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Showing content from https://github.com/pgRouting/pgrouting/pull/2635 below:

pgr_LineGraph ignores directed flag and use negative values for identifiers. · Issue #2635 · pgRouting/pgrouting · GitHub

ignores the directed flag

When the graph is undirected, it ignores the directed flag, and has negative identifiers for the source and target

SELECT * FROM pgr_lineGraph(
  'SELECT id, source, target, cost, reverse_cost
  FROM edges WHERE id IN (2,4,5,8)',
  false);
 seq | source | target | cost | reverse_cost
-----+--------+--------+------+--------------
   1 |     -8 |     -4 |    1 |            1
   2 |     -8 |      8 |    1 |            1
   3 |     -4 |      4 |    1 |            1
   4 |     -2 |      4 |    1 |           -1
   5 |      5 |     -8 |    1 |           -1
(5 rows)

Expected

 seq | source | target | cost | reverse_cost
-----+--------+--------+------+--------------
   1 |      2 |      4 |    1 |           -1
   2 |      2 |      5 |    1 |           -1
   3 |      4 |      8 |    1 |           -1
   4 |      5 |      8 |    1 |           -1

Works well when the input is directed and with unique edges

CREATE TABLE edges_unique (
    id BIGINT,
    source BIGINT,
    target BIGINT,
    cost FLOAT,
    geom geometry
);
CREATE TABLE
INSERT INTO edges_unique (id, source, target, cost, geom) VALUES
  (102, 1, 2, 1, ST_MakeLine(ST_POINT(0,  2), ST_POINT(2,  2))),
  (104, 1, 4, 1, ST_MakeLine(ST_POINT(0,  2), ST_POINT(0,  0))),
  (301, 3, 1, 1, ST_MakeLine(ST_POINT(2,  0), ST_POINT(0,  2))),
  (203, 2, 3, 1, ST_MakeLine(ST_POINT(2,  2), ST_POINT(2,  0))),
  (304, 3, 4, 1, ST_MakeLine(ST_POINT(2,  0), ST_POINT(0,  0))),
  (302, 3, 2, 1, ST_MakeLine(ST_POINT(2,  0), ST_POINT(2,  2))),
  (403, 4, 3, 1, ST_MakeLine(ST_POINT(0,  0), ST_POINT(2,  0)));
INSERT 0 7
SELECT seq, source, target, cost, reverse_cost
FROM pgr_lineGraph(
  'SELECT id, source, target, cost FROM edges_unique',
  true);
 seq | source | target | cost | reverse_cost
-----+--------+--------+------+--------------
   1 |    102 |    203 |    1 |           -1
   2 |    104 |    403 |    1 |           -1
   3 |    203 |    301 |    1 |           -1
   4 |    203 |    304 |    1 |           -1
   5 |    301 |    102 |    1 |           -1
   6 |    301 |    104 |    1 |           -1
   7 |    302 |    203 |    1 |            1
   8 |    304 |    403 |    1 |            1
   9 |    403 |    301 |    1 |           -1
  10 |    403 |    302 |    1 |           -1
(10 rows)

expected 12 edges total

 seq | source | target | cost | reverse_cost
-----+--------+--------+------+--------------
   1 |    102 |    203 |    1 |           -1
   2 |    104 |    403 |    1 |           -1
   3 |    203 |    301 |    1 |           -1
   4 |    203 |    304 |    1 |           -1
   5 |    301 |    102 |    1 |           -1
   6 |    301 |    104 |    1 |           -1
   7 |    302 |    203 |    1 |            1
   8 |    304 |    403 |    1 |            1
   9 |    403 |    301 |    1 |           -1
  10 |    403 |    302 |    1 |           -1
(10 rows)

Edges with negative identifiers

The same graph as above but represented with shared identifiers on the edges

CREATE TABLE edges_shared (
    id BIGINT,
    source BIGINT,
    target BIGINT,
    cost FLOAT,
    reverse_cost FLOAT,
    geom geometry
);
CREATE TABLE
INSERT INTO edges_shared (id, source, target, cost, reverse_cost, geom) VALUES
  (102, 1, 2, 1, -1, ST_MakeLine(ST_POINT(0,  2), ST_POINT(2,  2))),
  (104, 1, 4, 1, -1, ST_MakeLine(ST_POINT(0,  2), ST_POINT(0,  0))),
  (301, 3, 1, 1, -1, ST_MakeLine(ST_POINT(2,  0), ST_POINT(0,  2))),
  (203, 2, 3, 1,  1, ST_MakeLine(ST_POINT(2,  2), ST_POINT(2,  0))),
  (304, 3, 4, 1,  1, ST_MakeLine(ST_POINT(0,  0), ST_POINT(2,  0)));
INSERT 0 5
SELECT seq, source, target, cost, reverse_cost
FROM pgr_lineGraph(
  'SELECT id, source, target, cost, reverse_cost FROM edges_shared',
  true);
 seq | source | target | cost | reverse_cost
-----+--------+--------+------+--------------
   1 |   -304 |   -203 |    1 |            1
   2 |   -304 |    301 |    1 |           -1
   3 |   -203 |    203 |    1 |            1
   4 |    102 |    203 |    1 |           -1
   5 |    104 |   -304 |    1 |           -1
   6 |    203 |    301 |    1 |           -1
   7 |    301 |    102 |    1 |           -1
   8 |    301 |    104 |    1 |           -1
   9 |    304 |   -304 |    1 |            1
(9 rows)

Expected original ids

 seq | source | target | cost | reverse_cost
-----+--------+--------+------+--------------
   1 |    102 |    203 |    1 |           -1  
   2 |    104 |    304 |    1 |           -1    
   3 |    203 |    203 |    1 |            1
   4 |    203 |    301 |    1 |           -1
   5 |    203 |    304 |    1 |            1
   6 |    301 |    102 |    1 |           -1
   7 |    301 |    104 |    1 |           -1
   8 |    304 |    301 |    1 |           -1
   9 |    304 |    304 |    1 |            1
(9 rows)

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