[pgrouting-users] TRSP: rule vs via_path and other documentation

[Stephen Woodbridge]

Hi all,

For those people that are using or trying to use the new trsp function, 
I want to highlight the difference between rule and via_path.

I unfortunately implemented via_path to be a list of edges in the 
reverse order of rule.

IF for RULE:
target_id: 1, rule: 2,3,4,5

THEN for via_path
target_id: 1, via_path: 5,4,3,2

What I do in my turn restriction tables is to have both columns and if 
you already have a rule column here is some SQL to create a via_path column:

-- add a new column via_path
alter table my_turns add column via_path text;

-- need a helper function for the update
CREATE OR REPLACE FUNCTION array_reverse(anyarray)
   RETURNS anyarray AS
$BODY$
SELECT ARRAY(
     SELECT $1[i]
     FROM generate_series(
         array_lower($1,1),
         array_upper($1,1)
     ) AS s(i)
     ORDER BY i DESC
);
$BODY$
   LANGUAGE sql IMMUTABLE STRICT
   COST 100;

-- populate the via_path column
update my_turns set 
via_path=array_to_string(array_reverse(string_to_array(rule,',')),',');

One more common issue that is easy to get caught up in, is consistency 
of edge ids. When data is imported via shapefiles, you get a handy "gid" 
as the primary key and a lot of people use this to reference records. If 
you also load a restriction table or assemble it from your vendors data 
edges will typically be referred to be some edge_id that is NOT the 
"gid" the the shapefile loader dynamically adds when the data is loaded.

So if you vendor data uses a column "object_id" as the edge identifier, 
then it is likely that your restriction data will be defined in terms of 
"object_id" also. Therefore when you construct your query it needs to be 
something like:

select * from turn_restrict_shortest_path(
    'select object_id as id, source, target, ...',
    1234, -- start NODE_ID
    5678, -- end NODE_ID
    true,
    true,
    'select to_cost, object_id as target_id, via_path from my_turns');

I would also recommend the people use the following version of trsp 
because it can handle oneway conditions and restrictions at the start 
and end of the route that can not be detected using the above.

select * from turn_restrict_shortest_path(
    'select object_id as id, source, target, ...',
    1234, -- start EDGE_ID
    0.5,  -- position along edge
    5678, -- end EDGE_ID
    0.5,  -- position along edge
    true,
    true,
    'select to_cost, object_id as target_id, via_path from my_turns');

Notice that this uses EDGE_IDs and not NODE_IDs as the first does. Also 
position along the edge is a float from 0.0 to 1.0 where 0.0 is the 
start of the edge and 1.0 is the end of the edge. The postGIS linear 
referencing functions will return a value suitable for this by dropping 
a point on and edge and returning the percentage along that edge.

I hope this helps people use this new function.

Thanks,
   -Steve