lwt_GetFaceEdges()

int lwt_GetFaceEdges	(	LWT_TOPOLOGY *	topo,
		LWT_ELEMID	face,
		LWT_ELEMID **	edges
	)

Return the list of directed edges bounding a face.

For ST_GetFaceEdges

Parameters

topo	the topology to operate on
face	identifier of the face
edges	will be set to an array of signed edge identifiers, will need to be released with lwfree

Returns

the number of edges in the edges array, or -1 on error (liblwgeom error handler will be invoked with error message)

Definition at line 3098 of file lwgeom_topo.c.

{
  LWGEOM *face;
  LWPOLY *facepoly;
  LWT_ISO_EDGE *edges;
  uint64_t numfaceedges;
  int fields;
  uint32_t i;
  int nseid = 0; /* number of signed edge ids */
  int prevseid;
  LWT_ELEMID *seid; /* signed edge ids */
 
  /* Get list of face edges */
  numfaceedges = 1;
  fields = LWT_COL_EDGE_EDGE_ID |
           LWT_COL_EDGE_GEOM |
           LWT_COL_EDGE_FACE_LEFT |
           LWT_COL_EDGE_FACE_RIGHT
           ;
  edges = lwt_be_getEdgeByFace( topo, &face_id, &numfaceedges, fields, NULL );
  if (numfaceedges == UINT64_MAX)
  {
          lwerror("Backend error: %s", lwt_be_lastErrorMessage(topo->be_iface));
          return -1;
  }
  if ( ! numfaceedges ) return 0; /* no edges in output */
  LWDEBUGF(1, "lwt_GetFaceEdges: lwt_be_getEdgeByFace returned %d edges", numfaceedges);
 
  /* order edges by occurrence in face */
 
  face = _lwt_FaceByEdges(topo, edges, numfaceedges);
  if ( ! face )
  {
    /* _lwt_FaceByEdges should have already invoked lwerror in this case */
    _lwt_release_edges(edges, numfaceedges);
    lwerror("Corrupted topology: unable to build geometry of face %"
      LWTFMT_ELEMID " from its %"PRIu64" edges", face_id, numfaceedges);
    return -1;
  }
 
  if ( lwgeom_is_empty(face) )
  {
    /* no edges in output */
    _lwt_release_edges(edges, numfaceedges);
    lwgeom_free(face);
    return 0;
  }
 
  /* force_lhr, if the face is not the universe */
  /* _lwt_FaceByEdges seems to guaranteed RHR */
  /* lwgeom_force_clockwise(face); */
  if ( face_id ) lwgeom_reverse_in_place(face);
 
#if 0
  {
  size_t sz;
  char *wkt = lwgeom_to_wkt(face, WKT_EXTENDED, 6, &sz);
  LWDEBUGF(1, "Geometry of face %" LWTFMT_ELEMID " is: %s",
              face_id, wkt);
  lwfree(wkt);
  }
#endif
 
  facepoly = lwgeom_as_lwpoly(face);
  if ( ! facepoly )
  {
    _lwt_release_edges(edges, numfaceedges);
    lwgeom_free(face);
    lwerror("Geometry of face %" LWTFMT_ELEMID " is not a polygon", face_id);
    return -1;
  }
 
  nseid = prevseid = 0;
  seid = lwalloc( sizeof(LWT_ELEMID) * numfaceedges );
 
  /* for each ring of the face polygon... */
  for ( i=0; i<facepoly->nrings; ++i )
  {
    const POINTARRAY *ring = facepoly->rings[i];
    int32_t j = 0;
    LWT_ISO_EDGE *nextedge;
    LWLINE *nextline;
 
    LWDEBUGF(1, "Ring %d has %d points", i, ring->npoints);
 
    while ( j < (int32_t) ring->npoints-1 )
    {
      LWDEBUGF(1, "Looking for edge covering ring %d from vertex %d",
                  i, j);
 
      int edgeno = _lwt_FindNextRingEdge(ring, j, edges, numfaceedges);
      if ( edgeno == -1 )
      {
        /* should never happen */
        _lwt_release_edges(edges, numfaceedges);
        lwgeom_free(face);
        lwfree(seid);
        lwerror("No edge (among %d) found to be defining geometry of face %"
                LWTFMT_ELEMID, numfaceedges, face_id);
        return -1;
      }
 
      nextedge = &(edges[edgeno]);
      nextline = nextedge->geom;
 
      LWDEBUGF(1, "Edge %" LWTFMT_ELEMID
                  " covers ring %d from vertex %d to %d",
                  nextedge->edge_id, i, j, j + nextline->points->npoints - 1);
 
#if 0
      {
      size_t sz;
      char *wkt = lwgeom_to_wkt(lwline_as_lwgeom(nextline), WKT_EXTENDED, 6, &sz);
      LWDEBUGF(1, "Edge %" LWTFMT_ELEMID " is %s",
                  nextedge->edge_id, wkt);
      lwfree(wkt);
      }
#endif
 
      j += nextline->points->npoints - 1;
 
      /* Add next edge to the output array */
      seid[nseid++] = nextedge->face_left == face_id ?
                          nextedge->edge_id :
                         -nextedge->edge_id;
 
      /* avoid checking again on next time turn */
      nextedge->face_left = nextedge->face_right = -1;
    }
 
    /* now "scroll" the list of edges so that the one
     * with smaller absolute edge_id is first */
    /* Range is: [prevseid, nseid) -- [inclusive, exclusive) */
    if ( (nseid - prevseid) > 1 )
    {{
      LWT_ELEMID minid = 0;
      int minidx = 0;
      LWDEBUGF(1, "Looking for smallest id among the %d edges "
                  "composing ring %d", (nseid-prevseid), i);
      for ( j=prevseid; j<nseid; ++j )
      {
        LWT_ELEMID id = llabs(seid[j]);
        LWDEBUGF(1, "Abs id of edge in pos %d is %" LWTFMT_ELEMID, j, id);
        if ( ! minid || id < minid )
        {
          minid = id;
          minidx = j;
        }
      }
      LWDEBUGF(1, "Smallest id is %" LWTFMT_ELEMID
                  " at position %d", minid, minidx);
      if ( minidx != prevseid )
      {
        _lwt_RotateElemidArray(seid, prevseid, nseid, minidx);
      }
    }}
 
    prevseid = nseid;
  }
 
  lwgeom_free(face);
  _lwt_release_edges(edges, numfaceedges);
 
  *out = seid;
  return nseid;
}

References _lwt_FaceByEdges(), _lwt_FindNextRingEdge(), _lwt_release_edges(), _lwt_RotateElemidArray(), LWT_TOPOLOGY_T::be_iface, LWT_ISO_EDGE::edge_id, LWT_ISO_EDGE::face_left, LWT_ISO_EDGE::face_right, LWT_ISO_EDGE::geom, lwalloc(), LWDEBUGF, lwerror(), lwfree(), lwgeom_as_lwpoly(), lwgeom_free(), lwgeom_is_empty(), lwgeom_reverse_in_place(), lwgeom_to_wkt(), lwline_as_lwgeom(), lwt_be_getEdgeByFace(), lwt_be_lastErrorMessage(), LWT_COL_EDGE_EDGE_ID, LWT_COL_EDGE_FACE_LEFT, LWT_COL_EDGE_FACE_RIGHT, LWT_COL_EDGE_GEOM, LWTFMT_ELEMID, POINTARRAY::npoints, LWPOLY::nrings, LWLINE::points, LWPOLY::rings, and WKT_EXTENDED.

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