PK_TOPOL_fctab_fin_fin_t

struct PK_TOPOL_fctab_fin_fin_s
        {
        int     *fin;
        int   length;
        };
typedef struct PK_TOPOL_fctab_fin_fin_s
               PK_TOPOL_fctab_fin_fin_t;

Used in:

PK_TOPOL_facet_r_t
PK_TOPOL_facet_table_t

This is a support structure for PK_TOPOL_facet_r_t and PK_TOPOL_facet_2_r_t.
It is an indexed table which maps fins onto co-fins. Fins without co-fins
(laminar edges on sheet bodies or edges that lie on loops that are being
ignored on one face) have their co-fin value set to -1. Fins with a single
co-fin share the same start & end vertices as their co-fin but in reversed
order.  This mapping is commutative.

A fin is represented by an ordered pair of vertices. Each fin follows an
anti-clockwise direction (from its tail to its head) consistent with
the ordering of fins in a facet or the boundary of a facet_strip. Each fin is
assigned an integer value from 0 to ('number_of_fins' - 1).

Each fin can be associated with a co-fin (one belonging to an adjacent facet
or facet strip).  In this case, the co-fin will have a value from 0 to
('number_of_fins' - 1).

If a fin lies on the perimeter of the facet mesh, it has no co-fin. A null
co-fin value of -1 is used to represent such a case.  -1 is also returned
for fins that lie on edges between facetted faces in the case where geometry or
trimmed matching have been requested.

A co-fin value of -2 will be used to represent cases where the associated
co-fin for a particular fin has not been found but where a co-fin should exist
(i.e. -1 was not returned in the 'fin_fin' table).  This may be the result of
facetting failing on a face (indicated by an entry in the 'error_object'
table).  Otherwise a co-fin value of -2 indicates that an error has occurred in
fin-fin matching and so the 'fin_fin' table is not correct.

When a general body is facetted using PK_TOPOL_facet_2 with 'match' set to
PK_facet_match_topol_c then the fin_fin table contains rings of fins around
non-manifold edges (those with more than two model fins).  A facet fin at a
non-manifold model edge will have as its co-fin a fin from a facet from
the mesh of another face which meets at that model edge.  This co-fin will in
turn have as its co-fin a fin of a facet from a mesh of yet another face which
meets at the model edge.  Eventually by following these entries in the fin_fin
table, the original fin will be found as the co-fin of a facet fin of a facet
from the mesh of some face which meets at the edge. Thus the fin-fin
relationship around a non-manifold model edge forms a ring.

The ordering of this ring is the same as the ordering of the model fins around
the model edge as returned by PK_EDGE_ask_fins.  Although model fins form a
ring around a model edge, and facet fins form a corresponding ring there is no
favoured start fin for these rings.  Thus although the ordering of the two
rings is the same it can not be assumed that the first fin encountered in the
fin_fin table from a ring of fins around a non-manifold edge is a fin from the
mesh of the face of the first model fin returned by PK_EDGE_ask_fins.  When
facetting a list of faces which does not contain all of the faces at a
non-manifold edge, the fin-fin table entries still form a ring around the edge;
these occur in the same order as if all the faces were being facetted, but the
fins from unfacetted faces are omitted.