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spectre_adjacent: optionally report dst_edge.

Previously, you'd ask this function 'What lies on the other side of
edge #i of this Spectre tile?' and it would tell you the identity of
another Spectre. Now it will also tell you which _edge_ of that
Spectre adjoins the specified edge of the input one. This will be used
in the extra spectre-test mode I'm about to add.

--- a/spectre-internal.h

+++ b/spectre-internal.h

@@ -243,7 +243,7 @@

  * delivering both plane and combinatorial coordinates as they go */

 Spectre *spectre_initial(SpectreContext *ctx);

 Spectre *spectre_adjacent(SpectreContext *ctx, const Spectre *src_spec,

-                          unsigned src_edge);

+                          unsigned src_edge, unsigned *dst_edge);

 /* For extracting the point coordinates */

 typedef struct Coord {

--- a/spectre.c

+++ b/spectre.c

@@ -154,7 +154,7 @@

 }

 Spectre *spectre_adjacent(SpectreContext *ctx, const Spectre *src_spec,

-                          unsigned src_edge)

+                          unsigned src_edge, unsigned *dst_edge_out)

 {

     unsigned dst_edge;

     Spectre *dst_spec = snew(Spectre);

@@ -162,6 +162,8 @@

     spectrectx_step(ctx, dst_spec->sc, src_edge, &dst_edge);

     spectre_place(dst_spec, src_spec->vertices[(src_edge+1) % 14],

                   src_spec->vertices[src_edge], dst_edge);

+    if (dst_edge_out)

+        *dst_edge_out = dst_edge;

     return dst_spec;

 }

@@ -454,7 +456,7 @@

         for (edge = 0; edge < 14; edge++) {

             Spectre *new_spec;

-            new_spec = spectre_adjacent(ctx, spec, edge);

+            new_spec = spectre_adjacent(ctx, spec, edge, NULL);

             if (find234(placed, new_spec, NULL)) {

                 spectre_free(new_spec);