AsTopoJSON
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## Name

AsTopoJSON — Returns the TopoJSON representation of a topogeometry.

## Synopsis

`text AsTopoJSON(`topogeometry tg, regclass edgeMapTable`)`;

## Description

Returns the TopoJSON representation of a topogeometry. If `edgeMapTable` is not null, it will be used as a lookup/storage mapping of edge identifiers to arc indices. This is to be able to allow for a compact "arcs" array in the final document.

The table, if given, is expected to have an "arc_id" field of type "serial" and an "edge_id" of type integer; the code will query the table for "edge_id" so it is recommended to add an index on that field.

Arc indices in the TopoJSON output are 0-based but they are 1-based in the "edgeMapTable" table.

A full TopoJSON document will be need to contain, in addition to the snippets returned by this function, the actual arcs plus some headers. See the TopoJSON specification.

Availability: 2.1.0

Enhanced: 2.2.1 added support for puntal inputs

## See Also

ST_AsGeoJSON

## Examples

    CREATE TEMP TABLE edgemap(arc_id serial, edge_id int unique);

    -- header
    SELECT '{ "type": "Topology", "transform": { "scale": [1,1], "translate": [0,0] }, "objects": {'

    -- objects
    UNION ALL SELECT '"' || feature_name || '": ' || AsTopoJSON(feature, 'edgemap')
    FROM features.big_parcels WHERE feature_name = 'P3P4';

    -- arcs
    WITH edges AS (
      SELECT m.arc_id, e.geom FROM edgemap m, city_data.edge e
      WHERE e.edge_id = m.edge_id
    ), points AS (
      SELECT arc_id, (st_dumppoints(geom)).* FROM edges
    ), compare AS (
      SELECT p2.arc_id,
             CASE WHEN p1.path IS NULL THEN p2.geom
                  ELSE ST_Translate(p2.geom, -ST_X(p1.geom), -ST_Y(p1.geom))
             END AS geom
      FROM points p2 LEFT OUTER JOIN points p1
      ON ( p1.arc_id = p2.arc_id AND p2.path[1] = p1.path[1]+1 )
      ORDER BY arc_id, p2.path
    ), arcsdump AS (
      SELECT arc_id, (regexp_matches( ST_AsGeoJSON(geom), '\[.*\]'))[1] as t
      FROM compare
    ), arcs AS (
      SELECT arc_id, '[' || array_to_string(array_agg(t), ',') || ']' as a FROM arcsdump
      GROUP BY arc_id
      ORDER BY arc_id
    )
    SELECT '}, "arcs": [' UNION ALL
    SELECT array_to_string(array_agg(a), E',\n') from arcs

    -- footer
    UNION ALL SELECT ']}'::text as t;

    -- Result:
    { "type": "Topology", "transform": { "scale": [1,1], "translate": [0,0] }, "objects": {
    "P3P4": { "type": "MultiPolygon", "arcs": [[[-1]],[[6,5,-5,-4,-3,1]]]}
    }, "arcs": [
     [[25,30],[6,0],[0,10],[-14,0],[0,-10],[8,0]],
     [[35,6],[0,8]],
     [[35,6],[12,0]],
     [[47,6],[0,8]],
     [[47,14],[0,8]],
     [[35,22],[12,0]],
     [[35,14],[0,8]]
     ]}

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