How do I correctly map U-turns in the SQL restriction data?

[luening-ct]

Hi everyone, I hope someone here can help me with my problem. I’m trying to set up a routing algorithm that can calculate routes on a custom forest road network. It should also be able to reflect turn restrictions.

My problem is that I can’t properly handle U-turns. Using the path array, I can specify a sequence of edges that must not be used. If I were to model a U-turn, this would result in the same ID in both the from and to fields [edge_id, edge_id]. However, this doesn’t indicate which node this restriction applies to.

Example
If I enter the constraint [1,1], it's not clear whether it applies to a U-turn at node A or B.

A <---1---> B <---2---> C

To Reproduce
Here is a short code snippet you can use to test the behavior.

DROP TABLE IF EXISTS test_edges;
CREATE TEMP TABLE test_edges (
    id           BIGINT PRIMARY KEY,
    source       BIGINT,
    target      BIGINT,
    cost        DOUBLE PRECISION,
    reverse_cost DOUBLE PRECISION
);

INSERT INTO test_edges VALUES
    (1,  1, 2,  10.0, 10.0),  -- e1: 1 <-> 2
    (2,  2, 3,  10.0, 10.0),  -- e2: 2 <-> 3
    (3,  3, 4,  10.0, 10.0),  -- e3: 3 <-> 4
    (4,  4, 1,  50.0, 50.0);  -- e4: 

DROP TABLE IF EXISTS test_restrictions;
CREATE TEMP TABLE test_restrictions (
    path BIGINT[],
    cost DOUBLE PRECISION
);

INSERT INTO test_restrictions VALUES
    (ARRAY[1,1],99999.0);

SELECT *
FROM pgr_trsp_withpoints(
    'SELECT id, source, target, cost, reverse_cost FROM test_edges',
    'SELECT path, cost FROM test_restrictions',
    'SELECT pid, edge_id, fraction, side
     FROM (VALUES 	(1::BIGINT, 1::BIGINT, 0.5::DOUBLE PRECISION, ''r''::CHAR),
	 				(2::BIGINT, 4::BIGINT, 0.9::DOUBLE PRECISION, ''b''::CHAR))
          AS p(pid, edge_id, fraction, side)',
    ARRAY[-1]::BIGINT[],
    ARRAY[-2]::BIGINT[],
    directed := true,
	details => true
);

Expectation
I can't imagine being the first person to encounter this problem, so I hope there's a solution. I would have expected it to be possible to specify a via node to improve the assignment (similar to specifying OSM restrictions).

Platform/versions
Unfortunately, I'm stuck with pgrouting 3.5, since that's the latest version available for postgresql for azure.