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.

2 files changed, 5 insertions, 3 deletions

diff --git a/spectre-internal.h b/spectre-internal.h
index bbb2561..e43d30b 100644
--- a/spectre-internal.h
+++ b/spectre-internal.h
@@ -243,7 +243,7 @@ void spectrectx_step_hex(SpectreContext *ctx, SpectreCoords *sc,
  * 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 {
diff --git a/spectre.c b/spectre.c
index 2ad32ff..ba72304 100644
--- a/spectre.c
+++ b/spectre.c
@@ -154,7 +154,7 @@ Spectre *spectre_initial(SpectreContext *ctx)
 }
 
 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 @@ Spectre *spectre_adjacent(SpectreContext *ctx, const Spectre *src_spec,
     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 @@ void spectrectx_generate(SpectreContext *ctx,
         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);