pgr_trspViaEdges miscalculates costs for link segments in some cases

[FrankyBoy]

Problem
When routing with pgr_trspViaEdges from and to the same link, the costs are miscalcuated, resulting in wrongly calculated routes. Only if the distance between the points is below 50% the correct route is calculated.

To Reproduce

SELECT * FROM pgr_trspViaEdges(
    'SELECT 123::int4 id, 234::int4 source, 235::int4 target, 1::float8 cost, 1::float8 reverse_cost',
    ARRAY[123, 123]::INTEGER[],
    ARRAY[0.9, 0.1]::FLOAT[],
    true,
    true)

Result:

seq	id1	id2	id3	cost
1	1	-1	123	0.1
2	1	234	123	0.1
3	1	-2	-2	0

As can be seen the cost is calculated as 2 times 0.1 and the route lads through node 234 instead of directly. Properly calculated one of the costs would have to be 0.9, and not 0.1.

Expectation

seq	id1	id2	id3	cost
1	1	-1	123	0.8
2	1	-2	-2	0

Testcases that work correctly:

• switching the percentages to 0.1, 0.9
• reducing the distance to below 0.5 (so eg. 0.9 0.4)

Platform/versions
Linux, pgrouting 3.6.1

"PostgreSQL 14.10 (Debian 14.10-1.pgdg110+1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 10.2.1-6) 10.2.1 20210110, 64-bit"
"POSTGIS=""3.4.2 c19ce56"" [EXTENSION] PGSQL=""140"" GEOS=""3.9.0-CAPI-1.16.2"" PROJ=""7.2.1 NETWORK_ENABLED=OFF URL_ENDPOINT=https://cdn.proj.org USER_WRITABLE_DIRECTORY=/var/lib/postgresql/.local/share/proj DATABASE_PATH=/usr/share/proj/proj.db"" LIBXML=""2.9.10"" LIBJSON=""0.15"" LIBPROTOBUF=""1.3.3"" WAGYU=""0.5.0 (Internal)"" (core procs from ""3.4.0 0874ea3"" need upgrade)"
"3.6.1"

[cvvergara]

The function was deprecated in v3.4.0:
https://docs.pgrouting.org/latest/en/release_notes.html#id39
There is this migration guide:
https://docs.pgrouting.org/latest/en/migration.html#migration-of-pgr-trspviaedges
For v4.0.0 it will be removed:
#2686

[FrankyBoy]

sadly that doesn't help me at all considering I am stuck with a pgrouting version below 3.4 in some legacy environments.