rebase" would change its start/end point to any of the existing vertices. The new vertex may be expressed in terms of a vertex index from current start, in which case we'd need a function to extract a vertex index from a point (closest vertex). Or it could be expressed as a "location" so that ST_Locate_Point can do its thing.

[strk]

Stumbled upon "scroll" in GEOS/JTS, so might be another candidate for a name... ST_Scroll(linestring geometry, first_point geometry)

[robe]

Milestone renamed

[strk]

I've started work on a scroll function: ​https://gitlab.com/postgis/postgis/-/merge_requests/27

[Sandro Santilli <strk@…>]

In ec89011/git:

Implement ptarray_scroll_in_place

Includes unit tests
References #2175

[Sandro Santilli <strk@…>]

In f4a1ce5/git:

Avoid creating two nodes when adding same closed line to topology

Closes #2175

[Sandro Santilli <strk@…>]

In 04aa7f6/git:

Fix scrolling of closed lines on topology population

Includes test for adding same closed line with first point slighly
off but within tolerance.

Use of tolerance moves the first point, which is later NOT
found by the scroll call, unless we recomputed as in this
commit, which recomputes start point after snap.

References #2175