[Processing] "Points to path" algorithm calculates incorrect attributes values

[agiudiceandrea]

What is the bug or the crash?

The "Points to path" (native:pointstopath) C++ processing algorithm calculates incorrect values of the begin and end fields in the attribute table of the output line layer in some circumstances.

The issue didn't occur using the "Points to path" (qgis:pointstopath) Python processing algorithm in QGIS 3.16.

Steps to reproduce the issue

Using the following GeoJson layer (also provided as Points.zip zipped file):

{
"type": "FeatureCollection",
"name": "Points",
"crs": { "type": "name", "properties": { "name": "urn:ogc:def:crs:OGC:1.3:CRS84" } },
"features": [
{ "type": "Feature", "properties": { "ID": "A", "ord": 1 }, "geometry": { "type": "Point", "coordinates": [ 11.97, 43.32 ] } },
{ "type": "Feature", "properties": { "ID": "A", "ord": 3 }, "geometry": { "type": "Point", "coordinates": [ 13.18, 42.69 ] } },
{ "type": "Feature", "properties": { "ID": "A", "ord": 2 }, "geometry": { "type": "Point", "coordinates": [ 12.79, 43.52 ] } },
{ "type": "Feature", "properties": { "ID": "A", "ord": 5 }, "geometry": { "type": "Point", "coordinates": [ 14.71, 41.43 ] } },
{ "type": "Feature", "properties": { "ID": "A", "ord": 4 }, "geometry": { "type": "Point", "coordinates": [ 13.94, 42.04 ] } },
{ "type": "Feature", "properties": { "ID": "B", "ord": 1 }, "geometry": { "type": "Point", "coordinates": [ 15.94, 40.53 ] } },
{ "type": "Feature", "properties": { "ID": "B", "ord": 3 }, "geometry": { "type": "Point", "coordinates": [ 17.07, 41.63 ] } },
{ "type": "Feature", "properties": { "ID": "B", "ord": 2 }, "geometry": { "type": "Point", "coordinates": [ 16.44, 41.11 ] } }
]
}

as "Input point layer" of the "Points to path" (native:pointstopath) processing algorithm and setting the ID field as "Groups field" and the ord field as "Order field", then the output line layer contains the correct feature geometries but the incorrect values of the begin and end fields of the attribute table:

ID	begin	end
A	3	4
B	1	2

while it should be:

ID	begin	end
A	1	5
B	1	3

Versions

The issue occurs using the latest QGIS 3.28.14, QGIS 3.34.2 and QGIS 3.35.0-Master

QGIS version
3.35.0-Master
QGIS code branch
master
Qt version
5.15.11
Compiled against Python
3.11.7
Running against Python
3.11.6
GDAL/OGR version
3.8.3
PROJ version
9.3.1
EPSG Registry database version
v10.098 (2023-11-24)
GEOS version
3.12.1-CAPI-1.18.1
SQLite version
3.44.1
PostgreSQL client version
unknown
SpatiaLite version
5.1.0
QWT version
6.2.0
QScintilla2 version
2.13.4
OS version
Windows 7 Version 6.1 (Build 7601: SP 1)

This copy of QGIS writes debugging output.

Active Python plugins
db_manager
0.1.20
grassprovider
2.12.99
processing
2.12.99

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

The processing algorithm has been ported to C++ with PR #40014 since QGIS 3.18.